Engine Pressure Indicating Handbook

Preface

Preface

Pressure indicating was used as long ago as in the development of steamengines and is therefore at least as old as the internal combustion engine itself.Whereas purely mechanical indicators were employed in the early days ofinternal combustion engines, today it is mainly piezoelectric transducers that areused to measure combustion chamber pressure and pressure curves in intake,exhaust and injection systems. Specially grown crystals are virtually the onlythings used nowadays as the piezo-materials (e.g. quartz, galliumorthophosphate, etc.). Apart from their excellent dynamic behaviour, their highmeasurement quality, their high stability and very good linearity, themetrological properties of these transducers above all are largely independentof temperature, which is of special importance in measurements on IC engines.The cyclic temperature drift in particular can be kept small with thesetransducers, which very often represents the decisive error factor in accuratethermodynamic observations of the engine phenomena. State-of-the artdevelopment methods, such as FE simulation especially, must also be used toensure low error using modern piezoelectric pressure transducers.

Not least because of the extensive possibilities of digital signal conditioning haspressure indicating become a standard development tool that, in addition toaccurate thermodynamic analysis, also permits wide-ranging evaluation on theengine test bed itself. Despite, or rather because, of this sophistication inindicating technology, users need highly specialised know-how. This manual isintended to provide such basic know-how and to be used as a reference work inpractical applications with piezoelectric engine instrumentation.

I wish you every success in your measurement tasks.

Graz, January 2002 O.Univ.-Prof. Dipl.-Ing. Dr.techn. Rudolf PischingerHead of the Institute for Combustion enginesand Thermodynamics, University of Technology Graz

Engine Indicating

Contents I

Contents

Preface I

1 Pressure Indicating in IC Engines 1-11.1 High Pressure Indicating 1-3

1.1.1 Indicating Parameters 1-31.1.2 Indicating Parameters Available on the Test Bed 1-41.1.3 Thermodynamic Analysis 1-5

1.2 Low Pressure Indicating 1-61.2.1 Gas Exchange Analysis 1-61.2.2 Development Work Based on Gas Exchange Analysis 1-7

2 The Piezoelectric Pressure Measurement System 2-12.1 The Piezoelectric Pressure Transducer 2-3

2.1.1 The Piezoelectric Measuring Principle 2-32.1.2 Piezoelectric Measuring Element Designs 2-42.1.3 Piezoelectric Materials for Using Pressure Transducers 2-7

2.1.3.1 Quartz (SiO2) 2-82.1.3.2 Gallium Orthophosphate (GaPO4) 2-92.1.3.3 Other Piezo Materials 2-11

2.1.4 Construction of Piezoelectric Pressure Transducers 2-122.1.4.1 Water-cooled Pressure Transducers 2-122.1.4.2 Uncooled Miniature Pressure Transducers 2-14

2.1.5 Pressure Transducer Cooling 2-142.1.5.1 Pressure Transducer Cooling Systems 2-152.1.5.2 Influences of the Cooling System on the Pressure Signal 2-15

2.2 Charge Amplifier 2-162.2.1 Operating Principle 2-162.2.2 Electrical Drift 2-172.2.3 Counteracting Electrical Drift 2-202.2.4 Electrical Filters 2-22

2.3 Measurement Cabling 2-232.4 Preventing Electrical Interference Signals 2-24

2.4.1 Ground Loop 2-242.4.2 Interference due to Electromagnetic Fields 2-25

3 Characteristics of the Piezoelectric Pressure Measurement System 3-13.1 Characteristics of Piezoelectric Pressure Transducers 3-1

3.1.1 Operating Conditions 3-13.1.2 Piezoelectric Pressure Transducer Parameters 3-3

3.1.2.1 Permissible Operating Conditions 3-3

II Engine Indicating

3.1.2.2 Transmission Behaviour 3-43.1.2.3 Temperature and Heat Flow Influences 3-73.1.2.4 Acceleration Influence 3-153.1.2.5 Deformation impact 3-163.1.2.6 Chemical Influence and Deposits 3-16

3.2 Properties of the Charge Amplifier 3-183.3 Properties of the Measurement Cabling 3-21

4 Selection of Piezoelectric Pressure Transducers 4-14.1 Measurement Task 4-3

4.1.1 Test Engine and Operating Conditions 4-34.1.2 Requirements of the Measurement Task 4-3

4.1.2.1 Accuracy 4-34.1.2.2 Type of Evaluation 4-54.1.2.3 Stability 4-64.1.2.4 Installation Effort and Costs 4-6

4.2 Influence of the Transducer Installation 4-74.2.1 Glow Plug/Spark Plug Adaptors 4-84.2.2 Installation Using a Suitable Installation Bore 4-8

4.2.2.1 Site of the Measuring Position in the Combustion Chamber 4-84.2.2.2 Design of the Measuring Position 4-124.2.2.3 Access to Measuring Point 4-17

4.3 Pressure Transducers 4-184.3.1 Categories 4-184.3.2 Measurement Properties 4-20

4.4 Guidelines for Pressure Transducer Selection 4-22

5 Installing Piezoelectric Pressure Transducers 5-15.1 Pressure Indicating with no Intervention in the Test Engine 5-1

5.1.1 Glow Plug Adaptation 5-15.1.2 Spark Plug Adaptation 5-2

5.2 Pressure Indicating with Intervention in the Test Engine 5-55.2.1 Installation Variations 5-5

5.2.1.1 Direct Installation 5-55.2.1.2 Installation Using Adaptor Sleeve 5-6

5.2.2 Instructions for Machining the Mounting Bore 5-95.2.2.1 Example of Direct Installation 5-95.2.2.2 General examples of Installation Using Adaptor Sleeves 5-125.2.2.3 Inclined bore axis single sealing sleeve 5-135.2.2.4 Perpendicular bore axis multiple sealing sleeves 5-15

6 Calibration 6-16.1 General 6-16.2 Type of Calibration 6-26.3 Calibration Using Dead Weight Tester 6-3

Contents III

6.3.1 Construction and Function of Dead Weight Testers 6-36.3.2 Interference during Calibration 6-5

6.4 Important Instructions 6-7

7 Zero-line Detection 7-17.1 Reference Method 7-1

7.1.1 Fixed Point (Empirical) 7-17.1.2 Measured Pressure Curve in the Intake Pipe 7-2

7.2 Thermodynamic Zero-line Coefficient 7-27.2.1 Assumption of a Constant Polytropic Exponent 7-27.2.2 With the Aid of the Integral of the Heat Release 7-37.2.3 Comparison of Measured and Calculated Pressure Curve 7-4

7.3 Comparison of Methods 7-5

8 Maintenance, Repair and Cleaning 8-18.1 Insulation resistance 8-1

8.1.1 Maintaining High Insulation Resistance 8-18.1.2 Restoring High Insulation Resistance 8-2

8.2 Descaling Cooling Water Galleries 8-28.3 Cleaning the Outside of Pressure Transducers 8-38.4 Repairing Pressure Transducers 8-4

9 Crank Angle Encoder 9-19.1 Digital Signal Recording 9-19.2 Function Principle and Construction of Crank Angle Encoders 9-4

9.2.1 Principles of Signal Generation 9-49.2.1.1 Optical Sensors 9-49.2.1.2 Inductive Sensors 9-4

9.2.2 Mounting Position of Crank Angle Encoders 9-59.2.3 Crank Angle Encoder Designs 9-79.2.4 Influence of Angle Encoder on Engine Behaviour 9-8

9.3 Crank Angle Encoder Characteristics 9-99.4 Crank Angle Errors and their Causes 9-119.5 Choice of Measuring Site and Mounting Instructions 9-169.6 Assigning Angle Mark Trigger to Engine TDC 9-17

9.6.1 Influence of TDC Errors on Evaluation Accuracy 9-179.6.2 Methods for TDC Determination 9-18

9.6.2.1 Static Top Dead Centre Determination 9-189.6.2.2 Top Dead Centre Determination Based on the pressure curve

of a motored engine 9-199.6.2.3 Mathematical TDC Determination 9-209.6.2.4 TDC Determination with TDC Sensor 9-219.6.2.5 Comparison of TDC Determination Methods 9-23

10 Trouble shooting 10-1

IV Engine Indicating

10.1 Pressure Transducer 10-410.2 Measurement Cabling 10-610.3 Charge Amplifier 10-710.4 Crank Angle Encoder 10-810.5 Data Acquisition and Evaluation (Indicating Equipment) 10-810.6 Calibration 10-8

11 LINK List 11-1

12 Bibliography 12-1

13 List of Figures 13-1

14 List of Tables 14-1

15 Index 15-1

Pressure Indicating in IC Engines 1-1

I1 Pressure Indicating in IC Engines

Before Nikolaus August Otto first put his engine into operation, he alreadycalculated the anticipated pressure chart and expected work based on the gaslaws. He was then later able to confirm them by measuring the cylinder pressureon his test engine. The mechanically recorded pressure traces were calledindicator charts. Figure 1-1 shows an indicator chart of the first four-strokeengine recorded on 18 May 1876.

Figure 1-1: Indicator chart recorded by Nikolaus August Otto (from Friedrich Sass:“Geschichte des deutschen Verbrennungsmotoren-baus von 1860-1918") [22]

Although engine instrumentation has changed considerably since then, theconcept of “engine indicating” has remained.

In the old days the term “engine indicating” was applied to the measurement ofin-cylinder pressure only. Today, however, the term is generally taken to meanthe measurement of crank angle-based parameters, such as in-cylinder pressure,

pressure in the intake and exhaust system, rapidly changing temperatures, andneedle lift and line pressure, to name but a few. The one thing all thesemeasurement parameters have in common is the fact that they create a directreference to the instantaneous position of the piston, which permits assignmentto the relevant cycle phase of the engine.

Indicator chart

Concept of “engineindicating”

Classic

Advanced

1-2

Thalloacc

Thcylco

Fig

Ovvalthesopmepocu

•

•

•

Subject of themanual

Engine Indicating

e special significance of indicating technology for engine developers is that itws excellent insight into the instantaneous events inside the engine at aneptable cost

e pressure inside the cylinder is still the central parameter that describes the in-inder phenomena. This manual therefore deals almost entirely withmbustion chamber pressure measurement.

Abstimmung AGR

Grenzwertüberwachung

Wirkungsgradbestimmung

Verbrennungsgeräusch

Aussetzererkennung

Energiebilanzen

automat. Kennfeldoptimierung

Reibungskennfeld

Klopferkennung

Schwingungsanregung

Einspritzverlaufuntersuchung

mechanische Beanspruchung

Verbrennungskennwerte

Restgasermittlung

ure 1-2: Application areas of indicating technology

er recent years, high and low pressure indicating has developed into auable, highly sophisticated analysis method for combustion optimisation. Both sensors used and the computer-supported data acquisition have reached ahistication today that not only allows us to use indicating as an operationalasurement technology but also satisfies the accuracy demands that make itssible to obtain extensive information from the analysis of measured pressurerves (see Figure 1-2).

Indicating is the development tool for quick and high-quality optimisation ofthe engine combustion sequence.

No other measurement procedure delivers so much information about thein-cylinder phenomena.

When applied properly, indicating is a reliable and repeatable measurementprocedure and can therefore be used as standard measurement technologyon development test beds. [20]

Tuning EGR

Limit value monitoring

Efficiency determination

Combustion noise

Misfire detection

Engergy balance

Automatic eng. mapping Knock detection

Vibration excitation

Injection analysis

Mechanical stress

Combustion parameters

Friction mapping Residual gas verification


I

1.1 High Pressure Indicating

High pressure indicating measures the combustion chamber pressure in thecylinder of an IC engine, see Figure 1-3. The measurement range generallyincludes the complete engine cycle but can also be restricted to a limited crankangle range depending on the measurement task.

-180UT ZOT

Kurbelwinkel [°KW]

Dru

ck [b

ar]

UT LOT UT-90 0

0

10

20

30

40

50

60

70

90 180 270 360 450 540

Figure 1-3: Cylinder pressure curve over an engine cycle

High pressure indicating in the combustion chamber is usually carried out withpiezoelectric pressure transducers, which are either installed in the combustionchamber direct or in a spark or glow plug adaptor.

1.1.1 Indicating Parameters

The analysis of pressure curves measured in the combustion chamber allows usto make various comprehensive judgements of the in-cylinder phenomena. Awhole range of important data can be calculated on the basis of the measuredpressure curve. These results are generally known as characteristic indicatingparameters, which can basically be divided into two categories:

• direct and

• indirect indicating parameters.

Direct indicating parameters are determined straight from the curve of thecylinder pressure (p) over the cycle.

Direct indicatingparameters

BDC

Pres

sure

[bar

]

Crank Angle [°CA]

BDC BDCGasexchange TDCIgnition TDC

1-4 Engine Indicating

Indirect indicating parameters are values that need other parameters (e.g.braking torque, crank gear geometry, etc.) for calculation in addition to the basicpressure curve. Before the start of combustion can be calculated, for example,the heat release (burn rate) curve first has to be calculated from the pressurecurve. Table 1-1 shows a selection of direct and indirect indicating parameters.

Table 1-1: Indicating parameters

Direct indicating parameters Indirect indicating parameters

- Peak pressure pmax - Indicated mean effective pressures IMEPIMEPHP, IMEPGE

- Position of peak pressure αpmax - Friction mean effective pressure FMEP

- Pressure rise dp/dα - Start of combustion

- Position of max. pressure rise αdp/dαmax - Duration of combustion

- Speed of the pressure rise dp/dα2 - Energy conversion- Mass burned fractions- Combustion noise

The indicating parameters determined from high pressure indicating provide abasis for direct assessment of the engine.

1.1.2 Indicating Parameters Available on the Test Bed

State-of-the-art indicating systems are now able to calculate certain indicatingparameters in real-time (i.e. during the engine cycle). Such values are thenavailable for closed loop control of the next cycle (e.g. knock control and misfiredetection).[5].

Other indicating parameters will be calculated after the measurement hasfinished so that the engine developer can access them together with othermeasurement parameters on the test bed data acquisition system (i.e. on-line).That means that it is now possible to set up comprehensive automatic engineoptimisation for

• Enleanment tuning

• Exhaust gas recirculation tuning

• Combustion noise

• etc.

Indirect indicatingparameters

Real-time/closedloop controlOn-line/fast

optimisation


I

1.1.3 Thermodynamic Analysis

One very comprehensive and computer-intensive application area for highpressure indicating is thermodynamic analysis of the pressure curve. It is basedon the calculation of the rate of heat release which describes the instantaneousreleased heat energy from the chemical energy of the fuel per degree C.A.. Thisallows important conclusions about the process of combustion.

There is a whole range of calculation models for thermodynamic analysis.Today’s evaluation programs usually offer numerous options. Important results ofthermodynamic analysis are:

• Efficiency

• Rate of heat release and thus ignition delay, start of combustion, duration ofcombustion and mass burned fractions

• Gas condition and thus the basis for pollutant formation

For a detailed description of the basic equations required for thermodynamicanalysis, see for example [21].


1.2 Low Pressure Indicating

Low pressure indicating is generally used to describe the crank angle-relatedacquisition of pressures in the intake manifold and exhaust pipe of IC engines,see Figure 1-4. As in high pressure indicating, the measurement is usually carriedout over a complete cycle. In some cases, the term low pressure indicating isalso used for measurements of the low pressure range of the combustionchamber pressure with a special transducer of adaptors.

-180UT ZOT

Kurbelwinkel [°KW]

Brennraum

Auspuff

Saugrohr

Dru

ck [b

ar]

UT LOT UT-90 0

0

1

2

3

4

5

6

90 180 270 360 450 540

Figure 1-4: Low pressure curves over one cycle

1.2.1 Gas Exchange Analysis

The base parameters for gas exchange analysis are

• the measured pressure curves in the intake manifold, combustion chamberand exhaust

• the exact valve lift curves and

• flow coefficients.

It is important for gas exchange analysis that low pressure indicating is carriedout together with high pressure indicating. In other words, three pressures aremeasured simultaneously on each cylinder: the intake manifold pressure, thecombustion chamber pressure and the exhaust pressure. Only with such aconfiguration can complete heat release and gas exchange analysis of the enginebe carried out.

Crank Angle [°CA]

Pres

sure

[bar

]Combustion chamber

Exhaust

Intake

BDC Ignition TDC Gasechange TDC BDCBDC


I

1.2.2 Development Work Based on Gas Exchange Analysis

The gas exchange analysis is used as the basis for:

• the design of the gas exchange devices (intake manifold, exhaust),

• the design of the control devices (valve timing, cam shapes),

• the assessment of the gas exchange work, and

• the analysis of the intake and outlet mass flows (charge, residual gas,backflows).

If the gas-dynamic processes in the “intake manifold/combustionchamber/exhaust” system are not taken into consideration by the calculationroutines used for the gas exchange analysis, it is important for the low pressuretransducers to be placed as close as possible to the valves.

Piezoresistive and piezoelectric pressure transducers are used for low pressureindicating. [13]


The Piezoelectric Pressure Measurement System 2-1

2

2 The Piezoelectric Pressure Measurement System

As Figure 2-1 shows, the piezoelectric pressure measurement system basicallyconsists of the following components:

Verbrennungs-motor

Ladungsverstärker

Druckaufnehmer

Meßverkabelung

RAM

Daten-erfassung

AuswertungDarstellung

Winkelaufnehmer

A/D

CD

M-C

Druckaufnehmer-kühlung

2222

2222

IndiziergerätAnzeige

Figure 2-1: Structural diagram of the piezoelectric pressure measurement system withadditional devices

• Piezoelectric pressure transducer

Piezoelectric pressure transducers work on the principle of electrostaticcharge output of certain crystals under mechanical load. They thereforerepresent an active measuring element with the output charge beingproportional to the load, i.e. to the pressure applied.

• Charge amplifier

The charge generated by a piezoelectric pressure transducer is convertedto a voltage signal by means of a charge amplifier. The signal is then fed tothe data acquisition and evaluation equipment (Indicating System) forfurther processing.

• Measurement cabling

The measurement cabling is used to transmit the charge and voltagesignals. Due to the low electrical charge output of pressure transducers, theconnection between the transducer and the charge amplifier is of criticalimportance.

Transducercooling

Charge amplifier

Analogue Display Indicating system

Dataacquisition evaluation

Transducer

Combustionengine

Measurementcables

Angle Encoder


Very high insulation values and low noise are required both for themeasurement cabling and the charge amplifier.

The following equipment is also required before measurements can be carriedout on the IC engine:

• Crank angle encoder

A crank angle encoder mounted on the crankshaft of an IC engine deliversthe time/angle basis for crank angle-related acquisition of the pressurecurve.

• Indicating equipment

The analogue output voltage signal of the charge amplifier is digitized andrecorded by means of a so called indicating equipment. Indicatingequipment comprises at least an Analogue/Digital converter (ADC), a dataacquisition unit based on the crank angle (CAM-C Crank Angle MarkerControl Unit) and a fast memory for temporary storage of the raw data.The data is taken from the memory and the indicating parameterscalculated, stored, and displayed by means of a computer that, in somesystems, is already integrated.

Other equipment that may be required includes:

• Pressure transducer cooling

Water-cooled pressure transducers are often used for precision pressuremeasurements. They have to be continuously cooled during operation andtherefore require an appropriate cooling system to be installed.

• Display

The measurement signals can be visually checked on the indicatingequipment itself or, if the indicating equipment has no display, via anoscilloscope.

Chapter 9

AVL ProductInformation:

[9]Indicating Technology


2

2.1 The Piezoelectric Pressure Transducer

Piezoelectric pressure transducers are characterized by the fact that theirtransducer element is made of a piezoelectric material whereby the pressure istransmitted to it via a diaphragm. Piezoelectric pressure transducers areeminently suitable for dynamic measurements. Because of their inherent workingprinciple, however, they cannot be used for static pressures.

2.1.1 The Piezoelectric Measuring Principle

Generally speaking, piezoelectricity denotes an interaction between themechanical and the electrical state in certain types of crystal [16], [23]. As showninFigure 2-2, a distinction is made between the reciprocal and the directpiezoelectric effect relevant to the pressure measurement.

Piezoelectricity: in 1880 Pierre and Jacques Curie discovered first of all the directpiezoelectric effect on tourmaline crystals. They established that the pressure appliedin certain directions to opposing crystal faces produces reverse-poled electric chargeson the surfaces, which are proportional to the applied pressure. Later they found thesame effect on quartz and other asymmetric crystals. The reciprocal electrical effectwas first predicted by Lippmann based on thermodynamic considerations and alsolater discovered by the Curie brothers in experiments.

Direct piezoelectric effect Reciprocal piezoelectric effect

Description A mechanical deformation of apiezoelectric body causes a changein the electric polarisation that isproportional to the deformation.

++++++++++++++++++

F

F

- - - - - - - - - - - - - - - - - -

An external electrical Field Ecauses mechanical stresses

proportional to the field, whichalter the size of the piezo-crystal.

EE

Application For measuring mechanicalparameters, especially of forces,

pressures and accelerations

In ultrasonic andtelecommunications engineering

Figure 2-2: Piezoelectric effects


The phenomenon of the direct piezoelectric effect can described for theelectrically free state of a piezo-crystal (achieved in experiments most easily byshort-circuiting the transducers electrodes) by Equation 2-1.

Di = diµµµµ . Tµµµµ (2-1)

Di (i = 1 to 3) Vector of the electric flow density

diµ Tensor of the piezoelectric coefficient according to Equation 2-2

diµ =

d d d d d dd d d d d dd d d d d d

11 12 13 14 15 16

21 22 23 24 25 26

31 32 33 34 35 36

(2-2)

Tµ (µ = 1 to 6) Tensor of the mechanical stresses (with T1 to T3 for normalstresses σx, σy, σz and T4 to T6 for tangential stresses τyz, τxz and τxy)

Each piezoelectric coefficient (dIµ) determines the relationship of a specific stresstensor coordinate (Tµ) with a specific vector coordinate of the electrical flowdensity.

Equation 2-3 then applies to charge output (Q) of the face of the crystal elementcovered by the electrodes.

Q = A . Di . ni (2-3)

A face area

ni (i = 1 to 3) components of the normal vector of the face

2.1.2 Piezoelectric Measuring Element Designs

We make a distinction between different types of piezoelectric effect dependingon the direction of the piezoelectric polarisation in relation to the direction of theapplied force. The longitudinal and transversal effects are primary used forpressure transducers.

Flow density

Charge output


2

• Longitudinal Effect

The measuring elements are usually disc-shaped and the charge outputoccurs on the face itself where the force is applied, see Figure 2-3. If thecrystallographic x-axis and the direction in which the force is applied arethe same, the charge output is as shown in Equation 2-4 assuming auniaxial stress condition.

Q = A . d11 . σx = A . d11 . FA

= d11 . F (2-4)

Figure 2-3: Measuring element for thelongitudinal effect

The charge output resulting from the longitudinal effect does not thereforedepend on the geometry of the measuring element but purely on the force(F) applied.

Several discs can be connected force-wise in series and electrically parallelto increase the charge output (sensitivity), see Figure 2-4. One advantageof this design is that it permits a compact and resistant measuring element.Not only that but the charge is output straight from the pressed faces sothat contact problems in the charge pick-up are virtually excluded.

Figure 2-4: Increasing the charge output with thelongitudinal effect

F

+ + + + + + + + + +

- - - - - - - - - -

+ + + + + + + + + +

- - - - - - - - - -

+ + + + + + + + + +

- - - - - - - - - -

+ + + + + + + + + +

- - - - - - - - - -

FQ+ Q-

F

F

r

x

Charge output


• Transversal effect

The measuring elements are bar-shaped and the charge is outputperpendicular to the faces to which the force is applied, see Figure 2-5.Assuming a uniaxial stress condition, the charge output can be determinedusing Equation 2-5 for a purely transversal cut (i.e. the longitudinal axis ofthe bar lies in exactly the direction of the crystallographic y-axis, one sideface is perpendicular to the z-axis and the charge is picked-up on the facesperpendicular to the x-axis).

Q = A . d12 . σy = l . b . d12 . F

a . b = d12 . F . l

a(2-5)

Figure 2-5: Measuring element for the transversal effect

In addition to the electrical properties of the piezo material used, thecharge yield is determined in particular by the thinness of the measuringelement (l/a). With a favourable edge ratio (l/a), greater polarisationcharges can be achieved with the transversal effect but the dimensionaldesign constitutes a practical limit due to the limited mechanical strengthof the piezo material.

x

y

z

l

a

bF F

Charge output


22.1.3 Piezoelectric Materials for Using Pressure Transducers

Materials used for piezoelectric measuring elements (i.e. piezo materials) inpressure transducers must above all have the following properties:

• Good measuring behaviour

High output signal

The piezoelectric sensitivity, which is determined by piezoelectriccoefficient (diµ), is decisive for the amplitude of the output signal. Thecoefficients relevant to the generation of the measurement signal in thetransducer should be as large as possible.

Good linearity

There should be the best linear ratio possible between themeasurement parameter (i.e. pressure) and the charge output.

High natural frequency

High natural frequencies require high mechanical rigidity.

Good insulation

A high electrical insulation resistance is essential for low electrical driftand allows similar-to-static measuring with piezoelectric transducers.

• Good resistance

High mechanical strength

High mechanical strength is essential when large forces, pressures andaccelerations are to be measured and for the transducer to be resilientto mechanical impact.

High temperature resistance

• Stability of the measuring properties and their immunity againstexternal influences

Temperature and mechanical load

Achieving a piezoelectric sensitivity that does not depend ontemperature and mechanical load is a very difficult task. The choice ofmaterial and the orientation of the piezoelectric elements in relation to

Chapter 2.2.2


the crystallographic axes can help to find the solution. Basically nopyroelectric effect should occur with the piezo materials used.

Pyroelectric effect: denotes the effect of the charge output of piezo materialsunder the influence of temperature.

• Low price

Low material costs

Easy machining

2.1.3.1 Quartz (SiO2)

The classic piezoelectric material for pressure transducers is quartz (SiO2),see Figure 2-6. Quartz occurs in several modifications based on silicium-

oxygen-tetrahedons. Its low temperature modificationknown as α-quartz (lowquartz), which occurs below573 °C is used for piezoelectric applications. When thetemperature is increased above 573 °C a phasechange occurs and the resultant modification is calledβ-quartz (highquartz).

Figure 2-6: Quartz crystal

Because of the unavoidable flaws and impurities in naturally grown quartz,nowadays we only use cultured quartzes (using hydrothermal synthesis) toobtain consistently high quality.

Hydrothermal synthesis: using this process quartz crystals are created in thick-walled steel autoclaves at pressures between 0.3 and 1.3 kbar and temperaturesof around 400 °C. Water with small additives of Na2CO3 or NaOH is used as thesolvent. The material is transported primarily by convection. Large quartzcrystals with a mass of more than 1kg take several weeks to grow.

Temperature significantly influences the piezoelectric properties of quartz.Figure 2-8 shows piezoelectric constant (d11) as a function of thetemperature. (d11) already starts to significantly decrease in the mostfrequently used temperature range up to about 250 °C and finallydisappears completely at the conversion temperature of 573 °C. In

z

x

y


2

addition, the load limit decreases with rising temperature due totwin formation.

Twin formation: in α-quartz so called Dauphiné twins (i.e. secondary twin formation) canform at high loads in certain areas of the crystal where the mathematical sign of thepiezoelectric coefficients sometimes changes resulting in lower sensitivity. Whereastwins form at room temperature only at pressures of approx. 5.108 to 9.108 Pa, as thetemperature increases, twin formation starts to occur at lower loads, and twinformation can be observed even in unloaded quartz just below the conversiontemperature of 573 °C. It has been found that twins, which formed under load cancompletely disappear again when the load is removed. But if the mechanical load isapplied for a longer period, stable twins can also develop which result in a permanentreduction of the piezoelectric sensitivity.

Conventional measuring elements made of quartz can therefore only be used upto temperatures of about 200 to 250 °C, which means that the measuringelement requires appropriate cooling for applications in IC engines where tempe-ratures much higher than 400 °C can occur at the measuring position.

To improve the thermal behaviour with the transversal effect may be utilized (bycontrast to the longitudinal effect) by defining the cut of the crystal in such a waythat the effective piezoelectric coefficient remains relatively independent of thetemperature within a certain temperature range. At the same time, such crystalcuts do not usually have a strong tendency to form twins. They can therefore beused to make measuring elements suitable for temperatures up to approx.350 °C.

2.1.3.2 Gallium Orthophosphate (GaPO4)

In recent years the piezo material Gallium Orthophosphate (GaPO4) has beendeveloped especially for high temperature applications. It has distinguished itselfin particular with its high piezoelectric sensitivity that is largely independent oftemperature.

The crystal structure of Gallium Orthophosphate can be derived from α-quartz byreplacing silicium alternatively with gallium and phosphorus, see Figure 2-7:Crystal structure of Gallium Orthophosphate. α-Gallium Orthophosphate is stableup to a temperature of 933 °C and above that changes into the high cristobalitetype.

Thermally stablecut quartz

AVL ProductInformation:[7]GalliumOrthophosphate(GaPO4)

2-10

Figu

Ga

Engine Indicating

Figure 2-7: Crystal structure of Gallium Orthophosphate

re 2-8: Temperature dependency of piezoelectric constant (d11) for quartz and GalliumOrthophosphate

α-Quartz β-Quartz

α-GaPO4 Highcristobalit

Quartz

lliumortho-phosphat

573 870

933Temperature [°C]

200 400 600 800 1000 1200

6

5

4

3

2

1

0

Galliumortophosphate

Quartz

d 11[

pC/N

]

Tridymit


2

The following properties represent the main advantages of GalliumOrthophosphate

• temperature-resistant up to more than 900 °C

• almost twice the sensitivity of quartz which remains virtually unchanged upto far in excess of 500 °C (see Figure 2-8)

• high electrical insulation resistance up to high temperatures

• stable against stress-induced twin formation

• no pyroelectric effect

The excellent thermal behaviour and high sensitivity of Gallium Orthophosphatehave made great advances over quartz possible especially when buildinguncooled miniature pressure transducers [24].

2.1.3.3 Other Piezo Materials

In addition to quartz and Gallium Orthophosphate, there is a whole range ofother piezo materials that, however, have various disadvantages that make themonly suitable to a certain extent for use in piezoelectric pressure measurements.They include:

• Tourmaline

Tourmaline is an aluminium borosilicate. Tourmaline crystals have theadvantage that they cannot form twins, but have a relatively strong pyro-electric effect.

• Langasite

The disadvantage of langasite is that its piezoelectric sensitivity dependsrelatively heavily on the temperature.

• Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3)

The disadvantage of lithium niobate and lithium tantalate is above all thefact that the electrical resistance decreases with rising temperature.

• Piezoceramics (barium titanate, etc.)

In piezoceramics the material properties also depend to a relatively largeextent on the temperature.

AVL ProductInformation: [6]Pressure transducersfor engineinstrumentation


2.1.4 Construction of Piezoelectric Pressure Transducers

2.1.4.1 Water-cooled Pressure Transducers

A wide range of piezoelectric pressure transducer designs is available for use inIC engines. We will illustrate the basic construction first by using the example ofa water-cooled quartz pressure transducer using the longitudinal effect, shownin Figure 2-9.

Figure 2-9: Construction of a piezoelectric pressure transducer based on the longitudinaleffect (from AVL) – Mounting thread M14x1.25

The pressure (p) to be measured acts via a pliable diaphragm and a short, rigidpressure plate on the piezoelectric measuring element. The function of thepressure plate is to create as even a mechanical stress state as possible in themeasuring element. The compensation disc between the measuring element andpressure plate compensates for differences caused by thermal expansion. Themeasuring element and the diaphragm are surrounded by a water jacket and


[6] Pressure transducers

for engineinstrumentation

Diaphragm

Pressure

Pressure Plate

Compensation Disk

Measuring element

Insulator

Connector

Electrode

Cooling water nipples

-


2

intensively cooled during operation. That ensures that the measuring elementonly becomes a little warmer than the cooling water when used in the engine(typically up to about 10°C, in extreme cases 20 °C above the cooling watertemperature).

To increase the charge output the measuring elements usually comprise severaldisc-shaped elements. Each disc (here made of quartz) is coated with metal sothat the electrical parallel connection is guaranteed by bridges with contacttongues and insulation zones (Figure 2-9 right).

The pressure transducer housing is electrically connected to the positiveelectrode of the measuring element and thus represents the electrical ground.For the electric charge output, the negativeelectrode is connected to a connector,which is highly insulated against thehousing.

Figure 2-10 shows by comparison a cross-section of a water-cooled quartz pressuretransducer based on the transversal effect.Since with the transversal effect, thepolarisation charges occur on the unloadedlateral faces, those faces are fitted withvapour-deposited electrodes contacted bya spiral spring.

Figure 2-10: Construction of piezoelectric pressure transducers based on the transversaleffect (from Kistler) – Mounting thread M14x1.25

Chapter 2.1.2

Measuring Element

Coolingwaternipples

Electrode

Diaphragm


2.1.4.2 Uncooled Miniature Pressure Transducers

With the compact designs of modern IC engines the use of multiple valvetechnology, less and less installation space is available and miniature pressuretransducers are now playing a more and more significant role in engineindicating. These transducers can also be installed on test engines withoutrequiring any mechanical interventions (i.e. spark or glow plug adaptors). Figure2-11 shows the construction of such a miniature pressure transducer (transversaleffect). Miniature pressure transducers usually have to manage without water-cooling which places extremely high demands on the piezo materials (i.e. use ofGaPO4) and on thetransducer design.

Figure 2-11: Uncooledminiature pressure transducer (from AVL) – Mounting thread M5x0.5

2.1.5 Pressure Transducer Cooling

Generally speaking, cooling increases the stability and heat flow load-bearingcapability of pressure transducers. The direct water cooling of the diaphragm andmeasuring element mean the following advantages for the pressure transducer:

• Overheating of the measuring element is avoided

• The temperature has less influence on the sensitivity (quartz) and thermaldrift

• The insulation resistance is not decreased by high temperatures

• Direct water cooling permits the pressure transducer to be installed flushwith the combustion chamber even in measurement positions subject tohigh thermal load

Chapter 5

Chapter 3.1.2.3

Measuring element


2

2.1.5.1 Pressure Transducer Cooling Systems

Pressure transducer cooling systems usually have a closed cooling circuit with atank, pump and return cooling. The pressure transducer should be cooled withdistilled or de-ionised water because deposits can form in the transducer ifcalcium-containing water is used which can result in blocked cooling channels.The latest cooling systems have temperature control and flow rate monitoring.

2.1.5.2 Influences of the Cooling System on the Pressure Signal

It is important that the cooling of the pressure transducer is constant and freefrom pulsations, i.e. that no vibrations are transmitted to the cooling mediumeither from the coolant pump itself or from external devices (e.g. movement ofpumps, hoses, etc.). Any change in the cooling water pressure is superimposedon the measuring signal. The term cooling water crosstalk is often applied towater-cooled pressure transducers in this context, which is a measure of thechange in the pressure transducer’s output signal as a function of fluctuation inthe cooling water pressure.

AVL ProductInformation:[6]Pressure transducersfor engineinstrumentation

Chapter 3.1.2.2


2.2 Charge Amplifier

The charge output from the pressure transducer is converted to a voltage signalin the charge amplifier (see Figure 2-12).

Figure 2-12: Charge amplifier (e.g. from AVL)

2.2.1 Operating Principle

As can be seen from the circuit diagram shown in Figure 2-13, a chargeamplifier basically consists of an amplifier (V) with very high inner voltagegain and a negative feedback capacitor (CG).

V

DA

CG

CK

UE

RG

R ISO

Reset

Ladungsverstärker

Short

Long

UA

IEI

IIso

Figure 2-13: Circuit diagram of a charge amplifier

Charge Amplifier

PT CC

II

UI

A

VO

CF

RF

RINS

IINS


2

When a charge is delivered from a piezoelectric pressure transducer (PT), there isa slight voltage increase at the input of the amplifier (A). This increase appears atthe output substantially amplified and negative – in other words, the outputvoltage has a negative polarity as opposed to the input voltage. The thusnegatively biased negative feedback capacitor (CF) correspondingly taps chargefrom the input and therefore keeps the voltage rise small at the amplifier input.

At the output of the amplifier (A) precisely the voltage (VO) sets itself that picksup enough charge through the capacitor to allow the remaining input voltageresults in exactly (VO) when amplified by (A).

Because the gain factor of A is very large (up to about 100 000), the inputvoltage (VI) remains virtually zero. The charge output from the pressuretransducer is not used to charge, i.e. to increase the voltage at the inputcapacitances, but is drawn off by the feedback capacitor.

Changes in the input capacitance - e.g. due to different cables with differentcable capacitance (CC) – therefore have virtually no effect on the measurementresult.

The output voltage (VO) of the amplifier is directly proportional to the chargeoutput (Q) of the transducer and inversely proportional to the capacitance of thenegative feedback capacitor (CF,), see Interrelationship 2-7:

VO ∼∼∼∼ - Q / CF (2-7)

By activating appropriate capacitance values many measurement ranges (RANGEsetting) may be realized.

2.2.2 Electrical Drift

If the measurement parameter stays constant, i.e. if the pressure on thetransducer stays constant, we would expect a constant voltage at the amplifieroutput. In the piezoelectric measurement system, however, there is always aninherent drift in the output signal due to the working principle of the system. Ifthe cause of the drift is electrical, we call it electrical drift (see also Temperaturedrift).

Chapter 3.1.2.3

2-18

Reasons for electrical drift are basically:

• Discharge of the negative feedback capacitor

The full output voltage (VO) is applied to the negative feedback capacitor.A leakage current is caused due to the only finitely high insulation value ofthe capacitor, which results in the gradual discharge of the capacitor, seeFigure 2-14 which shows the discharge curve of a capacitor. The dischargeprocess is determined by the time constant (τ).

The time constant is an important yardstick for assessing the capability of apie oelectric measurement system for so-called similar-to-static measuring,i.esig

Timerdth

T(Ris(Cc

Figure 2

• Inp

A inpvothe

z

Engine Indicating

. for permitting the measurement of very slow phenomena without anynificant errors due to the discharge of the capacitor.

constant: denotes the characteristic time period for an RC unit (i.e. combination ofesistor - capacitor) in which the capacitor would just be fully discharged if theischarge current stayed at its initial value (see Figure 2-14). In practice the voltage athe capacitor is still always the eth part of the initial value after the time constant (τ)as expired.

he time constant can be determined mathematically from the product of resistance) responsible for the discharge and the capacitor capacitance (C). The time constant 1000 s for typical values for the insulation resistance of 1013 Ω and a capacitance) of 100 pF. The time constant can be increased to 10000 s at the same capacitor

apacitance (C) by increasing the insulation resistance to a value of 1014 Ω.

-14: Definition of the time constant when discharging a capacitor

ut offset voltage

certain input offset voltage (VE) and an input current (II) occur at theut of the amplifier (A) due to its non-ideal properties. The input offset

ltage also acts on the terminal resistor (RINS) (see Figure 2-13) and causes insulation current (IINS). The total current (I) made up of the input

U

~0.37

U0

U0

tU

t.τt= =R C t

V

VO

~0.37VO

Vt


2

current (I) and insulation current (IINS) causes a drift in the output voltagesignal. The result can be an inflow or outflow of charges depending on thepolarity of the input offset voltage.

The consequence of the individual drift effects can best be illustrated by meansof a simple pressure jump, see Figure 2-15, top. In an ideal situation, the curve ofthe voltage at the amplifier output labelled would be produced in response tothe pressure jump, which represents an analogue mapping of the pressure curve.

However, the discharge process of the negative feedback capacitor that is due tothe insulation resistance not being infinite, means that the result is a drift curvethat looks like curve . The voltage at the output drops towards zero inaccordance with an exponential function.

Figure 2-15: Electrical drift effect

Linear drift curves like curves and occur when input currents at the amplifierinput and/or leakage currents flow through the terminal insulation resistors of thecable and/or pressure transducer due to an input offset voltage. Without suitablecountermeasures, such as Short mode, drift compensation etc. (see Chapter2.2.3), the output voltage signal will drift in both cases until the amplifier issaturated (see for example).

Chapter 2.2.3

Endladung CG

Input offset > 0

Input offset < 0

Ideal

Input offset < 0and RG (Short)

U

p

t

Sättigung

t

6

1

2

A5

3

4Saturation

Input-offset <0Input-offset <0and RF (Short)

Input-Offset >0Discharge CF

Ideal

VO


The output voltage can then only be returned to zero by closing the reset switch(see Figure 2-13) that effectively discharges the negative feedback capacitor.

In practice, the drift effects described above rarely occur on their own but areusually superimposed on one another.

2.2.3 Counteracting Electrical Drift

Basically the following measures can be taken to counter electrical drift:

• High insulation values

Both the amplifier input and the pressure transducer and measurementcable including the connectors must be highly insulated (insulation valuesat room temperature in the order of 1013 Ω) otherwise unacceptably highleakage currents can flow and it is impossible to obtain meaningfulmeasurement results.

• SHORT operating mode

By connecting an additional resistor for negative feedback in parallel (RF)(SHORT mode - see Figure 2-13), the drift due to input offset can berestricted to a certain value and drifting into saturation can be prevented,see curve in Figure 2-15. This means that measurements are alsopossible in SHORT mode even when the insulation is not so good. It isusually only suitable, however, for monitoring purposes because thepressure signal is phase-offset due to the resultant dramatically reducedtime constant depending on speed and RANGE setting and has a smalleramplitude.

To illustrate this influence, Figure 2-16 shows the pressure differentialcurves determined using a reference transducer on a test engine at aspeed of 2000 rpm. The signal from the transducer being tested was firstacquired in SHORT mode and then in LONG mode - i.e. withoutconnecting the negative feedback resistor, under the same load conditions.Measurements in SHORT mode cause errors similar to those due to cyclictemperature drift. Precision measurements are thus only possible in LONGmode and when maintaining the required insulation values.

Source of errors

Chapter 3.1.2.3


2

-90

-90

ZOT

ZOT

UT

UT

Kurbelwinkel [°KW]

Zylinderdruck [bar]

Druckdifferenz [bar]

LONG

SHORT

-60

-60

-30

-30

0

0

0

-0.8

-0.4

10

20

30

0

0.4

40

50

60

30

30

60

60

90

90

120

120

150

150

180

180

210

210

240

240

270

270

Figure 2-16: Typical effect of SHORT mode on the measurement result at low speed andwith low transducer sensitivity

• Drift compensation

Some charge amplifiers on the market have an activatable electronic driftcompensation. One possible type of drift compensation is implemented byfeeding a compensation current to the input of the operation amplifierwhich is as large as the current discharged via the insulation resistors.

The drift compensation must be deactivated during calibration of thepiezoelectric pressure measurement system.

AVL ProductInformation:[11]3066A02 PiezoAmplifier

Source of errors

Crank Angle [°CA]

Cylinder pressure [bar]

Differential Pressure Ignition TDC [bar]

Ignition TDC BTC

Ignition TDC BTC


2.2.4 Electrical Filters

Electrical filters are used to eliminate certain frequencies from the measurementsignal:

• High pass filter

With high pass filters, high frequencies are transmitted unchanged, whilelow frequencies experience attenuation and phase lead (i.e. skew). Atypical application example for high pass filters is in the analysis of knockphenomena where the knock oscillations themselves are of primary interestwithout the superimposed cylinder pressure signal.

• Low pass filter

With low pass filters, low frequencies are transmitted unchanged, whilehigh frequencies experience attenuation and phase lag (i.e. skew). Lowpass filters are used mainly to remove high frequency, interference signalcontent from the measurement signal, e.g. structure-borne noise signalsfrom the engine that are transmitted to the transducer. What are known asBessel filters are often used for this purpose in engine instrumentationapplications.

Bessel filter: the major advantage of the Bessel filter is its linear phase shift-to-frequencyratio. That means that any signal only experiences a single skew (apart from thedesired amplitude attenuation of high frequencies). The signal shape is not thendistorted. A Bessel filter is ideal for engine measurements because a skew can becompensated by shifting TDC.

When using electrical filters, it should be borne in mind that a certain skew willalways result, which can cause errors. For example, a skew has a negative effecton the accuracy of the IMEP determination when a low pass filter is used. Thehigher the engine speed, the higher the lowest permitted filter frequency (rule ofthumb: the main frequency of the cylinder pressure signal should not be morethan 1 % of the filter frequency to avoid unacceptable skew).

Source of errors


22.3 Measurement Cabling

The charge produced by piezoelectric pressure transducers is very low which iswhy special demands are made on the connection cable between the pressuretransducer and the charge amplifier:

• High insulation resistance

The piezoelectric measurement system as a whole requires a very highinsulation resistance in the pressure transducer/cable/connector systemand at the charge amplifier input in order to avoid excessive electricaldrift. A value > 1013 Ω at room temperature is a guideline.

• Good screening

Adjacent cables and devices may cause interference signals in themeasurement lines. Measurement cables should therefore never be placednear mains supply lines or other sources of interference and they should bekept as short as possible.

• Freedom from "motion noise"

When subject to vibration, conventional coaxial cables generate frictionelectricity due to relative motion between the screen mesh and theinsulation material (i.e. triboelectrification). The "useful charge" is thensuperimposed by an "interference charge" generated by vibration in thecable. Low-noise cables are necessary therefore that have an additionalconductive layer of carbon or conductive plastic.

• Short cable length

The cable length basically has little effect on the measurement signal whena charge amplifier is used. Very long cables, however, have the effect ofreducing the upper cut-off frequency, see Figure 2-17. Also, the longer thecable, the lower the insulation resistance. We recommend therefore thatcables no longer than 15 m should be used. If a longer cable isunavoidable, you should check the effect it has.

Chapter 3.3

Chapter 2.2.2

AVL ProductInformation:[8]IFEM Indicating FrontEnd Module

AVL ProductInformation:[11]3066A02 PiezoAmplifier

Source of errors


Figure 2-17: Basic effect of cable length on the upper cut-off frequency

2.4 Preventing Electrical Interference Signals

Experience has shown that interference signals occur mainly due to ground loopsand the effect of electromagnetic fields. The more favourable the signal/noiseratio, the less effect there is from this type of interference, which can above allbe achieved by using pressure transducers with high sensitivity.

2.4.1 Ground Loop

Piezoelectric pressure transducers are usually a single-pole design, i.e. one of thetwo poles is electrically conductive with the transducer housing and thusconnected to the engine block. Each potential difference between the engineblock and the measurement ground therefore drives a current through the shieldof the input cable and can cause interference signals.

Experience has shown that ground loop problems occur more significantly whenmore than one pressure transducer is connected to the same engine and sameevaluation unit.

Remedies include:

• Laying a low-ohm connection between the engine block and evaluation unit(the best way is with copper mesh of at least 10 mm2 cross-section)

• Inserting a differential amplifier (or isolated amplifier) between the chargeamplifier output and the evaluation unit. The intermediately connectedamplifiers must then be operated with their own, electrically segregated


2

power supplies. With some amplifiers available on the market, suchdifferential amplifiers are already installed with appropriate supplies.

2.4.2 Interference due to Electromagnetic Fields

Possible sources of electrical and magnetic fields on the test bed are the engine’signition system, the electric dynamometer or other current-conducting systems.Interference occurs primarily when there are interference sources near the inputcircuit of the charge amplifier.

Remedies include:

• Using shielded input cables

• Laying the input cable separated from AC voltage-conducting lines andother interference sources

• Keeping the input cable as short as possible


Characteristics of the Piezoelectric Pressure Measurement System 3-1

3

3 Characteristics of the Piezoelectric PressureMeasurement System

This chapter deals with the characteristics of the various components of thepiezoelectric pressure measurement system, which on the one hand form thebasis on which they are selected and on the other allow us to assess the effectsof influences during operation.

3.1 Characteristics of Piezoelectric Pressure Transducers

3.1.1 Operating Conditions

The external influences that act on the piezoelectric pressure transducer when inuse can substantially affect the measurement result. Figure 3-1 shows the maininfluences.

Apart from the pressure itself, these primarily include:

• Temperature and heat flow

Due to the very high working gas temperatures (approx. 2400 °C) duringcombustion, pressure transducers are subject to very high heat flow loadsduring operation. In extreme cases, cyclic heat flows occur with anamplitude of more than 1000 W/cm2 (for example, during knockingcombustion) and average heat flows occur at up to 50 W/cm2.

The very high heat flow load produces very high temperatures in thetransducer. In uncooled pressure transducers, temperatures of up to500 °C in the front area, up to 400 °C at the measuring element and up to200 °C at the connector are not unusual. In cooled transducers, thetemperature level is by their very nature substantially lower. If the coolingis sufficient, the temperatures in the front area are usually about 100 °C,the temperatures at the measuring element are about 20 °C higher thanthat of the coolant temperature of the pressure transducer cooling systemand the connector reaches about the coolant temperature.


Tcool / 200 °C

Tcool + max. 20°C/400 °C

Temperatur in thetransducer

cooled / uncooled

DeformationStress

Vibration / Shock1000g

Chemical / soot deposits

Heat Flux500 W/cm2 – permanent

1000 W/cm2 - cyclic

Figure 3-1: External influences on the pressure transducer

• Accelerations

Structure-borne noise and also general vibrations of the engine causeaccelerations at the pressure transducer. Acceleration values from enginevibrations can reach up to 1000 g at the transducer (or in extreme caseseven up to 2000 g).

• Deformation stresses

Gas and mass forces, and the thermal load result in deformation stresses ofup to 200 N/mm2 at the pressure transducer mounting position.

• Chemical influence and deposits

Combustion products occur in the engine, which can damage the pressuretransducer due to corrosion.


3

In addition, deposits occur on the surface of the combustion chamber inboth petrol and diesel engines and therefore also on the pressuretransducer. That can also affect the measurement result.

3.1.2 Piezoelectric Pressure Transducer Parameters

To be able to predict the reaction to the influences described in Chapter 3.1.1and their effect on the measurement, a whole series of other parameters apartfrom the basic properties are specified for piezoelectric pressure transducers thatare to be used in IC engines, such as permissible operating conditions andtransmission behaviour. Special test methods have been developed for some ofthese parameters.

The main piezoelectric pressure transducer parameters are described below.

3.1.2.1 Permissible Operating Conditions

Measurement range [bar]

The measurement range defines the pressure range in which the pressuretransducer fulfils the defined specifications (see Figure 3-2).

Druck [bar]25 50 75 100 125 150

1000

800

600

400

200

0

Meßbereich

Q [pC]

Überlastbereich

FSO

Figure 3-2: Measurement and overload range

Overload range [bar]

The specification limits may be exceeded in the overload range but that must nothave any lasting effect on the measurement properties of the pressure transducer(see Figure 3-2). In the overload range, however, no clear relationship can beexpected between the pressure and the output quantity.

Measuring range Overload range Pressure [bar]


Operating temperature range [°C]

Temperature range in which the pressure transducer fulfils the defined specifi-cations.

Connection cables lead to components that remain cool during operation.They therefore have a lower operating temperature range, as do transducerparts that only serve the purpose of signal transmission in cooler ranges (e.g.measurement probes).

Lifetime [ ]

Number of load cycles over which the transducer retains its technicalmeasurement properties.

This represents a purely comparative number that is achieved on highlyloaded engines. Favourable installation conditions prolong a transducer’slifetime while extreme operating conditions, e.g. combinations of very hightemperatures, pressure rises, deformations of the mounting position andcorrosive environment, can shorten it.

3.1.2.2 Transmission Behaviour

Sensitivity [pC/bar]

Ratio of the change in the output signal (i.e. charge) to the associated changein the measurement quantity (i.e. pressure).

Linearity deviation [±% FSO]

Deviation of the pressure transducer characteristic from the “best straight linewith forced zero point“.

Sensitivity and linearity are quantities, which are determined by means ofcalibration. The sensitivity and linearity values always depend on themethod used to determine them. Piezoelectric pressure measurementtechnology usually uses the “best straight line with forced zero point“method. This is the centre line which travels through the zero point and liesbetween two parallel straight lines that enclose the pressure transducercharacteristic as closely as possible, see Figure 3-3.

Chapter 6

Characteristics of the Piezoe

100

80

60

40

20

0

FSO [%]

Figure 3-3: Sensitivity and li

The mean sensitivity forced zero point“. TEquation 3-1 as a pOutput).

Linearity deviati

A ......... Distancthe best

Qmax ....Maximum val

Piezoelectric pressurthey are manufacturpressure transducer rsensitivity.

Before being despamanufacturer and theon a calibration sheeat very high temperatthe order of about 1

Natural frequency 1

The lowest frequencelement of a fully asse

By contrast with thdefines the frequenc

bwp

lectric Pressure Measurement System 3-5

3

Meßbereich [%]20 40 60 80 100

beste Gerade mitZwangsnullpunkt

+A

-A

Qm

ax

nearity

is defined as the gradient of the “best straight line withhe linearity (deviation) is specified in accordance withercentage of the full-scale signal (% FSO - Full Scale

on [% FSO] = ±A

Qmax. 100 (3-1)

e of the straight lines enclosing the characteristic from straight line with forced zero point

ue of the output signal (FSO)

e transducers show sensitivity scatter due to the wayed. The sensitivity specified for the individual types ofepresents a mean value, or what is called the nominal

tched, each pressure transducer is calibrated by the result (characteristic, sensitivity, linearity) documented

t. It should be borne in mind that “running-in” processesures and pressures can result in changes in sensitivity of%.

st order [kHz]

y of free (non-forced) oscillations in the measuringmbled transducer.

e natural frequency, the basic resonance frequencyy of the measurement quantity at which the pressure

est straight lineith forced zerooint

Measuring range

3-6

transducer outputs the output signal with the greatest amplitude. Wherethere is little attenuation, as is generally the case in piezoelectric pressuretransducers, the basic resonance frequency is the same as the naturalfrequency 1st order.

Figure 3-4 shows the result of a measurement of an uncooled miniature pr

100 200 300 400

A0

Amplitude

AR

Figure 3-4: Resonance frequency spectrum

A high natural frequency is important ispeeds or knock measurements. The fror the parts of the signal of the meacquired with great accuracy, should nfrequency of the pressure transducer us

Insulation resistance [Ω]

This is the ohmic resistance between thecharges and the pressure transducer ground.

The insulation resistance should be atemperature) to keep the influence of t

Chapter 2.2.2

F

Engine Indicating

resonance frequency determinationessure transducer as an example.

Frequenz [kHz]

n particular for measurements at highequency of the measurement quantityasurement quantity that have to beot be more than 20 % of the naturaled.

connection for picking up the electrichousing, which represents electrical

s high as possible (> 1013 Ω at roomhe electrical drift as low as possible.

requency


3

Natural capacitance [F]

Piezoelectric pressure transducers have a natural capacitance due to theirconstruction that is primarily due to the electrodes of the measuring elementand the capacitances of the connector and the line to the measuring element.

This natural capacitance can be ignored when a charge amplifier is used.When an electrometer amplifier1 is used, on the other hand, not only thenatural capacitance of the pressure transducer but also the cablecapacitance and the input capacitance of the amplifier itself have to be takeninto account.

Electrometer amplifier: in an electrometer amplifier, the voltage that occurs in thetransducer as a result of the charge transfer is measured direct. The total capacitancein the transducer circuit is decisive for the output voltage. An electrometer system istherefore influenced by the cable length between the transducer and the amplifier.Because of its greater universality, however, the charge amplifier has becomestandard equipment in laboratory applications. But with its simplicity theelectrometer amplifier has the advantage that it fits in the smallest spaces and istherefore integrated, for example, in many piezoelectric accelerometers.

Cooling water crosstalk

The cooling water crosstalk is a yardstick of the change in the output signalfrom the pressure transducer as a function of the cooling water fluctuation.

A positive pressure fluctuation in the cooling water pressure results in areduction in the measured pressure. A value (factor) of –0.35 cooling watercrosstalk indicates that a change of +0.1 bar in the cooling water pressurecauses a change of –0.035 bar in the pressure display.

3.1.2.3 Temperature and Heat Flow Influences

Change in sensitivity over the temperature [%/°C]

Temporary change in sensitivity when the operating temperature changeswithin the specified range.

The change in sensitivity in piezoelectric pressure transducers is usuallydescribed by the temperature coefficient of the sensitivity which indicates

Chapter 2.1.4


the actual change in sensitivity as a percentage of the nominal sensitivityper °C within a specific temperature range.

With small temperature changes, e.g. when water-cooled pressuretransducers are used, the change in sensitivity is negligible or allowed for bythe mean temperature coefficient. Otherwise the effect of temperature canbe allowed by calibrating the pressure transducer at operating temperature,i.e. at the mean measuring element temperature that occurs duringoperation.

Temperature drift

By temperature drift we mean the "pressure indicating" that is caused solely bythe temperature changes at the pressure transducer and mounting position.

Temperature drifts represent critical measurement errors in manymeasurements. In addition to the design of the pressure transducer, thetemperature drift is mainly caused by the magnitude and sequence in whichthe pressure transducer is heated which also strongly depends on theinstallation position of the pressure transducer.

In pressure indicating on IC engines, a clear distinction can be madebetween two temperature drift phenomena:

• the cyclic temperature drift and

• the load change drift.

Cyclic temperature drift (short term drift, thermoshock) [bar]

The maximum error pressure reading within a cycle related to a point at thestart of the heating phase caused by the cyclic heating of the pressuretransducer.

This problem is more severe at low speeds. In other words, this is the erroredpressure reading that occurs due to cyclic heating of the pressure transducerwithin a cycle.

To illustrate the cyclic temperature drift, Figure 3-5 shows the result of anFEM simulation of an uncooled miniature pressure transducer. Thetemperature distribution and deformation of the transducer is resulting fromthe cyclic heat flow load is illustrated for three different points in the cycle

1 Electrometer amplifiers are virtually no longer used nowadays for measurements with

piezoelectric pressure transducers on IC engines.

Chapter 4


3

(25 °CA before ignition TDC as well as 25 °CA and 180 °CA after ITDC).Pronounced deformations in the vicinity of the pressure transducerdiaphragm can clearly be seen at the 25 °CA after ITDC point. The loads onthe measuring element due to the deformation finally result in the cyclictemperature drift shown at the bottom of the figure. [4]

Figure 3-5: Temperature distribution and deformation of a pressure transducer and curve ofthe cyclic temperature drift with cyclic heating at the front face (result of anFEM simulation)

Since in many cases the cyclic temperature drift acts over a large crank anglerange, the influence on quantities that are integrated over a cycle (e.g. theindicated mean effective pressure (IMEP)) is considerable. Figure 3-6 showsthe effect of a characteristic cyclic temperature drift curve on thedetermination of the indicated mean effective pressure and the energy

Cyc

lic T

empe

ratu

re D

rift

Crank Angle [°CA]Ignition TDC Gasexchange TDC


balance. By contrast, the effect of a pressure-proportional error of the samemagnitude, as caused by a change in sensitivity, is much smaller.

-90ZOT UT

Kurbelwinkel [°KW]

Druckdifferenz [bar]

-60 -30 0-0.8

-0.4

0

0.4

30 60 90 120 150 180 210 240 270

-90ZOT

Zylinderdruck [bar]

-60 -30 00

10

20

30

40

50

60

30 60

0

20

40

80

60

Abweichung im indizierten MitteldruckAbweichung in der Energiebilanz[%]

DruckproportionalerFehler

ZyklischeTemperaturdrift

ITDC

Figure 3-6: Deviations in indicated mean effective pressure and in the energy balance due toan error that is proportional to the pressure and due to cyclic temperaturedrift

Characteristic values to describe the cyclic temperature drift are determinedfirstly by unpressurised, cyclic heating of the transducer in a special testerand secondly in real engine operation by comparative measurement with areference transducer.

• Determination by unpressurised, cyclic heating

Here, the tester [18] shown schematically in Figure 3-7 is used whichallows the unloaded pressure transducer to be exposed to a cyclicheating of a similar magnitude and frequency to that which occurs in theengine. Opposite the transducer being tested is a radiant heating surface,which is alternately covered and uncovered by a rotating chopper wheelso that a heat flow as shown in Figure 3-8 is produced at the transducer.The radiant heat surface is electrically heated to temperatures of over


Pressure difference [bar]

Crank Angle [° CA]Ignition TDC BDC

CyclicTemperature Drift

Deviations in indicated mean effective pressureDeviations in the energy balance

pressureproportionalerror


3

2000 °C, which means that heat flows of more than 100 W/cm2 can beproduced. The pressure transducer is inserted in a solid, cooled steelplate, flush with the plate surface.

Graphitheizfläche

Aufnehmer

Blendenrad

wassergekühlt

Drehzahl (n)

Winkelaufnehmer

Stromversorgung

UIN2 - Atmosphäre

Figure 3-7: Tester for determining the cyclic temperature drift (diagram)

Graphite heatingsurface

Chopperwheel

Transducer

Angle Encoder

Power Supply

Speed (n) Graphite radiant heaterChopper Wheel N2 Atmosphere

Water cooled

Transducer


0 30 60 90 120 150 180

0

Drehwinkel [°]

Blende offen Blende geschlossen

qm.

qu.

qo.

∆q

Figure 3-8: Curve of heat flow density in the tester

The maximum pressure deviation of the pressure transducer that occursin a cycle (in relation to the value of the measurement signal immediatelybefore the heating phase) at a specific frequency of the chopper wheeland at a specific radiant heating is used as the characteristic value for thecyclic temperature drift.

• Determination in real engine operation

The characteristic value for the cyclic temperature drift of a pressuretransducer can be determined in real-life engine operation bysimultaneous measurement with a reference transducer anddetermination of the difference between the pressure curve of thetransducer being tested and the reference transducer. The referencetransducer must satisfy very high accuracy requirements. Usually used forthis purpose are water-cooled transducers with very low thermoshocksensitivity. The diaphragm of the reference transducer is furthermorecoated with silicone rubber, which means that the heat flow into thediaphragm (caused by the combustion) is significantly reduced.

The maximum pressure deviation (in relation to a pressure value beforethe start of heating) within an engine cycle at a specific load state of theengine is specified as the characteristic value for the cyclic drift.

Surfa

ce H

eat F

lux

Chopperorifice open

Chopperorifice closed

Rotation Angle


3

Load Change Drift 2

This denotes the slow drift of the pressure signal after a load change, i.e. achange in the heating over a series of cycles.

The load change drift manifests itself as a relatively slow pressure level shift,which is caused by a temperature change in the entire pressure transducer.This shift in level will only stop when the mean temperature in the pressuretransducer no longer changes.

The characteristic value for the load change drift is determined in real engineoperation, by first running the engine at a specific load point and thenchanging to motored mode by shutting off the fuel supply thus producing aquick change in the mean heating effect on the pressure transducer (by asudden load change), see Figure 3-9.

[bar]

-2.0

-1.0

0

1.0

Lastwechseldrift

Zeit [sec]0 2 4 6 8 10 12 14 16 18 20

max. Nullpunkts-gradient dp/dt

bleibende Abweichung des Druckniveaus

Lastwechsel

[bar]

0

10

Zeit [sec]0 2 4 6 8 10 12 14 16 18 20

20

30

40

50

Zyl

inde

rdru

ckN

ivea

uver

schi

ebun

g

Figure 3-9: Load change drift

2 The load change drift is called long-term drift or long- and medium-term drift in older

publications.

Time [sec]

Cyl

inde

r pre

ssur

eLe

vel s

hift

Permanent Zero-line Deviation

Connecting line of one point each per cycle at sameCrank Angle position in the low pressure rangeLoad change

Load Change Drift

Time

Max. Zero-lineGradient dp/dt


The following characteristic values may be derived from the measurementsignal thus acquired as shown in the bottom half of Figure 3-9:

Maximum Zero-line Gradient dp/dt [mbar/ms]

Describes the change in the pressure level per time unit caused by the heatflow, i.e. the maximum drift gradient after the load change..

The Zero-line Gradient causes a clear incline in the pressure curve of thesingle cycle which is noticeable in the evaluation (IMEP, rate of heat release,etc.) just like other distortions of the pressure curve (e.g. cyclic drift, electricalinterference, etc.).

Permanent Zero-line Deviation [bar]

Zero point deviation 20 s after the change in heat flow.

In this range, the load change drift has virtually no gradient any more theindividual cycles are thus only shifted in level. Since zero-line determinationis in any case necessary in piezoelectric measurements, this deviation has noeffect. [3]


3

3.1.2.4 Acceleration Influence

Acceleration sensitivity [bar/g]

Pressure transducer output signal due to accelerations affecting the transducer.

The acceleration sensitivity denotes the apparent pressure change thatappears to take place at an acceleration of 1 g. The acceleration sensitivity ofwater-cooled pressure transducers is also influenced by the mass of thecooling water in the pressure transducer and feed hoses and is usuallysignificantly higher than in non-cooled mode. For pressure measurements atmeasuring positions with high acceleration load, such as in intake or exhaustlines or in racing engines at high speed, pressure transducers with lowacceleration sensitivity should be used.

The extent of the influence of acceleration also depends to a large extent onthe installation site, on the direction of the accelerations that occur inrelation to the pressure transducer axis, and on the engine speed. Pressuremeasurements on revving racing engines often reflect a strong influence ofstructure-borne noise over the entire cycle.

Figure 3-10 shows an example of the influence of acceleration. The high-frequency oscillations superimposed on the pressure signal are caused in thisspecific measurement arrangement by the impact of the intake and outletvalves on the valve seat and transmitted by structure-borne noise.

Figure 3-10: Influence of structure-borne noise in the measured pressure signal


Kurbelwinkel [°KW]

Zylinderdruck [bar]

0

10

20

30

40

50

60

Auslassventil schließt

Einlassventil schließt


Inlet valve closes

Exhaust valve closes

Crank Angle [°CA]


3.1.2.5 Deformation impact

Deformation sensitivity

Deformation stresses at the measuring position result in load on the pressuretransducer (and measuring element), which can produce errors in thepressure signal. The decisive thing for deformation sensitivity, apart from thedesign, is also the installation principle used for the pressure transducer. Plug-in type pressure transducers are usually less sensitive to deformation thanthreaded types. [2]

3.1.2.6 Chemical Influence and Deposits

Depending on the fuel used, various combustion products form in the enginewhich can result in corrosive damage to pressure transducers. Thanks to the useof corrosion-resistant materials and special coatings, such problems occur withtoday’s state-of-the-art pressure transducers under very unfavourable conditionsonly (e.g. when the fuel used has a very high sulphur content).

Deposits build up on the combustion chamber walls of both petrol and dieselengines. Combustion residues (soot particulate etc.) tend to form deposits oncooler surfaces in particular. Sooting occurs to a varying extent on the transducerdiaphragm depending on the operating conditions and type of pressuretransducer. The diaphragm becomes particularly rapidly sooted in measurementsin the exhaust system using cooled pressure transducers.

These deposits can have negative effects on the measurement result dependingon the type of pressure transducer used. In particular when pressure transducerswith heat shield are in operation for longer periods, the gap between the heatshield and diaphragm can be filled up with combustion residues, which changesthe rigidity in the diaphragm area and thus the sensitivity. In extreme cases anIMEP error of 10% or more can occur.

Information about the sensitivity of pressure transducers to sooting can begained from the characteristic value: IMEP stability.

Corrosion

Deposits

Source of errors


3

IMEP Stability [%]

Percentage of change in the indicated mean effective pressure (IMEP) over adefined runtime.

IMEP stability is the percentage of the change in the indicated meaneffective pressure (IMEP) over a defined runtime in relation to valuesdetermined with a reference pressure transducer. The engine is operated at aconstant speed and load and the IMEP values derived from the pressuretransducer being tested and a simultaneously operated reference transducerare then compared to determine the IMEP stability. The reference transduceris cleaned and calibrated at regular intervals to verify the continuity of theperipheral conditions.


3.2 Properties of the Charge Amplifier

The essential properties for the application and choice of charge amplifiers are:

Polarity

Positive or negative sign of the output voltage with reference to the inputcharge

Charge amplifiers have an inverting action, i.e. a negative input chargeproduces a positive output voltage.

Sensitivity [mV/pC]

Ratio of the change in the output signal (voltage) to the associated change inthe input quantity (i.e. charge).

Especially when the output signal is to be digitized a sensitivity range shouldbe selected that allows if possible the full input range of the A/D converterto be used.

Different sensitivity ranges can be implemented in most charge amplifiers byactivating different capacitance values for the negative feedback capacitor(i.e. RANGE setting). There are usually 4 measurement ranges (10, 100, 500and 1000 bar).

Charge amplifiers are usually designed in such a way that when thesensitivity of the pressure transducer used is set at the amplifier in pC/bar, apressure change corresponding to the chosen measurement range (RANGE)produces a 10 V change in the output voltage. That means that the outputvoltage of the charge amplifier can be directly assigned to the pressure to bemeasured without separate calibration of the measurement system. Theresulting error, however, can be up to one per cent which is why thismethod cannot be used for precision measurements.

Sensitivity error [%]

Deviation of the actual transfer factor from the measurement value set at thecharge amplifier in % of the measurement value.

This error can be avoided by calibrating the entire measurement systemusing a dead weight tester.

Chapter 2.2.1

Chapter 2.2.1

Source of errors

Chapter 6


3

Linearity error [%]

Deviation from the ideal straight charge-voltage characteristic in relation to themaximum output voltage

The linearity error of charge amplifiers is minimal (in the order of 0.01%) andtherefore usually negligible.

Lower cut-off frequency [Hz]

-3 dB cut-off frequency of the charge amplifier in SHORT mode.

The lower cut-off frequency essentially depends on the time constant of theamplifier. It is determined by the value of the negative feedback capacitorand the resistor connected in parallel to it. The lower cut-off frequency (fl) for-3 dB drop is calculated in accordance with Equation 3-2.

fl = FFCRπ2

1 (3-2)

RF ......... Resistance of negative feedback resistor

CF ......... Capacitance of negative feedback capacitor

The longest time constant and therefore the lowest possible lower cut-offfrequency is given when the parallel resistance consists only of the insulationresistance of the negative feedback capacitor (LONG mode).

Upper cut-off frequency [kHz]

The upper frequency at which the amplitude is decreased by 3 dB for asinusoidal signal.

The upper cut-off frequency of modern charge amplifiers is in the order ofabout 100 kHz. Although higher values can be achieved, they are notnecessary due to the limitation set by the usual values for the natural orresonance frequency of piezoelectric pressure transducers.

The upper cut-off frequency can be limited by a plug-in low pass filter todefined, lower values.

Chapter 2.2.2

Chapter 2.2.3

Chapter 3.1.2.2

Chapter 2.2.4


Ripple and noise [mV RMS] or [mVpp]

Interference signal that occurs at the output without an input signal.

The ripple and noise of the charge amplifier superimpose on the wantedsignal and should be as small as possible. Values for ripple and noise arespecified either as a root-mean-square value [mV RMS] or as a peak-to-peakvalue [mVpp].


3

3.3 Properties of the Measurement Cabling

The following properties and parameters are relevant for V:

Insulation resistance [Ω]

Resistance between the inner conductor and the shield of the input circuit(cable)

As for the pressure transducer and amplifier input, very high insulation valuesare required for the measurement cables used in the input circuit (> 1013 Ω),see Chapter 2.2.2.

Capacitance [F]

Capacitance between the inner conductor and the shield of the input circuit(transducer including input cable)

The capacitance affects the upper cut-off frequency of the measurementsystem (see Chapter 2.3). The influence of the pressure transducercapacitance is nearly always negligible and the influence of cables that areless than 15 m long is also insignificant.

Noise charge [pC]

The noise charge in the cable caused by movement

For the cables directly connected to the transducer and thus being subject tosubstantial movement special low noise versions must be chosen (values inaccordance with MIL C17: < 2pC).

Screening

Protective sleeve around the signal lead

Screening is used to prevent electromagnetic interference (the braidedscreen normally used in standard coaxial cables is usually sufficient).


Selection of Piezoelectric Pressure Transducers 4-1

4

4 Selection of Piezoelectric Pressure Transducers

Pressure transducers for high and low pressure indicating are available indifferent designs and sizes. The wide variety of transducers availableunderscores the fact that no transducer is usually capable of providing all theproperties demanded by users, such as:

• very small,

• no measurement error,

• resistance to all external influences and

• very long lifetime.

As shown in Figure 4-1: Interrelation between measurement task, pressuretransducer and installation

, the pressure transducers available are basically distinguished by the type ofcooling, the design, the installation possibilities and of course, their technicalmeasurement characteristics.

The decisive thing for users is to select a pressure transducer which meetsthe requirements for the relevant measurement task in terms of accuracy,stability, costs, etc.

An important criterion in pressure indicating is the correct installation of thetransducer on the test engine (defined by the type of installation, design andlocation of the measuring position) because it defines the operatingconditions during the measurement (heat flow and temperature load,accelerations, etc.) and any effects on the measurement object itself (e.g.change in combustion chamber volume, change relating to the coolingsituation, etc.).

Since different transducers also permit different installations, the selection ofa pressure transducer is in fact a complex weighing up of the influencingfactors shown in Figure 4-1.

On the whole, the aim when selecting or defining a pressure transducer andits installation is to find the best possible compromise to meet themeasurement requirements.

Range oftransducersavailable

Measurement task

Installation


Test engine

Operational conditions

Requirements regarding

Accuracy Type of evaluation Stability/meas. duration Possibility and

cost of installation

Measurement task

With mounting bore

Position Design Access

Glow plug /spark plugadaptors

Design

Installation

Cooling Fluid cooling Heat conduction

Transducer design Plug-in sensor Scew-in sensor Probe

Adaptation/componentintegratability

Measurement properties

Measurement range Sensitivity Linearity Natural frequency Lifetime Cyclic

Temperature drift Stability ...

Pressure transducer

Figure 4-1: Interrelation between measurement task, pressure transducer and installation


4

4.1 Measurement Task

Before the right pressure transducer and type of installation can be selected, themeasurement task and measurement requirements must be formulated in detail.In this context, it is vital to determine the operating conditions that occur duringthe measurement as accurately as possible because that is what definitivelyaffects the selection.

4.1.1 Test Engine and Operating Conditions

The operating conditions during the measurement are determined in particularby the test engine itself and the engine operating states in which themeasurement is to be carried out. As described in Chapter 3.1.1, this includesinformation about

• the anticipated pressure range,

• the temperature and heat flow load,

• the expected accelerations forces and

• any chemical loads.

The test engine also determines the possibilities for the installation of thepressure transducer. Engine manufacturers rarely allow for the mounting situationof pressure transducers in the design of cylinder heads because they are usuallyonly used during the development phase and ought not to affect the optimumshape of the series product. That is why the compact design of modern enginesoften makes compromise decisions regarding the site of the measuring positionand type of installation necessary. Because the installation position has a majorinfluence on the quality of the measurement, the user should pay due regard tothis fact (see also Chapter 4.2.).

4.1.2 Requirements of the Measurement Task

4.1.2.1 Accuracy

Different demands are made on the accuracy of the pressuring indicatingdepending on whether the measurement task involves determining direct orindirect indicating parameters.


• Determining direct indicating parameters

Measurement tasks, such as peak pressure measurements for determiningthe mechanical load or the determination of the pressure gradient for noiseanalysis are easy to carry out on the basis of the pressure curve only. Thedefinition of the accuracy requirements in these cases is usually noproblem or only a relatively small problem.

Pressure curve:Pressure curve:Pressure curve:Pressure curve: pressure curve means that the pressure is measured over a time base or, as iscustomary in pressure indicating, over the corresponding crank angle position. That allowscorrect correlation of the instantaneous pressure values. The accuracy of the pressure curvemeasured in this way over the crank angle depends, of course, on the two measurementparameters, pressure and crank angle position.

• Determining indirect indicating parameters

As described in Chapter 1, the indirect indicating parameters aredistinguished by the fact that they can only be determined from thepressure curve when other parameters are available and often requirecomprehensive evaluations.

When defining the accuracy requirements, the methods for determiningthese additional parameters should also be considered accordingly. Forexample, the accuracy when determining the indicated mean effectivepressure (IMEP) is decisively influenced how reliable the determination ofthe top dead centre position (TDC position) is.

The pressure indicating therefore only represents a part of themeasurement task in the determination of indirect indicating parameters.The requirement for a specific accuracy of the result cannot therefore bedescribed simply by the accuracy of the pressure indicating.

Indicated mean effective pressureIndicated mean effective pressureIndicated mean effective pressureIndicated mean effective pressure: the indicated mean effective pressure (IMEP) is determinedfrom the integral of the volume change work done – the relevant pressure multiplied by thechange in the work volume – in relation to the swept volume. In addition to the influence thatthe accuracy of the determination of the TDC position has, it should also be noted thatrelatively small errors in the pressure curve can result in large errors in the indicated meaneffective pressure (e.g. 0.1% error in the pressure results in an error in the IMEP of approx. 1%- depending on the engine load state).

The mean effective pressure (MEP) has to be determined first before the friction mean effectivepressure (FMEP) can be determined. Determining the FMEP to an accuracy of 10% requires notonly the accurate determination of the mean effective pressure but also the determination ofthe indicated mean effective pressure depending on the load state to an accuracy of up to 1-2% because the proportion of the frictional power is only a fraction of the indicated power.


4

4.1.2.2 Type of Evaluation

The type of evaluation, i.e. the criteria that are necessary to derive the requiredinformation from the pressure signal, plays a significant role in defining therequirements for the pressure measurement. Table 4-1 shows the necessaryevaluation criteria for important measurement tasks and the additionalmeasurement parameters required [19].

Table 4-1: Important measurement tasks with evaluation criteria and additionalmeasurement parameters

Task area Measurement task Combustionchamberpressure-evaluation

Additionalmeasurementparameters

Mechanicalengine load

Peak pressure pmax Maximum,minimum,amplitude (cycle)

Noise analysis Pressure gradients dp/dα derivation

Dire

ct

Monitoring Misfire detection Qualitative curvebefore and afterTDC

(TDCdetermination)

Knock detection Frequencyamplitude ofhigh-frequencyoscillations

(TDCdetermination)

Friction analysis Friction meaneffective pressureFMEP

IMEP TDCdetermination,mean effectivepressure MEP

Gas exchangeanalysis

Charge mass,residual gas, gasexchange work, ...

Pressure curve,IMEP-gas exchange

TDCdetermination,p-intake pipe,p-exhaust

Combustionanalysis

Combustionchamber pressure,start ofcombustion,ignition delay, endof combustion,centre of gravity

Internal work overcrank angle

TDCdetermination,wall heat,leakage, fuel andair volume, ...

Energy conversionpoints

Integrated internalwork versus crankangle

Indi

rect


The effects of the pressure transducer properties and the influences at themeasuring point on the quality of the measurement result vary depending onthe evaluation criterion, see Chapter 4.3.

4.1.2.3 Stability

The term stability is taken to mean quite generally the ability of a transduceror a measurement system to maintain its metrological properties over arelatively long period of time. Measurement tasks that place high demandson the stability are, for example, long-term monitoring and control of thework process based on the pressure curve or the use of pressure indicatingas part of automatic engine optimisation.

Where high demands are made on the stability of the measurement signal,cooled transducers should preferably be used and the transducers shouldonly be used in the lower load range. Unfavourable installation positionsshould also be avoided.

The characteristic value of the IMEP stability gives an important indication ofthe stability of the pressure transducer behaviour, especially its response tothe effect of deposits on the diaphragm of the transducer, see Chapter 3. Inthe case of piezoelectric pressure measurement systems in particular,stability is also determined by the properties of the amplifier andmeasurement cabling.

In addition to ensuring that the fewest possible changes occur in themetrological properties during very long measurements, the operationalreliability in particular (e.g. certainty that the cooling system will not fail incooled sensors) must be guaranteed. Here uncooled pressure transducershave advantages if they are installed in a correspondingly cool position.

4.1.2.4 Installation Effort and Costs

The pressure indicating costs are mainly determined by the time and effortrequired for installation of the transducer. As shown in Table 4-2, a pressuretransducer installation using glow plug or spark plug adaptors requiring nomachining of the test engine represents a cost-efficient and at the same time,space-saving solution. It should be taken into account, however, that thispredefines the measuring point. In addition, you should also check whetherthe requirements regarding accuracy, stability etc. can be met with the


4

uncooled pressure transducers that have to be used in such an adaptationprinciple.

Table 4-2: Adaptation with and without intervention in the test engineIntervention intest engine

No interven-tion in engine

Installation time and effort substantial littleCosts high lowPossible to selectmeasuring position

yes no

Accuracy high medium high

4.2 Influence of the Transducer Installation

Not only is the pressure transducer important for reliable operation and highmeasurement accuracy but also the type of installation and the site of themeasuring position (influence of measuring position) are decisive factors.

Basically it should be noted that the pressure in the combustion chamber isnot the same at any position (due to divisions in combustion chambers,squish areas, etc.). The installation position must therefore be selectedsomewhere where a representative pressure prevails for the sought quantity .Furthermore, the pressure transducer is subject to different loads at differentplaces in the combustion chamber (i.e. temperature, heat flow, deformation,acceleration) which are in effect interference influences that can causemeasurement errors.

The choice of the installation position and the design of the mountingposition must also always ensure that

• the very pressure is measured that is decisive for the measurement task,

• errors due to the installation are kept to a minimum (e.g. pipeoscillations, etc.),

• the permissible operating temperature and heat flow load of thepressure transducer are not exceeded,

• the temperature fluctuations in the pressure transducer are kept as lowas possible and

• the engine behaviour remains unaffected.

“The rightpressure”

Requirements forthe installation


4.2.1 Glow Plug/Spark Plug Adaptors

Basically the question has to be answered as to whether it is acceptable tomechanically modify the test engine and if so, whether such a modification ispossible in terms of time and/or costs. As mentioned above, the advantagesof being able to mechanically modify the engine are the fact that you canchoose the measuring position and that highly accurate pressure indicating isachievable.

Although by comparison an installation using a glow plug or spark plugadaptor involves lower costs, the site of the measuring position cannot thenbe chosen freely and the measuring accuracy is usually lower. This is mainlydue to the fact that the measuring position is often located in a highlythermally loaded environment so that the usual quartz pressure transducerscan only be installed at a significant clearance from the combustionchamber (with the associated problems of pipe oscillations, overheating ofthe measuring elements, etc.). Significant improvements can only beachieved here with state-of-the-art piezo-materials (e.g. GalliumOrthophosphate).

4.2.2 Installation Using a Suitable Installation Bore

4.2.2.1 Site of the Measuring Position in the Combustion Chamber

The ideal measuring position is a place where the local pressure isrepresentative for the measurement task and where the interferenceinfluences are minimal.

Installation positions above the squish gap, above the impact site of theinjection spray and in highly thermally loaded positions should be avoided ifpossible. Cool installation positions (for example, near the intake valve etc.)are preferable.

Since in practice it is often impossible to fully achieve the ideal measuringposition, resulting influences of unfavourable measuring positionarrangements usually have to be taken into account.

• Installation above the squish gap

The gases in the squish gap are substantially accelerated by the motion ofthe piston around top dead centre (TDC). Resulting from these gas flows

AVL ProductInformation:[7]GalliumOrthophosphate(GaPO4)

The idealmeasuring position

Selection of Piezoel

4

are substantial differences in pressure in the squish gap, which can result inerrored information when a pressure measurement is evaluated.

Generally speaking, the following can be said about the installation of thepressure transducer in the squish gap:

Gas oscillations

The strength of the squish gap flow varies in terms of time and space inparticular if the position of the piston bowl is eccentric and if the cross-section creases in the area of the valve reliefs. The gas in thecombusta coupleDependtransducand amphas a hig

kurzerQuetsch

muldeKolben-

muldeKolben-

Figure 4-2: Influenin the

To illustmeasurepiston recogniz

Basicallythe side bowls.

Pistonbowl

Shortsquish ga

Pistonbowl

in

ectric Pressure Transducers 4-9

ion chamber recess and the gas in the squish gap can manifestd gas oscillation under excitation (e.g. start of combustion).

ing on the site of the measuring position therefore, the pressureer can be subject to gas oscillations of differing frequencieslitudes which can then significantly develop if the combustionh pressure rise speed.

spalt

rand

ZOT0 5 10 15 20

randKolben-

randKolben-

Kolben-

Kurbelwinkel [°KW]

Druck [bar]

muldeKolben-

65

60

55

50

langerQuetschspalt

ce on the measured pressure curve of a measuring position arrangement squish gap

rate these influences, Figure 4-2 shows the pressure curvesd on a DI diesel engine at measurement positions above thebowl and piston edge. Significant gas oscillations areable at the measuring position above the piston edge.

an arrangement whereby the pressure transducer is installed atwith the short squish gap is advantageous in eccentric piston

Pistonedge

Pistonedge

Longsquish gapp

Pressure [bar] Piston edge

Pistonbowl

Crank Angle [°CA]Ignition TDC


Heat flow load

When the pressure transducer is installed in the area of the piston bowl,a much higher heat flow load can occur due to the effect of thecombustion than when the pressure transducer is installed at the pistonedge, which results in higher cyclic temperature drift. If on the otherhand, the pressure measurement error from the squish gap flow isacceptable, a pressure transducer can be installed at the outer edge ofthe cylinder head so that it remains in the shadow of the combustionradiation for longer. Installing the transducer recessed from the pistonsurface is advantageous in both arrangements because this can reducethermoshock errors [4].

• Installation near the valve

Arranging the measuring position near the valve affects the measurementresult as follows:

Marginal local pressure differences occur due to the high flow rates inthe gas exchange phase and it is not the pressure that is representativeof the total combustion chamber that is then measured. This canslightly distort the low pressure loop.

High flow rates occur on the surface of the pressure transducerdiaphragm especially in the immediate vicinity of the outlet valvesresulting in an increase heat flow load (cyclic temperature drift).

The cylinder head ceiling is usually hottest near the outlet valve.Figure 4-3 shows the temperature distribution in the cylinder head of adirect injection diesel engine calculated using the FE method. Thetemperature maximum occurs in the vicinity of the outlet valve seat. Ifthe measuring position is selected somewhere here, the pressuretransducer also reflects a high temperature, which can cause a changein sensitivity and a reduction in the pressure transducer’s loadability.


4

Figure 4-3: Temperature distribution at the cylinder head of a 2-valve DI diesel engine

• Installation above the impact site of the injection spray

When the measuring position is arranged directly above the impact site ofthe injection spray, the impact of cold fuel (which changes the heat flowload) can cause errors due to cyclic temperature drift. That is why suchmeasuring position arrangements should be avoided.

• Pressure measurement in the prechamber and swirl chamber

For various reasons in some chamber-type engines indicating can only becarried out in the prechamber or swirl chamber, even though the pressurecurve in the main combustion chamber is the curve that is of interest. Suchmeasurements only permit a rough estimate of the thermodynamicphenomena in the main combustion chamber (for accurate information thetransducer must be installed in the main combustion chamber).


4.2.2.2 Design of the Measuring Position

When designing the measuring position care should be taken to ensure thatthe combustion chamber is not unacceptably modified by the adaptation ofthe pressure transducer in the test engine and that no measurement errorsare created.

Errors due to the design of the measuring position can include:

• Dead volume when the transducer is installed in an inclined position orrecessed,

• Pipe oscillations when the transducer is installed recessed,

• Interference to the gas flow,

• Fuel deposits in the combustion chamber,

• Creation/machining of the measuring position, or

• Cool/hot positions due to cooled/uncooled transducers.

There are various types of transducers and adaptors that can be used tokeep these influences to a minimum.

In general, the pressure transducer diaphragm should follow as far aspossible the contour of the cylinder head ceiling (flush or approx. 0.5 mmrecessed). The longitudinal axis of the pressure transducer should if possiblebe at right-angles to the cylinder head ceiling.

• Flow recess

Usually however, it is only possible to install the pressure transducer in aninclined position due to the design of the cylinder head.

A very inclined position is associated with the following negative effectsdue to the flow recess it generates:

a slight increase in the combustion chamber volume

interference to the flow conditions at the installation site

danger of fuel collecting (influence on emissions)

• Indicating channel

The ideal design

Selection of Piezoelectric Pressure Transducers

When used in conjunction with a spark plug or glo ptor, it isoften necessary to recess the pressure transducer to kand heat flow load at the transducer as low as possible

Figure 4-4 shows the heat flow load (top) and the cy(bottom) at the “piston bowl” and “piston edge“ metwo different installation depths.

Figure 4-4: Heat flow load and cyclic temperature drift as a function and depth

Hea

t Flu

x

Install

Ignition TDC

Crank Angle [°CA]

Diff

eren

tial p

ress

ure

Ignition TDC

Install

Crank Angle [°CA]

w plug ada

4-13

4

eep the temperature.

clic temperature driftasuring positions for

of installation position

ation position

Bowl-Flush

Bowl – 35 mm

Edge – flash

Edge – 35 mm

ation Depth


The indicating channel resulting from recessed mounting can, however,have the following negative effects on the measurement result:

Change in engine behaviour

The increase in the dead volume produces slight changes in thecompression ratio, which can affect the engine behaviour. Fuel can alsocollect in these areas, which has a negative effect on the emissions (HCin particular).

Pipe oscillations

The indicating channel represents an acoustic resonator, which isexcited by changes in pressure and produces oscillations. This effect isillustrated in Figure 4-5 where the measured pressure curves relate toindicating channels of different lengths. Five pressure curves from singlecycle measurements are shown for each indicating channel length. Theyhave been shifted in level to provide a clearer overview.

Figure 4-5: Influence of the length of the indicating channel on the measured pressure curve

The frequency of this interference depends not only on the length ofthe indicating channel but also on the gas state, which makes the use offrequency filters for eliminating pipe oscillations difficult. Furthermore itis not easy to distinguish pipe oscillations from combustion chamberoscillations.

Pressure [bar]

0

10

20

30

40

50

60

70

80

90

Druck [bar]

-20 0 20 40 60 -20 0 20 40 60 -20 0 20 40 60ZOT

Kurbelwinkel [°KW]ZOT

Kurbelwinkel [°KW]ZOT

Kurbelwinkel [°KW]

LKanal > 37 mm LKanal = 25 mm LKanal = 2,7 mm

Crank Angle [°CA] Crank Angle [°CA]Crank Angle [°CA]Ignition TDC Ignition TDCIgnition TDC

LChannel >37 mm LChannel >25 mm LChannel >2,7 mm


4

As shown in Figure 4-6, for design reasons there is usually an additionalvolume (V) in front of the pressure transducer in an installation with anindicating channel. This arrangement can be regarded as a simpleHelmholtz resonator. In simplified terms, the following equation can bespecified for the frequency of the pipe oscillation (f) [kHz]:

f = lV

r2

TR ππ

κ 2(4-1)

κ Isentropic exponentR Gas constant [J/kg K]T Gas temperature [K]cV Specific thermal capacity at constant volume [J/kg K]r Radius of the indicating channel [m]l Length of the oscillating gas column (in indicating channel) [m]V Volume [m3]

Figure 4-6: Indicating channel with additional volume

Figure 4-7 illustrates the relation to the length of the indicating channeland to the gas temperature (500, 1000 and 2000 K) for estimating thelevel of the frequency of pipe oscillations. The displayed values relate toan indicating channel radius (r) of 1.5 mm and a volume (V) ofapprox.11.8 mm3, which corresponds to the volume for a miniaturepressure transducer installed front-sealed (mounting thread M5x0.5).

Combustion Chamber

Druckaufnehmer

Volume

Indicating channel (r,l)

DruckaufnehmerPressure Transducer


Figure 4-7: Pipe oscillation frequency as a function of indicating channel length and gastemperature

Delays in the pressure signal

This effect occurs because the pressure wave first has to propagatethrough the indicating channel before it reaches the pressuretransducer. However, it is only relevant in extremely long indicatingchannels (e.g. ship’s engines with indicating cock).

Increased temperature load

In an unfavourable indicating channel arrangement, an increased heatflow load on the pressure transducer can occur due to high flow ratesat the diaphragm.

• Creation and design of the measuring position

In pressure indicating, the optimum design and workmanship quality of theadaptor is a vital part of the measurement accuracy (e.g. in glow plugadaptors: gap around the glow element, access bores to the sensor,replication of the exact geometric shape of the glow element, etc.; or inspark plug adaptors: access bores to the sensor, etc.), see Chapter 5.

The instructions provided by the (glow plug) manufacturer relating to thepermissible plays/gaps and roughnesses, and the requirements for thegeometrical accuracy must be complied with exactly.

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Länge des Indizierkanals [mm]

Frequenz[kHz]

2000 K

1000 K

500 K

Freq

uenc

y [k

Hz]

Length of indicating channel [mm]


4

4.2.2.3 Access to Measuring Point

When making the mounting bores for pressure transducers in cylinder headsensure that

• when transversing oil and water jackets, the oil and coolant flow is notinterfered with (i.e. the cooling function is not affected),

• no component is diminished in strength and

• the leakproofness of the adapted engine is guaranteed.

A detailed study of the cylinder head design is indispensable and technicalmanufacturing criteria should also be taken into account. See Chapter 5 forinstructions on creating mounting bores and examples of installations.


4.3 Pressure Transducers

4.3.1 Categories

Pressure transducers can be evaluated on the basis of their metrologicalproperties. Moreover it is useful to make a distinction between typicalcategories of pressure transducers with specific characteristics, see Table 4-3.A sensible preselection can therefore be made if the characteristics specificto each category are taken into account.

Table 4-3: Pressure transducers for engine instrumentationType of cooling Heat conduction (uncooled) Fluid cooling

Piezo material GaPO4 SiO2

Design Screw-in Probe Plug-in Plug-in Screw-in

M5 x 0.5 ∅ 4.3 ∅ 6.2 ∅ 9.9 M 10 x 1M14 x 1.25

Mounting principle Direct (in cylinder head or components) or using an adaptor

Spark plug/glow plug Requires extra space

Metrologicalcharacteristics as per specifications in the data sheets

Below is a summary of points pertinent to the selection of pressure transducers:

• Type of cooling/piezo material

As shown in Table 4-3, pressure transducers for engine instrumentation canbasically be divided into two categories.

One group is made up of the water-cooled transducers. They include thewidely used M10 and M14 pressure transducers with their screw-indesign and the ∅ 9.9 mm plug-in models. The measuring elements ofthese transducers only reach temperatures of 20 °C to 80 °C duringoperation.

The other group comprises the uncooled pressure transducers, whichare much smaller so that they can fulfil measurement tasks even in thetightest of spaces. This group includes in particular pressure transducerswith M5 thread design, ∅ 6.2 mm plug-in models and miniature pressureprobes (∅ 4.3 mm) which are used mainly in glow plug and spark plugadaptors. Using GaPO4 technology means that these transducers can be


[6]Pressure transducers

for engineinstrumentation


4

used at temperatures up to 400 °C at the measuring element. Tests haveeven been successfully carried out at up to 500 °C.

Cooled pressure transducers are normally larger than uncooled ones andusually have a higher sensitivity, i.e. a better signal-to-noise ratio. Due to thevery restricted space for mounting sensors in modern multiple valve engines,however, uncooled pressure transducers with good thermodynamicproperties are being used more and more. Using GaPO4, which has abouttwice the sensitivity of quartz has now made it possible even with smalluncooled pressure transducers to achieve similarly high sensitivities aspreviously achieved in larger transducers, and thus adequate signal-to-noiseratios.

Signal-to-noise ratioSignal-to-noise ratioSignal-to-noise ratioSignal-to-noise ratio: the lower the sensitivity is, the smaller the signal-to-noise ratio. Inextreme cases, the wanted signal can be superimposed on by the same magnitude ofinterference so that the pressure curve cannot be evaluated. Generally speaking, it can besaid that transducers with a sensitivity >10 pC/bar are unproblematic in measurementoperations.

Uncooled pressure transducers usually have higher natural frequencies dueto their design and size and are therefore more suitable than cooledtransducers for certain measurement tasks (e.g. knock detection,measurements at high speeds etc.).

Natural frequencyNatural frequencyNatural frequencyNatural frequency:::: the frequency (frequency spectrum) of the measuring signal, that has to beacquired with great accuracy, should be less than 20 % of the natural frequency of thepressure transducer used. The natural frequency of a typical pressure transducer forcombustion chamber indicating is between 50 and 200 kHz depending on the model.

Uncooled transducers generally have a higher cyclic temperature drift andabove all a higher load change drift than cooled transducer.

Furthermore, greater changes in sensitivity can be expected due to thehigher operating temperatures for uncooled pressure transducers despite theuse of GaPO4 as the piezo material. Cooled pressure transducers are morestable in their behaviour in terms of load and operating duration.


• Design

As mentioned above, a distinction is made between pressure transducerswith plug-in and threaded design, and the special designs. Plug-intransducers have the basic advantage that the achievable measurementaccuracy is largely independent of the tightening torque and of thermo-mechanical stresses in the cylinder head during operation. Thedisadvantage of plug-in transducers, however, is that they are more time-consuming to mount and remove.

• Mounting principle

Pressure transducers are mounted either directly in the test engine or bymeans of an adaptor. Uncooled pressure transducers with threaded designand the miniature pressure measurement probe designs have the particularadvantage that they can be used in spark plug or glow plug adaptors. Adetailed description of the installation principles is given inChapter 5.

4.3.2 Measurement Properties

The manufacturer’s specifications given in the data sheets should be used fordetailed pressure transducer selection. To make the choice easier for theuser, Table 4-4 shows an evaluation of the metrological properties ofpressure transducers and how their measuring position influences themeasurement task (characterised by the type of evaluation, see Table 4-1).


4

Table 4-4: Effect of pressure transducer properties and installation on signalevaluation

Type of evaluation(measurement task)

Effect of Max

imum

, min

imum

,am

plitu

de (c

ycle

)

Qua

litat

ive

curv

e be

fore

and

afte

r TD

C

Freq

uenc

y, a

mpl

itude

of h

igh-

freq

uenc

yos

cilla

tions

.

Inte

gral

val

ues

(ene

rgy

conv

ersi

on, m

ean

Diff

eren

t. va

lues

(hea

tre

leas

e, d

p /d

α

Measurement range W W W W WLifetime (cycles) W W W W WSensitivity C W C C CLinearity WNatural frequency W W WAcceleration sensitivity C W C CShock resistance W W W W WTemperature resistance (transducer) W W W W WChange in sensitivity over temperature C C C CCyclic temperature drift (heat flow pulse) WZero-line gradient (load/heat flow change) WZero point deviation (load change)

Pres

sure

tran

sduc

er p

rope

rty

IMEP stability (behaviour in continuousoperation)DeformationIndicating channel W

Inst

alla

tion

Gas flow W W

Key

No effect

Warning! Effect only avoidable through careful choice oftransducer, measuring position design, handling etc.

Calculable effect (e.g. 1% change in property means 1% changein pressure signal)

Significant effect

Substantial effect

W

C


4.4 Guidelines for Pressure Transducer Selection

This chapter finishes with a summary of the pressure transducer selectionprocedure based on the flow chart shown in Figure 4-8.

As mentioned above, the right way to start selecting a pressure transducer iswith a detailed formulation of the measurement task and the requirementsfor accuracy, stability, etc. that derive from it.

The next important step is the decision as to whether the measurement is tobe carried out with or without intervention in the test engine. The maincriteria for the decision are the accuracy requirements, the availability of atest cylinder head and the availability of time and funds.

If the decision is taken not to make any intervention in the test engine, onlyuncooled miniature pressure transducers or pressure measuring probes canbe used which are installed by means of spark plug or glow plug adaptorsdepending on the type of engine (petrol or diesel engine).

No or very little choice of measuring position and pressure transducer isthen possible.

Such an arrangement can fulfil the measurement requirements, themeasurement can be carried out. If not, one conceivable approach is toreduce the requirements. If that is not possible, it should be reconsideredwhether installation via a mechanical intervention in the test engine mightnot after all be possible which permits much better results. Otherwise, ameasurement is not possible.

If the decision is in favour of an intervention in the test engine, it is sensiblefirst of all to select a pressure transducer from one of the categories shownin Chapter 4.3.

All the transducers of that category can then be considered for the detailedselection based on pressure transducer properties.

The next step is to define the type of installation, the measurement site, thedesign of the measuring position, etc. which very considerably determinethe operating conditions and thus the effects on the pressure transducer.

With nointervention in the

test engine

With interventionin the test engine


4

The information about the extent of the effects and the metrologicalproperties of the selected transducer should be used to check whether therequirements can be fulfilled and, if so, the measurement can be started.

If the requirements cannot be met, other optimisation loops have to beimplemented for the selection of pressure transducers and their installationuntil a suitable combination is found. If that is not possible, the measurementrequirements should be reconsidered. Otherwise, accurate measurementsare just not possible.


Figure 4-8: Flow chart for pressure transducer selection

Eingriff amVersuchsträger

?

Measurement task

Define requirements resp.accuracy, stability, costs etc.

Preselection accordingpressure transducer category

(kind of cooling, design etc.)

Transducer according thechoosen category

Operation conditions (kind ofInstall., measuring position, shaping

of the measuring position ...)

Anforderungenerfüllt?

ja

Small uncooled pressureTransducer

nein

Spark plug - orGlow plug adaption

AndereKombination

Aufnehmer / Ein -bau?

Veränderung derAnforderungen

möglich?ja

nein

nein

Anforderungenerfüllt?

Veränderung derAnforderungen

möglich?

Möglichkeitmechanischer

Eingriff?

Performing of measuremnt

No Measurement possible No Measurement possible

ja

ja

ja

nein

nein

nein

nein

ja

ja

Intervention inthe test engine

?

Requirementsfullfilled?

yesno

-

Othercombination of

Transducer install.?-

Change ofrequirements

possible?yes

no

no

Requirementsfullfilled?

Change ofrequirements

possible?

Possibility ofmechanical

Intervention?

yes

yes

yes

no

no

no

no

yes

yes

Installing Piezoelectric Pressure Transducers 5-1

5

5 Installing Piezoelectric Pressure Transducers

5.1 Pressure Indicating with no Intervention in the Test Engine

Modern combustion chamber indicating is carried out, if possible, in boresthat already exist in the cylinder head. Adaptors are used which for spacereasons contain uncooled miniature pressure transducers or pressuremeasurement probes and are inserted in the place of original components,such as glow plugs or spark plugs. This makes it possible to minimize theadaptation work and thus costs.

5.1.1 Glow Plug Adaptation

Glow plug bores are the preferred measuring position in diesel engines withglow plugs. The adaptation type of is then primarily determined by thegeometric dimensions of the bore. If possible, a position should be aimed forwhere the pressure transducer diaphragm is close to the combustionchamber in order to minimize any pipe oscillations.

G M 1 2 D

AG 0 1

Figure 5-1: Glow plug adaptor with pressure transducer

Chapter 4.2.2.2


Long, thin glow plugs are best catered for specially developed pressuremeasuring probes to achieve a position close to the combustion chamber andthus an undistorted measurement signal (see Figure 5-2).

If the diameter of the bore is so small(< 4.3 mm) that the pressure measuringprobe has to be recessed behind thesealing surface, specially designed dampingvolumes in the adaptor ensure that possiblesignal distortion due to pipe oscillations inthe gas channel are significantly reduced.

The highest measurement quality can onlybe achieved with optimally matched adap-tors. The gap between the glow plug boreand the adaptor is of great significance interms of the thermal load on thetransducer. The larger the gap, the greaterthe temperature increase in the area of thetransducer and the greater the effect on themeasurement signal. That is why glow plugadaptors are manufactured to customerspecifications to the exact dimensions ofthe glow plug bore. Even with adaptorsoptimized in this way, temperatures of400 °C or more can still occur at thetransducer, which makes it necessary to usesuitable piezo materials (GalliumOrthophosphate ).

Figure 5-2: Glow plug adaptor with pressure measuring probe

Glow plugs improve cold start characteristics in diesel engines. Problems canarise during cold starts when a glow plug adaptor is installed instead of theoriginal glow plug. The start characteristics are hardly affected, however, atnormal ambient temperatures. [15]

5.1.2 Spark Plug Adaptation

Indicating measurements can be carried out in petrol engines withoutintervention in the cylinder head if a spark plug adaptor is used. Spark plug

Temperatures atthe pressure

transducer

Cold start


5

adaptors fulfil a dual function. On the one hand, like standard spark plugs theyignite the mixture in the cylinder, and on the other, they hold the pressuretransducer.

The requirement to bring the diaphragm of the transducer (Pressure MeasuringProbe) as close as possible to the combustion chamber necessitates a very sliminsulator. The assembly of the adaptor / transducer unit and the subsequentmounting to the engine should be carried out by engineers familiar with handling/ mounting sensitive pressure transducers

Figure 5-3: Different designs of spark plug adaptors

There are basically two designs for spark plug adaptors:

• Spark plug adaptor with miniature pressure transducer

A still widely used method is to install standard uncooled miniature pressuretransducer recessed in the spark plug head at the level of the hexagon

GM12D

GU12P


Figure 5-3, left). The combustion chamber is connected to the pressuretransducer by a relatively long connection channel.

Advantage: low cyclic temperature drift due to low heat flow load

Disadvantage: pipe oscillations

• Spark plug adaptor with pressure measuring probe

Since the advent of GaPO4 technology, a pressure measuring probe can nowbe mounted flush in the spark plug adaptor (Figure 5-3, right).

Advantage: minimum pipe oscillations even at high engine speeds

Disadvantage: cyclic temperature drift is greater than with recessedinstallation (corresponding to the usual pressure transducer installationclose to the combustion chamber)

When using spark plug adaptors, ensure that the suitable Heat range is selectedfor the engine. That is why spark plug adaptors are available with different heatvalues.

Heat range: the spark plug should be so hot during operation that deposits on parts thatprotrude into the combustion chamber burn without self-ignition occurring (i.e. self-cleaning). The heat range is a measure of the heat conductivity of the spark plug. Wemake a distinction between so-called cold and hot spark plugs. Cold plugs are usedprimarily for high-performance engines and for ranges close to full load whereas hotspark plugs are used mainly in engines that are predominantly operated in partialload state. Different heat values can be achieved by suitable selection of design andmaterials.

Furthermore, the Spark position (particularly in direct-injection petrol engines instratified partial load operation) and the electrode gap of the spark plug adaptormust be the same as the values for the original spark plug.

Spark position: spark position is taken to mean the position of the electrode in relationto the plug face.

Spark plug indicating is used especially for knock tests, engine map optimisationand for monitoring mode (e.g. peak pressure monitoring).


5

5.2 Pressure Indicating with Intervention in the Test Engine

This type of installation offers many possible ways of combining transducers andinstallation variations at different measuring positions. The right choice oftransducer, installation variant and measuring position is, however, highlycomplex due to the mutual dependencies. Many potential sources of error (e.g.indicating channel, high heat flow loads, etc.) must be taken into account inorder to obtain meaningful measurement results.

5.2.1 Installation Variations

The ideal installation position of a pressure transducer is almost flush with thecylinder head ceiling, i.e. slightly recessed. A distance of 0.5 to 1 mm betweenthe pressure transducer diaphragm and cylinder head ceiling should prevent thepiston from hitting the diaphragm, when being coated with deposits,.

A distinction is made between direct installations and installations using anadaptor sleeve. Adaptor sleeves are primarily required when an existing coolingwater jacket and/or oil channel has to be traversed for the pressure transducerinstallation.

5.2.1.1 Direct Installation

If the installation position permits, we recommend that transducers be installedstraight into the cylinder head of an IC engine, see Figure 5-4. That guaranteeseasy installation and removal of the transducer.

As mentioned above, the aim is to install the transducer fairly flush with thecombustion chamber. Experience has shown, however, that a slightly recessedtransducer installation with an oscillation-optimized indicating channel can haveadvantages over a flush installation. This applies in particular when thetransducer is subject to high thermal loads (e.g. in diesel engines). The diameterof the indicating channel should be the same as or greater than its length inorder to prevent undesirable pipe oscillations (see Figure 5-4, left).If two mounting principles are possible with a particular transducer type, thefront-sealing option is preferable to the shoulder-sealing installation. This ensuresthat a minimum pressure transducer surface is heated and also the sensitivity todeformation is reduced.

Chapter 4

The idealinstallation position

Chapter 4.2.2.2


Figure 5-4: Front and shoulder-sealing direct installation

5.2.1.2 Installation Using Adaptor Sleeve

If the requirements are not met for direct installation, the pressure transducer canbe installed in an adaptor sleeve, which seals the transducer off from coolingwater jackets or oil galleries. Often the camshaft cavity also has to be traversed,which may even necessitate further sealing with a second sleeve.

Unlike a direct installation, a sleeve installation involves more time and effort formachining and adaptation. The machining of the mounting bore in particularrequires very careful and exact workmanship. Adaptor sleeves are usuallyscrewed in with high-temperature resistant bonding agent (e.g. LOCTITE 648,UHU-plus) and sealed off from the cooling water and oil channel with O-rings.

G M 1 2 D Q C 3 3 DQ H 3 3 D

M 5 x0 . 5

M 1 0 x1

4 G

G,

Al

1.5

Sta

hl

10

.5m

in

0.5

min

7 . 5 min 1 4 min

3


5

Figure 5-5 shows an installation with adaptor sleeve for a small uncooledpressure transducer and Figure 5-6 for a water-cooled one.

Figure 5-5: Example of the installation of an uncooled pressure transducer with adaptorsleeve

M 7 x0 . 7 5

20

21

0m

ax

127.5

min

G M 1 2 D

AH 0 1

1 8 min

4 G

G,

Al

1.5

Sta

hl

1.5

stee

l


Figure 5-6: Direct installation of a cooled transducer with adaptor sleeve

If O-rings are used, the dimensions, tolerances and surface quality specified bythe O-ring manufacturer must be observed when preparing the measuringposition in order to ensure the necessary leakproofness. The function of the O-ring seal is to compensate any possible inaccuracies that can occur whenmachining the cylinder head bore which is usually inclined. An O-ring alsocompensates changes in length due to the thermal expansion of the cylinderhead and adaptor sleeve. As an alternative, a bond can be created using anelastic bonding agent (e.g. LOCTITE 290), but in that case, the gap between thecylinder head and the adaptor sleeve should be smaller than 0.05 mm.

0.5

min

7m

in

20

15

1

Ada pterhülse

M o nta gehülse

M 1 4 x1 . 2 5

2 6 min

Q C 3 3 CQ H 3 3 C

Adapter sleeve

Mounting sleeve


5

Once the installation is complete and the adaptor sleeve is sealed, the part thatprotrudes from the cylinder head into the combustion chamber should bemachined or cut off to restore the contour and surface quality of the cylinderhead ceiling.

Before a cylinder head with an adaptor sleeve fitted can be used formeasurements, the sleeve installation must always be checked for leaks. This isdone in a water bath. The cooling water and/or oil channel of the cylinder headmust be cleared out with compressed air before the sleeve is installed.

Leakage problems can still arise later during measurements, however, which inextreme cases can cause damage to the engine due to Water slap.

Water slap: water gets into the combustion chamber due to leaks in the adaptor sleeve.If the volume of water entering the combustion chamber is greater than thecompression volume, the forces that then occur can kink the conrod and thusirreparably damage the engine.

Water slap can also occur with a direct-installed water-cooled transducer if thetransducer’s cooling water gets into the combustion chamber as a result of a faultytransducer.

Reasons for possible leaks of adaptor sleeves are:

• Sealing surfaces have not been machined properly

• Damage to the O-rings during installation or removal of the sleeves

• Sleeve has worked loose due to improper installation or removal of thetransducer

• Different thermal expansion coefficients between cylinder head andadaptor sleeve

5.2.2 Instructions for Machining the Mounting Bore

A series of special tools and accessories (e.g. stepped drill, screw tap, drill jigsockets, sealing kits, mounting tools etc.) is used to make creating the installationbore as easy as possible.

5.2.2.1 Example of Direct Installation

On the left of Figure 5-7 is an uncooled pressure transducer ready to be installed and on theright the machined bore ready for a front-sealing installation. If the external contour of the

Source of errors



cylinder head is the same as in the example, i.e. at right-angles to the bore axis, the bore canbe drilled centred at the defined measuring position.

Figure 5-7: Pressure transducer and bore ready for transducer installation

The bore is then finished in one go using a stepped drill (Figure 5-8 left). Specialattention must be paid to the exit situation of the drill. If the measuring position isin a curvature or if the angle between the exit surface and the bore axis deviatessignificantly from 90 °, material must be clamped to the combustion chamberend to stop the drill from running off centre as it exits. Figure 5-8 centre showsthe next process, i.e. thread tapping, with a drilling socket being used so that thedrill can be accurately guided.

The bore has to be deburred and cleaned of all chips and shavings by means ofthe mounting tool before the transducer is installed as shown on the right ofFigure 5-8.


5

Figure 5-8: Work stages for installing a transducer

5-12

5.2.2.2 General examples of Installatio

Figure 5-9 shows two uncooled pressureusing adaptor sleeves, where one transvethe other transverses both the cooling situation like this, it should be rememberedisturbed by the installation of transduceffects on the cooling capacity due to alte

Figure 5-9: Cylinder head with two installed pre

The work stages necessary for the instFigure 5-14.

Engine Indicating

n Using Adaptor Sleeves

transducers installed in a cylinder headrses the cooling water jacket only and

water jacket and the oil channel. In ad that the cooling water circuit may beers which may result in local negativered flow conditions.

ssure transducers

allation are shown in Figure 5-10 to


5

5.2.2.3 Inclined bore axis single sealing sleeve

Arbeitsschritt 1

Arbeitsschritt 3

Arbeitsschritt 2

Arbeitsschritt 4

Figure 5-10: Work stages for single sleeve and inclined bore axis

Because the surfaces of the material to be drilled are not at right-angles to thebore axis an initial centering must be machined with a single-lip drill, see WorkStage 1. If a single-lip drill is not available, a starter cut has to be made on thesurface at right-angles to the bore axis to prevent the stepped drill from runningoff centre.

Since the measuring position also lies in a curved surface, a piece of materialshaped to the contour of the cylinder head has to be clamped to the head fromthe combustion chamber side before Work Stage 2 is carried out so that thestepped drill does not break or run off centre when it exits.

In Work Stage 3 the thread is tapped for securing the adaptor sleeve. A drillsocket is used to ensure that the screw tap is accurately guided. It is advisable toleave the counter piece clamped to the cylinder head for the tapping work aswell because otherwise the tool can become damaged. Work Stage 4 shows

Workstage 1 Workstage 2



how a single-lip drill is then used again to re-drill the sealing surfaces. Afterwardsthe bore has to be deburred and cleaned of all chips and shavings. The quality ofthe O-ring sealing surface must also be checked.

Arbeitsschritt 5 Arbeitsschritt 6

Figure 5-11: Work stages for installing the adaptor sleeve

Work Stages 5 and 6 describe the installation of the adaptor sleeve. It consists ofthe adaptor itself in which the pressure transducer is later installed, and aseparate sealing sleeve, which can be slipped over the adaptor. The next thing isto decide the correct position of the sealing sleeve on the adaptor. To do that,the adaptor is temporarily installed and the sealing sleeve pushed into the correctposition. The position is marked so that the adaptor and sealing sleeve can bebrazed or bonded together in exactly that position after removal from the bore.Next step is to shorten the part of the sleeve, which protrudes the hexagon.

Before the finished adaptor sleeve is installed, the O-ring is inserted in the grooveprovided for it and the bonding area carefully cleaned of all grease ready forbonding. For perfect sealing the adhesive should be a high-temperature resistant,low-viscosity one-component adhesive (e.g. LOCTITE 648) or an appropriate two-component adhesive (e.g. UHU plus or ARALDITE). It has to be applied evenly tothe thread and sealing surface of the cylinder head bore, and to the thread of theadaptor sleeve before the sleeve is mounted in the bore.

When the adhesive has hardened, the part of the adaptor protruding into thecombustion chamber has to be shortened and shaped to the contour of thecombustion chamber. (Work Stage 6). Figure 5-12 shows how the pressuretransducer is then installed using a suitable mounting tool.



5

Figure 5-12: Installing the pressure transducer

5.2.2.4 Perpendicular bore axis multiple sealing sleeves

Figure 5-12 and Figure 5-14 show the Work Stages from drilling the installationbore to installing the pressure transducer when cooling water jackets and oilchannels have to be transversed perpendicular. In this example, the initial drillingwith a single-lip drill is not necessary because of the favourable drilling angle.However, due to the pronounced curvature at the exit, a suitably shaped pieceof material has to be clamped to the cylinder head at the combustion chamberend.

Work Stage 1 involves drilling with a stepped drill. In Work Stage 2, the thread istapped using a drill socket. Work Stage 3 shows how the two O-ring sealingsurfaces are re-drilled.

Special Box Spanner


Arbeitsschritt 1 Arbeitsschritt 2 Arbeitsschritt 3

Figure 5-13: Work Stages for multiple seals and perpendicular bore axis

The bore is then deburred and cleaned of chips and shavings. The adaptor istemporarily installed and the correct positions of the sealing sleeves are marked.After the adaptor has been removed from the bore again, the sealing sleeves arebrazed or bonded to the adaptor and the O-rings are fitted into the groovesprovided for them. The bonding surfaces should then be meticulously cleanedbefore the adhesive is applied evenly to the thread and the sealing surface of theadaptor and cylinder head bore and the adaptor sleeve is screwed into thefinished bore (Work Stage 4).

Work Stage 5 involves cutting off the adaptor at the combustion chamber end.The cut has to be processed to match the contour and surface quality of thecylinder head ceiling. Finally in Work Stage 6, the pressure transducer is installedusing a suitable mounting tool.

Workstage 1 Workstage 2 Workstage 3

Installing Piezoelectric Pressure Transducers

Arbeitsschritt 4 Arbeitsschritt 5 Arbeitsschrit

Special Box Spanner

Figure 5-14: Installing adaptor sleeve and transducer

Workstage 4 Workstage 5 Workstage 6

5-17

5

t 6


Calibration 6-1

6

6 Calibration

6.1 General

Calibration is used to determine the relationship between output and inputparameters (i.e. transmission function). Known values for the input quantity arepredefined for this purpose and the corresponding output signals are recorded.

Each component of the piezoelectric pressure measurement system (i.e. pressuretransducer, charge amplifier and A/D converter) has a certain transfer function,see Figure 6-1. Basically it is possible to determine the transfer function of theentire measurement system from the transfer functions of the individualcomponents.

Druckaufnehmer A/D-WandlerLadungsverstärker

Druck[bar]

elektr. Ladung[pC]

Spannung[V]

steps

pC / bar steps / VV / pC

V / bar

steps / bar

Übertragungsfunktion derEinzelkomponenten

ÜbertragungsfunktionDruckaufnehmer und Ladungsverstärker

ÜbertragungsfunktionDruckaufnehmer, Ladungsverstärker und A/D-Wandler

Figure 6-1: Calibration methods

Unfortunately, this procedure has the disadvantage that the individual errors canadd up to a large sum error. Furthermore, possible changes in the characteristics

Componentcalibration/measurementsystem calibration

Display onindicatingequipment in V orbar (calculatedfrom Step-values)

Source of errors

Pressure Transducer Charge Amplifier A/D Converter

Pressure[bar]

Charge[pC]

Voltage[V]

Transferfunctionof the single components

Transferfunctionof Transducer and Charge Amplifier

TransferfunctionTransducer, Charge Amplifier and A/D converter


(i.e. sensitivity, linearity) of the various components are not taken into account. Inpractice the situation where charge amplifiers or A/D converters are calibratedseparately for lack of calibration devices hardly ever arises – usually themanufacturer’s specifications tend to be used which can change during theoperating time of the devices. For measurements with high accuracyrequirements therefore, it is absolutely necessary to calibrate the entiremeasurement system.

6.2 Type of Calibration

A distinction is made between two types of calibration depending on thenumber of calibration points:

• Multi-point-calibration

A multi-point calibration is always carried out when particularly highaccuracy requirements are placed on the measurement or there is asuspicion that changes have occurred in the linearity (for example, in thepressure transducer due to overheating).

The pressure transducer is subjected to a series of pressure values. Theresult of a multi-point calibration is the so-called characteristic from whicha mean sensitivity and the linearity can be determined.

• Single-point calibration

In most applications, however, a single-point calibration is sufficientbecause the linearity deviations of the pressure transducer and the othercomponents of the pressure measurement system are minimal. Here thecalibration is carried out at a value that is about 80 % of the maximumvalues (expected from the measurement).

Calibration 6-3

6

6.3 Calibration Using Dead Weight Tester

Defined pressure values (pressure jumps) are usually applied with what is knownas a dead weight tester.

6.3.1 Construction and Function of Dead Weight Testers

The diagram in Figure 6-2 shows the basic construction of a dead weight tester.A dead weight tester essentially consists of the following components:

• a hydraulic cylinder with weighted plunger

• a device for producing pressure (e.g. hand pump, hand wheel)

• a 3-way changeover valve for creating the calibration jump

• a (heatable) holder for mounting the pressure transducer

• a pressure gauge for visually checking the system pressure

• a system of hydraulic lines for connecting the above components

• a set of weights for loading the plunger

The primary plunger is actuated by a hand wheel and spindle or a hand pump tocreate pressure. As of a certain system pressure the movable secondary plungerwith the weights on it is raised because the hydraulic system is completely sealedat one end by the pressure transducer and the 3-way changeover valve. Thepressure in the system is determined only by the total secondary plunger massplus the added weight. The pressure is usually displayed on a pressure gauge toprovide a visual check.

Construction

Functioningprinciple


3-WegeUmschaltventil

Druckauf-nehmer

Manometer

beheizbareAufnahme

Hydraulik-system

gewichtsbelasteterSekundärkolben

Primärkolben

Handrad mit Spindel

entlasteterZustand

belasteterZustand

Figure 6-2: Schematic representation of the construction of a dead weight tester design

The diaphragm of the pressure transducer is subjected either to the systempressure (Figure 6-2, loaded state) or ambient pressure (Figure 6-2, unloadedstate) with the aid of a 3-way changeover valve. Each time the system is switchedto ambient pressure, a little hydraulic fluid escapes into an expansion vessel (notshown in the diagram). This causes the secondary plunger to move downwardsslightly longitudinally. When the transducer is then subjected again to systempressure, it is important to ensure that not too much hydraulic oil has escapedand that the secondary plunger is resting against the contact surface. Ifnecessary, the level should be adjusted by pumping in more hydraulic oil usingthe hand pump or hand wheel.

The secondary plunger is made to rotate during the calibration process in orderto minimise the influence of friction between it and the cylinder.

weight loadedsecondary piston

loadedcondition

Transducer

Heatablehousing

3-wayswitch-over

valve

Hydraulic-system

Primary Piston

Handwheel with shaft

dischargedcondition

Calibration 6-5

6

The calibration pressure (pk) is determined by Equation 5-1:

p gA

(M + M )KSK

SK Z= = ⋅FASK

(5-1)

F ............... plunger forceASK ........... secondary plunger area,MSK .......... secondary plunger mass,MZ ........... added mass (weights)g .............. mean gravitational acceleration (9.81 m/s²)

Some dead weight testers have a heatable receptacle for mounting the pressuretransducer so that the calibration can be carried out at the probable operatingtemperatures and thus temperature-based changes in sensitivity can be takeninto account.

6.3.2 Interference during Calibration

The top diagram in Figure 6-3 represents the perfect calibration jump in whichthe loading and unloading jump are exactly the same. In practice, however, aninterfering overshoot occurs during the loading jump, which is caused by theoscillations in the hydraulic system, see bottom diagram in Figure 6-3. Theunloading jump should therefore always be used for the calibration.

Belastung EntlastungZeit

Dru

ck

Zeit

Dru

ck

Zeit

Dru

ck

p

P

p

Umgebung

Umgebung

K

P K

Figure 6-3: Calibration jump

Temperaturecalibration

el 3.1.2.1

Pres

sure

Pres

sure

P Surrounding

P Surrounding

Loading Relief

Time

Time


The effect of electrical drift during calibration is shown in Figure 6-4. When theRESET switch on the charge amplifier opens, a significant drift in the output signalcan be observed which is superimposed over the entire calibration process. Ifduring the unloading jump not difference (∆AB) is determined exactly butdifference ∆AC is determined by time-offset acquisition, the error caused by theelectrical drift is included in the calibration result.

To prevent such errors, state-of-the-art data acquisition systems allow thecalibration jump to be recorded exactly from which the calibration value canthen be calculated.

Belastung EntlastungZeit

Aus

gang

ssig

nal

RESETöffnen

RESETschließen

∆

B C

A

AC∆AB

Fehler

Figure 6-4: Influence of electrical drift during calibration

Due to the leakage problems in the changeover valve, high pressure dead weighttesters have an arrangement as shown in the top half of Figure 6-5 with (non-switchable) needle valves for producing the pressure jump. The loading valve isclosed before the unloading valve is opened. The pressure at the pressuretransducer also changes in an undefined way due to the change in volume thatoccurs, see bottom half of Figure 6-5. To determine the calibration valuecorrectly, it is imperative to ensure that the difference is used between the valuedisplayed immediately before the loading valve closes which corresponds to thepressure defined by the load of the applied weights, and the value immediatelyafter the unloading valve opens.

Source of errors

Source of errors

Out

put s

igna

l

open

close

ErrorTime

Loading Relief

Calibration 6-7

6Belastung Entlastung

Zeit

Aus

gang

ssig

nal

p K

Belastungsventil schließen Entlastungsventil öffnen

Druckaufnehmer

Figure 6-5: Unloading jump in high pressure dead weight testers with loading and unloadingvalve

6.4 Important Instructions

The following points should be observed during calibration:

• Calibration of the entire measurement system

The entire measurement system must be calibrated to achieve highaccuracy (i.e. pressure transducer, cables used, charge amplifier and A/Dconverter).

• Calibration pressure

Piezoelectric pressure transducers are usually calibrated at about 80 % ofthe anticipated measurement pressure (except in measurements in the gasexchange system where they are calibrated at 100 % of the expectedmeasurement pressure). In any case, the selected calibration pressure mustalways be in the upper half of the measurement pressure range, otherwisethe measurement accuracy will be too low, particularly in the peakpressure range.

• Keep the electrical drift small

As during the measurement itself, it is also necessary during the calibrationto keep the electrical drift as small as possible.

Out

put s

igna

l

Loading Relief

Time

Loading valve closes Unloading valve opens

Transducer


• Disable drift compensation

If a charge amplifier with drift compensation is used for the calibration, it isabsolutely necessary to deactivate it during the calibration.

• Use the voltage range of the A/D converter

The gain should be set on the charge amplifier so that the largest possiblepart (70 - 90 %) of the available voltage range of the A/D converter is usedso as to keep the digitization stages as small as possible. A safety gapshould be maintained, however, at both the lower and upper limit of themeasurement range.

• When is it sufficient to set the gain rather than carry out calibration?

If the quality requirements for the measurement result are not high or nodead weight tester is available, the assignment of pressure to output signalcan be determined by setting the sensitivity value of the pressuretransducer used on the charge amplifier. In that case, a pressure jumpcorresponding to the RANGE setting produces a voltage of 10 V at thecharge amplifier output.

Zero-line Detection 7-1

7

7 Zero-line Detection

Due to their working principle, piezoelectric pressure transducers can onlymeasure the changing pressure content (pMeas), i.e. only the pressure changes inthe cylinder, and not the physically correct absolute pressure (p). The measuredpressure curve (pMeas) therefore has to be adjusted in accordance withEquation 7-1 by the amount (∆pn) (zero-line shift).

nMess p)(p)p( ∆+= ϕϕ (7-1)The technical literature features a whole range of methods for determining thezero-line level. They differ mainly only in terms of accuracy and necessarycomputational work. Since a correct pressure curve forms the basis of anaccurate heat release calculation, correct zero-line detection is of centralimportance. The chapters below deal with the most frequently applied methods.

7.1 Reference Method

7.1.1 Fixed Point (Empirical)

Under this method, the measured cylinder pressure (pMeas) at a specific crankangle (ϕRef) (i.e. reference angle) is adjusted to a predefined pressure value (pFix )(i.e. reference point). The signal for the measured pressure curve (pMeas) in therange of reference angle (ϕRef) is averaged to minimise the effect ofinterferencing signals/spikes on the measured cylinder pressure curve, seeEquation 7-2.

)(1

1 2/

2/

Re

Re

ϕϕ

ϕϕ

+

−=+−=∆

N

NMessFixn

f

f

pN

pp (7-2)

• Ambient pressure

Often the ambient pressure (e.g. 1 bar) serves as the reference value,which is set to the cylinder pressure at gas exchange TDC (GETDC). Thisapproach, however, only provides adequately accurate zero-line detectionin unthrottled naturally aspirated engines. In other types of engine andengine map points, this type of adjustment can cause errored results


because the cylinder pressure at gas exchange TDC no longer correspondsto the ambient pressure.

• Intake pipe pressure

Another way to adjust the fixed point quickly is based on the assumptionthat the cylinder pressure approximates fairly closely to the intake pipepressure during the intake phase. The cylinder pressure in the range of thegas exchange bottom dead centre (GEBDC) can therefore be set to themean pressure in the intake pipe.

The accuracy of this method depends to a large extent on the quality of theintake pipe pressure measurement. The pressure in the intake pipe isusually measured with piezoresistive pressure transducers the accuracy ofwhich varies heavily with temperature. In rare cases, U-tubes are still usedfor the intake manifold pressure measurement.

The advantage of fixed point adjustment is that it is quick and easy to carry out.

7.1.2 Measured Pressure Curve in the Intake Pipe

The accuracy of zero-line detection using the intake pipe pressure can besignificantly increased by using the crank-angle-based, cylinder-specific intakepipe pressure curve instead of the mean intake pipe pressure for zero-linedetection. The pressure between the maximum inlet valve lift and the gasexchange BDC is then set to the same value as the intake pipe pressure curvemeasured in that range.

7.2 Thermodynamic Zero-line Coefficient

7.2.1 Assumption of a Constant Polytropic Exponent

Thermodynamic zero-line detection is based on a comparison of the measuredand a calculated pressure curve. A relatively simple method for doing this is themethod devised by Hohenberg [17] which assumes a constant polytropiccoefficient in a certain crank angle range.


7

Equation 7-3 applies to the polytropic compression:

CVV

pppp

n

nMess

nMess =

=

∆+∆+

2

1

1

2 (7-3)

This then produces Equation 7-4:

CppCp MessMess

n −−⋅=∆

121 (7-4)

The following values are recommended as the polytropic coefficient (n):

• for diesel engines and for petrol engines in motored operation:n = 1.37...1.40

• for petrol engines with combustion:n = 1.32...1.33

The following ranges are recommended for the two crank angle values (ϕ1) and(ϕ2):

ϕ1 = 100 °CA 80 °CA before Ignition TDC

ϕ2 = 70 °CA 60 °CA before Ignition TDC

This type of zero-line detection is very often used in practice because it offersgood accuracy, despite of its simplicity and quickness of calculation. The mainuncertainty of this method is associated with the use of a constant polytropiccoefficient (n) which in fact changes over the crank angle range. The crank angleinterval should therefore not be too large in order to minimise this influence.

Another weakness of this method is the fact that interference noise is oftensuperimposed on the pressure curve in this crank angle range.

In some cases, the interference signal from valve impact (structure-borne noisewhen the inlet valve hits the valve seat at inlet closes) or the effects of structure-born noise at very high speeds (e.g. in racing engine) affect thermodynamic zero-line detection so severely that the results can be fairly wrong.

7.2.2 With the Aid of the Integral of the Heat Release

This method is based on the assumption that no heat may be released in thecompression area [21].


02

1

=⋅ ϕϕ

ϕ

ddQB (7-6)

The zero-line shift has to be estimated first as initial value using one of themethods described above, before the necessary parameters can be calculated.The zero-line shift is then adjusted iteratively.

This principle using the integral heat offers high accuracy but is not suitable forreal-time zero-line detection due to the necessary computation times.

7.2.3 Comparison of Measured and Calculated Pressure Curve

This method is based on the calculation of the compression line. The pressurelevel is varied until the area between the measured and the calculatedcompression line is at a minimum [14]. The calculation is usually carried out inthe range between inlet closes and ignition point or start of injection.

The advantage of this method is that the compression ratio and angle positioncan also be adjusted.

The relatively large calculation time and effort, however, means that the methodis unsuitable for real-time applications. Its high accuracy, on the other hand,makes it highly recommendable for thermodynamic analyses with high accuracyrequirements.


7

7.3 Comparison of Methods

In addition to the system-based source of errors in each method, the result ofzero-line detection is also affected by “external” factors, such as

• short term drift/thermoshock in the pressure transducer

• interference signals due to acceleration excitation of the pressuretransducer (at valve closes, high speeds)

• interference signals due to electrical effects (ignition voltage, injectorcontrol)

When deciding on the method, it should therefore be borne in mind that anapparently suitable one can easily lead to errored results (Table 7-1).

Table 7-1: Assessment of zero-line detection methods

Fix

poin

t adj

ustm

ent w

itham

bien

t pre

ssur

e

Fix

poin

t adj

ustm

ent w

ithm

ean

inta

ke p

ipe

pres

sure

Fix

poin

t adj

ustm

ent w

ithC

A-re

solu

tion-

base

din

take

pip

e pr

essu

re

Con

stan

t pol

ytro

pic

coef

ficie

nt

Inte

gral

of H

eat R

elea

se /

com

paris

on m

easu

red-

calc

ulat

ed p

ress

ure

curv

e

Peak pressure pmax o o + + +Duration of combustion - o o o +Combustion delay - o + + +Position of 50% energyconversion

- o + + +

Energy balance - o o o +Additional metrological effort no (yes) yes no yesAccuracy moderate moderate good good very good

Method suitable for realtime yes yes yes yes no

+ ...... suitableo ...... suitable to a limited extent- ....... not sufficient


Maintenance, Repair and Cleaning 8-1

8

8 Maintenance, Repair and Cleaning

Piezoelectric pressure transducers are precision devices. The manufacturer’sspecifications can only be guaranteed if they are handled with care. Thediaphragms and sealing areas of a pressure transducer must always be protectedagainst mechanical damage. Apart from that, particular attention should also bepaid to maintaining the insulation resistance. Furthermore, the cooling watershould always be able to flow unimpeded in cooled pressure transducers (i.e.there should be no scaling).

8.1 Insulation resistance

Low insulation resistances in the pressure transducer, connector and cable and atthe input to the charge amplifier cause significant measurement errors, especiallyduring similar-to-static calibration of the measurement system. The followingminimum insulation values are recommended depending on the charge output:

• Q < 1000 pC…RIns > 5x1012 Ω• Q > 1000 pC…RIns > 1x1012 Ω

Charge (Q) is calculated from the product of the pressure transducer sensitivityin pC/bar and the pressure jump in bar.

8.1.1 Maintaining High Insulation Resistance

High insulation resistance can be maintained by observing the following points:

• Take care to keep oil and water away from the connector.

• Do not touch the connector insulator with your hand and do not blow it outwith compressed air (i.e. always place the cap on the connector beforecleaning the cooling water galleries with compressed air).

• Pressure transducers and piezo-input cables should preferably be treated asa single assembly. The cap should always be placed onto the connector atthe free end of the piezo-input cable to close it off. If the piezo-input cableis unscrewed from the pressure transducer, the cap should be screwed onto the connector of the pressure transducer to close it off.


• The transducer’s cooling water gallery should be emptied after a water-cooled pressure transducer is removed from its mounting position.

• At relative humidity levels higher than 50 %, pressure sensors and piezo-input cables should be kept in a moisture-proof container (desiccator)together with a drying agent (e.g. blue gel).

8.1.2 Restoring High Insulation Resistance

The following measures can be taken to restore high insulation resistance:

• Clean the connectors of pressure transducer and connecting cable (theymust be completely clean, free of grease and dry). Use purified petrol,isopropanol or ethanol, a fine-hair brush and a non-fraying cloth to cleanthem. Pulling them in an ultrasonic bath for about 10 minutes improves thecleaning effect, but the solvent should only ever be used once. If necessary,the pressure transducer should be dried without the piezo-input cable at120°C (normally for a period of 8 hours, or a maximum of 72 hours). If thatdoes not restore the high insulation resistance, we recommend returningthe pressure transducer to the manufacturer for inspection and/or repair.

• Piezo-input cables can be dried in accordance with the manufacturer’sinstructions.

8.2 Descaling Cooling Water Galleries

Special attention should also be paid to the cooling water galleries of pressuretransducers. If they become blocked by scaling, they can be cleaned by pumpinga standard descaler solution through them (e.g. amino acid, ethanoic acid, formicacid) or leaving it in the galleries for a while to dissolve the scale (until thecharacteristic flow rate for the transducer is restored) and then rinsed out withdistilled water. After rinsing, the cooling water galleries should be cleaned bycentrifuging or blowing out – as should also be done when a pressure transduceris removed from its mounting position.

CautionFollow the

manufacturer’s safetyand disposal

instructions for thechemicals used.

Maintenance, Repair and Cleaning 8-3

8

8.3 Cleaning the Outside of Pressure Transducers

In the various operating states of diesel and petrol engines, deposits occur on thesurface of the combustion chamber and therefore also on the surface of thepressure transducer (i.e. soot, burnt-in oil) which can distort the measurementsignal. Pressure transducers should therefore be cleaned of such depositsregularly but as carefully as possible. Chemicals should be used to clean thembut if cleaned mechanically you should only use methods will cause no damageto the sensor surface.

We recommend the following cleaning procedure:

(1) First check whether the connector between the piezo-input cable and thepressure transducer is tight enough (manual check).

(2) Spray the complete pressure transducer several times with iso-propanol ina spray bottle (from about 3 to 4 mm below the cable connector). Placethe pressure transducer at an inclined angle to do this (with the end facepointing downwards). Ensure that no solvent gets into the connector/cablegap as it may contain residual impurities.

(3) Then clean the pressure transducer as well as possible over the entire areadescribed in (2) with a cloth soaked in iso-propanol.

(4) This is followed by a 5 to 10-minute dip in a solution of 20% DeconexUV11 in distilled water or 20% Extran-alkal. in distilled water. Ultrasoniccleaning is not absolutely necessary but can be a good idea.

(5) Rinse the pressure transducer once with distilled water and dip it twice indistilled water (for a minute each time).

(6) The pressure transducer should then be rinsed with iso-propanol asdescribed in (2) and (3) and wiped dry.

CautionThis cleaningprocedure does notapply to connectors.Follow themanufacturer’s safetyand disposalinstructions for thechemicals used.

CautionDo not treat thepressure transducerwith iso-propanol in anultrasonic bath.


8.4 Repairing Pressure Transducers

Indicating sensors are made of high-precision, mechanical components that areso well attuned and connected to each other during manufacture that it is notnormally possible to replace any of the components. Such a repair would also betoo time-consuming. Only very few types of damage can be remedied by arepair that would be of acceptable time and cost:

• Decrease in the high insulation resistance (clean the transducer, see alsoChapter 8.1)

• Rupture or damage to the cooling water nipple (replace the nipple)

Slight mechanical damage to the sensor housing (machine to restore theinstallation dimensions)

Crank Angle Encoder 9-1

9

9 Crank Angle Encoder

Measurements of dynamic quantities, i.e. quantities that change over time, bytheir very nature require a time base. The virtually time-proportional crank angleis here the obvious answer and is more or less the only solution used in practice.

In engine indicating therefore measurements are carried out on an angle basis.The reason for that is firstly that the angle assignment of pressure and otherrapidly changing quantities to the position of the crankgear largely determinesthe behaviour of an IC engine, and secondly, the crank angle is relatively simpleto pick up.

9.1 Digital Signal Recording

Only digital angle measurement principles can be considered due to thecurrent type of data acquisition and processing:

• Absolute angle measurement

Digital angle measurement principles producing absolute values alwaysdeliver the absolute value of the current crank angle, for example, over adigital pattern (bit pattern) of the respective value. These relativelycomplicated, absolute-measuring principles must, however, be discountedbecause of their sensitivity to vibrations, high temperatures and electricalscatter.

• Relative angle measurement

Crank angle encoders are usually based on relative, digital measurementprinciples and thus deliver just pulses. For this purpose, a trigger mark isused for the synchronisation (or triggering) after each revolution and anumber of angle marks (usually 360 or 720 marks per revolution) for theangle information.

In addition to discretization of the basis (i.e. time, crank angle ...), the amplitudeof the signal also has to be discretized, i.e. divided into steps. The step width andtherefore the number of steps used for the discretization represents a majorinfluence on the quality of the signal obtained.

Anglediscretization

Signaldiscretization


Figure 9-1 illustrates the principle of the discretization of analogue signals. Thesignal amplitude is represented by the next lowest discretization level at eachdiscretization step of the basis (i.e. crank angle) (in the AVL scheme). If wecompare the coarse crank angle and amplitude resolution with the fine one, wecan see how important it is to choose an adequate resolution and sampling rate.Whereas at a coarse resolution the high-frequency parts of the analogue signalare not acquired, they can be rendered well when the signal is discretized at afine resolution.

Analogsignal „grobes“ Digitalsignal„feines“ Digitalsignal

grobe Winkelauflösung

feine Winkelauflösung

fein

e A

mpl

itude

nauf

lösu

ng

grob

e A

mpl

itude

nauf

lösu

ng

Figure 9-1: Principle of signal discretization

Analogsignal „grobes“ Digitalsignal„feines“ Digitalsignal

grobe Winkelauflösung

feine Winkelauflösung

fei n

e A

mpl

itude

nauf

lösu

ng

grob

e A

mpl

itude

nauf

lösu

ng

Figure 9-2: Influence of angle and amplitude resolution on the discretization

Figure 9-2 shows that choosing an insufficient sampling rate can in extremecases even result in completely distorted representation of the actual analoguesignal and also of the integral value (e.g. indicated mean effective pressure).When a coarse sampling rate is used, only the local minima are acquired and the

Analog signal „coarse“ digital signal„fine“ digital signal

Coa

rse

ampl

itude

reso

lutio

n

Fine

am

plitu

de re

solu

tion

Fine angle resolution

Coarse angle resolution

„coarse“ digital signal„fine“ digital signal

Coa

rse

ampl

itude

reso

lutio

n

Fine

am

plitu

de re

solu

tion

Fine angle resolution

Coarse angle resolution

Analog signal


9

analogue signal is then incorrectly discretized as a rising curve trace. At a fineresolution, however, the analogue signal is discretized correspondingly well.

Figure 9-3 shows another example of the effects of an inadequate crank angleresolution when indicating measurements are carried out on knockingcombustion cycles. Whereas the high-frequency pressure oscillations are clearlyshown at a sufficiently fine resolution, (fig. at the left) they are only justdiscernible or completely unrecognisable at a crank angle resolution that is toocoarse.

0

10

20

30

40

50

60

70

80

90

100

-20 0 20 40 60

0

10

20

30

40

50

60

70

80

90

100

-20 0 20 40 60

fine angle resolution coarse angle resolution

Figure 9-3: Influence of the resolution on the display of knocking combustion

In practice, a 12 bit resolution (4096 steps) or a 16 bit resolution (65536 steps) isusually used to discretize the signal amplitude. High-frequency parts of the signal(e.g. pressure oscillations due to knocking combustion) require sampling rates of0.1 °CA for discretization of the basis.


9.2 Function Principle and Construction of Crank AngleEncoders

9.2.1 Principles of Signal Generation

Different signal-generating principles are applied to produce the angle signal,depending on the requirements:

• Optical sensors

• Inductive sensors

• Other: Hall-type sensors, capacitive sensors, etc.

9.2.1.1 Optical Sensors

The measurement principle is based on light barriers. The backlighting orreflection lighting method is used depending on the configuration. Optical crankangle encoders satisfy high accuracy requirements (even with small dimensions)due to the possibility of highly accurate structuring of specially developed markerdisks. Another advantage is that they are less susceptible to interference,especially when using optical fibres working in the vicinity of large electricalinterference fields. The crank angle resolution is usually less than 1°CA.

9.2.1.2 Inductive Sensors

Inductive sensors comprise at least a permanent magnet and a coil. If you movea magnetically conductive mark carrier (e.g. toothed wheel) in front of the sensoras shown in Figure 9-4, a voltage is induced in the coil. The output voltage ofinductive sensors, depends very much on the speed, low speeds are thereforedifficult to detect.

Inductive sensors (but also Hall-type and capacitive sensors) are usually used inconjunction with toothed wheels that already exist in the engine. The achievableaccuracy and crank angle resolution tends to be low, which is why this type oftransducer is mainly used for monitoring purposes and/or speed measurements,not least because relative movements between sensor head and mark carrieraffect the signal.

AVL ProductInformation:[12]Crank Angle Encoder


9

Inductive OpticalFigure 9-4: Inductive and optical transducers

9.2.2 Mounting Position of Crank Angle Encoders

The task of the angle encoder is to measure as accurately as possible thecrankshaft rotational motion in relation to the crankcase. Figure 9-5 shows thetwo mounting positions that are basically possible for angle encoders:

• Free crankshaft end

• Crankshaft end at the drive output

Figure 9-5: Possible mounting positions for sensor head and marker disk


The following mounting principles may be derived therefrom:

• Crank angle encoder with integrated sensor

The sensor head sits floating via a bearing arrangement on the same shaftas the marker disk and is usually supported by a long arm to the engineblock, see Figure 9-6 left. The direct mounting of the angle encoderhousing (secured so that it cannot rotate towards the crankcase) on thefree end of the crankshaft minimises any relative movements between themarker disk and the sensor head. Possible crank angle errors in this casedepend not on the marker disk diameter but on the support radius. Theadvantage of this design is that it allows very small marker disks.

• Crank angle encoder with no integrated sensor

The marker disk in this case is mounted on the crankshaft and the sensor isfixed to the engine block by a support. This mounting principle is usuallyused for the drive shaft side of the engine because it allows the shaft to befed through to the dynamometer.

Relative movements between the marker disk and the sensor head have adisadvantageous effect. A marker disk with the largest possible diametershould therefore be used to minimise the resultant crank angle error, seeFigure 9-6 right. In view of possible relative movements (X), the diameterof the marker disk should be the same as the support radius of the angleencoder with integrated sensor to obtain the same crank angle error (α).

R

R1

X 1

α α

Sensor-kopf

Markenscheibe

Markenscheibe

Sensor-kopf

Figure 9-6: Crank angle encoder with and without integrated sensor

Marker disk

Marker disk

Sensor head

sensorhead

with integrated sensor without integrated sensor


9

9.2.3 Crank Angle Encoder Designs

The two basic mounting options result in two generally different angle encoderdesigns, as shown in Figure 9-7.

Sensorkopf

Arm

Markenscheibe(Glass)

Lagerung Markenscheibe (Steel)

Sensorkopf

ohne Eigenlagerungmit Eigenlagerung

Gehäuse

Figure 9-7: Examples of crank angle encoder designs with and without integrated sensor(from AVL)

Angle encoders are more or less sensitive to soiling depending on their design.Angle encoders without integrated sensor are generally more sensitive to soilingbecause of their open design.

To prevent measurement errors with such encoder versions therefore, thebackground lighting method is used for scanning the slots in the marker disk.With regard to the material for the marker disk, steel is virtually the only materialthat can be considered for manufacturing and strength reasons (because largedisk diameters are necessary for sufficient accuracy).

Crank angle encoders with integrated sensor generally use the reflection methodin which marks lithographically applied to a glass disk are scanned. The smallglass disk with this encoder version can easily handle even the severest loads atthe angle encoder mounting site.

with integrated sensor without integrated sensor

Marker diskSensor head

Sensor head

Marker disk (Steel)Bearing

Housing

Supporting arm


9.2.4 Influence of Angle Encoder on Engine Behaviour

It should be borne in mind that a mounted angle encoder has some effect on theengine operation. In small engines in particular, the angle encoder can affect theengine behaviour due to:

• weight,

• friction and

• moment of inertia.


9

9.3 Crank Angle Encoder Characteristics

Speed range

The speed range within which the angle encoder’s specifications areguaranteed.

Crank angle encoders can normally be used for speeds up to 20,000 rpm ormore for special applications (e.g. racing engines).

Crank angle resolution

Crank angle resolution means the smallest step width to be represented in thediscretization of the crankshaft motion.

High-frequency parts of the signal require a crank angle resolution ≤ 0.1°CA.

Accuracy

The accuracy is a measure of the extent to which the measurement valuedeviates from the reality.

The achievable accuracy without taking into account errors relating tomounting or the engine itself is better than 0.05°CA in dynamic mode.

Lifetime

Describes the number of revolutions over which the specified characteristicsare maintained.

The lifetime is purely a comparative parameter, which can be exceeded if theoperating conditions are favourable. Conversely extreme operatingconditions can shorten a device’s lifetime. However, a lifetime of more than106 revolutions can be expected, even under unfavourable conditions.

Thermal resistance

Resistance of the angle encoder to thermal loads at the mounting surfaces andin the environment without affecting its function.

Crank angle encoders have to have the following thermal resistancecapabilities to cope with their mounting site:


Temperature at the mounting surfaces: -30°C to 120°C(determined by the sensor)

Ambient temperature: -30°C to 60°C(determined by the converter electronics)

Vibration resistance

Resistance to sensor accelerations in an axial and/or radial direction withoutimpairing the angle encoder’s function.

Typical values are ± 250 g, with peaks of ± 1000 g.

Electromagnetic compatibility (EMC)

Defines the interference environment in which a device can still be operatedwithout impairment to its functions.

Crank angle encoders usually have to satisfy the interference immunityrequirements specified in Standard EN1326

Contamination resistance

Resistance to functional impairment due to contamination from externalcomponents.

A device must be resistant to all standard operating materials (i.e. fuels,coolants and lubricants) and cleaning agents.

Leakproofness

Resistance to media penetrating the device and impairing its function underthe conditions stated in the Standard.

The relevant Standard is Degree of Protection IP54.


9

9.4 Crank Angle Errors and their Causes

Errors and inaccuracies can occur during the manufacture, mounting andoperation of crank angle encoders. To minimise those errors, it is important tounderstand their possible causes.

• Manufacturing errors (XM)

Manufacturing errors relate to the position of the angle marker track inrelation to the centre of the marker disk (XM1), the quality of the allocationof the marks division (XM2) and any possible eccentricity of the marker diskcentre to the angle encoder axis (XM3 only in angle encoders withintegrated sensor). Such errors are usually beyond the control of the user,see Figure 9-8.

XH1 XH2

Mitte der Markenscheibe(Anschluss)

Center of the marks

XH3

Winkelaufnehmerachse

Winkelaufnehmer mit Eigenlagerung: X=X+X+XH1H2H3

Winkelaufnehmer ohne Eigenlagerung: X=X+XH1H2

Actual position

Soll-Lage

Mitte derMarkenscheibe

(Anschluss)

Figure 9-8: Manufacturing errors

• Installation errors (XI)

Installation errors are within the control of the user. They occur due todeviations in the marker disk axis from the crankshaft axis. The influence ofthis error becomes smaller with rising support radius or marker diskdiameter, see Figure 9-9.

Angle encoder without integral bearing X = XM1+XM2

Desired-position

Center of theMarker disk(connection)

Angle Encoder Axis

Center of theMarker disk (connection)

Center of the marks trace

Angle encoder with integral bearing X = XM1+XM2+XM3

XM1 XM2 XM3


RR

Xi

X i

Figure 9-9: Installation error

• Engine-based error

Engine-based errors occur due to geometric and positional deviations ofthe crankshaft, due to crankshaft torsion or due to relative motion in thestabilising support or sensor head in relation to the crankcase. These typesof error can be limited by selecting the measuring site accordingly.

Geometric and positional deviations of the crankshaft (XG)

These are radial deviations of the marker disk axis from the “ideal”engine axis at the mounting point of the angle encoder or marker disk,see Figure 9-10.

In angle encoders without integrated sensor, this error can beinfluenced by the choice of mounting position for the sensor head (pickup). Only in sensor head position 2 do relative motions of the markerdisk in the direction shown by the arrow result in angle errors, seeFigure 9-10. The same motion produces no errors in sensor headposition 1.

Axial shifts within the specified limits are unimportant.

Causes of possible deviations may be a crankshaft that is alreadydeformed, a sag in the crankshaft caused by the combustion pressureor a displacement of the crankshaft in the main bearings. This errorshould be borne in mind in particular in long or flexible crankshaft ends.


9

XG

Rideale

Motorachse

verformteKurbelwellenachse

R

XG

Figure 9-10: Errors due to geometric and positional deviations of the crankshaft

Crankshaft torsion

Torsion in the crankshaft has a direct effect on the crank angle error. Tominimise this error, it is advisable to mount the crank angle encoder asclose as possible to the cylinder where the indicating measurement is tobe done. Crankshaft torsion results in the same crank angle error inboth angle encoders with and without integrated sensor.

ideal engineaxis

deformed crankshaft axis

with integrated sensor

without integrated sensor


Relative movements of the stabilising support or sensor head (XV) inrelation to the crankcase

In a similar way to the geometric and positional deviations of thecrankshaft, relative movements of the stabilising support or the sensorhead in relation to the crankcase also result in measurement errors andin extreme cases, in synchronization errors (Figure 9-11). These occuramong other things when the oscillation velocity is greater than that ofthe marker disk and thus one or more angle marks are detected morethan once. Vibrations at the mounting site of the stabilising support orthe sensor head are the commonest cause of such errors.

Xv

Xv

Xv

RR

Figure 9-11: Error due to relative movement of stabilising support or sensor head in relationto the crankcase

Figure 9-12 shows how angle error depends on positional error (X) for differentmarker disk diameters. The positional error is made up of installation errors (XI),errors due to geometric and positional deviations of the crankshaft (XG) anderrors due to relative movement of the stabilising support or sensor head (XV).The major advantage of angle encoders with integrated sensor is obvious. Thisencoder design obtains the large support radius necessary for good accuracy bymeans of a supporting arm of appropriate length (with an angle encoder that canbe as small as you like). On the other hand, in an angle encoder withoutintegrated sensor, the diameter of the marker disk determines the accuracy,which means that the disk diameter has to be very large for good accuracy.

Sensor windowshifted due tovibration

Sensor windowin correctposition


9

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

Positionsfehler X [mm]

Winkelfehler[°KW]

D=140mm

D=220mm

D=280mm

R=225mm

D

R

X=XI +XG+XV

Figure 9-12: Effects of positional errors on the accuracy of crank angle encoders

Furthermore, the crank angle error resulting from crankshaft torsion has to beadded to this crank angle error. This additional angle error is identical for bothangle encoder versions.

Ange

l erro

r [°C

A]

Position error X [mm]


9.5 Choice of Measuring Site and Mounting Instructions

The following points must always be observed when mounting crank angleencoders:

• Where possible always use an angle encoder with integrated sensor

A crank angle encoder with integrated sensor should be used wherepossible due to its lower sensitivity to mounting and engine-based errors.

• Mounting position as close to the indicated cylinder as possible

Another important point is to mount the angle encoder as close as possibleto the cylinder where the indicating measurement is to be done. That is theonly way to minimise crankshaft torsion and thus to keep the crank angleerror small.

This requirement can, however, conflict with the point above, namelyalways try to use a crank angle encoder with integrated sensor. If that is thecase, choose a mounting position for which the expected sum of angleerrors caused by positional errors (dependent on the magnitude of thepositional error and the type of angle encoder, see Fehler! Verweisquellekonnte nicht gefunden werden.) and by crankshaft torsion is minimal.

• Low-vibration support of the stabilising support or sensor head

In order to further minimize measurement errors, you should ensure thatthe stabilising support (crank angle encoder with integrated sensor) or thesensor head (crank angle encoder without integrated sensor) is secured tothe engine block in a position that is subject to as little vibration as possibleand that it is mounted as rigidly as possible.

• Mounting the angle encoder on the camshaft

An angle encoder mounted on the camshaft or an intermediate drive shaftcan only render the angle position of that shaft correctly and is unsuitablefor representing the exact crankshaft position because of deformations andplay.


9

9.6 Assigning Angle Mark Trigger to Engine TDC

In order to correctly correlate the angle based measuring values to the top deadcentre of the engine, angle encoders provide a trigger mark in addition to thecrank angle marks, the position of which must be precisely assigned to the TDC.

9.6.1 Influence of TDC Errors on Evaluation Accuracy

The exact determination of top dead centre is of vital importance especially inconjunction with the thermodynamic evaluation of pressure curves in internalcombustion engines. (e.g. for determining IMEP). Figure 9-13 shows the effect ofa crank angle error on the energy balance and the friction mean effectivepressure of a diesel engine.

-2

-1.5

-1

-0.5

0.5

1

1.5

2

-0.2 0.1 0.2-0.1

∆∆∆∆pf [bar]

∆ϕ∆ϕ∆ϕ∆ϕ [°KW]

OT liegt zu frühOT liegt zu spät

εεεε=14

εεεε=14

εεεε=22

εεεε=22

-20

-15

-10

-5

5

10

15

20

25

30

-2 1 2-1

∆∆∆∆QB [%]

∆ϕ∆ϕ∆ϕ∆ϕ [°KW]

εεεε=14,LL

εεεε=14,VL

εεεε=22,LL

εεεε=22,VL

εεεε=14,22

εεεε=14 εεεε=22

LL ... LeerlaufVL ... Volllast

OT liegt zu spät OT liegt zu früh

Figure 9-13: Effect of an angle error on the energy balance and friction mean effectivepressure (diesel engine)

If the TDC position is too early, which means a shift to the right in the cylinderpressure curve, it has the effect that there is too low a pressure as the piston risesand too high a pressure as the piston descends. An apparently longer afterburnphase and a higher energy conversion (∆QB) are the result. The converse occurswhen the TDC position is too late.

The effects of angle shifts are also felt in the indicating work (i.e. indicated meaneffective pressure IMEP), which is greater when TDC is too early and vice versa.This results also in larger (smaller) friction mean effective pressures (∆ pf ).

LL ... idleVL ... full load

TDC too late TDC too lateTDC too early TDC too early

[°CA] [°CA]


9.6.2 Methods for TDC Determination

Various methods are known for assigning the trigger to the top dead centre.

9.6.2.1 Static Top Dead Centre Determination

A flagging mark is first applied to the engine block, which continues over arotating part of the engine with the largest diameter (e.g. flywheel), seeFigure 9-14. The crankshaft is then cranked until the crank throw and the conrodof the cylinder being measured, are approximately at right-angles (position a).The height of the piston is measured in this position by placing a micrometerfeeler (2) on the piston surface through the spark plug bore, for example. Thisposition is marked on the flywheel (A) facing the flagging mark. The piston isthen lowered and raised again by turning the crankshaft further until the samereading as appeared before is registered on the micrometer (position b). Thisposition, too, is marked on the flywheel (B). The distance between the two markson the flywheel is then halved. When the crankshaft is turned until the half-waypoint stops exactly at the flagging mark, the piston is at top dead centre.

2

Stellung a

Stellung b

190°

2

Stellung a

Stellung b

190°90°

Figure 9-14: Method for static TDC determination

The statically determined top dead centre deviates from the TDC that prevailsduring engine operation (i.e. dynamic TDC) because the mechanical structure ofIC engines is not ideally rigid. Therefore, dynamic TDC determination methodsare to be preferred.

Position a

Position b


9

9.6.2.2 Top Dead Centre Determination Based on the pressure curve of amotored engine

This dynamic TDC determination method is based on the cylinder pressure curvemeasured in the motored engine mode with subsequent angle determination ofthe curve maximum. Due to the flat signal curve in the vicinity of TDC, however,and also because of the limited resolution steps of the ADC, this cannot be donesimply by taking the pressure maximum.

Mathematical methods must be applied for more precise determination of theangle position of the curve maximum in order to achieve an acceptableaccuracy. A well functioning algorithm for this purpose is described in chapter9.6.2.4, Figure 9-18 (AVL algorithm).

• Influence of the thermodynamic loss angle

In a real engine, the pressure maximum in a motored engine comes beforethe real top dead centre because of heat and leakage losses, Figure 9-15.This differential angle between the pressure maximum and TDC is knownas the thermodynamic loss angle. In addition to the fact that the loss angledepends on heat losses and leakage as already described, it also signi-ficantly depends on the speed. The loss angle rises at low speeds due tothe longer time available for heat loss and leakage.

-10 -5 0 5 10Kurbelwinkel [°KW ]

Schl

eppd

ruck

[bar

] verlustfreierSchleppdruckverlauf

Schleppdruckverlauf mit Wärme- und Leckageverlusten

thermodynamischer Verlustwinkel

Figure 9-15: Thermodynamic loss angle

• Taking the thermodynamic loss angle into account

When the loss angle is known, TDC determination can be corrected byadding to the relevant angle value to the pmax position of the (motored)

Crank Angle [°CA]

„mot

ored

“ pr

essu

re [b

ar]

Thermodynamic loss angle

corrected pressure curve

measured pressure curve (withheat and leakage losses)


pressure curve, thus shifting the pressure curve until the distance betweenthe pressure maximum and the engine TDC corresponds to thethermodynamic loss angle.

• Determination of the thermodynamic loss angle

The accuracy of the above procedure depends primarily on how the lossangle was determined. The specifications given in the technical literaturefor the loss angle of typical engines are usually imprecise. Once the lossangle of a particular engine has been determined reliably, however, (e.g. bytop dead centre determination using a TDC sensor) it can then be used inthe future to determine the TDC position quickly and reliably for the sametype of engine.

• Application in prechamber engines

If indicating measurements are carried out in the main combustionchamber of prechamber engines, the distance between the pressuremaximum and TDC is increased further.

Measured prechamber pressure curves of pre-chamber and swirl chamberengines are not suitable for TDC determination using the thermodynamicloss angle because of the flow conditions between the main combustionchamber and the pre-chamber.

9.6.2.3 Mathematical TDC Determination

The measured curve can be assigned to TDC by comparing the angle position ofthe compression line of a measured combustion chamber pressure with that of acalculated pressure curve [14]. To do this the area between the two curves in acertain crank angle range is minimised (iteration) by shifting the measured curveand thus determining the TDC position, see Figure 9-16.


9

0

10

20

30

40

50

60

70

80

90

100

-150 -100 -50 0 50 100

Crank Angle [°CA]

Pres

sure

[bar

]

Alignement range

calculated“motored”pressure

measuredcylinder pressure

Figure 9-16: TDC determination by means of thermodynamic adjustment

Heat transfer, leakage and the mass in the cylinder have to be taken into accountin the calculation of the pressure curve in the compression phase. The accuracyof the angle adjustment depends on how accurately these quantities are takeninto account. Commercial programs for pressure curve analysis offer plenty ofways to determine the TDC position using this approach.

9.6.2.4 TDC Determination with TDC Sensor

A particularly accurate way of determining top dead centre in IC engines is touse a TDC sensor, see Figure 9-17. That is mainly because the sensor measuresthe piston motion directly and can thus achieve accuracy levels of better than±0.1°CA.

TDC determination is carried out in motored engine mode. In multi-cylinderengines, the determination can also be carried out in fired-engine mode in a nonfired cylinder.

AVL ProductInformation:[10]TDC sensor

Intake valvescloses


Elektronics

Sensor

Adaptor

min. distance 1.5mm inthe Gas Exchange TDC

Clamping

Figure 9-17: Construction and mounting of a TDC sensor

• Measurement principle

The functional principle is based on a capacitive measurement procedurein which the sensor measures the changes in capacitance between thepiston and the sensor head. The capacitance changes in inverse proportionto the distance between the piston and the sensor head.

• TDC sensor mounting

The TDC sensor consisting of the adaptor, clamping piece, the actualsensor and evaluation electronics is mounted in the cylinder head, seeFigure 9-18. It is installed in existing spark plug, injection nozzle orpressure transducer bores.

• Evaluation of the TDC signal (AVL algorithm)

For a 4-stroke engine, the output signal of the TDC sensor indicates amaximum both at gas exchange TDC and at ignition TDC. Because of theplay in the friction bearings and the low cylinder pressure at gas exchange,the signal of the gas exchange TDC will be higher. Despite the highersignal at gas exchange TDC, the signal of the ignition TDC should be usedfor maximum accuracy. For one thing, this is so that the deformations thatoccur in real operation can be allowed for, and for another, because the


9

valve motions during gas exchange can affect the capacitance betweenengine ground and sensor tip and therefore distort the result.

-20 -10 0 10 20Crank Angle [°CA]

-

TDC

-Sen

sor s

igna

l

αAi+ αBi

2αAi+ αBi

2

αA1,2, ... αB1,2, ...

Bisection points TDC-position determined by averageof the Bisection points

Figure 9-18: TDC signal evaluation

Determination of the actual TDC position from the output signal of thesensor cannot be done using the signal maximum due to the flat signalcurve in the vicinity of TDC and due to the limited resolution of the signalsampling. Rather the following algorithm is used:

The amplitude is measured in the rising branch at angle (αA1,2, ...) and thecorresponding angle (αB1,2, ...) in the falling branch with the same signalamplitude, see Figure 9-18. This process is repeated a number of times andthe position of top dead centre determined from the results (centre line). Asuitable angle range for (αA1, 2, ...) is, for example, from 15°CA to 5°CAbefore TDC.

9.6.2.5 Comparison of TDC Determination Methods

The methods described for determining TDC differ mainly in terms of time andeffort, and achievable accuracy, see Figure 9-19. The classifications shown in thefigure are based on the assumption that the necessary equipment is available forall the different TDC determination methods. The most accurate but the mosttime consuming one is TDC determination with a TDC sensor. Thethermodynamic TDC determination requires a little less time and effort, but hasslight disadvantages in terms of accuracy. The static TDC determination requiresconsiderable time and effort and is significantly less accurate than the first twomethods. TDC determination using the pressure curve of a motored engine


offers the least accuracy, but the method is significantly more accurate if reliablevalues are available for the loss angle. [1]

( )

Accuracy

Effort

high

small

medium

highmediumsmall

TDC-Sensor

Thermodynamicalignement

staticTDC-detemination

“motored” pressure curve

(Accuracy depending from the quality of the loss angle)

Figure 9-19: Time and effort, and accuracy of different TDC determination methods

Trouble shooting 10-1

10

10 Trouble shooting

Table 10-1 lists the problems that frequently occur in the measurement signaland according to the listed significance their possible causes. We recommendproceeding to remedy the problem quickly. The effects of problems that can beso small that they are not visible until the measurement values are evaluated, areidentified by .

The listed problem codes and measures for trouble shooting are described inChapters 10.1 to 10.6, the problem codes being divided into the followinggroups:

• Pressure transducer 101 - 118

• Measurement cabling 201 - 206

• Charge amplifier 301 - 310

• Crank angle encoder 401 - 402

• Data acquisition and evaluation 501 - 502

• Calibration 601 - 604

The following category is also included to cover influences (see Table 10-2) thatdo not derive from measurement errors, but which nevertheless have an effecton the measurement signal:

• Real effects (can be confused with error) 001 – 003

Chapter 10.1

Chapter 10.2

Chapter 10.3

Chapter 10.4

Chapter 10.5

Chapter 10.6


Table 10-1: Effects on the measurement signal and possible causes of errorEffect Possible causes of error in order

of significance

1 2 3 Other

No measurement signal 301 202 117 310

Output voltage of charge amplifier atmaximum value

303 304 206

Occasional jumps or irregularities in themeasurement signal

202 401 118

Sudden change in level of themeasurement signal up or down with noload change

104 103 202

Truncated measurement signal 304 502 113 201

Angle-misaligned measurement signal 402 401

Proportional error over entiremeasurement range, or expected peakpressure or IMEP not reached

306 102 601 602, 603, 604,101, 103, 110,116, 501, 102,115, 117

Changes in measurement signal or IMEPat unchanged operating point (i.e.continuous operation)

111 104 103 101, 113, 117,200, 401

Measurement signal superimposed byhigh-frequency oscillations

204 302 107 109, 205, 108,001

Measurement signal superimposed by low-frequency oscillations synchronous withthe mains

302 203

Measurement signal superimposed by low-frequency oscillations synchronous withthe engine

108 105 110

Slow drift of measurement signal atunchanged load

201 113 305


10

Measurement signal changes level after anengine load change

106

Mean level change at load change is notproperly reflected

106 307 308

“Inclined” gas exchange curve section 105 307 201 113, 305

Measurement values in the vicinity of TDCnot plausible

109 002 003

Pressure proportional error to the in thegas exchange range

116 309

Table 10-2: Real effects – Description and how to avoid themCode Effect Description and how to avoid them

001 Combustion chamberoscillations (high-frequency)

It is not the task of the measurement toavoid combustion chamber oscillations buttry selecting a less “troublesome”measuring point or use filters.

002 Squish-induced flow(dynamic pressure changesaround TDC)

Measuring positions with a strong squish-induced flow are only suitable to a limitedextent for accurate measurements andshould be avoided. Their influence onlyaffects a small angle range around TDC,however, and may therefore not present aproblem with every measurement task

003 Influence of a dividedcombustion chamber(pressure difference due toflow phenomena)

Pressure measurements in prechamber orswirl chamber provide only veryapproximate information about thephenomena in the main combustionchamber


10.1 Pressure Transducer

Code Description of error Remedy

101 Change in pressuretransducer characteristicsduring “running in”

Re-calibrate

102 Change in sensitivity due toinfluence of temperature

Use cooled pressure transducers ortransducers with GaPO4 technology.Calibrate the pressure transducer at theexpected operating temperature.Maintain cooling water at a constanttemperature (return cooling).

103 Permanent change inmetrological characteristicsof the pressure transducerdue to overheating

Replace transducer.Use pressure transducer, which canwithstand high temperatures. Reduce thethermal load on the transducer by usingrecessed-type installation, by improvingthe cooling or choosing a coolermeasuring position.

104 Failure of pressuretransducer cooling

Shut down engine immediately and donot switch on cooling; let pressuretransducer cool down with the engine,then remove it and check it.

105 Cyclic temperature drift Use pressure transducers with low cyclictemperature drift.Reduce the heating or thermal load onthe transducer by using recessed-typeinstallation, by improving the cooling orchoosing a cooler measuring position.

106 Load change drift Use pressure transducers with low loadchange drift.Reduce the heating or thermal load onthe transducer by using recessed-typeinstallation, by improving the cooling orchoosing a cooler measuring position.

107 Mechanical oscillationexcitation (e.g. caused byvalve impact, speeds> 10000 rpm)

Use pressure transducers with lowacceleration sensitivity.

Chapter 3.1.2.1

Chapter 3.1.2.1

Chapter 3.1.2.1

Chapter 3.1.2.2


10


108 Effect of acceleration forcesdue to vibrations onpressure transducer, cable,cooling water hoses andcooling system

Use pressure transducers with lowacceleration sensitivity.Lay or position cables, cooling waterhoses and cooling system on vibrationfree parts of the test bed.

109 Pipe oscillations Mount the pressure transducer flush ormount it with short, oscillation-optimisedindicating channel.

110 Deformations of themounting position

Use plug-in type pressure transducers orchoose a suitable measuring position.

111 Change in the metrologicalcharacteristics of thepressure transducer due tosooting

Clean the pressure transducer.Use pressure transducers that are notsensitive to soot.

112 Scaling of cooling watergalleries

Descale;Use decalcified cooling water.

113 Insulation problems due tomoisture, contamination ortoo high operatingtemperatures

Restore high insulation resistance bycleaning the connections on the pressuretransducer or by heating it to dry it.Reduce the thermal load on thetransducer by using recessed-typeinstallation, improving the cooling orchoosing a cooler measuring position.Use charge amplifiers with driftcompensation;Use SHORT mode on the chargeamplifier.

114 Squish-induced flow Choose a suitable measuring position.115 Influence of a divided

combustion chamberChoose a suitable measuring position.Mathematical compensation.

116 Non-linearity of thepressure transducer

Check calibration range. Use multi-pointcalibration.

117 Faulty pressure transducer Replace pressure transducer.118 Contact problems in

pressure transducerReplace pressure transducer

Chapter 3.1.2.2

Chapter 4

Chapter 3.1.2.3

Chapter 3.1.2.4

Chapter 2.2

Chapter 4 Chapter 4


10.2 Measurement Cabling


201 Insulation problems due tomoisture, contamination ortoo high operatingtemperatures

Restore high insulation resistance bycleaning the connectors or by heating.Lay cables where it is cool.Use charge amplifiers with driftcompensation.Use SHORT mode on the chargeamplifier.

202 Ruptured cable, contactproblems or loose cableconnection

Check connections on the pressuretransducer and the various measurementsystem components.

203 Ground loop problems dueto different potentialbetween engine, groundand measuring ground(potential difference)

Use an equipotential bonding line (withlarge cross-section) between engine andmeasuring ground.Use an isolating transformer.Segregate the supply grounds of thecharge amplifiers when more than onepressure transducer is used.

204 Influences due toelectromagnetic fields

Use shielded cables.Change the way cables are laid.

205 Triboelectrical effect due tocable movement

Use low-noise cables.Secure cables to non-vibratingcomponents.

206 Cable short circuit Replace cable.

Chapter 2.3

Chapter 2.4.1

Chapter 2.4.2

Chapter 2.3


10

10.3 Charge Amplifier


301 RESET switch of chargeamplifier is activated

Deactivate the RESET switch.

302 Interference in supplyvoltage

Use mains filters and isolatingtransformers.

303 The charge amplifier hasreached saturation

Ground the charge amplifier by pressingthe RESET switch.

304 Charge amplifier isoverloaded

Select correct measurement range.Ground the charge amplifier by activatingthe RESET switch.Use drift compensation.Improve the insulation.

305 Input offset is not zero Correct the charge amplifier.306 Sensitivity set incorrectly on

charge amplifierAdjust sensitivity setting.

307 Charge amplifier is inSHORT mode

Use LONG mode.

308 Drift compensation isactivated on the chargeamplifier

Deactivate drift compensation.

309 Non-linearity of chargeamplifier

Ground the charge amplifier by pressingthe RESET switch.Use drift compensation.Use SHORT mode on charge amplifier.

310 Faulty charge amplifier Replace charge amplifier.

Chapter 2.2.2

Chapter 2.2.2

Chapter 2.2.3

Chapter 2.2.3


10.4 Crank Angle Encoder


401 Number of pulsesmeasured per revolutiondoes not agree withnumber of angle marks(electromagnetic scatter,torsional vibration, etc.)

Check angle encoder and its mounting;eliminate scatter sources.Use state-of-the-art indicating equipmentwhich outputs an error message when thenumber of pulses per revolution is notcorrect.

402 Errored TDC determination Determine the TDC correctly, e.g. byusing a TDC sensor for dynamic TDCdetermination

10.5 Data Acquisition and Evaluation (Indicating Equipment)


501 Sensitivity is set incorrectlyon indicating equipment

Correct sensitivity setting.

502 Indicating device hasovershot range

Select correct measurement range; useindicating equipment with automaticmeasurement range monitoring.

10.6 Calibration


601 Pressure transducer wasnot calibrated together withthe components used forthe measurement

Calibrate the entire measurement system.

602 Pressure transducer wasnot calibrated at operatingtemperature

Calibrate in heated holder.

603 Calibration pressure notconstant

Check dead weight tester;Vent the hydraulic lines.

604 Incorrect calibration values Check dead weight tester and transducer(insulation, linearity). Deactivate driftcompensation on charge amplifier

Chapter 5

Link List 11-1

11

11 LINK List

AVL LIST GmbH

AVL is the world's largest privately owned and independent company for thedevelopment of powertrain systems with internal combustion engines as well asinstrumentation and test systems.http://www.avl.com/

AVL Indicating Technology

For engine R&D a comprehensive knowledge of the processes in an internalcombustion engine necessary. For these purpose AVL have developed a wholefamily of indicating systems, each tailor-made to suit a particular application.http://www.avl.com/indicating

AVL GaPO4 crystals

Gallium Orthophosphate (GaPO4) is a piezoelectric crystal material withoutstanding properties in respect of its application on engine test sensors. AVLhave established the only industrial production of GaPO4 in the world.http://www.gapo4.com

http://www.avl.com/

http://www.avl.com/

http://www.gapo4.com/


University of Technology Graz

Institute for Combustion engines and ThermodynamicsThe Institute was founded in 1920, and under its heads Prof. Dr. Julius Magg,Prof. Dr.Dr.h.c. Hans List, and Prof. Dr.Dr.h.c. Anton Pischinger it developed intoone of the renowned research establishments for internal combustion engines.Its laboratories are equipped with most advanced test and research facilities forinternal combustion engines, motor vehicles and investigation of pullantdispersion.http://www.tugraz.at/einrichtungen/

http://www.tugraz.at/einrichtungen/

Bibliography 12-1

12

12 Bibliography

[1] Angström H-E.: “Cylinder pressure indicating with multiple transducers,accurate TDC-evaluating, zero levels and analysis of mechanicalvibrations”, 3rd International Indicating Symposium, Mainz 1998

[2] AVL Application Notes MI-006-2000: “Zylinderkopf undMontagebohrung Deformation” (Cylinder Head and Installation BoreDeformation), AVL LIST GmbH, 2000-

[3] AVL Application Notes MI-007-2000: “Lastwechsel und Lastwechseldrift”(Load Change and Load Change Drift), AVL LIST GmbH, 2000

[4] AVL Application Notes MI-008-2000: “Zyklische Temperaturdrift undEinfluss auf das pi “ (Cyclic Temperature Drift and Influence on IMEP),AVL LIST GmbH, 2000

[5 AVL Product Information MI-002-96: “AVL Echtzeit-Klopfanalyse fürOttomotoren“ (AVL Realtime Knock Analysis for SI Engines), AVL LISTGmbH, 1996

[6] AVL Product Information: Druckaufnehmer für die Motormesstechnik(Pressure Transducers for Engine Instrumentation), AVL LIST GmbH,2001

[7] AVL Product Information: Gallium Orthophosphat GaPO4, AVL LISTGmbH, 2000

[8] AVL Product Information: IFEM – Indizier-Frontendmodul (IFEMIndicating Front End Module), AVL LIST GmbH, 2000

[9] AVL Product Information: Indiziertechnik (Indicating Technology), AVLLIST GmbH, 2000

[10] AVL Product Information: OT-Sensor (TDC Sensor), AVL LIST GmbH,2001

[11] AVL Product Information: Piezoverstärker 3066A02 (3066A02 Piezo-Amplifier), AVL LIST GmbH, 2000

[12] AVL Product Information: Winkelaufnehmer (Crank Angle Encoder), AVLLIST GmbH, 2001


[13] Beran R., Figer G., Wimmer A., Glaser J., Prenninger P., “Möglichkeitenfür die genaue Messung von Ladungswechseldruckverläufen”(Possibilities for the Exact Measurement of Gas Exchange PressureCurves”, 4th International Indicating Symposium, Baden-Baden 2000

[14] Feßler, H.: Berechnung des Motorprozesses mit Einpassung wichtigerParameter (Calculation of the Engine Process with Adjustment ofImportant Parameters), Dissertation, Technical University of Graz, 1988

[15] Glaser J., Winkler J.: “Indizieren mit Glühkerzenadaptern“ (Indicatingwith Glow Plug Adaptors”, AVL Product Information MI-020-95, AVLLIST GmbH, 1995

[16] Glaser, J.: Fehler von Quarzdruckaufnehmern und Probleme bei derDruckindizierung von Verbrennungsmotoren (Quartz PressureTransducer Errors and Problems in Pressure Indicating on IC Engines),Dissertation, Technical University of Graz, 1983

[17] Hohenberg, G.: “Experimentelle Erfassung der Wandwärme inKolbenmotoren” (Experimental Acquistion of the Wall Heat in PistonEngines), Habilitation Thesis, Technical University of Graz, 1980

[18] Klell M., Wimmer A.: Ein Verfahren zur thermodynamischen Bewertungvon Druckaufnehmern (A Method for Thermodynamic Evaluation ofPressure Transducers), Motortechnische Zeitschrift MTZ 50, 1989

[19] Merzhäuser T.: “Motorüberwachung- und Regelung auf Basis einerkontinuierlichen Zylinder-Spitzendruckmessung” (Engine Monitoring andControl based on Continuous Cylinder Peak Pressure Measurement), 3rd

International Indicating Symposium, Mainz 1998

[20] Mühlögger M., Teichmann R., “Indizieren – eine Philosophie?Anforderungen, Wünsche und Möglichkeiten aus der Sicht desAnwenders” (Indicating – a Philosophy? Requirements, Wishes andPossibilities from the Point of View of the User), 4th InternationalIndicating Symposium, Baden-Baden 2000

[21] Pischinger, R., Kraßnig, G., Taucar, G., Sams, Th.: “Thermodynamik derVerbrennungskraftmaschine” (Thermodynamics of Internal CombustionEngines), Springer Publishing Vienna, 1988

[22] Sass, F.: “Geschichte des deutschen Verbrennungsmotorenbaus von1860 – 1918“ (History of German Internal Combustion EngineEngineering from 1860 – 1918”

Bibliography 12-3

12

[23] Tichy, J., Gautschi G.: Piezoelektrische Meßtechnik (Piezoelectric EngineInstrumentation), Springer Publishing Berlin Heidelberg New York, 1980

[24] Wimmer, A., Glaser, J.: Welche thermodynamischen Aussagen sind mitMiniaturdruckaufnehmern möglich? (What Thermodynamic Informationcan Miniature Pressure Transducers Give Us?), Paper for 2nd IndicatingSymposium in Offenbach/Main, May 1996


List of Figures 13-1

13

13 List of Figures

Figure 1-1: Indicator chart recorded by Nikolaus August Otto (from FriedrichSass: “Geschichte des deutschen Verbrennungsmotoren-baus von1860-1918") [22] .................................................................................1-1

Figure 1-2: Application areas of indicating technology............................................1-2Figure 1-3: Cylinder pressure curve over an engine cycle........................................1-3Figure 1-4: Low pressure curves over one cycle ......................................................1-6Figure 2-1: Structural diagram of the piezoelectric pressure measurement system

with additional devices ........................................................................2-1Figure 2-2: Piezoelectric effects ...............................................................................2-3Figure 2-3: Measuring element for the longitudinal effect ......................................2-5Figure 2-4: Increasing the charge output with the longitudinal effect .....................2-5Figure 2-5: Measuring element for the transversal effect .........................................2-6Figure 2-6: Quartz crystal.........................................................................................2-8Figure 2-7: Crystal structure of Gallium Orthophosphate ......................................2-10Figure 2-8: Temperature dependency of piezoelectric constant (d11) for quartz

and Gallium Orthophosphate .............................................................2-10Figure 2-9: Construction of a piezoelectric pressure transducer based on the

longitudinal effect (from AVL) – Mounting thread M14x1.25..........2-12Figure 2-10: Construction of piezoelectric pressure transducers based on the

transversal effect (from Kistler) – Mounting thread M14x1.25 .........2-13Figure 2-11: Uncooled miniature pressure transducer (from AVL) – Mounting

thread M5x0.5....................................................................................2-14Figure 2-12: Charge amplifier (e.g. from AVL) .....................................................2-16Figure 2-13: Circuit diagram of a charge amplifier ................................................2-16Figure 2-14: Definition of the time constant when discharging a capacitor ...........2-18Figure 2-15: Electrical drift effect ..........................................................................2-19Figure 2-16: Typical effect of SHORT mode on the measurement result at low

speed and with low transducer sensitivity..........................................2-21Figure 2-17: Basic effect of cable length on the upper cut-off frequency...............2-24Figure 3-1: External influences on the pressure transducer ......................................3-2Figure 3-2: Measurement and overload range ..........................................................3-3Figure 3-3: Sensitivity and linearity .........................................................................3-5Figure 3-4: Resonance frequency spectrum..............................................................3-6Figure 3-5: Temperature distribution and deformation of a pressure transducer

and curve of the cyclic temperature drift with cyclic heating at thefront face (result of an FEM simulation)..............................................3-9


Figure 3-6: Deviations in indicated mean effective pressure and in the energybalance due to an error that is proportional to the pressure and dueto cyclic temperature drift ..................................................................3-10

Figure 3-7: Tester for determining the cyclic temperature drift (diagram) .............3-11Figure 3-8: Curve of heat flow density in the tester................................................3-12Figure 3-9: Load change drift.................................................................................3-13Figure 3-10: Influence of structure-borne noise in the measured pressure signal...3-15Figure 4-1: Interrelation between measurement task, pressure transducer and

installation............................................................................................4-2Figure 4-2: Influence on the measured pressure curve of a measuring position

arrangement in the squish gap..............................................................4-9Figure 4-3: Temperature distribution at the cylinder head of a 2-valve DI diesel

engine.................................................................................................4-11Figure 4-4: Heat flow load and cyclic temperature drift as a function of

installation position and depth ...........................................................4-13Figure 4-5: Influence of the length of the indicating channel on the measured

pressure curve ....................................................................................4-14Figure 4-6: Indicating channel with additional volume ..........................................4-15Figure 4-7: Pipe oscillation frequency as a function of indicating channel length

and gas temperature ...........................................................................4-16Figure 4-8: Flow chart for pressure transducer selection........................................4-24Figure 5-1: Glow plug adaptor with pressure transducer..........................................5-1Figure 5-2: Glow plug adaptor with pressure measuring probe................................5-2Figure 5-3: Different designs of spark plug adaptors ...............................................5-3Figure 5-4: Front and shoulder-sealing direct installation ........................................5-6Figure 5-5: Example of the installation of an uncooled pressure transducer with

adaptor sleeve ......................................................................................5-7Figure 5-6: Direct installation of a cooled transducer with adaptor sleeve...............5-8Figure 5-7: Pressure transducer and bore ready for transducer installation............5-10Figure 5-8: Work stages for installing a transducer................................................5-11Figure 5-9: Cylinder head with two installed pressure transducers ........................5-12Figure 5-10: Work stages for single sleeve and inclined bore axis.........................5-13Figure 5-11: Work stages for installing the adaptor sleeve ....................................5-14Figure 5-12: Installing the pressure transducer.......................................................5-15Figure 5-13: Work Stages for multiple seals and perpendicular bore axis .............5-16Figure 5-14: Installing adaptor sleeve and transducer ............................................5-17Figure 6-1: Calibration methods...............................................................................6-1Figure 6-2: Schematic representation of the construction of a dead weight tester

design...................................................................................................6-4Figure 6-3: Calibration jump ....................................................................................6-5Figure 6-4: Influence of electrical drift during calibration .......................................6-6Figure 6-5: Unloading jump in high pressure dead weight testers with loading

and unloading valve .............................................................................6-7

List of Figures 13-3

13

Figure 9-1: Principle of signal discretization............................................................9-2Figure 9-2: Influence of angle and amplitude resolution on the discretization.........9-2Figure 9-3: Influence of the resolution on the display of knocking combustion.......9-3Figure 9-4: Inductive and optical transducers...........................................................9-5Figure 9-5: Possible mounting positions for sensor head and marker disk...............9-5Figure 9-6: Crank angle encoder with and without integrated sensor.......................9-6Figure 9-7: Examples of crank angle encoder designs with and without integrated

sensor (from AVL)...............................................................................9-7Figure 9-8: Manufacturing errors ...........................................................................9-11Figure 9-9: Installation error ..................................................................................9-12Figure 9-10: Errors due to geometric and positional deviations of the crankshaft .9-13Figure 9-11: Error due to relative movement of stabilising support or sensor

head in relation to the crankcase ........................................................9-14Figure 9-12: Effects of positional errors on the accuracy of crank angle encoders 9-15Figure 9-13: Effect of an angle error on the energy balance and friction mean

effective pressure (diesel engine).......................................................9-17Figure 9-14: Method for static TDC determination................................................9-18Figure 9-15: Thermodynamic loss angle ................................................................9-19Figure 9-16: TDC determination by means of thermodynamic adjustment ............9-21Figure 9-17: Construction and mounting of a TDC sensor.....................................9-22Figure 9-18: TDC signal evaluation .......................................................................9-23Figure 9-19: Time and effort, and accuracy of different TDC determination

methods..............................................................................................9-24


List of Tables 14-1

14

14 List of Tables

Table 1-1: Indicating parameters 1-4Table 4-1: Important measurement tasks with evaluation criteria and

additional measurement parameters 4-5Table 4-2: Adaptation with and without intervention in the test engine 4-7Table 4-3: Pressure transducers for engine instrumentation 4-18Table 4-4: Effect of pressure transducer properties and installation

on signal evaluation 4-21Table 7-1: Assessment of zero-line detection methods 7-5Table 10-1: Effects on the measurement signal and possible causes of

error 10-2Table 10-2: Real effects – Description and how to avoid them 10-3


Index 15-1

15

15 Index

Acceleration....................... 3-2, 3-15, 10-5Accuracy requirements..............4-24, 9-4Adaptor sleeve .............................5-6, 5-12Analogue/Digital converter ................ 2-2Angle measurement.............................. 9-1Bessel filter ............................................2-22Cable length..........................................2-23Calibration.............................6-1, 6-3, 10-4Capacitance ..........................................3-21Capacitive..............................................9-23Change in sensitivity ...................3-7, 4-23Characteristic........................................... 3-4Charge amplifier............................2-1, 2-16, 3-18, 6-1, 10-7Charge amplifiers.................................3-18Charge output ........................................ 2-4Chemical influence .....................3-2, 3-16Cleaning.................................8-1, 8-3, 10-5Combustion chamber pressuremeasurement.......................................... 1-2Combustion noise ................................. 1-4Construction of piezoelectricpressure transducers...........................2-12Contamination .......................... 10-5, 10-6Contamination resistance..................9-10Cooling water crosstalk........................ 3-7Crank angle encoder ..................9-1, 10-8Crank angle encoder withintegrated sensor ................................... 9-6Crank angle error.................................9-17Crank angle errors...............................9-11Crank angle resolution ......................... 9-9Crankshaft torsion ...............................9-13Cut-off frequency.................................3-19Cyclic heating.......................................3-10Cyclic temperature drift .. 3-8, 3-12, 4-14Data acquisition...........................2-2, 10-8Dead volume............................. 4-13, 4-15Dead weight tester......................6-3, 10-8Deformation impact ...........................3-16Deposits .................................3-16, 5-4, 8-3Descaling .......................................8-2, 10-5Design ......................................... 4-20, 4-22

Diaphragm ............................................2-12Digital signal recording ........................ 9-1Discharge ..............................................2-18Drift compensation........2-21, 10-5, 10-7Duration of combustion .............. 1-5, 7-5Electrical drift ..............................2-17, 2-20Electrical filter .......................................2-22Electrodes................................................ 2-4Electromagnetic compatibility..........9-10Electromagnetic fields ..............2-25, 10-6Electrometer amplifier .......................... 3-7Energy conversion points .................... 4-5Evaluation........................... 4-5, 9-17, 10-8Flow recess ...........................................4-13Forced zero point ...................................3-4Friction analysis...................................... 4-5Friction mean effective pressure.........................................................4-4, 9-17Front-sealing............................................ 5-5Gallium orthophosphate.............. 2-9, 5-2Gas exchange analysis ......... 1-6, 1-7, 4-5Gas oscillations ...................................... 4-9Glow plug adaptator ..........................4-14Ground loop...............................2-24, 10-6Heat flow......................................... 3-1, 3-7Heat range .............................................. 5-4Helmholtz resonator...........................4-16High pass filter .....................................2-22High pressure indicating...................... 1-3Hydrothermal synthesis ....................... 2-8IMEP stability ........................................3-17Indicated mean effective pressure..9-17Indicating channel....................................... .................................4-13, 4-15, 4-18, 4-23Indicating equipment .................2-2, 10-8Indicating parameters... 1-3, 1-4, 4-3, 4-5indicator chart ........................................ 1-1Influence of measuring position ........ 4-7Input offset............................................2-18Input voltage ........................................2-17Installation bore ..................................... 4-8Installation error...................................9-11Installation position............................... 5-5


Insulation resistance........... 2-23, 3-6, 8-1Interference signals .....................2-24, 7-5Knock detection.....................................4-5Leakproofness....................4-19, 5-8, 9-10Lifetime.................................. 3-4, 4-23, 9-9Linearity............................................ 2-7, 3-5Linearity deviation ......................... 3-4, 3-5Load change drift...................... 3-13, 10-4LONG.....................................................2-20Longitudinal effect.......................2-5, 2-12Low pass filter.......................................2-22Low pressure indicating .......................1-6Machining the mounting bore ...........5-9Maintenance...........................................8-1marker disk..............................................9-7Marker disk....................................9-6, 9-12Measurement cabling......2-1, 2-23, 10-6Measurement range....................3-3, 4-23Measurement task .......................4-3, 4-23Measuring element ............ 2-4, 2-12, 3-9Measuring point...................................4-19Measuring position...........4-8, 4-13, 4-18Mechanical engine load.......................4-5Miniature pressure transducer.................. .....................................2-14, 3-8, 4-24, 5-3Misfire detection....................................4-5Monitoring...............................................4-5Motion noise.........................................2-23Mounting principle................... 4-20, 4-22Multi-point-calibration...........................6-2Natural frequency........................3-5, 4-21Noise.......................................................3-20Noise analysis .........................................4-4Noise charge.........................................3-21Operating conditions............................4-3Operating temperature range ............3-4Output signal ..................................2-7, 3-4Overload range......................................3-3Parameters...............................................3-3Peak pressure....................... 4-5, 7-5, 10-2Permanent zero-line deviation .........3-14Piezo material...............................2-7, 2-11Piezoelectric effect................................2-3Piezoelectric pressuretransducer......................................2-1, 2-12Piezoelectricity .......................................2-3Pipe oscillations......................... 4-15, 10-5Plug-in transducer ................................4-22

Polarity ...................................................3-18Polytropic exponent ............................. 7-2Potential difference.............................10-6Prechamber................................ 4-12, 10-3Pressure indicating...................................1-1, 3-8, 4-1, 5-1, 5-5Pressure jump...............................2-19, 6-6Pressure measuring probe.........4-24, 5-2Pressure rise............................................ 1-4Pressure transducer cooling .. 2-14, 10-4Pressure transducer selection................... ..................................................... 4-20, 4-24Pyroelectric effect ................................. 2-8Quartz ...............2-3, 2-8, 2-12, 2-14, 4-21Reference transducer .........................3-12Repair ....................................................... 8-1Representative pressure....................... 4-7RESET..............................................6-6, 10-7Resistor for negative feedback.........2-20Resonance frequency.................3-6, 3-19Ripple .....................................................3-20Sampling rate.......................................... 9-2Screening...............................................2-23Sensitivity.................................... 3-18, 4-23Shock resistance ..................................4-23SHORT........................................ 2-20, 10-7Short term drift ....................................... 7-5Shoulder-sealing..................................... 5-5Signal discretization .............................. 9-1Signal-to-noise ratio.............................4-21Similar-to-static measuring.................2-18Single-point calibration......................... 6-2Sooting ........................................ 3-16, 10-5Spark plug adaptator ............................ 4-8spark plug adaptor ................................ 5-3Spark position......................................... 5-4Speed of the pressure rise................... 4-9Speed range............................................ 9-9Squish-induced flow................. 10-3, 10-5Stabilising support .................... 9-14, 9-16Stability............................................. 2-7, 4-6Start of combustion ...................... 1-4, 1-5Step........................................................... 6-1Structure-borne noise ...... 2-22, 3-15, 7-3Supporting arm ....................................9-14Swirl chamber..................4-12, 9-21, 10-3TDC determination .............................9-20TDC Determination ................. 9-19, 9-24

Index 15-3

15

TDC sensor ................................ 9-22, 9-23Temperature ................................... 2-8, 3-1Temperature calibration....................... 6-5Temperature drift................................... 3-8Temperature resistance......................4-23Test engine....................................4-3, 4-24Thermodynamic analysis ..................... 1-5Thermodynamic loss angle ...............9-20Time constant.......................................2-18Transmission behaviour ....................... 3-4

Transversal effect.........................2-6, 2-13Triboelectrical effect ...........................10-6Twin formation....................................... 2-9Uncooled pressure transducer.........4-22Vibration resistance ............................9-10Water slap ............................................... 5-9Water-cooled pressure transducer..2-12Zero-line detection ....................... 7-1, 7-5Zero-line gradient................................3-14

Documents

Engine Pressure Indicating Handbook