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Global Training – The finest automotive learning
Cars, Trucks, Vans · Overall Vehicle - Engine Combustion · Go Participant Document
T0298F Issue date 09.05.2012
This document is intended for training purposes only. The exercises performed in the course cannot simply be implemented in practice without regard to various considerations. Country-specific laws, regulations and specifications must always be observed.
The training documents are not subject to the ongoing update service. When working at the vehicle, always use the most up-to-date workshop aids (e.g. EPC net, WIS net, DAS, special tools) provided by the manufacturer for the vehicle in question.
Printed in Germany
© 2012 Copyright Daimler AG
Publisher: Global Training
This document, including all its parts, is protected under the laws of copyright. Any further processing or use requires the previous written consent of Daimler AG. This applies in particular to reproduction, distribution, alteration, translation, microfilming and storage and/or processing in electronic systems, including databases and online services.
Note: The term "employee" always refers to both female and male members of staff.
1511 3162 - 1st edition 09.05.2012 76
Contents
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Contents
1 Orientation .................................................................................................................2 1.1 Welcome and introduction.................................................................................................... 2
2 Introduction to mixture formation..............................................................................3 2.1 Mixture formation .............................................................................................................. ... 3
3 Mixture formation in spark ignition engines...............................................................6 3.1 Mixture formation in spark ignition engines.......................................................................... 6 3.2 Injection systems in spark ignition engines .......................................................................... 8 3.3 Direct injection ............................................................................................................... .... 15 3.4 Exercises on mixture formation .......................................................................................... 17
4 Ignition in spark ignition engines .............................................................................22 4.1 Ignition in spark ignition engines ........................................................................................ 22 4.2 Student exercise on ignition in spark ignition engines ........................................................ 32
5 Exhaust treatment in spark ignition engines............................................................36 5.1 Exhaust treatment in spark ignition engines....................................................................... 36 5.2 Student exercise on exhaust treatment in spark ignition engines....................................... 41
6 Mixture formation in diesel engines.........................................................................43 6.1 Mixture formation in diesel engines.................................................................................... 43 6.2 Student exercise work on mixture formation in diesel engines........................................... 56
7 Exhaust treatment in diesel engines ........................................................................59 7.1 Exhaust treatment in diesel engines ................................................................................... 59
8 Increasing output by forced induction .....................................................................61 8.1 Increasing output by forced induction ................................................................................ 61 8.2 Student exercise on increasing output................................................................................ 67
9 Power and torque.....................................................................................................69 9.1 Power and torque ............................................................................................................... 69 9.2 Student exercise work on power and torque ...................................................................... 70
10 Exhaust system........................................................................................................71 10.1 Exhaust system................................................................................................................. .. 71
1 Orientation 1.1 Welcome and introduction
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1 Orientation
1.1 Welcome and introduction
Welcome to the self pace training course T0298F.
During the self paced training course T0298F you will obtain an overview of all basic topics on
engine combustion.
You'll learn about these topics by completing student exercises.
We wish you success in the course, and hope this self paced training will
prove to be informative!
Fundamentals of engine combustion
Whether combustion occurs by the diesel or the spark-ignition principle, there must always be
mixture formation, compression, ignition and combustion. This unit of the course introduces the
combustion process and the systems involved. It enumerates alternative methods of boosting
the power of internal combustion engines and clarifies issues of exhaust production and
treatment. These topics are covered separately according to the spark ignition and diesel
engine combustion principles.
2 Introduction to mixture formation 2.1 Mixture formation
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2 Introduction to mixture formation
2.1 Mixture formation
The combustible mixture
In order for the fuel to burn completely a certain quantity of oxygen or air is required. The blend
of fuel and air is termed the mixture. We will examine the fuel first.
Simple and reliable formation of a combustible fuel/air mixture which ignites readily and quickly and burns without residues.
Low weight and small space for the energy unit and for the fuel tank.
Reliable power transmission.
Immediate readiness for operation.
Convenient and safe accommodation and carriage in the vehicle.
Fuel
Fuels and their accommodation in the vehicle must satisfy certain requirements with regard to
suitability for combustion and transportation:
Fuel types: There are three types: solid, liquid and gaseous fuels. Today, however, only liquid
fuels are significant and to a limited extent gaseous fuels. The requirements listed above are
best satisfied with liquid fuels. The most common liquid fuels are gasoline and diesel. These
consist of different hydrocarbons from the same origin (crude oil). Hydrocarbons are
compounds of carbon (C) and hydrogen (H2). The manufacturing process begins with the
distillation of crude oil into its constituent parts. Further processes refine the components to
produce gasoline and diesel.
In addition there are other fuels based on regenerative (reusable) energy sources. Some of the
following so-called alternative fuels are already in use; others are in discussion:
Methane, obtained in part from animal by-products
Methanol, obtained from wood biomass
Ethanol, obtained from sugarcane biomass
Vegetable oil, obtained from rapeseed biomass
Hydrogen, obtained from nuclear or solar energy
The use of alternative fuels requires modifications to the engine, the vehicle and the fuel
distribution infrastructure. Their use in the near future can only be expected in certain niche
sectors. In the medium term methanol appears to be the most promising alternative, while the
chances of hydrogen technology may only appear in the long term.
Properties of fuels
The main property of fuels, and the reason they are used for combustion, is that they store
chemical bond energy. The internal combustion engine converts the chemical energy into
2 Introduction to mixture formation 2.1 Mixture formation
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mechanical energy (power). The fuel's chemical energy is expressed as its so-called net heating
value in joules per kilogram (J/Kg).
In addition, fuels have certain properties with regard to their readiness to ignite under high
pressures and at high temperatures. This readiness is referred to as the combustibility. The
measure for the combustibility is the octane number (spark ignition engine) or the cetane
number (diesel engine).
Spark ignition fuel (gasoline) should have a low combustibility so that residual mixture does not
detonate by itself. As a reminder: The spark ignition engine ignites the fuel in a controlled
manner by means of a spark.
The octane number therefore indicates the resistance of gasoline to the occurrence of
undesirable autoignition. Undesired autoignition is also referred to as combustion knocking.
Engine knock is feared because it can cause irreparable damage to the engine. Knocking
mostly occurs at low engine speeds in full load mode (acceleration knock) or at high engine
speeds (high-speed knocking).
The higher the octane number, the more resistant to knocking and the higher the quality of the
fuel. At filling stations the octane number is stated with the letters RON. RON stands for
Research Octane Number and is an international standard. In contrast to gasoline, diesel fuel
must have a high combustibility. As a reminder: The diesel principle is based on autoignition.
The key figure for the combustibility of diesel is the cetane number. Diesel fuel also possesses
another property. Diesel becomes viscous and therefore unusable at low temperatures. This
viscosity begins to occur at approx. –24°C.
The table below lists the octane and cetane numbers for common fuels.
Fuel Octane/cetane numbers
Gasoline 91
Premium 95
Super Plus 98-100
Diesel 50
Air requirement
The air requirement is the volume of air that is required for the complete combustion of a fuel.
It is also referred to as the stoichiometric air requirement.
In engine combustion the fuel/air mixture usually varies from the stoichiometric mixture ratio
to a greater or lesser extent. The ratio of the actual air mass mL to the stoichiometric air mass
is called the air/fuel ratio .
Three mixture states are differentiated in engine operation:
A mixture with air deficiency has a < 1 and is referred to as a “rich mixture”.
A mixture with a stoichiometric mixture ratio has a = 1 (ratio of fuel : air = 1 : 14.8) and is termed the stoichiometric mixture.
A mixture with excess air has a > 1 and is known as a “lean” mixture.
2 Introduction to mixture formation 2.1 Mixture formation
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Nearly all spark ignition engines today are equipped with a regulated catalytic converter. This
must be operated with an approximately stoichiometric mixture.
Diesel engines are always operated with excess air in order to avoid excessive soot formation.
3 Mixture formation in spark ignition engines 3.1 Mixture formation in spark ignition engines
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3 Mixture formation in spark ignition engines
3.1 Mixture formation in spark ignition engines
Spark ignition fuels are conditioned outside the cylinder with the aid of a carburetor (not
covered here because no longer state of the art), by injection into the intake manifold or by
direct injection into the combustion chamber.
The pictures illustrate intake manifold injection and direct injection.
Mixture formation and metering must fulfill the following tasks:
The formation of a gaseous fuel/air mixture from finely distributed fuel.
The accurate metering of the fuel for the desired air/fuel ratio.
The adjustment of the mixture volume by flow restrictors in order to regulate the power output.
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1. Injection nozzle
2. Spark plug
3. Intake manifold
4. Exhaust port
3 Mixture formation in spark ignition engines 3.1 Mixture formation in spark ignition engines
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The mixture formation system is involved not only in optimizing combustion in the context of
the stoichiometric air ratio, but also is influencing other critical operating variables:
Enabling quick starting.
Enabling stable idling.
Operating economically in the partial-load range.
Reacting rapidly to load changes, e.g. acceleration.
Metering the fuel so that the pollutant exhaust emissions remain as low as possible in all operating states.
Enriching the mixture with fuel in order to obtain the maximum power output.
Operating regardless of atmospheric conditions.
Mixture composition: The mixture composition has a great influence on the main operating
variables. It is described by the fuel/air mixing ratio or the air/fuel ratio.
The air/fuel ratio range in which spark ignition engines can be operated is defined by the rich
and lean misfire limits or by corresponding ignition limits.
Air/fuel ratio range of a spark ignition engine: 0.5-0.7 < < 1.3-1.7
The mixture ratio basically depends on:
Engine temperature
Engine speed
Engine load
The following engine operating states require a rich mixture:
Cold start
Warm-up
Acceleration (full load)
Full power (full load)
Full load is the state where the vehicle is driven with the throttle valve wide open. The engine
speed is immaterial.
When the engine is warm and the engine load is low (so-called partial load), a lean mixture is
used to drive economically.
3 Mixture formation in spark ignition engines 3.2 Injection systems in spark ignition engines
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3.2 Injection systems in spark ignition engines
Mixture formation systems in spark ignition engines – Intake manifold injection
In spark ignition engine injection, a pump delivers the fuel and injection valves inject it either in
front of each cylinder individually (multi-point injection) or centrally in the zone of highest air
speed (single-point injection). In the case of single-point injection, the fuel is injected by only
one injection valve (single point); in multi-point injection by one injection valve per cylinder
(multiple points).
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Essentially, intake manifold injection is either continuous or discontinuous.
Continuously operated systems are mechanical systems that inject fuel constantly (continuously). •
Discontinuous systems are regulated by electromagnetic valves. These are actuated on demand by an electronic control unit and therefore do not always inject fuel (discontinuous).
Certain control systems are required in order the adjust the fuel stream to suit the air flow to
achieve the desired air/fuel ratio. These are the mixture formation systems. They must be able
to measure the air volume or mass as well as control the allocation of the fuel quantity
according to the measured air volume. Other components of the mixture formation systems
include the injection valves.
On-demand mixture formation can be imagined as follows: The driver demands a certain power
output from the engine via the accelerator pedal position. The accelerator pedal is linked to the
throttle valve in the engine. The throttle valve controls the load state in the mixture formation
system. When more power is required, the throttle valve opens further. This allows a greater
flow of air into the combustion chambers. This air volume is measured. With the greater
measured air volume a greater quantity of fuel can be injected, and the engine receives more
power.
We will now discuss exactly how these mixture formation systems operate.
Mechanically controlled continuous single fuel injection (K-Jetronic)
In the so-called “K-Jetronic” from Bosch the fuel is injected continuously and metered by
measuring the intake air volume.
3 Mixture formation in spark ignition engines 3.2 Injection systems in spark ignition engines
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The opening of the throttle valve also causes the baffle plate to open. This happens because of
the intake vacuum: When the throttle valve is open, the engine can draw in a large air volume,
pulling the baffle plate far into the air funnel.
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The deflection of the baffle plate is directly proportional to the air volume. In the fuel distributor
the control plunger alters the cross section of rectangular slots according to the deflection of
the baffle plate. In this way the fuel quantity to the injection nozzles is metered. The number of
slots is the same as the number of cylinders in the engine. The control plunger is connected to
the baffle plate by a linkage. The pressure differential valve ensures that the pressure across
the metering slots is constant. The injection valves atomize the fuel very finely even at low flow
rates and open at a fuel pressure of approx. 3.3 bar.
An electric pump (fuel pump) supplies the system with a pressure that is kept constant at
about 5.7 bar by the pressure regulator. A pressure reservoir maintains the pressure for a short
time after the engine is switched off. This is necessary to facilitate restarting when the engine is
hot.
The basic setting of the air/fuel ratio is set at idle by an adjusting screw and then remains
practically constant throughout the entire partial-load range. The mixture is enriched for wide
open throttle by changing the contour of the air funnel.
For cold starts (cold start enrichment = rich mixture) a special cold start valve is used. The
warm-up compensator (control pressure regulator) and ab auxiliary air valve are used for warm-
up. The warm-up compensator regulates the control pressure acting on the control plunger
according to temperature and regulates the additional fuel volume in warm-up by increasing or
reducing the control pressure.
Electromechanically controlled continuous single fuel injection (KE-Jetronic)
The K-Jetronic was developed to produce the KE-Jetronic by adding an electronic control (the
basic system is the same as the K-Jetronic). This allows intelligent functions to be implemented,
such as regulating to = 1 for catalytic converter cleaning or warm-up enrichment. The
advantage of the KE-Jetronic over purely electronic systems is its limp-home ability. The basic
system continues to operate mechanically if the electronic system fails.
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Electronically controlled single fuel injection (L-Jetronic)
Advances in semiconductor technology have made it possible to control the injection quantity
electronically. This provides a number of correction capabilities to improve fuel economy,
running characteristics and exhaust emissions. In the L-Jetronic system the air volume drawn
in by the engine is measured directly and the quantity of fuel to be injected is calculated
electronically.
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Injection valves
The fuel is injected by electromagnetically actuated injection valves.
The injection valves consist of a nozzle needle with solenoid armature and a valve body with
magnet coil. When de-energized, a spring presses the nozzle needle into its seat. When excited
by electrical pulses from the control unit, the nozzle needle lifts approx. 0.1 mm off its seat and
fuel can escape through an annular gap. The fuel supply comes from the side or the rear end of
the valve.
3 Mixture formation in spark ignition engines 3.2 Injection systems in spark ignition engines
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The fuel is delivered from the tank via an electric pump and a fuel filter. The fuel filter holds
back impurities and must be replaced in line with the maintenance intervals.
All the injection valves in an engine are actuated with pulses by the control unit simultaneously.
They therefore inject fuel in different phases of the operating cycle of each cylinder.
In order to create approximately identical conditions in all cylinders, the injection quantity is
split into 2 halves and injected twice per operating cycle of each cylinder (intermittent injection
in groups). The resulting displacement of part of the fuel in front of the closed intake valve
improves mixture formation.
The control unit calculates the injected fuel quantity from the pulse duration of the valves. The
control pulses are triggered by the contact breaker in the distributor.
A cold start facility is also integrated. A finely atomizing starting valve guarantees a dependable
cold start.
A bank of contacts at the throttle valve shuts of the fuel supply completely in deceleration
mode above the engine idle speed and also controls wide open throttle enrichment.
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Electronically controlled fuel injection (LH-Jetronic)
The basic system is the same as the L-Jetronic. One of the advances in this system is a change
in the way the air volume is measured. Instead of measuring the air volume with a pressure
governing flap, the air mass is measured by a hot wire mass air flow sensor, or after 1987 by
the hot film MAF sensor. A thin, heated platinum wore or a platinum film resistor is cooled by
the mass air flow. This changes its electrical resistance. The change is registered in an amplifier
and the current through the wire or film resistor is increased until there is a constant
temperature in the hot wire or film resistor. The current required for this is a measure of the
intake air mass.
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Another advance is the cold start enrichment. This is now integrated in the control unit and is
achieved by lengthening the injection period or increasing the injection frequency. This does
away with the cold start valve and the thermo-time switch
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Motronic engine control (ME)
The Motronic features a single control unit to provide complete electronic control of the engine
timing. The Motronic is available in a wide variety of variants. The table below provides an
overview of the Motronic variants and their installation in the various Mercedes engines:
3 Mixture formation in spark ignition engines 3.2 Injection systems in spark ignition engines
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ME version
Engine Models Market launch
Remarks
ME 1.0 119
120
129/140
210/129
140
September 1995
New introduction
ME 2.0 112
113
129/163
202/208
210/215
220
March 1997 New introduction
ME 2.1 104
111
129/140
170/202
208/210
August 1996 New introduction
ME 2.7 137 220/215 January 2000 New introduction
ME 2.7.1 275
285
215/220
230/240
01.10.2002 Replaces ME 2.7
ME 2.8 112
113
163/170
202/203
208/209
210/211
215/220
230
June 2000 Replaces ME 2.0
ME 2.8.1 112
113
170/203
209/211
215/220
230
March and October 2001
As ME 2.8,
but with specific AMG elements
The modern generation of Motronic comprises the following functions:
Electronically controlled injection system with mass air flow sensor.
Electronically controlled ignition system.
Idle speed control: As the idle speed drops and fluctuates when consumers are switched on (radiator fan, A/C compressor, etc.) during idling, the Motronic works to counteract this, in order to achieve a stable idle speed.
Lambda control.
Knock control: At each cylinder there is a knock sensor (microphone) that detects combustion knock. As soon as knocking occurs, the sensor reports it to the Motronic. The firing point is then retarded. If this stops the knocking, then the ignition timing is gradually advanced again in stages.
Electronic boost pressure control (only in combination with a turbocharger).
Control of the exhaust gas recirculation.
Camshaft timing.
Control of the variable-geometry intake manifolds. In some vehicles the length and geometry of the intake ports can be changed in order to improve torque according to the operating point.
3 Mixture formation in spark ignition engines 3.2 Injection systems in spark ignition engines
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Cruise control.
Adaptive adjustment of the opening speed of the throttle valve to suit the driver's driving style (ME 2.8).
Pressure sensor in the intake manifold for altitude detection (the air pressure is less at altitude) (ME 2.8).
The Motronic also assists the control units of the other vehicle systems. For example, it is
linked to the control unit of the automatic transmission. During a gearshift the torque must be
reduced in order to protect the transmission and to improve comfort. The link to the ABS
control unit allows the acceleration skid control (ASR) to operate to increase driving safety.
Other links exist in vehicles with ESP between the vehicle dynamics control unit and the
Motronic. These enable specific drive and braking intervention to be performed.
Block diagram of the ME Motronic
Input (sensor) Processing Output (actuators)
Engine rpm / inductive sensor Main relay
Ignition TDC of first cylinder / Hall sensor
Fuel pump relay / fuel pump
Air mass / mass air flow sensor Injection valve
Throttle valve position / throttle valve potentiometer
Electronic accelerator actuator motor
Engine temperature / engine NTC
Fuel tank vent valve
Residual oxygen upstream of CAT / oxygen sensor I
Exhaust gas recirculation valve
Intake manifold pressure / pressure sensor
Shutoff valve
Pressure differential / pressure sensor
Oxygen sensor I heater
Residual oxygen downstream of CAT / oxygen sensor II
Oxygen sensor II heater
Ignition TDC of first cylinder / Hall sensor
Secondary air valve
Accelerator pedal position / accelerator pedal potentiometer
Control unit
Basic adjustment via characteristics map
Startup control
Post-start, wide open throttle, acceleration enrichment
Deceleration fuel shutoff
Rpm limitation
Lambda control
Idle speed control
Tank ventilation system
Exhaust gas recirculation
Torque control
Electronic accelerator function
Vehicle speed regulation
Load change regulation
Secondary air injection
EOBD II
CAN bus system Secondary air injection pump
3 Mixture formation in spark ignition engines 3.3 Direct injection
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3.3 Direct injection
Spark ignition direct injection
This is a relatively new method of internal mixture formation, which was introduced by
Mitsubishi in 1997. In direct injection systems the fuel is injected directly into the combustion
chamber by electromagnetically operated injection valves. One injection valve is allocated to
each cylinder. The mixture is formed inside the cylinder. When operating, the engine draws in
only air, and not the fuel/air mixture as in conventional injection systems. This is one
advantage of the new system: Fuel can no longer condense in the intake manifold and produce
high exhaust emission values. With external mixture formation the fuel/air mixture is generally
homogeneous (finely and evenly mixed) throughout the combustion chamber in the
stoichiometric ratio. Mixture formation in the combustion chamber, on the other hand, allows
two completely different methods of operation:
Stratified charge operation: Swirling movements form different layers in the fuel/air mixture, so that mixture with different air/fuel ratios is stratified in the combustion chamber. In stratified operation the mixture only needs to be ignitable in the area around the spark plug. The rest of the combustion chamber contains only fresh gas and residual gas with no unburnt fuel. At idle and in the partial-load range the mixture as a whole is very lean (fuel/air ratio about 1:40). The result is a reduction in the fuel consumption. However, as the system is not operating at = 1, a new catalytic converter technology is necessary.
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Homogeneous operation: In homogeneous mode the mixture is homogeneous throughout the combustion chamber, as with external mixture formation. All the fresh air in the combustion chamber is involved in combustion. For this reason this operating mode is used in the wide open throttle range.
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3 Mixture formation in spark ignition engines 3.4 Exercises on mixture formation
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Exercise 1
a)
b)
c)
Exercise 2
Exercise 3
Exercise 4
a)
b)
c)
3.4 Exercises on mixture formation
What is meant by mixture formation in internal combustion engines?
The mixing of regular gasoline and Super gasoline
The formation of the fuel/air mixture
The formation of the air/fuel ratio and octane number
What do gasoline and diesel consist of?
What are alternative fuels? Name 3!
What is meant by the mixture ratio of the fuel/air mixture? The ratio of ...
Fuel to air.
Diesel fuel to gasoline and air.
Basic gasoline to additives and air.
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Exercise 5
Exercise 6
Exercise 7
a)
b)
c)
d)
Exercise 8
a)
b)
What does combustibility mean? How is combustibility designated in the case of spark ignition
and diesel engines?
What is the meaning of = 1?
Which of the following statements are true?
Spark ignition fuels should have low combustibility.
Spark ignition fuels should have high combustibility.
Diesel fuels should have low combustibility.
Diesel fuels should have high combustibility.
Which statement is true?
A rich mixture has > 1.
A rich mixture has < 1.
3 Mixture formation in spark ignition engines 3.4 Exercises on mixture formation
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Exercise 9
Exercise 10
a)
b)
c)
Exercise 11
a)
b)
Exercise 12
When is the throttle valve of the engine wide open? Is this related to rpm?
Which engine operating states require a rich fuel/air mixture?
Cold start, warm-up, acceleration, full power
Only at full power
Only at cold start and full power
The mixture ratio of fuel to air depends on...
Engine rpm, temperature and load.
Engine rpm, load and piston weight.
How many injection valves are there in a 4-cylinder engine with central intake manifold
injection?
3 Mixture formation in spark ignition engines 3.4 Exercises on mixture formation
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Exercise 13
Exercise 14
Exercise 15
Is the fuel quantity adjusted to suit the air volume, or vice versa?
Describe a mixture formation system of your choice!
What are the differences between a Motronic and a K-Jetronic?
3 Mixture formation in spark ignition engines 3.4 Exercises on mixture formation
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Exercise 16
Which variable tells the controller in the hot wire mass air flow sensor how much air has been
taken in?
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4 Ignition in spark ignition engines
4.1 Ignition in spark ignition engines
Ignition system in spark ignition engines
The purpose of the ignition system is to ignite the compressed air/fuel mixture at the right
instant and thereby initiate combustion. The right instant is called the firing point. In a spark
ignition engine this is accomplished by an electrical spark between the electrodes of the spark
plug. An ignition system that functions reliably under all circumstances is a prerequisite for
proper operation of the engine and catalytic converter. Misfiring results in poor engine
performance, higher consumption and degradation of the catalytic converter due to
overheating. Afterburning of unburnt fuel in the catalytic converter causes it to overheat. High-
voltage systems are generally used to produce the ignition spark. These consist of the spark
plug(s) and the actual ignition system for generating the high voltage required.
Firing point
The instant at which the mixture should be ignited depends primarily on:
Engine speed
The time it takes for a uniform fuel/air mixture to burn completely is constant. About 2 milliseconds pass from the instant of ignition until combustion is complete. As the engine speed rises, the time for this cycle decreases. For this reason ignition must occur earlier at higher engine speeds.
Load
At low loads the mixture leans out, the residual gas quantity increases and the volumetric efficiency decreases. This effect produces a longer ignition delay and a lower combustion rate in the mixture, so ignition must take place earlier.
For these reasons the ignition system includes an adjustment facility to regulate the firing point
according to load and engine speed.
It is usual to reference the firing point to the position of the crankshaft relative to top dead
center (TDC). It is stated as an angle in degrees before TDC. Two adjustment variants exist:
Ignition retard: Adjustment of the ignition angle towards TDC.
Ignition advance: Adjustment away from TDC.
The choice of firing point influences a number of combustion processes and results:
Maximum engine power
Fuel consumption
Driveability
Emissions
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Tendency to knock: Knocking is uncontrolled combustion in the engine. It occurs due to the sudden burning of mixture particles that have not yet been reached by the flame. If this happens, the firing point is too far advanced. Knocking leads to excessive temperatures and steep pressure rises in the combustion chamber. Pressure oscillations occur which superimpose themselves on the normal pressure curve. The cylinder head, cylinder head gasket, bearings and pistons may be damaged. A distinction is drawn between knocking during acceleration (at low rpm and high loads) and high-speed knocking (high rpm and high loads).
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1. Firing point at the correct instant
2. Firing point too early
3. Firing point too late
Spark plug
The spark plug is screwed into each combustion chamber and has the task of bringing the
ignition energy into the combustion chamber. An electrical spark between the electrodes
initiates combustion of the cylinder charge. A ceramic insulator is installed in the plug housing
with gas-tight seals. Usual electrode gaps are 0.6 to 0.9 mm. A longer electrode gap activates a
greater mixture volume, requires a higher ignition voltage and therefore increases the demands
on the ignition system and insulation. With a smaller electrode gap there is the danger of
combustion misfiring as the volume activated is too small. Together with other components of
the engine, the spark plug plays a crucial role in ensuring that the engine operates correctly.
It must enable a reliable cold start and must always guarantee that the engine runs without
misfiring.
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1. Insulator
2. Electrode gap
The demands on a spark plug are enormous:
Electrical: Insulation of high voltages in excess of 30 kV.
Mechanical: Pressure spikes of over 100 bar while remaining gas-tight.
Chemical load: At high temperatures the plug is exposed to the chemical reactions of combustion. Aggressive residues cause deposits that can change the properties.
4 Ignition in spark ignition engines 4.1 Ignition in spark ignition engines
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Thermal load: Extreme temperature cycles: Combustion up to 2800°C, gas exchange at 60-100°C. For a spark plug to function reliably, 2 limit temperatures must not be violated.
Lower limit temperature 500°C: The insulator nose must be hot enough to burn off carbon residues and thus avoid electric shunts.
Upper limit temperature 900°C: The temperature should not be higher if glow ignition is to be avoided. Otherwise the mixture ignites not due to the controlled initiation of an ignition spark, but by the high temperature in excess of 900°C.
The operating temperature of a spark plug is a temperature equilibrium between heat
absorption and heat release. Heat is absorbed through the combustion temperatures. Heat is
released to the fresh gas and via the center electrode, the insulator nose and the plug housing
through the cylinder head to the coolant. The input of heat depends on the engine design.
Engines with higher specific outputs generally have higher combustion chamber temperatures.
The temperature is maintained within the limits by varying the insulator nose accordingly.
The thermal load capacity of a spark plug is identified by its heat rating:
Low heat-range code numbers: Short insulator nose and thus a small heat-absorbing surface area, rapid heat dissipation, low resistance to carbon fouling, high resistance to glow ignitions (“short plug”).
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High heat-range code numbers: Long insulator nose and thus a large heat-absorbing surface area, slow heat dissipation, high resistance to carbon fouling, low resistance to glow ignitions (“hot plug”).
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Spark plug designs are distinguished not only by the heat rating (long or short insulator nose),
but also by the material, the shape and the number of the electrodes used. According to the
shape and number of electrodes a distinction is made between:
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Overhead electrode
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Side electrode
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Surface gap spark plug without ground electrode
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Ignition system
The energy required for ignition is provided by the ignition system and allocated to the spark
plug of the relevant cylinder at the firing point. The high voltage necessary for ignition is not
generated until the firing point. The electrical energy for this comes from an energy buffer. The
type of ignition system depends on the nature of this energy storage:
Coil ignition
Capacitor-discharge ignition
Coil ignition
The breaker-controlled coil ignition described here is the simplest version of ignition system in
which all the functions are implemented.
Its elements are:
Ignition coil as inductive energy storage
Distributor with mechanical contact breaker (triggers high-voltage generation at the firing point), timing gear (adjusts the firing point by means of centrifugal and vacuum timing adjusters), distributor finger (distributes the high-voltage pulse to the ignition cables of the individual cylinders) and ignition capacitor.
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Circuit diagram and function of the elements
The ignition energy source for the coil ignition system is the ignition coil. It stores the energy in
the magnetic field and releases it at the moment of ignition as a high-voltage pulse (ignition
pulse) via the ignition lines to the relevant spark plug. The storage method is based on an
induction process (magnetic field with coil). The ignition coil is therefore referred to as an
“inductive energy storage device”.
The ignition coil consists of 2 mutually insulated wire coils, one on top of the other: The primary
coil has few windings of thick copper wire, and the secondary coil has many windings. One end
of the primary coil is connected via the ignition switch to the positive terminal of the battery.
The other end is grounded via the contact breaker. The ignition capacitor is connected in
parallel with the contact breaker. It is operated mechanically so that the contact breaker opens
whenever ignition is supposed to take place. The secondary coil also has one end connected to
ground. The other end is connected via the distributor and ignition lines to the center electrode
of the spark plug.
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When the ignition switch is closed, the primary coil is connected to the positive terminal of the
battery. If the contact breaker is closed, a current flows – the primary current. It rises to a limit
value, the so-called “no-load current”. In conventional coil ignition systems with mechanical
contact breaker, this no-load current is limited to 3 to 4 amps.
At the moment of ignition the contact breaker opens the primary circuit and stops it. In this
instant the magnetic field collapses and induces a voltage in the primary and secondary
windings made of thin copper wire. The primary and secondary windings surround an iron core.
The purpose of this iron core is to intensify the magnetic field and therefore boost the stored
energy.
An important factor in the production of the ignition spark is the magnitude of the secondary
voltage. It is higher,
the faster the magnetic field collapses,
the greater the winding ratio between the primary and secondary coils and
the stronger the primary current.
Because the winding ratio used is 1:100, a high voltage is generated on the secondary side.
Before ignition commences, the gap between the electrodes of the spark plug is completely
non-conductive. The ignition pulse is carried by the ignition cables to the center electrode of
the spark plug, where it causes a steep voltage rise. When a certain voltage (ignition voltage
> 15000 V) is reached, the gap becomes conductive and the spark can cross. Immediately
afterwards the voltage drops steeply down to the burn voltage. This burn voltage is sufficient to
maintain the spark. This ensures that the mixture can still be ignited if this did not already
occur due to the high ignition voltage.
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1. Brief spark head 2. 3. 4. 5.
2. Spark tail as post-discharge with slightly fluctuating burn voltage
3. Spark line
4. Decay phase
5. Primary current flows
As soon as the energy supplied by the storage device drops below a given minimum, the
ignition spark breaks. The spark gap becomes non-conductive again. When the contact breaker
opens, a voltage of 300-400 V is also induced in the primary winding of the ignition coil. If
precautions were not taken, this voltage would produce a strong spark at the contact breaker.
This would lead to energy consumption, severe contact erosion and high transition resistances
at the contacts. The ignition capacitor prevents such negative phenomena. It is connected in
parallel with the contact breaker. At the instant the primary current is interrupted, it takes on
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the electrical charge and thus forms a secondary path to the opening contact breaker. It is
charged to the induced peak voltage. This takes a certain amount of time. The contacts are now
too far apart for the spark to cross. At spark rates below 3000/min the lift speed of the
breaker lever is so small that weak sparking still occurs in spite of the capacitor. This means
that with frequent urban driving the wear on the contact breakers is high and they must be
replaced more frequently.
Further developments of coil ignition
Further developments of the conventional coil ignition are:
Breaker-controlled transistorized coil ignition (TSZ-k),
Breakerless transistorized coil ignition (TSZ).
These developments were necessary because the conventional coil ignition system had the
following shortcomings:
Wear of the contact breakers
Insufficient energy capacity of the ignition coil in some cases.
In the breaker-controlled transistorized coil ignition system (TSZ-k) the function of the contact
breaker as the switch for the primary current is replaced by a transistor. The control current is
interrupted on the basis of the transistor by the mechanical contact breaker in the distributor.
In other words, the transistor here acts as a switch with “on/off” positions. When the contact
breaker is closed, the control current flows and the transistor is conductive (“on” position). This
allows the primary current to flow to charge the ignition coil. When the contact breaker opens,
the control current is interrupted and the interrupted is conductive (“off” position). The primary
current in the ignition circuit is interrupted, initiating the generation of high voltage in the
secondary circuit. The transistor can switch up to 9 amps (higher energy capacity of the ignition
coil)!
In the breakerless transistorized ignition system (TSZ) the mechanical contact breaker for the
control current of the transistor is replaced by electrical switches. Both elements are housed in
the distributor. Because there is no mechanical contact breaker, the firing point remains
constant across the operating duration.
Otherwise the TSZ-k and TSZ systems are identical to conventional coil ignition systems.
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Timing gear in coil ignition systems
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The ignition angle is an important factor in controlling combustion in spark ignition engines.
As the ignition delay and combustion duration change sharply according to the mixture
composition and engine speed, the ignition timing must be adjusted accordingly. Several
different systems are available for timing adjustment based on rpm and load.
Centrifugal timing adjuster: The purpose of the centrifugal timing adjuster installed in the distributor is to advance the firing point as the engine speed increases.
Two pivoting centrifugal weights are mounted on the carrier plate which is connected to the drive shaft. As the engine speed increases, centrifugal force swings them increasingly further away from their rest position against the force of the extension springs. The drivers adjust the pivoting breaker cams in the drive shaft relative to the drive shaft
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Vacuum timing adjuster: The vacuum adjuster changes the firing point in relation to the intake manifold vacuum. A law of physics dictates that a vacuum is produced in the intake manifold due to the high flow rates of the air. In the partial-load range, the spark ignition engine is operated with a less ignitable, slower burning mixture which must therefore be ignited earlier. The vacuum in the intake manifold is used as a measure of the engine load. The vacuum-dependent firing point advance function is integrated in the distributor and operates additively to the rpm-dependent centrifugal adjustment. The vacuum immediately after the throttle valve acts on the membrane. In the partial-load range this is pressed against the effect of the spring and rotates the breaker plate in the opposite direction to the breaker cam via the pull rod. The spring is preloaded so that no adjustment takes place at wide open throttle. The adjustment range is limited by end stops on the pull rod.
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Electronic ignition systems – Capacitor-discharge ignition (CDI)
A further stage of development was the integration of electronics in ignition systems and the
transition to capacitor-discharge systems. These are characterized by the fact that the ignition
energy is stored in a capacitor. They may be breaker-controlled or breakerless. The CDI
consists of a switching unit and an ignition transformer. The switching unit contains a charging
device, the capacitor and the necessary electronic circuitry. The charging device is a voltage
converter, which transforms the battery voltage into the considerably higher charging voltage
for the capacitor. The capacitor is charged to 300-400 volts. The charging currents (up to
100 amps) are switched by a thyristor (electronic switch). The ignition transformer transforms
the discharge current coming from the storage capacitor into the high voltage required to
generate the ignition spark.
The advantages of CDI over coil ignition are:
Higher ignition voltages
Almost complete insensitivity to fouled spark plugs
Timing adjustment in electronic ignition systems
Ignition systems with centrifugal and vacuum-controlled firing angle adjustment can only follow
simple adjustment characteristics in relation to engine speed and load. However, these
systems take inadequate account of different operating states and mixture compositions. By
using electronic ignition systems it is possible to obtain the optimum ignition of the fuel/air
mixture in all engine operating states. The mechanical centrifugal or vacuum-controlled timing
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adjuster is replaced by an ignition map stored in the control unit. With the aid of input factors
such as the engine speed/crankshaft position, load (intake manifold vacuum) and other
parameters (e.g. engine temperature, data from the knock sensor etc.), the control unit
determines the optimum firing point to each operating point.
Maintenance note
on ignition systems
The maintenance operations commonly carried out on modern electronic ignition systems
without mechanical distributor are limited to the replacement of spark plugs according to
the specified maintenance intervals (since the distributorless ignition and factory-set firing
points mean that there are no mechanisms susceptible to failure). On conventional
systems work is also performed to adjust the ignition timing, renew the contact breakers
and replace the capacitors. Before replacing the spark plugs, the areas around the plugs
must be cleaned and then the ignition cables disconnected. Care must be taken to ensure
that the ignition cables are reconnected in their original positions. This can be done using
an adhesive label if there is not already a cylinder marking on the cable. The spark plugs
can then be unscrewed using a spark plug wrench. The exact heat rating of the spark plug
for the vehicle must be noted. The new spark plug is then screwed in and the ignition
cables are connected. Observe the tightening torques (see Workshop Information System
WIS).
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Exercise 17
Exercise 18
Exercise 19
Exercise 20
4.2 Student exercise on ignition in spark ignition engines
What is meant by mixture formation in internal combustion engines?
a The mixing of regular gasoline and Super gasoline
b The formation of the fuel/air mixture
c The formation of the air/fuel ratio and octane number
What do gasoline and diesel consist of?
What are alternative fuels? Name 3!
What is meant by the mixture ratio of the fuel/air mixture? The ratio of ...
a Fuel to air.
b Diesel fuel to gasoline and air.
c Basic gasoline to additives and air.
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Exercise 21
Exercise 22
Exercise 23
Exercise 24
Exercise 25
What does combustibility mean? How is combustibility designated in the case of spark ignition
and diesel engines?
What is the meaning of = 1?
Which of the following statements are true?
a Spark ignition fuels should have low combustibility.
b Spark ignition fuels should have high combustibility.
c Diesel fuels should have low combustibility.
d Diesel fuels should have high combustibility.
Which statement is true?
a A rich mixture has � > 1.
b A rich mixture has � < 1.
When is the throttle valve of the engine wide open? Is this related to rpm?
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Exercise 26
Exercise 27
Exercise 28
Exercise 29
Exercise 30
Which engine operating states require a rich fuel/air mixture?
a Cold start, warm-up, acceleration, full power
b Only at full power
c Only at cold start and full power
The mixture ratio of fuel to air depends on...
a Engine rpm, temperature and load.
b Engine rpm, load and piston weight.
How many injection valves are there in a 4-cylinder engine with central intake manifold
injection?
Is the fuel quantity adjusted to suit the air volume, or vice versa?
Describe a mixture formation system of your choice!
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Exercise 31
Exercise 32
What are the differences between a Motronic and a K-Jetronic?
Which variable tells the controller in the hot wire mass air flow sensor how much air has been
taken in?
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5 Exhaust treatment in spark ignition engines
5.1 Exhaust treatment in spark ignition engines
The formation of pollutants
As a reminder: The fuel is composed of hydrocarbon compounds. If the carbon (C) and
hydrogen (H) components in the fuel could be completely burned with the oxygen (O2) from the
combustion air in the combustion chamber, the exhaust gas would principally consist of water
(H2O) and carbon dioxide (CO2). Unfortunately, this so-called ideal combustion is not possible
for technical reasons.
Rather, we are dealing with real-life combustion in which a number of by-products are
produced.
The by-products (exhaust gases) produced by spark ignition combustion are classified as:
Non-pollutants: Nitrogen (N2), carbon dioxide (CO2), oxygen (O2), water vapor (H2O). However, the non-pollutant carbon dioxide (CO2) has negative effects in the context of global warming.
Pollutants: Carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX).
Carbon monoxide (CO) is particularly hazardous as it acts as a respiratory poison and can be
fatal in higher concentrations. Hydrocarbons (HC) occur increasingly when the combustion
conditions are not good, for example with a rich mixture and in the “hidden” spaces in the
combustion chamber (top land).
Nitrogen oxides (NOX) occur at high combustion temperatures when the nitrogen (N2) in the
combustion air combines with the oxygen (O2). The action of sunlight breaks nitrogen dioxide
(NO2) down into nitrogen oxide (NO) and in one atom of oxygen (O). This free oxygen atom
bonds with the oxygen (O2) in the air to form ground-level ozone (O3). In conjunction with the
hydrocarbons, ground-level ozone is responsible for the formation of smog.
A great deal of effort is therefore invested in minimizing pollutant emissions. The concepts
extend from engines designed for optimum fuel consumption through to technical concepts for
exhaust treatment. These technical concepts can be broken down into:
Engine-related measures
Exhaust aftertreatment measures using catalytic converters
Engine-related measures
These are always compromises. What is good for reducing one pollutant may encourage the
formation of another:
The problem is to reduce the quantity of hydrocarbons (HC) as much as possible. This is done by means of small combustion spaces where there is little opportunity for the mixture to creep into “hidden” locations, from which it would be exhausted again unburnt.
A high compression ratio improves thermal efficiency. This reduces fuel consumption and the quantity of pollutants. However, greater efficiency leads to higher combustion temperatures and thus to higher nitrogen oxide (NOX) levels.
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The location of the spark plug in the combustion chamber, the spark temperature and duration: The ignition of the mixture and the heat release depend on these factors. And these are accompanied by the formation of pollutants. A central location for the spark plug is therefore preferred. The reason is that this results in very short flame propagation distances and allows almost complete combustion of the mixture.
The firing point and the valve timing also have a very strong influence on the pollutant level and fuel consumption.
Exhaust gas recirculation: This system feeds a small percentage of the exhaust gas back to the intake system and mixes it with the inducted fuel/air mixture. The recirculated gas lowers the combustion temperature, which causes a reduction in the NOX levels in the exhaust gas. A maximum of 8-10% of the fresh gas volume is recirculated gas.
Only the map-controlled exhaust gas recirculation of the Motronic operates perfectly. The control unit contains a characteristics map for exhaust gas recirculation. This lists the air mass required for each operating point. From the mass air flow sensor the control unit receives information about the air mass currently being inducted. It then compares this with the specified values in the map and allocates the exhaust gas recirculation quantity accordingly. This is done by controlling the EGR valve.
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Exhaust aftertreatment measures using catalytic converters
The combination of oxygen sensor and 3-way catalytic converter is the most efficient method of
keeping the pollutant levels of spark ignition engines low.
3-way catalytic converter
The 3-way catalytic converter derives its name from the breakdown (Greek “katalysis”) of the
3 pollutants carbon monoxide, hydrocarbons and nitrogen oxides. But this breakdown is more
of a conversion.
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Catalytic converters exist in ceramic and metal catalyst versions. The ceramic catalyst is
discussed here. The only difference is the material used for the substrate.
The ceramic catalytic converter is composed of the following elements:
Substrate: The substrate consists of a ceramic honeycomb structure, the channels and cells of which provide a high surface area for the chemical conversion. One liter of ceramic honeycomb provides a total surface area of about three liters.
Washcoat: To further increase the surface ares, the cell walls of the honeycomb structure are coated with an intermediate layer (washcoat) of aluminum oxide and so-called promoters. The promoters intensify the catalytic effect of the precious metal coating (catalytically active layer, see below). They increase the surface area by about 7000 times.
Catalytically active layer: This is applied to the washcoat and consists of the precious metals platinum, rhodium and/or palladium.
Housing with insulation: The ceramic substrate is extremely brittle and also has a different thermal expansion rate from the housing. For this reason it is embedded in an insulation layer, a wire mesh or ceramic fiber matting. Metal substrates do not require an insulation layer.
The task of the catalytic converter is to bring about the parallel oxidation and reduction
processes. Oxidation means that oxygen (O) is attached to the exhaust gases. Reduction means
that oxygen is taken away from the pollutant. The carbon monoxide (CO) and hydrocarbon (HC)
pollutants are converted into harmless carbon dioxide (CO2) and in water vapor (H2O) through
oxidation. The nitrogen oxide (NOX) pollutants are reduced by reduction into nitrogen (N2) and
oxygen (O2). The conversion rates on new catalytic converters is about 90-98% at a
temperature between 400 and 800° C.
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Lambda ( ) control
The conversion rates of the pollutants in the catalytic converter are most effective at around
= 1. It must be attempted to keep the fuel/air mixture in this range. This range is also
designated as the so-called lambda window.
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This is what the Lambda sensor is for. It is located after the exhaust manifold and before the
catalytic converter and determines oxygen content of the exhaust. This allows conclusions to
be made about the lambda ratio of the fuel-air mixture. It must determine whether the fuel/air
mixture is richer or leaner than = 1. A lean mixture contains more air, a rich mixture, in
contrast, less air. Each deviation from Lambda =1 must be transmitted to the control unit so
that it can make a corresponding change of the injection volume within the lambda window.
Mode of operation and design of the lambda control
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The oxygen sensor is a gas-permeable ceramic body made of zirconium dioxide. This becomes
permeable to oxygen at temperatures of 350°C and above. Above this temperature the
difference in the electrical charge between the outside air and exhaust sides allows a voltage
comparison to be made which provides an indication of the oxygen content of the exhaust gas.
As the measurement procedure is highly complex, it will not be covered any further here.
One more thing: Since the sensor can only measure, and thus regulate, at a temperature of
350°C and above, these sensors are usually heated so that the catalytic converter is able to
operate approx. 20-30 seconds after a cold start even at low temperatures.
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Problems in the use of catalytic converters
There are also problems in the use of catalytic converter technology:
Degradation of efficiency due to poisoning (filling with leaded fuel)
Thermal aging due to overheating
Aging of the oxygen sensor
Increased exhaust back pressure
Use of rare materials
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Exercise 33
Exercise 34
Exercise 35
5.2 Student on exhaust treatment in spark ignition engines
What measures can be taken to reduce pollutants?
What is the function of the oxygen sensor?
What does an EGR valve do?
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Exercise 36
Exercise 37
Exercise 38
Is complete combustion of the air/fuel mixture possible? Explain your answer!
In which air/fuel ratio range does the catalytic converter operate most effectively? What is it
called?
Complete the following sentence: “If the combustion temperature drops, the following is
reduced”:
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6 Mixture formation in diesel engines
6.1 Mixture formation in diesel engines
The diesel engine has an internal mixture formation system, i.e. the fuel/air mixture is only
produced and rendered ignitable in the combustion chamber. Ignition occurs due to
autoignition of the mixture, in other words no ignition device is necessary.
In the 2nd stroke (compression) at about 15-20° crank angle before TDC the fuel delivered by
an injection pump is injected into the combustion space via the injection nozzle. But the fuel,
injected into air at 500-900°C, does not ignite immediately. It first has to warm up, then
evaporate and mix with the combustion air. Only then is a combustible mixture formed, which
ignites at several points simultaneously. Because of the ignition the temperature in the
combustion chamber exceeds 2000°C, so the fuel injected afterwards ignites immediately
without a delay. The injection period is up to about 20° crankshaft angle.
Ignition delay
The time period from the start of injection to the start of ignition is the ignition delay, and is
usually approx. 1 ms.
Under the following conditions, however, the ignition delay can last for up to 2 ms:
Low engine temperature
Injection nozzles not atomizing
Injection pump start of delivery set too early
Bad fuel (e.g. gasoline)
This has certain consequences: A hammering noise known as combustion knock occurs. The
reason for this is that, due to the extended ignition delay, more fuel has accumulated in the
combustion chamber, which then ignites all the more violently. This sudden ignition sounds like
hammering or knocking.
Combustion knock can have the same consequences as in a spark ignition engine (see relevant
chapter).
The ignition delay can be influenced by changing the start of injection. When injection occurs
later, the temperatures and pressure in the combustion chamber are higher and the ignition
delay is shorter.
Mixture formation processes
The shape of the combustion chamber together with the positions of the injection jets
characterize the mixture formation process. The mixture formation processes are therefore
classified according to these features,
A distinction is made between:
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Chamber process (indirect injection): In the chamber combustion process the combustion chamber is divided into at least 2 chambers which are connected by relatively narrow transfer ports. The gas flow between the chambers speeds up the mixture formation process considerably. Such processes include the prechamber process and the swirl chamber process. •
Direct injection: In direct injection systems the combustion chamber is not divided, or any division is not significant. The chamber is formed from flatter or deeper cavities in the cylinder head or in the pistons. The task of mixture formation in these systems is often transferred largely to the injection nozzle. Mixture formation may be assisted by rotating air movements generated during the induction process. These systems include central injection, the M-process and the D-process.
The important variants of these combustion systems are outlined below:
Prechamber process
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The picture illustrates a typical layout with eccentrically arranged prechamber. One or more
transfer ports connect the prechamber with the main combustion chamber. A single-hole nozzle
(pintle nozzle) injects fuel directly into the prechamber with the spray aligned in the longitudinal
axis of the prechamber. As the piston rises, a violent air flow is created in the prechamber,
which activates mixture formation in the chamber. Because the walls of the prechamber are
hot, ignition occurs in the prechamber itself after a relatively short ignition delay. When
combustion has started, the majority of the rich (inert) mixture formed in the prechamber is
ejected into the main combustion chamber where it mixes with the air.
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Swirl chamber process
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The swirl prechamber is a spherical or cylindrical chamber in which turbulence is produced by
the tangential entry of the air flow from the transfer port. Fuel is injected by the nozzle in the
same direction as the air flow. Assisted by the centrifugal effect, the mixture is stratified in the
swirl chamber, with rich, heavy inert mixture around the periphery. After ignition in the
chamber, the gas containing the unburnt or only partially burnt rich mixture is ejected into the
main combustion chamber.
Direct injection
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The illustration shows a combustion system with one centrally located multi-hole nozzle.
The fuel jets are aimed radially outwards towards the cylinder wall so that as much of the
combustion cavity volume as possible is covered. The nozzle is primarily responsible for mixture
formation. An extremely high injection pressure ensures good atomization.
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M-process
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The M-process arose from the efforts to improve smooth running characteristics with low
consumption. The fuel jet from the eccentrically arranged nozzle is directed against the wall of
a spherical combustion cavity located deep in the piston. The jet sprays in the direction of the
air turbulence so that atomization is reduced. This produces a film of fuel on the wall of the
combustion chamber. Only a small proportion of the fuel is used for ignition, which means that
combustion is gentle. Accelerated evaporation of the film from the chamber wall, final mixing
and combustion proper are achieved by means of the hot combustion gases only after
combustion has commenced.
D-process
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In the D-process, too, the nozzle is eccentrically arranged in the cylinder head. Two jets are
aimed approximately parallel to the cylindrical wall of the oblique combustion cavity in the
piston. A vortex about the combustion chamber axis, which is produced as normal during
induction, creates an intense transverse flow at the injection jets and therefore guarantees
intensive atomization of the injected fuel. Again, a layer of rich mixture is formed, not on the
chamber wall, but in the vortex. At the end of the ignition delay, this inert layer is initially
immune from rapid combustion, which ensures that the engine runs smoothly.
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Comparison of chamber processes with direct injection
The chamber (indirect injection) processes are characterized by remarkably low combustion
noise levels. The reasons for this are the short ignition delay (hot chamber walls) and the rich
mixture in the chamber at the start of combustion.
A particular disadvantage of the indirect injection processes is the longer combustion durations
compared to the direct injection processes. Other negative aspects are the losses due to
transfer between the parts of the combustion chamber, meaning that fuel consumption is
5-10% higher than in engines with direct injection. In addition, the high flow rates increase
thermal losses. For cold starts a starting aid (starter glow plug) is necessary to heat the
contents of the prechamber.
Injection systems
Since good combustion requires that the fuel be distributed as finely as possible, the pressures
upstream of the injection valves should be as high as possible. A variety of injection system
variants are available for this, which will be described in more detail below.
The conventional diesel injection systems are:
In-line injection pumps (PE)
Individual injection pumps (PF)
Distributor injection pumps (VE)
More recent variants are:
Common rail systems
Pump-Nozzle Unit (PDE)
Pump-Line-Nozzle (PLD).
In-line injection pump
An in-line injection pump consists of two elements: a delivery pump and a high-pressure pump.
The delivery pump delivers the fuel from the fuel tank to the high-pressure pump at a pressure
of approx. 2 bar.
The high-pressure pump consists of piston pump elements. Piston pumps in turn consist of a
pump cylinder and a pump piston. There is one pump element for each cylinder.
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The pump piston is moved in the delivery direction by an engine-driven camshaft and back
again by the piston spring. Via the plunger the cam actuates the pump piston against the force
of the spring. The fuel is pumped via the pressure valve in the injection line to the nozzle.
Fuel delivery begins as soon as the top edge of the pump piston has closed the feed bore from
the delivery pump. It continues until the diagonal bottom edge of the pump piston opens the
feed bore again. The fuel displaced by the further movement of the piston can now flow back
through this bore. The end of delivery, and thus the injection quantity, can be adjusted by
rotating the pump piston with a toothed rack (control rod) and gear wheel to change the
position of the diagonal edge. The pump chamber is charged via the feed bore by filling the
cavity that is formed during the upward movement of the piston with the pressure valve closed
during the dwell period of the bore. To shut the engine off, the piston is rotated until its
longitudinal groove is constantly adjacent to the bore (zero delivery). These piston pumps are
connected in series in an injection pump block and are driven by a common camshaft and
rotated by a common control rod.
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Injection timing devices for in-line injection pumps
The higher the engine speed, the less time there is available for the combustion cycle. To
ensure that the mixture has sufficient time to combust, the fuel must be injected earlier. This is
why injection timing devices are used. Centrifugally controlled injection timing devices are
installed in the drive train between the engine and the injection pump. As the engine speed
increases, the centrifugal weights (1) turn the camshaft (2) of the injection pump against the
drive shaft in the “earlier delivery” direction.
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Individual injection pumps
To avoid the need for excessively long injection lines, individual pumps are used on large
engines. The pumps are flange-mounted on the crankcase itself as close as possible to the
associated cylinder. The pump pistons are operated by additional cams on the engine timing
camshaft. The modern Pump-Nozzle variants were developed to do away with injection lines
entirely.
Distributor injection pumps
A distinction is drawn between distributor fuel injection pumps with mechanical control and
those with electronic diesel control; there are versions with rotary solenoid adjustment
mechanisms and versions with solenoid valve control.
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1 Regulating valve 4 Shutoff valve
2 Solenoid valve for injection timing device 5 Flow controller
3 Pump piston 6 Fuel rack position sensor
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Distributor fuel injection pumps have only one delivery unit for all cylinders, which supplies the
injection nozzles of the individual cylinders in sequence according to the firing order. The fuel is
pumped from the reservoir into the chamber of the injection pump by a vane-type supply pump
mounted on the drive shaft of the injection pump. Any surplus fuel delivered is directed back to
the reservoir by a return line connected to the highest point of the pump chamber for bleeding.
The fuel quantity circulating in this way is used to lubricate and cool the moving components of
the pump. The pump is fitted with a mechanical rpm governor which regulates the fuel quantity
via the movement of the regulating valve. The pump also possesses an injection timing device
operated by the rpm-proportional pressure, which turns the roller ring via an appropriate
adjustment mechanism. This influences the start of delivery and thus the start of injection. An
electromagnetic shutoff valve closes the fuel supply to the pump plunger when the engine is
switched off. The operating principle of the distributor fuel injection pump (for high-pressure
generation) and fuel distribution is described in detail below:
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The pump plunger is connected with the cam plate and the rolling motion of the cam plate on a
roller ring produces a combined linear/rotary movement of the plunger. The fuel is supplied via
grooves in the pistons. The rotary motion interrupts the fuel inflow and the linear travel opens
the bore to an injection line while the pressure builds up. The end of fuel delivery is determined
by the position of the regulating valve. After a quarter turn (in a four-cylinder pump) the process
starts again from the beginning. Compared with in-line injection pump, distributor pumps are
more susceptible to malfunctions. All injection pumps feature a governor for the low idle speed
and the maximum engine speed (governed maximum). Between these two speeds the load is
adjusted via the accelerator pedal, which moves the control rod, for example. To reduce the
exhaust emissions of diesel engines while keeping fuel consumption low, injection systems with
electronic control are increasingly being used. In these the injection process can be flexibly
adapted to suit the operating state of the engine by an electronic engine management system
as with a characteristics map.
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Radial piston distributor pump
A very high injection pressure (up to 1500 bar) can be produced by a so-called radial piston
distributor pump. The main components of this kind of pump are the feed unit, consisting of the
cam ring, the roller shoes, the rollers and the delivery piston, as well as the solenoid valve
needle operated by the pressure solenoids and the injection timing piston.
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Function description
Filling: When the solenoid valve is open, the fuel flows at low pressure to the feed unit of the radial piston pump.
The rotary motion of the drive shaft causes the rollers to roll up the cams of the cam ring and push the pistons inwards. The fuel between the pistons is compressed. When the pressure solenoid is energized, the high-pressure system is closed by the solenoid valve needle, the fuel is subjected to high pressure and conveyed to the injection nozzle. The injection timing device can turn the cam ring and change the position of the delivery process on the delivery cam. In combination with the solenoid valve control it is therefore possible to adjust the delivery rate within certain limits independently of the delivery time and quantity allocation by shifting the position of the delivery process on the delivery cam.
Defining the injection quantity: When the solenoid valve is opened, the high-pressure side of the pump is connected with the low-pressure side, interrupting the delivery process.
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Common rail system
In the common rail injection system the injection pressure is produced by a high-pressure
pump, stored in a pressure reservoir (common rail) and from there distributed by the solenoid
valve-controlled injectors.
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The main feature of the system is that the injection pressure can be generated regardless of
the engine speed and the injection quantity. The rail acts as a high-pressure reservoir for the
fuel. The storage capacity of the rail is such that neither the removal of fuel by the individual
injections nor the work of the high-pressure pump cause significant pressure fluctuations.
A high-pressure piston pump of the radial piston design produces the pressure. The desired rail
pressure is regulated by a pressure regulating valve located either at the pump or at the rail.
The high pressure generated by the high-pressure pump and regulated via a regulation circuit is
present at the injector. The injector is the core of the system and its task is to inject the fuel
correctly into the combustion chamber. A pulse given at the right moment by the control unit to
the solenoid valve in the injector initiates the injection process. The opening duration and fuel
pressure dictate the incoming fuel quantity.
Injector
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The fuel coming from the rail is constantly present at the high-pressure connection (1) of the
injector. It floods the chamber of the injection nozzle (3) and also, via a feed bore, the valve
control cavity (2). The same pressure prevails in the chamber of the injection nozzle and in the
valve control cavity as long as the solenoid valve is closed. To guarantee that the injection
nozzle remains leaktight, a pressure area ratio of approx. 1.5 of the control plunger surface
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area to the nozzle needle is generated. This means that the hydraulic force of the control
plunger predominates and the valve control plunger presses the nozzle needle into its seat.
When the solenoid valve is energized, the pressure in the valve control cavity is abruptly
reduced. The pressure in the chamber lifts the nozzle needle off its seat and the fuel is injected
into the combustion chamber.
Pump-Nozzle Unit
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The Pump-Nozzle Unit consists of a high-pressure injection pump and a multi-hole injection
nozzle. This is the so-called Pump-Nozzle Unit. There is also a gear pump that supplies the
Pump-Nozzle Unit with fuel under an initial pressure of 3.5 bar. The Pump-Nozzle Unit is
installed in the top of the cylinder head and there is one for each cylinder of the engine. It is
driven, i.e. the pump piston is operated, by the engine camshaft with a cam provided especially
for this purpose. The Pump-Nozzle Unit has no control rod and the pump piston has no control
edge. Instead there is a fast-switching solenoid valve actuated electrically by a control unit.
The solenoid valve is installed in the return duct between the high pressure chamber of the
pump element and the fuel tank. The Pump-Nozzle Unit can produce injection pressures of up
to 2000 bar.
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Pump-Line-Nozzle
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In this system each engine cylinder has its own feed unit, consisting of a single-cylinder high-
pressure injection pump, a short injection line and the injection nozzle. The injection pump is
driven by the engine camshaft with an additional injection cam. The pump has no control rod,
no pressure valve and the pump element has a piston with no diagonal control edge. Instead
there is a fast-switching, electrically controlled solenoid valve installed in the return duct to the
fuel tank. The Pump-Line-Nozzle system is installed in Mercedes-Benz commercial vehicle
engines and produces injection pressures of up to 1800 bar.
Injection nozzle and injection valve
The fuel delivered by the injection pump is sprayed into the combustion chamber under high
pressure by an injection nozzle. The most common type of nozzle features a spring-loaded valve
needle which is lifted off its seat by the fuel pressure and opens the nozzle hole.
Two different designs of nozzle may be distinguished:
Multi-hole nozzle
Multi-hole nozzles are mainly used in the direct injection processes.
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Throttling pintle nozzle
These have only one nozzle aperture and are mostly used for engines with divided combustion chamber.
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Injection nozzles are usually replaceably installed in the nozzle holder, which also contains the
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The injection nozzle is pressed directly against the nozzle holder by a union nut with flattened
surfaces. The compression spring acts on the nozzle needle via a spring plate. The whole unit is
known as the nozzle holder. The connection connects the nozzle holder with the injection
pump.
Injection nozzles have extremely high mechanical and thermal load capacities.
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Exercise 39
a)
b)
Exercise 40
a)
b)
c)
Exercise 41
a)
b)
c)
d)
Exercise 42
a)
b)
c)
d)
Exercise 43
a)
b)
c)
d)
6.2 Student exercise on mixture formation in diesel engines
Which statement is true?
The diesel engine has internal mixture formation.
The diesel engine has external mixture formation.
What is meant by the ignition delay? The time span from the start of injection until ...
the maximum combustion pressure.
the start of autoignition.
the fuel has burned completely.
Which control system meters the fuel quantity in the in-line injection pump?
Edge control by the pump piston
Pressure valve control in the feed bore
Solenoid valve in the injection line
Edge control by a control valve ring on the pump piston
What fresh gas charge does the diesel engine receive during induction?
Diesel/air mixture
Oxygen/fuel mixture
Clean air
Pure oxygen
Which pollutants result from combustion in a diesel engine?
Carbon dioxide, oxygen, soot particles
Unburnt hydrocarbons, soot particles, nitrogen
Soot particles, nitrogen oxides, unburnt hydrocarbons
Nitrogen oxides, carbon dioxide, lead
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Exercise 44
Exercise 45
Exercise 46
Is the air/fuel ratio in a diesel higher or lower than in a spark ignition engine?
Describe the advantages and disadvantages of the chamber combustion process and the direct
injection engine!
Why are direct injection diesel engines louder than chamber combustion engines?
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Exercise 47
How is the delivery rate regulated in a distributor fuel injection pump?
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7 Exhaust treatment in diesel engines
7.1 Exhaust treatment in diesel engines
In diesel engines, too, great importance is attached to reducing the 3 pollutants CO, HC and
NOX. With diesels engines soot formation is also a concern (soot has been proved to be
carcinogenic).
Pollutants from diesel engines can also be reduced by measures relating to the engine and by
an exhaust aftertreatment process.
Engine-related measures
The injection timing point plays a similar role to the firing point in spark ignition engines, as it can be used to influence the start and progress of combustion. If the injection timing point is retarded, soot emissions are increased and NOx emissions are reduced. There is hardly any effect on HC emission levels.
High injection pressures and electronic control (modern diesel technology) reduce the exhaust emissions.
Exhaust gas recirculation: Another measure to reduce nitrogen oxides (NOX) is exhaust gas recirculation as in gasoline engines. This can significantly reduce the NOx concentrations. Lowering the combustion temperature reduces the NOX component in the exhaust gas without any compromises in terms of efficiency. At the same time there are positive effects on HC emissions in the low partial-load range. Particulate emissions, however, remain unaffected. The design of the system is approximately the same as for spark ignition engines.
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Exhaust aftertreatment
Exhaust aftertreatment to reduce nitrogen emission levels is not yet possible for diesel
automotive engines due to the presence of oxygen in the exhaust gas, which is inherent in the
process. So-called DeNOx catalytic converters, which allow NOx reduction even with surplus
air, are still in the development stage. At present the following methods of exhaust
aftertreatment are available:
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Oxidation catalytic converters: With the aid of these, hydrocarbons (HC) and carbon monoxide (CO) can be oxidized to harmless carbon dioxide (CO2) and water vapor (H2O) at exhaust temperatures above 250°C. It is not possible to oxidize soot particles at these temperatures, but particulate emissions are reduced thanks to the afterburning of the hydrocarbons (HC). Some of the soot particles are attached to the HC. In the upper load range, an increase in particulate emissions can only be achieved by reducing the sulfur content of the diesel fuel.
Particulate filter: The greatest prospects of success in reducing particulate emissions are anticipated in regenerative trap oxidizers (particulate filters). On the basis of certain processes in the filter (diffusion and adsorption processes), the particles are retained by the filter bed and burned off after a certain period of time. Filter come in two types: surface-type filters and deep-bed filters. In surface-type filters, particles form a layer of soot on the surface of the filter material. Particles in deep-bed filters penetrate into the filter material through pores. When a certain charge level is reached, e.g. when there is an increase in the flow resistance, the particles are burned off (filter regeneration). Burn-off can be achieved either by heating electrically, by means of special burners or by increasing the exhaust temperature.
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8 Increasing output by forced induction
8.1 Increasing output by forced induction
The purpose of forced induction is to increase the power output and torque of the internal
combustion engine without increasing the engine speed. With diesel engines another aim can
be to reduce noise emissions.
There are a number of alternatives available to increase the power output of engines:
Increasing the displacement
Increasing the engine rpm
Improving the charge of fuel mixture in the combustion chamber (forced induction)
A larger displacement means more material, more weight and more bulk. Any rpm increase is
accompanied by higher mechanical stresses and shorter service life. Both measures therefore
mean less economic efficiency. In forced induction (charged) engines, air is compressed in a
compressor (the so-called charger) and forced into the cylinder. This increases the density of
the intake air, which increases the charge of the cylinder so that more fuel can be burned in
each working cycle. In spark ignition engines forced induction is limited by the start of
combustion knock. In diesel engines it is limited by the strength of the components.
Forced induction processes
The forced induction (charging) processes differ in terms of the method used to drive the
charger:
Utilizing the exhaust energy to compress the charge
Mechanical charging
Combined process
Utilizing the exhaust energy to compress the charge
In this process the compressor is driven by a turbine wheel with no mechanical connection to
the engine. This is called a turbocharger.
The shaft is installed so that the turbine wheel turns in the exhaust tract and the compressor
impeller in the intake tract. The two wheels are connected by a shaft so that they cannot rotate
separately and are lubricated by the oil circuit of the engine.
Operating principle
When the driver demands more power and torque from the engine via the accelerator pedal,
the throttle valve is instantly opened and more fuel is injected. The increases the combustion
emissions and the rate of combustion. The rotational speed of the turbine wheel rises. Because
of the non-rotating connection between the turbine and compressor sides, the intake air is
forced at higher speed towards the combustion chambers. This also increases the intake
pressure in the cylinders. With each intake process there is now more air in the cylinders than
without turbo compression. The mass/volume air flow meter registers this and allows the
engine control to inject more fuel. The vehicle thus now has more power available at
approximately the same engine speed. It takes a certain length of time for the boost pressure
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to build up. In other words, the power increase is only available a few seconds after the
accelerator pedal is depressed. This effect is also referred to as “turbo lag”. The direct form of
energy transfer from the exhaust to the fresh charge is more efficient than mechanical
supercharging. No additional mechanical energy from the combustion process is consumed in
its operation. On the contrary, it exploits the kinetic energy from the exhaust stroke which
would otherwise escape unused into the atmosphere from the exhaust pipe.
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1. Compressor impeller
2. Turbine wheel
3. Air outlet
4. Exhaust inlet
5. Air inlet
6. Exhaust outlet
7. Rotor shaft
Boost pressure regulation
In order to assure the best possible cooperation between engine and turbocharger in all
operating ranges (wide open throttle – partial load), it is necessary to regulate the boost
pressure.
For this there are 2 concepts available:
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Wastegate control
This is a valve in the exhaust tract upstream of the turbine wheel. When a defined boost pressure is reached, a part of the exhaust gases is directed into the exhaust pipe bypassing the turbine. The reference variable available to the wastegate valve via a pressure line is the intake pressure (boost pressure). In vehicles with mechanical boost pressure control, the boost pressure acts on a diaphragm in the wastegate and opens the valve when the boost pressures are too high. With electronic boost pressure control the motor electronics control unit controls the valve. This control guarantees that good performance is achievable even at low engine speeds (with only low exhaust pressure). It can also provide the high torque rise (overboost) which gives the engine high pulling power, e.g. when driving uphill. The wastegate valve is opened when the motor electronics control unit determines that the engine is overloaded, e.g. due to an excessively high oil temperature, a tendency to knock etc.
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Variable turbine geometry (VTG)
In this design the exhaust turbine side is additionally equipped with a guide vane system. It changes the cross section and angle of incidence at the turbine wheel. It is therefore possible to have the effect of a small or large turbocharger which varies its boost pressure by changing the geometry of the turbine. This concept has no need of a wastegate.
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Mechanical charging
In this process the compressor is coupled with the engine and is driven by it. Examples of
mechanical chargers are the Roots-type supercharger (“Kompressor”) and the scroll or spiral-
type supercharger (“G-Lader”).
Kompressor
The two two-lobe rotary pistons of the Roots supercharger turn with no contact between each
other or between them and the housing. The size of the pocket produced is dictated by the
design, the materials used and the manufacturing tolerances. The two rotary pistons are
synchronized by a pair of gear wheels outside the working chamber, which are usually driven by
the crankshaft via V-belts.
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G-Lader
The G-Lader is a scroll-type supercharger. The displacer is eccentrically mounted and guided on
the housing side so that it performs an oscillating motion of twice the eccentricity when the
drive shaft is rotated. In this way the working chambers are opened for charging, closed for
delivery, and reopened again for ejection at the hub in phases. This also compresses and heats
the inducted air.
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Dynamic pressure charger (Comprex)
A cell rotor is driven mechanically by the crankshaft (requiring about 1% of the engine power for
operation) and rotates at approx. 2 - 3.5 times the engine speed. The cell rotor turns in a
cylindrical housing. The processes are relatively complex, so they will not be described in more
detail here. Basically, the supercharger generates pressure waves (at the speed of sound)
which cause an energy exchange between the exhaust gas and the fresh air. This exchange
takes place in the cells of the rotor (hence the term cell rotor). Due to the very good boost
pressure curve even at low engine speeds, the torque curve is excellent. Because the pulses
are transferred directly, there is no delay effect during acceleration as there is, for example,
with the turbocharger.
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1. Air inlet
2. Exhaust exit
3. Charge line
4. Housing
5. Cell
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6. V-belt
7. Housing
8. Cell rotor
9. Exhaust line
Intercooling
So that the compressed air can be utilized even more effectively, the intake air is usually cooled
before the intake turbine by a so-called charge air cooler. Cooler air has a lower density, which
additionally improves the charge in the cylinders.
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Exercise 48
Exercise 49
Exercise 50
8.2 Student exercise on increasing output
How can the power output of engines be increased?
Describe wastegate control!
Name the components.
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Exercise 51
Exercise 52
What does this picture show?
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What limits the forced induction of engines?
9 Power and torque 9.1 Power and torque
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9 Power and torque
9.1 Power and torque
Power is expressed in kilowatts (kW) or in horsepower (hp). Torque is expressed in newton
meters (Nm). The two parameters are usually shown together in an engine graph. The power
and the torque are plotted against the engine speed (see below). The engine speed is
expressed in revolutions per minute (rpm).
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To explain the graph (“What can be read from an engine graph?”): The power curve is the one
that looks like a walking-stick. It is also referred to as an inverted J-curve.
The power in kW is read on the left.
The torque curve is the other one.
The torque in Nm is read on the right.
The engine has a peak output of 103 kW at a speed of 5600 rpm. The engine has a torque of
205 Nm at a speed of 4000 rpm. Supercharged engines can be recognized by the torque curve,
which is shifted up and to the left. This means that supercharged engines have a higher torque
that is available in a low rpm range.
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Exercise 53
9.2 Student exercise on power and torque
This engine graph illustrates 2 engines: a forced induction engine and a naturally aspirated
engine. Label the graph with the designations for power and torque and state which torque
curve belongs to the forced induction engine. Explain your statement!
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10 Exhaust system 10.1 Exhaust system
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10 Exhaust system
10.1 Exhaust system
The exhaust system includes the following subsystems; their functions are given in brackets
(in sequence from the engine exhaust to the rear of the vehicle in a front-engine vehicle):
Exhaust manifold or collector (collects the exhaust gases from all the cylinders in a cylinder bank and conveys them towards the catalytic converter)
Oxygen sensor (subsystem for exhaust treatment; see above)
Exhaust gas recirculation system (subsystem for exhaust treatment; see above)
Catalytic converter(s) (subsystem for exhaust treatment; see above)
Center muffler (soundproofing)
One or more rear mufflers (soundproofing)
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The primary function of the exhaust system is to convey the exhaust gases from the engine to
the exhaust outlet under the vehicle rear. The outlet is at the rear so that exhaust gases do not
enter the interior through the ventilation intakes when driving. The installation of silencers in
the form of the center and rear mufflers provides an acoustic improvement of the combustion
noise level. Pipes connect the mufflers to each other and to the catalytic converter and
manifold. These pipes are connected together by exhaust clamps and by bolted flange
connections with film seals. The entire exhaust system is mounted on rubber mounts at a
distance from the body. For this purpose the exhaust systems and the underside of the body
have special eyelets which are connected by rubber buffers. The rubber mounting and body
clearances are necessary to prevent the transfer of exhaust vibrations and heat to the body,
and to allow for changes in length due to thermal expansion.
Repair information
As the products of combustion consist of aggressive acids, the exhaust can be expected to
corrode during operation. For this reason, some manufacturers equip their vehicles with
exhaust systems made of stainless steel. Conventional exhausts are made of aluminized steel
and other materials. The mufflers are particularly susceptible to corrosion. The reason is their
bathtub shape. When the engine is switched off, condensation and acid accumulate in the floor
of the muffler, which accelerate the oxidation process. The exhaust is especially susceptible
when repeatedly operated “cold”. This occurs with frequent short-distance trips. The exhaust
does not heat up completely, and the gases are unable to evaporate after the engine has been
switched off. To replace a muffler, the clamps or bolted flanges are unfastened and the muffler
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is pulled out. With the clamp design the pipes overlap, which causes contact corrosion. This
means that these parts can only be separated with difficulty. In this case a grinder or similar
must be used to cut a slit in the overlapping pipe. The new muffler is always installed using new
clamps/bolts and new rubber elements.
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