9MTZ worldwide 3/2003 Volume 64

MATERIALSTitanium

Knocking combustion in spark-ignition (SI) engines is a phenomenon that has alwaysaccompanied the development of internal combustion engines. In addition to unwantednoises, this uncontrolled form of combustion can damage the engine severely. The pre-sent article reports on knock characteristics, modern knock control systems and IAVGmbH's methodology for the adaptation of these systems to production-vehicle engines.

1 Introduction

A number of development projects werecarried out in the 20th century, resulting inthe optimisation of fuel characteristics, thedevelopment of combustion chamberswith higher knock resistance and the intro-duction of knock control systems. The in-creasing use of more complex engine com-ponents calls for sophisticated knock signaldetection systems.

2 Thermodynamics

Extensive research projects [1] have helpedto determine the most important influenc-ing factors and processes that cause knock-ing combustion. The term “knocking com-bustion” describes self-ignition processes ofend-gas zones in the combustion chamberbefore the actual combustion flame. Theseself-ignition centres are often located in thedirect vicinity of the combustion chamberwalls, i.e. at a distance from the flame front,Figure 1, [2, 3].

The spontaneous propagation of the re-action fronts is very often inhomogeneouswith sequential, apparently uncontrolledignition of adjacent mixture pockets and

with a shock wave reaching propagationvelocities of up to 600 m/s. This means thereaction fronts reach the range of end-gassound velocity and can cause thermal ex-plosions which damage the engine's valves,cylinder head gasket, pistons or pistonrings, especially at high engine speeds. Atlower engine speeds, knocking combustionprimarily results in unwanted engine nois-es and low engine torque when driving off.

The knock events that take the form ofdetonations lead to high-frequency cylin-der pressure vibrations that decay expo-nentially. The excited natural frequenciesof the gas column enclosed in the hollowcylinder can be computed on the basis ofthe general wave equation [4,5]. Figure 2shows the frequencies computed for five vi-bration modes in relation to the cylinder di-ameter at constant sound velocity.

The occurrence of knocking combustionis strongly influenced by the cyclical cylin-der pressure variations that are typical of SIengines as well as by the inhomogeneousend-gas temperature spread [6]. If the endgas burns out as a consequence of heattransfer and diffusion processes, a numberof isolated self-ignition centres distributedover the whole end-gas zone is produced,

By Michael Fischer,

Michael Günther,

Karsten Röpke,

Michael Lindemann

and Rolf Placzek

Klopferkennung im Ottomotor

– Neue Tools und Methoden

in der Serienentwicklung

You will find the figures mentioned in this article in the German issue of MTZ 3/2003 beginning on page 186.

Knock Detection inSpark-Ignition EnginesNew Tools and Methods in Production-Vehicle Development

10 MTZ worldwide 3/2003 Volume 64

which does not necessarily result in anypressure waves [3].

In addition to compression and ignitiontime, fuel characteristics, the intake tem-perature of the combustion air, the com-bustion chamber geometry and charge con-dition are important boundary conditions.Furthermore, detailed knowledge about theevolution and development of knockingcombustion is helpful to be able to bettercontrol self-ignition processes.

3 Statistical Examination ofKnock Intensity

One reference value for the quantitativeevaluation of the current knock intensity isthe maximum amplitude dp̂ of the high-pass filtered cylinder pressure (frequencylimit: 5 kHz).

Knocking combustion is a stochasticphenomenon, i.e. identical engine condi-tions (e.g. engine speed, cylinder charge, ig-nition angle, etc) do not produce identicaldp̂ values. To define the knock phenome-non for an operating point under the sameengine conditions, statistical values such asdp̂ distribution must consequently be used.Figure 3 shows distributions for a non-knocking and a knocking engine condition.As can be clearly seen from the figure, thedistribution under knocking condition issimilar to an χ2 distribution density func-tion: it is limited by the zero reference pointto the left and phases out with higher val-ues. Under knocking condition, the distrib-ution is broader while its form remains un-changed. Theoretically, none of the twofunctions has a finite limit. When estab-lishing the presence or absence of knockingcombustion for a specific engine conditionin practice, it is not sufficient to evaluate asingle result; thus, the maximum valuecannot serve as a representative value.

In practice, distribution quantiles are of-ten used (e.g. the 1.5% or 15% quantiles). Thequantile indicates the threshold exceededby a specified number of readings (here, forexample 1.5% or 15%). The values are alsodescribed as dp̂98,5 (1.5% above threshold) ordp̂85 (15% above threshold). Depending onthe application, different quantiles can beused to assess the knock intensity. dp̂ val-ues with high indices (e.g. dp̂99,5) best indi-cate the load of individual components be-cause the majority of dp̂ values is below thespecified threshold. However, a large num-ber of combustion cycles (e.g. 9,000 fordp̂99,5) must be evaluated when establish-ing the characteristic value in order to guar-antee its stability. dp̂ values with lower in-dices (e.g.dp̂85) lend themselves to quick de-cisions on whether there is knocking com-bustion under specific engine conditions.

Here, a relatively low number of combus-tion cycles is sufficient to establish the val-ue (e.g. 350 cycles).

Figure 4 shows a widely distributed def-inition of the knock limit. According to thedefinition, knocking combustion occursabove the upper threshold, but not belowthe lower one. The thresholds determinedon the basis of the quantiles are adapted inrelation to the percentage values selected;they are within the boundaries of the corri-dor depicted. Besides the definition ofknock thresholds based on real-measure-ment quantiles, chemical-kinetic simula-tion models were developed alternatively[6], [7] in order to forecast knocking com-bustion with the help of a knock criterion.

4 Knock Detection Methodsand their Control Unit Application

Modern engines, and in particular strati-fied-charge engines designed for minimumfuel consumption, increasingly exhibithigh compression ratios of ε > 11. The hightorque gradients resulting from the igni-tion intervention increase the demandsplaced on efficient knock detection sys-tems. In general, the following detectionmethods are used, Figure 5:■ Evaluation of the pressure curve plottedin the combustion chamber■ Evaluation of the light curve plotted inthe combustion chamber■ Evaluation of the ionic current curveplotted in the combustion chamber■ Evaluation of acceleration curve on en-gine components (crankcase).

At present, the evaluation of the com-bustion chamber pressure curve is the mostsuitable method for a reliable detection ofknock events. However, the enormous ef-fort required for the adaptation of the pres-sure sensors and the high cost of the quartzpressure sensors do not allow this methodto be used in mass production. It serves,however, as a reference for the evaluationof detection quality.

As to the evaluation of the light curveplotted in the combustion chamber, basi-cally two possibilities have proven to bepracticable:■ Evaluation of light curve with IntegralLight-Conducting Measurement technique(ILM) [8]■ Detection of knock centres in the com-bustion chamber with the help ofvideoscopy/tomography [9]

None of the above-mentioned methodsis, however, a real alternative to pressureindication. It is true that ILM, just like theanalysis of the pressure curve, provides astatistically assessable coefficient for knock

intensity. In terms of process safety andadaptation effort, however, it cannot becompared with pressure indication.Videoscopy can serve as a complementarytool because it generates information onthe position of the knock centres, which canbe valuable for combustion chamber devel-opment.

The evaluation of the ionic current curvewill become more important in the future.At present, this method neither is a practi-cable alternative to pressure indication dueto the dynamic behaviour of the ionic cur-rent under different combustion conditionsand the resulting time-consuming signalpost-conditioning process. Thanks to sim-ple and low-cost adaptation possibilities inmass production as well as the multiple useof the obtained signal for misfire detection,combustion control and knock detection, itis safe to expect that these systems will berapidly developed in the foreseeable future.

As far as mass production is concerned,the evaluation of the acceleration curve onthe cylinder crankcase still is the most com-mon solution. One of the generally knowndisadvantages of this method is the over-lapping of the wanted signal by mechani-cal excitations of the crankcase caused bythe moving parts of the engine.

5 Signal Analysis

For correct sensing of the pressure modesdescribed in section 2, the position of thepressure sensor is vital [10]. The first twoenergy-rich modes are so-called circumfer-ence modes that are to be found on theboundary with a vibration node at the cen-tre. These modes which are important forknock detection cannot be sensed with acentrally located pressure sensor, e.g. whenthe sensor is integrated in the plug as it is in4V engines. This is clearly depicted in Fig-ure 6. Depending on the sensor position,the results obtained in the tests for dp̂ dif-fered by a factor of up to 4! The influence ofthe sensor position can only be corrected toa limited extent. For this reason, it is betterto introduce the pressure sensor decentral-ly into the combustion chamber by meansof additional indicator holes.

The frequency spectra of cylinder pres-sure of the knocking combustion cyclesclearly exhibit the speed-independenteigenfrequencies and the amplitudes thatdecrease synchronously with the frequen-cy, Figure 7 left. The analysis of the acceler-ation signal from the knock sensor is re-quired for the control unit filter design. Thenormalised FFT spectra (knocking combus-tion cycles in relation to non-knocking cy-cles) in Figure 7 right exhibit almost identi-cal knock-relevant frequencies as for com-

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bustion pressure. The amplitude ratio is notonly determined by the knock combustioncycles, but also by those cycles where noknock occurs (so-called basic noise). Com-pared to combustion pressure, this resultsin a different amplitude distribution, bothin relation to engine speed and frequency.Furthermore, there are considerable differ-ences of amplitude ratio between the cylin-ders as a result of the influence of parasiticnoises (e.g. the noise at valveclosing). Thisis why the filter parameters cannot be de-rived from the cylinder pressure signal oronly to a very limited extent (filter meanfrequency). Based on the results of the sig-nal analysis, the relevant filter parameterssuch as mean frequency, band width andedge steepness can be optimally adapted tothe knock signals.

6 Functions of Common Knock Control Systems andCalibration

The functions of knock control systems canbasically be divided into four blocks:■ Knock detection■ Quick response to correctly detectedknock tendency – control■ Slow response to correctly detectedknock tendency – adaptation■ Response in case of failure of the detec-tion system – diagnosis.

Depending on the number of accelera-tion sensors used, cylinder assignment andsignal limitation are effected by means ofcrankangle-related windowing with thehelp of the variables measurement win-dow start (MWS) and measurement win-dow length (MWL). Normally, the measure-ment windows are derived from the timescale of dp. When using an indication sys-tem, however, the position of the maxi-mum combustion chamber pressure αpmaxcan be used. The determination of αpmax inrelation to the engine's boundary condi-tions provides direct information on the po-sition of the knock centres. Figure 8 showsthe relation between the knock intensityand the start of the knock vibration as an-gular difference Δα to αpmax. It is obviousthat the stronger knock events occur be-tween Δα = 2 ... 4 °CA before the maximumcombustion chamber pressure pmax isreached. If the knock events start at a latertime, the knock intensity is considerablylower.

The configuration of the measurementwindow must meet all engine boundaryconditions, therefore “MWS = αpmax – Δα” isa suitable solution. In this respect, the in-fluence of the propagation time until thesignal reaches the acceleration sensor andparasitic noises caused by the engine's me-

chanics must also be taken into account.For this reason, the correlation betweenpressure and structure-borne noises mustbe checked in any case.

Frequency-dependent, mechanical exci-tations of the structure can have consider-able impact on the usability of the combus-tion chamber's eigenfrequencies describedin section 5. Modern engine managementsystems therefore permit the use of severalknock-related frequencies (multiple filterstrategy). By weighting the results of theindividual filters, the solution that is bestsuited for the respective engine conceptcan be obtained. Furthermore the qualityand order of up to three filters per bank canbe freely selected in relation to the operat-ing point.

The comparison of the current signal in-tegrated above the measurement windowwith the basic noise permits to detect un-typical cycles. With the help of the K factor,that depends on the operating point andcan be calibrated for each individual cylin-der, several decisions are possible:■ Slight overshooting of basic noise leadsto the suppression of basic noise determi-nation – detection line■ Significant overshooting of basic noiseadditionally results in requisite ignitioncorrection – correction line■ Stronger overshooting of basic noise canbe used for major ignition interventions.

This approach allows to react properly tothe respective knock condition and reducethe basic noise drift at the same time. Afterthe load dynamics have been established,the detection system can be adapted rapid-ly to changing engine boundary conditions.

The requisite ignition corrections aresaved for each cylinder in adaptation mapsthat depend on load, engine speed and dy-namics and are immediately availableupon any changes of engine conditions. Ad-ditive and multiplicative corrections makeit possible to react differently to quicklyand slowly changing engine conditions.

In order to use the engine's entire effi-ciency potential, the ignition pilot controlmust be adapted to the lowest knock ten-dency. In case of an increase of knock ten-dency, the failure of the knock detectionsystem can result in engine failure as aconsequence of knock-related damages.This is why the basic noise is plausibilisedand compared with calibratable thresholdsin order to detect wrong system reactions.With a view to protecting the engine, theignition pilot control is then corrected insuch a way that knock-free operation un-der any circumstances is guaranteed. Ide-ally, it should be possible to save the cor-rective value in a load- and speed-depen-dent map.

7 Effective Parameter Optimisation and Suitable Tools

Appropriate tools for the measurement ofknock activity are devices that are capableof computing the knock coefficient dp̂ inreal time and of imitating the functions ofcontrol units in terms of knock detection.

The Knock Indication System (KIS) de-veloped at IAV provides all these functions.A CAN interface and additional analogueinputs enable the measurement of furtherphysical variables and their mapping insynchronisation with the KIS data. The KISunit operates in conjunction with a PC orlaptop, making it suitable for both test-bench and in-vehicle measurements. TheCAN interface permits communicationwith test benches so that automated mea-surements with synchronous knock moni-toring are possible. The unit can be config-ured from the test bench and be activatedfor endurance run or single-shot measure-ments via the CAN interface. At the sametime, the physical variables measured byKIS, e.g. knock condition, pressure peaks,noise integral etc, can be transferred backto the test bench. Figure 9 shows the con-nection of the KIS unit to the test bench forthe measurement of physical variableswith the aim of offline optimisation of theknock detection system.

A test bench automation program isused to activate different operating pointsover the entire engine speed and loadrange. Subsequently, the ignition angle isadjusted until knock events occur. For eachoperating point, a large number of combus-tion cycles is measured. The pressure peaksare recorded with the KIS system and theraw signals of the knock sensors with adata recorder.

In practice, the parameter selection of anoptimum knock detection requires a sys-tematic approach. An optimum parameter-isation is guaranteed if the correlation ofpressure peaks and noise integral reachesthe “best possible” level. The term “bestpossible level” can either be quantifiedwith the help of the correlation coefficientor the integrated measurand ratio (= noiseintegral “knock”/noise integral “no knock”).Figure 10 (left) shows noise integral valuesas a function of the pressure peaks dp̂ withthe mean straight line. In order to obtainhigh correlation, the correlation coefficientshould converge to 1 (convergence of bi-variate point distribution and meanstraight line) or the measurand ratio be ashigh as possible (steep mean straight line).

By computing the noise integral valuesfor all parameters, optimum parameterisa-tions can be obtained offline for single orclustered operating points on the basis of

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DEVELOPMENT Knock Detection

the correlation coefficient or the measur-and ratio. Figure 10 (right) shows the curveof the correlation coefficient r in relation tothe measurement window start (MWS) andlength (MWL) for a filter mean frequency of14 kHz. With the help of these diagrams, itis very easy to find the maximum correla-tion coefficient and the respective mea-surement window parameters (e.g. MWS =10 °CA and MWL = 60 °CA).

8 Evaluation of a Knock Control System

When evaluating the performance of aknock control system, the only relevant cri-teria are thermodynamic aspects. The cali-bration of the knock control system meetsthe requirements if the following condi-tions are fulfilled:■ Avoidance of audible knock events – pri-marily in the lower engine speed segment■ Avoidance of destructive knock events –primarily in the upper engine speed seg-ment■ Avoidance of unnecessary ignition re-tards and, as a consequence, efficiency loss-es.

One evaluation criterion can be derivedwith the help of statistical methods, justlike the definition of the knock limit de-scribed above (section 3). With the knockcontrol system being active, an appropriaterandom sample of combustion cycles ismeasured and the intersection line of thedensity function with the same distribu-tion of the number of cycles is established.The comparison of the intersection line'sposition on the dp̂ axis with the desired po-sition according to the knock limit defini-tion can be used as a standard for evaluat-ing the quality of the knock control system,Figure 4. The knock intensity shall be inde-pendent of boundary conditions that influ-ence the knock activity, such as fuel quality,compression ratio, etc. The changingboundary conditions only reflect in the re-sulting control depth. The examination of alarge number of engines and the observa-tion of the control behaviour over the oper-ation period have confirmed that it is possi-ble to realise compliance with the knock in-tensity corridor shown in Figure 4.

9 Summary

Knocking combustion can damage spark-ignition engines. For this reason, modernvehicles are equipped with knock controlsystems in order to guarantee the request-ed operation at the knock limit in terms ofoptimum efficiency.

The present article describes the phe-nomenon of knocking combustion from the

thermodynamics aspect and shows possi-bilities for the measurement of knockevents. Cylinder pressure still serves as thereference for knock data. In order to provideknock control systems with optimum data,IAV relies on pressure-specific signal prop-erties and statistical methods. The use ofstatistical methods is supported by auto-mated measurements with specially devel-oped techniques and applied to the subse-

quent evaluation of the knock control sys-tem. Using the methods presented above,knock control systems can be calibrated ef-ficiently. New sensor signals such as ioniccurrent or combustion light will become in-creasingly important in mass production.

With a view to promoting the exchangeof information on “Knocking Combustionin SI Engines”, IAV will organise a technicalseminar on this issue in this year.

List of Abbreviations and Symbols

dp̂: Maximum amplitude ofhigh-pass filtered pressurecurve (frequency limit5 kHz), knock intensity inbar

dp̂98,5: 1,5% quantile of distribu-tion, knock intensity in bar

dp̂85: 15% quantile of distribu-tion, knock intensity in bar

ε: Compression ratioILM: Integral Light-Conducting

Measurement techniqueFFT: Fast Fourier TransformationFM: Mean frequencyOT: Top dead centre°KW: Degree crank angleαpmax: Position of pressure maxi-

mum in °CA after TDC°KWnach OT

Δα: Difference between theposition of pressuremaximum and start ofknock vibrations in °CA

pmax: Pressure maximum in barKIS: Knock Indication SystemGI: Noise integral in VsIWV: Integrated measurand ratio

= noise integral “knocking”/ noise integral“no knocking”

ρ: Correlation coefficientMFA: Measurement window start

in °CAMFL: Measurement window

length in °CA

References

[1] Pischinger, F.: Motorische Verbrennung,Abschlußbericht Sonderforschungsbere-ich 224, RWTH Aachen, 2001

[2] Kollmeier, H.-P.: Untersuchungen überdie Flammenausbreitung bei klopfenderVerbrennung, Dissertation RWTHAachen, 1987

[3] Stiebels, B.: Flammenausbreitung beiklopfender Verbrennung, Fortschritts-berichte VDI-Reihe 12, Nr. 311, Düssel-dorf VDI Verlag, 1997

[4] Heckl, M.; Müller, H.A.: Taschenbuch derTechnischen Akustik, Springer-VerlagBerlin, S. 22, 1994

[5] Adolph, N.: Messung des Klopfens anOttomotoren, Dissertation RWTHAachen, 1983

[6] Kleinschmidt, W: Zur Simulation desBetriebes von Ottomotoren an der Klopf-grenze, Fortschritts-Berichte VDI, Reihe12, Nr. 422, 2000

[7] Franzke, D.E.: Beitrag zur Ermittlungeines Klopfkriteriums der ottomotori-schen Verbrennung und zur Voraus-berechnung der Klopfgrenze, Dissertationder TU München, 1981

[8] Prilop, H.; Broll, H.; Halfmann, J.: ILM amOttomotor – optische Klopferkennung,Tagung “Motorische Verbrennung”, HDTEssen, 13./14.03.2001

[9] Winkelhofer, E.; Beidl, C.; Philipp, H.;Piock, W.: Erfahrung mit VisioFiber-Tech-niken bei der Entwicklung modernerBrennverfahren, 5. Int. Symposium fürVerbrennungsdiagnostik, Baden-Baden,2002

[10] Förster, J.; Günther, A.; Ketterer, M.;Wald, K.-J.: Ion Current Sensing for SparkIgnition Engines, Society of AutomotiveEngineers, 1999-01-0204, S. 21-32, 1999