400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

SAE TECHNICALPAPER SERIES 980139

Combustion Control with the Optical Fibre FittedProduction Spark Plug

Frank Geiser and Frank WytrykusSMETEC GmbH

Ulrich SpicherUniversität Karlsruhe, Institut für Kolbenmaschinen

Reprinted From: Analysis of Combustion and Flow Diagnostics(SP-1348)

International Congress and ExpositionDetroit, Michigan

February 23-26,1998

The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.

SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.

Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.

To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.

ISSN 0148-7191Copyright 1998 Society of Automotive Engineers, Inc.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.

Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.

Printed in USA

All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database

1

980139

Combustion Control with the Optical Fibre Fitted Production

Spark Plug

Frank Geiser and Frank WytrykusSMETEC GmbH

Ulrich SpicherUniversität Karlsruhe, Institut für Kolbenmaschinen

Copyright © 1998 Society of Automotive Engineers, Inc.

ABSTRACT

Optical measurement technique became more and morecommon for the last few years. Especially optical fibretechnique is often used to detect flame propagation. Withoptical sensors the ignition process can be investigatedwith high temporal and spatial resolution. An in-cylinderoptical sensor has been developed and tested to analyzethe ignition of mixture and luminous emission of burninggas. The sensor consists of eight optical probes fitted in aconventional spark plug.

The results show good correlation between measuredluminosity and combustion parameters such as load,engine speed, ignition timing and air-fuel mixture ratio. Acorrelation between development of light intensity andpressure was found. For evaluation of light signals differ-ent analysis methods are presented.

Furthermore it is shown that the luminosity of the flamecan be used to control the combustion process. With thedetected light signals it was possible to optimize ignitiontiming with respect to high engine performance.

INTRODUCTION

In-cylinder pressure measurements are the common toolto analyze and to improve the combustion process of SIengines. New engine concepts like direct injection SIengine, dual-plug ignition system and four or five valveengines require modern measurement techniques whichprovide further information about the combustion pro-cess. Optical fibre technique is more suitable to gainmore versatile data than conventional pressure transduc-ers.

Many optical in-cylinder sensor techniques have beenalready presented either to monitor flame propagation aspresented in /2,3,4/ or to analyze emission spectrum withregard on different molecules /9,10,11,12/.

In the present study, an optical sensor is described forthe use in a production engine to find engine controlstrategies.

The aim was to develop a sensor which is easy in han-dling and which could be fitted in any engine. For thatreason an optical combustion sensor that combine fibreoptics with a conventional spark plug was installed in aproduction engine. A modified conventional spark plugwas used for the investigations. Using a conventionalspark plug, the physical properties like heat value wereoriginal.

This kind of measurement had already been published byWitze /1/ who investigated the influence of charge motionon the early flame development. This study is focused onthe relationship between overall luminosity measured bythe probes in the spark plug and cylinder pressure, heatrelease, IMEP, knock detection and air-fuel ratio for thepurpose of combustion analysis and feedback enginecontrol.

2

Figure 1. Flow Chart of Fibre Optic Measurement Technique

MEASUREMENT TECHNIQUE

The measurement system includes the following compo-nents:

• Fibre Optic Spark Plug

• Optical Receiver

• Data Acquisition Board

• Software for Acquisition and Analysis

The signal processing is schematically shown in Figure1. The flame luminosity is transmitted via optical probesand their corresponding optical fibres to photomultipliers,which convert the light into proportional and amplifiedvoltage. The data acquisition board digitizes the ana-logue electrical signals. The analysis software is used tocontrol the combustion. The data acquisition system forthe detection of the light used here includes an 8-bit ana-log-digital converter. The system is capable of sampling128 channels of data at 1 MHz/channel. For the acquisi-tion of pressure data 12-bit analog-digital converters witha maximum sampling rate of 1 MHz/channel were used.This measurement system allows a resolution of 0.1degree crank angle (°CA) at engine speed up to 16000rpm.

SPARK PLUG – The configuration of the optical probesmounted in the spark-plug thread are shown in Figure 2.Depending on the spark-plug type six up to eight opticalprobes are installed uniformly spaced in the ring aroundthe electrodes. The optical probes operate like an aper-ture with a detection diameter of 0.8 mm.

Figure 2. Position of Optical Probes

The cone-shaped volume element observed by the sen-sor is limited by a 7 degree field of view. A sapphire win-dow is mounted at the front position in the combustionchamber to protect the optical fibres which transmit thelight from the combustion chamber to the photomultiplier.During the mechanical processing the ceramic insulatoris not damaged, so that the heat value remains constant.

3

Figure 3. Fibre Optic Fitted Production Spark Plug

Figure 3 shows the spark plug which was used for theinvestigations. The optical fibres inside the spark plug areprotected by a metal cover. At the end of the cover flexi-ble metal tubes separate the optical fibres. The compactdesign assures high stability and an easy handling.

TRANSMISSION – Figure 4 shows the spectra which istransmitted by all optical components used in the experi-mental setup. The emission spectra during combustionincludes radiation in a range of wavelength from 200 nmup to 600 nm /7,8/. As shown in Figure 4, all componentsused for the investigations are capable of transmitting thegas radiation, especially in the UV range. In order toavoid the detection of black body radiation the transmis-sion range of the photomultiplier ends at 650 nm.

OPTICAL SIGNALS AND EARLY FLAMEDEVELOPMENT – In Figure 5 a characteristic opticalflame signal is shown. The curve presents the normal-ized intensity of the light signal versus crank angle (CA).The intensity at highest possible amplification of the pho-tomultiplier with constant voltage supply was set to be 1.According to this normalization the voltage supply wasadjusted for higher or lower light intensities.

Figure 4. Transmission of Optical Components

At ignition timing some of the optical probes detect theradiation of spark discharge which appears as a smallpeak before the flame signal as shown in the Figures 5and 6b. The following rise of the signal is caused by theflame radiation when the flame kernel has grown and theflame front reaches the observation field of the sensors.

Figure 5. Evaluation of Early Flame Radiation

4

When the flame kernel leaves the spark plug area theprobes detect the gas radiation of the observation field inthe combustion chamber. The intensity of the gas radia-tion depends on the increase of temperature and pres-sure. The detected light signal grows up to its maximumafter the top dead center.

The flame front arrival (FFA) is defined by a signal thresh-old as shown in Figure 5. The signal remains on highlevel during the complete combustion process. The signalthreshold is set individually for each probe by averagingtheir corresponding peak values of all sampled cycles.The threshold level is then set to 2% of the average peakof each probe. This relative threshold level assures com-parable results for all probes despite different light trans-mission due to contamination with oil or soot particles.The rise of the signal depends on the development oftemperature and pressure.

With the FFA at each probe the flame propagation in theearly stage of combustion can be reconstructed asshown in the bottom picture in Figure 5. The flame area isreconstructed as follows:

The flame front shape is calculated by the interpolation ofthe FFA for all probes with the assumption that the mix-ture is ignited at the center electrode. These data yieldthe mean flame velocity Vi(α) of each probe as describedbelow with eq. 1,2,3. Assuming a half-spherical flamepropagation for the early stage of combustion the flamevolume V and the volume rate of flame growth xV(α) canbe determined additionally (eq.4).

Flame front position of each probe:

(Eq. 1)

Mean flame velocity of each probe:

(Eq. 2)

(Eq. 3)

Volume rate of flame growth:

(Eq. 4)

: Radius of optical probesα: Time of calculation in °CA ABDCα FFA: Time of flame front arrival point in °CA ABDCα IT: Ignition timing in °CA ABDCVF(α): Calculated flame volumeV(α): Engine displacementVC: Clearance volume

For most operating points the visualization of the earlyflame propagation at 10 Degree CA after ignition supplieswell information for analyzing the early flame develop-ment. A flame propagation evaluation is shown as anexample in Figure 5.

According to various operating conditions the curve ofthe light intensity shows different characteristic shapes.Therefore the signals provide much more informationthan the flame arrival timing.

Figure 6 represents five characteristic light signals mea-sured with the spark plug sensors. In Figures 6a and 6bcommon signals at normal operating conditions are illus-trated. In Figure 6b the ignition peak appears before theflame signal. A typical signal for part-load conditions isshown in Figure 6c where the main peak is very low. Thestrong fluctuations of the signal as shown in Figure 6doccurs for lean mixtures. Knocking combustion is charac-terized by oscillations of the intensities with high frequen-cies which is shown in Figure 6e.

INVESTIGATIONS

The measurements were performed in one cylinder of awater-cooled, four-cylinder, 4-stroke engine with fourvalves per cylinder and the following specifications:

Engine model: Opel C20XEBore: D = 86 mmStroke: S = 86 mmDisplacement 2.0 lCompression ratio: ε = 10.5Maximum power: P = 110 kW.

For the investigations a modified spark plug of the typeBosch FR8DCX was used.

Each result of the further Figures belongs to a meanvalue of 200 cycles. The pressure data were acquiredwith a temporal resolution of 1 °CA, the light signals witha temporal resolution of 0.1° CA. To analyze the knockingcombustion for both signals a temporal resolution of 0.1°CA is required.

( ) ( ) ( )xr

iFFA IT

IT

→=

−∗ −

( ) ( )V

xi

i→→

=∆

∆ = − IT

( ) ( )( )

xV

V VVF

C=

+

r→

5

Figure 6. Different Characteristics of Optical Flames

6

The investigations were ordered in three parts. First theinfluence of different engine parameters on flame propa-gation 10° CA after ignition time will be shown. In the nextstep the correlation between the development of pres-sure and flame radiation will be discussed. The third partdescribes investigations with an optical sensor used as aknock and misfire detector.

INFLAMMATION AND EARLY FLAMEDEVELOPMENT – To analyze the influences of differentoperation conditions on inflammation as well as flamepropagation the load and engine speed were varied.

In Figure 7 the inflammation and flame propagation at10° CA after ignition point for different loads is shown.The Figure shows a view through the cylinder head onthe piston, whereas in this presentation the location ofexhaust valves are below and of inlet valves above. Theengine was operated at engine speed of 2000 rpm, sto-ichiometric air/fuel ratio and indicated mean effectivepressure (IMEP) of 2, 4 and 8 bar. The ignition point wasoptimized to obtain maximum torque and minimum fuelconsumption.

As one can see the area of burned mixture increase withincreasing load. Due to different ignition points at variedload the responsible conditions for inflammation such astemperature, pressure and turbulence will change. At lowload the levels of temperature and pressure at early igni-tion point are much less then at full load. Because of thefact, that turbulent flame speed decrease with decreasinglevel of temperature and pressure the area of burnedmixture decrease with low load. If there is no difference inignition time at different loads the areas of burned gasare probably similar.

As a result of the inlet flow condition which generates atumble a deferral of flame propagation to the exhaustvalves can be seen. This effect is observed over thewhole engine map independent of load, engine speedand air/fuel-ratio.

In Figure 8 the flame propagation at 10° CA after ignitiontiming for different engine speeds is shown. The enginewas operated at a constant IMEP of 5 bar and stoichio-metric air/fuel ratio. The ignition timing was optimized,too. In addition to the previous Figures the flame velocityradial to spark plug is presented by arrows. Furthermorethe average of all probes of flame velocity is shown in thelegends. As one can see the flame propagation is influ-enced very strongly by varying the ignition timing. Withincreasing speed the flame velocity increases due to thehigher level of turbulence and temperature.

Figure 7. Early Flame Area; n = 2000 rpm; 10° CA After Ignition

7

Figure 8. Early Flame Area; IMEP = 5 bar; 10° CA After Ignition

Figure 9. Early Flame Area; n = 4000 rpm; 10° CA After Ignition

8

Figure 9 shows the influence of air/fuel ratio on inflamma-tion and flame propagation at 10° CA after ignition timing.These investigations were performed with constant igni-tion time and constant throttle position. Due to thedecreasing flame velocity and increasing inflammationtime by lean air/fuel mixtures the area of burned gasdecreases with lean air/fuel ratio.

In Figure 10 the volume rate of burned mixture 10° CAafter ignition timing versus engine speed at differentloads is presented. As one can see the volume rate ofburned gas increases with increasing load anddecreases with increasing engine speed. However, themean flame propagation velocity, pictured in Figure 11,increases with higher engine speed and load. Both thevolume rate as well as mean flame velocity are character-ized by a linear behavior over the engine map.

Figure 10. Volume Rate of Flame Growth; 10° CA after Ignition

Figure 11. Mean Flame Velocity 10° CA after Ignition

COMPARISON OF LIGHT SIGNALS AND PRESSUREDEVELOPMENT – As the results of the investigations offlame propagation short time after ignition point show, themodified spark plug used for these investigations is suit-

able to measure flame development close to the sparkplug. However, for this kind of investigation only the firstincrease of the light signals is used to identify the flamepropagation. Rembowski et al. /12/ and Achtleitner /13/found a strong correlation between measured gas emis-sion and cylinder pressure which can be used to charac-terize the combustion process and to predict magnitudeand location of peak heat release, IMEP and air/fuel ratio.

In this study a comparison of the light signals and thepressure development validate this correlation. Further-more it will be shown that it is possible to control and opti-mize engine operation over complete engine map and todetect knocking combustion and misfires.

In Figure 12 the cylinder pressure versus crank angle ofboth fired and motored cycles is shown. The develop-ment of flame luminosity and the difference of fired andmotored pressure behavior versus crank angle is plottedto compare gas emission and pressure characteristics.The pressure curve increases strongly to a maximumafter TDC. After the maximum the pressure valuesdecrease during expansion. Except of the delay untilstarting combustion process the development of gasemission increases to a maximum after TDC anddecrease during expansion also.

Figure 12. In-Cylinder Flame Radiation and Pressure

This similar behavior of both pressure and luminositydevelopment versus crank angle can be seen moredetailed in Figure 13. For these investigations the enginewas operated at an engine speed of 4000 rpm and IMEPof 6 bar. The ignition points were varied between 24° CABTDC and 14° CA BTDC. The maximum of both pres-sure and gas luminosity curves differs nearly 13° CAbetween the ignition point at 24° CA BTDC and 14° CABTDC. Nevertheless there is a good correlation betweenpressure data and intensity of gas radiation.

9

Figure 13. Variation of Ignition Timing

Figure 14. Correlation between maximum gas radiation intensity and maximum pressure

In Figure 14 the evaluation of the correlation betweenmaximum pressure and maximum radiation intensity fordifferent engine speeds and loads at stoichiometric air/fuel ratio is shown. In Figure 15 the coefficients versusIMEP to correlate the time delay of maximum radiationintensity and maximum pressure for different enginespeed is pictured. At this, the time delay between maxi-

mum radiation intensity and maximum pressure is lessthan 1° CA. The absolute difference between maximumradiation intensity and maximum pressure in °CA versusIMEP for different engine speeds is shown in Figure 16.

Figure 15. Correlation between time delay of maximum gas radiation intensity and maximum pressure

Figure 16. Absolute Difference of Timing of Radiationand Pressure Maxima

BURNING RATE – The mass fraction burned profiles orenergy-release fraction curves as a function of crankangle are important tools to characterize different stagesof the spark-ignition engine combustion process by theirduration in crank angle. To calculate the mass fractionburned profile from measured cylinder pressure two-zonemodels are used. Thereby one zone represents theunburned mixture ahead of the flame and the other zonethe burned mixture behind the flame. A functional formoften used to represent the mass fraction burned versuscrank angle is the Wiebe function /14/. In this paper alight integral function xL to characterize the combustionprocess is presented:

10

(Eq. 5)

L(α): Light intensityα1≤ α ≤ α2; α1; α2: Begin and end of acquisition

Figure 17. Light Integration Function and Heat Release Function

In Figure 17 a comparison between mass fractionburned, computed with a two-zone model, and the lightintegral function versus crank angle is shown. The time of50% mass burned, which is determined by the Wiebefunction, is an important value to control and optimize thecombustion process /6. However, there is a correlationbetween the time of 50% mass burned and the crankangle of the 50% point of the light integral function. Atime delay between these two points is observed.

The detection of burned and unburned gas radiation isresponsible for the time delay of the 50% point of the lightintegration function. In spite of this time delay one cansee in Figure 13 that the light curve is a very good mea-sure to determine the inflammation. In Figure 18 the tim-ing of the 50% points of both characteristics - determinedfor 50 cycles - for one operating point is shown. The dif-ference of both timing 50% points supplies a constantvalue which suggests a good correlation. The very goodcorrelation in Figure 18 shows the excellent suitability ofthe crank angle of the 50% point of the “Light IntegrationFunction“ to optimize the engine.

Figure 18. Correlation between Light Integration Function an Heat Release Function

KNOCK AND MISFIRE DETECTION – Due to the localcompression of the exhaust gas in the combustion cham-ber initiated by sonic wave during knocking combustionthe optical probes receive a more intense signal for ashort period. These waves have the resonance frequencyof the combustion chamber. The pressure and light sig-nals for knocking combustion are shown in Figure 19.The right chart represents the pressure signal with highfrequency fluctuations due to the knocking combustion.The left plot shows the corresponding light signals whichare much smother. The second curves in the charts arethe high pass filtered signals. Due to the fact that bothsignals show similar high frequency oscillation duringknocking combustion the knock intensity can be calcu-lated with both signals.

( )( )

( )x

L d

L d

L =∫

∫1

1

2

11

Figure 19. Knocking Combustion Cycles

12

The calculation of knock intensity is illustrated in Figure20. The signal is filtered with a high pass filter algorithm.The integration of the square of the high frequency signalyields the knock intensity.

Figure 20. Calculation of Knock Intensity

Figure 21. Knock Detection

Figure 21 represents the results of knock intensity for 50cycles. At cycle 18 the ignition timing was changed from7° CA BTDC to 14° CA BTDC in order to initiate knockingcombustion which leads to a rise of knock intensity. Dur-ing the non-knocking cycles the level of knock intensityobtained with the pressure signal is lower than the knockintensity which was calculated with the light signal. Dur-ing the knocking cycles the knock intensities are at thesame level for both signals. Hence, knock detection isfeasible with the light signal too.

Figure 22 shows misfiring detection for engine operationwith lean mixture (Air-Fuel ratio 1,4). Misfiring occurred incycle no. 8 which was detected by both signals. Theadvantage of the optical system is the fact that the infor-mation concerning the combustion is more detailed,because it is possible to detect combustion phenome-nons which are not possible to detect by pressure. In thiscase, a very light combustion at 195° CA. The peak at155° CA results from the ignition.

Figure 22. Misfire Detection

13

CONCLUSION

The multi-optical fibre technique for detection of the flamepropagation has been successfully applied in productionspark plugs. The results obtained with the optical fibre fit-ted spark plug are extremely helpful in analyzing theearly flame development. Especially, the excellent corre-lations to characteristics of pressure show the ability ofthe optical fibre fitted production spark plug to control theengine.

The investigation of the early flame development showsthat lean combustion and the variation of the ignition tim-ing have a great influence on the early flame area whichis determined by the flame front arrival points of the opti-cal probes. Therefore it is possible to draw a conclusionon charge motion in the combustion chamber. Influencesof swirl and tumble can be seen in the early flame areadistinctly.

In addition to the observation of the early flame areas theinvestigation of the flame radiation signals provide impor-tant results of the in-cylinder processes. The crank angleof the 50% point of the “Light Integration Function“ isextremely helpful to optimize the combustion similar tothe crank angle of 50% mass burned determined by theheat release function. In addition the analysis of theflame luminosity shows that the optical fibre fitted sparkplug is suitable for the detection of knocking and misfire.

REFERENCES

1. Witze, P.O., Hall, M.J., Wallace, J.S..: Fiber-Optic Instru-mented Spark Plug for Measuring Early Flame Develop-ment in Spark Ignition Engines, SAE 881638

2. Spicher, U., Bäcker, H.: Optical Fiber Technique as a Toolto Improve Combustion EfficiencySAE 902138

3. Spicher, U., Bäcker, H.: Correlation of Flame Propagationand In-Cylinder Pressure in a Spark Ignited Engine, SAE902126

4. Spicher, U., Schmitz, G., Kollmeier, H.-P.: Application of aNew Optical Fiber Technique for Flame Propagation Diag-nostics in IC Engines, SAE 881637

5. Spicher, U., Bach, M.: The Optical Fiber technique forInvestigate the early flame development in SI-Engines, 2. Indiziersymposium, Darmstadt Gemany, 1996

6. Bargende, M.: Most Optimal Location of 50% Mass Frac-tion Burned and Automatic Knock Detection Componentsfor Automatic Optimization of SI-Engine Calibrations; MTZ,Motorentechnische Zeitschrift 56 (1995) 10

7. Gaydon, A.G.: The Spectroscopy of Flames, Chapman andHall, London, 1960

8. Gaydon, A.G., Wolfhard, H.G.: Flames, Their Structure,Radiation and Temperature, Chapman and Hall, London,1960

9. Yoshishige Ohyama, Minoru Oshuga, Hiroshi Kuroiwa:Study on Mixture Formation and Ignition Process in SparkIgnition Engine Using Optical Combustion Sensor, SAE901712

10. Sohma, K., Yukitake, T., Azuhata, S., Takaku, Y.: Applica-tion of Rapid Optical Measurement to Detect the Fluctua-tions of the Air-Fuel Ratio and Temperature of a SparkIgnition Engine, SAE 910499

11. Moeser, P., Hentschel, W.: Development of a TimeResolved Spectroscopic Detection System and Its Applica-tion to Automobile Engine, SAE 961199

12. Rembowski, D.J., Plee, Jr., Plee, St. L., Martin, J.K.: AnOptical Sensor for Spark-Ignition Engine Combustion Anal-ysis and Control, SAE 890159

13. Achtleitner, E.: Über den Zusammenhang zwischen Licht-emission und Energieumsatz bei Verbrennungs-motoren und ihre Anwendung zu deren Regelung, Dissertation TU Wien, 1985

14. Heywood, J.B.: Internal Combustion Engine Fundamentals, 1988, McGraw-Hill International editions