Fundamental studies on the volkswagen stratified charge combustion process

COMBUSTION AND FLAME 25, 15-23 (1975)

15

Fundamental Studies on the Volkswagen Stratified Charge Combustion Process* W. R. BRANDSTETTER and G. DECKER VolkswagenwerkA G, I¢olfsburg, FederalRepublic of Germany

The object of this study of stratified charge combustion was to investigate various combustion chamber configurations and to evaluate their potential for automotive engines with low exhaust emissions. In single cylinder engine tests with a prechamber of approximately 30% of the total swept volume, numerous parameters that influence the combustion process markedly, such as air fuel ratio, cross-section of the passage, injector and spark plug position, air throttling, etc., were studied. These tests were supported with high speed photographs taken from the mixing and combustion process in a combustion bomb. Investigations of various combustion chamber configurations have shown that, with regard to NOxemissions, the divided combustion chamber comprising a main chamber and a prechamber is preferable. Greater control of mixture stratification can be achieved over a wide engine speed range. The reduction of the HC-emissionsis a more difficult matter, although tests with the multicylinder engine confirmed that thermal oxidation in the exhaust system is very effective. A significant reduction of emissions with a minimum of power loss can only be obtained by accurate control of the fuel quantities supplied to the main and prechamber. While combustion stability at very lean operation is guaranteed, intake air throttling in the part load range is necessary to keep combustion temperature high enough for effective oxidation of CO and HC. For good mixture preparation, a modest swirl in the precombustion chamber is advantageous, although too high a swirl results in an unacceptable combustion pressure gradient. The spark plug must be located further down the flow line than the injector to facilitate ignition of the mixture in a broader engine speed range. The injection timing can be kept constant over the entire operating range of the engine.

Introduction The application of the stratified charge principle as an effective means of reducing pollutants has recently acquired considerable significance. By definition, the mass ratio of air and fuel, in a stratified charge combustion process, varies throughout the combustion chamber. A number of studies have shown [1-4] that not only HCand CO-, but also NOx-emissions can be largely reduced by programmed stratified charge combustion. The achievement of low exhaust emissions without catalytic exhaust treatment devices again assumes increased importance. The expected relaxation of the United States NO x-emission standard and the increasing importance of low fuel consumption both play an important part in the optimization of the combustion process. In the course of our investigations of the stratified charge principle, fundamental studies were

carried out on a combustion bomb. A large number of tests were conducted on the single cylinder test bench, as, for example, the testing of different combustion chamber configurations. The program also included research on four-cylinder engines under stationary and transient conditions, and testing in vehicles.

Basic Investigations In view of the large number of parameters to be investigated, it is worthwhile to study the main *Part of the work covered in this report was assisted through funds made available by the Federal German Ministry for Research and Technology (Chapter 3108, Title 68321). The Federal German Ministry for Research and Technology accepts no liability for the correctness, accuracy or completeness of the details given, or that the private rights of third parties have been taken into consideration.

Copyright © 1975 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.

16

W.R. BRANDSTETTER and G. DECKER

parameters by methods of computer simulation and by basic experimental studies. For this purpose, a mathematical flow model was produced, thus allowing simulation of the processes taking place in the cylinder and prechamber, including the cylinder charging. Figure 1 is a schematic presentation of the model. The system represents a four-stroke single cylinder engine with a divided combustion chamber. Intake and exhaust pipes are represented by two surge tanks with equivalent volumes. Burning rate and heat transfer in the main combustion chamber and the prechamber were calculated by using formulations derived by other investigators [5-7]. ~ZV F 1;, ,

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Fig. 1. Mathematical flow model for the prechamber stratified charge engine. The mathematical model calculates gas pressures and temperatures, total mass and mass burned, and average air fuel ratio in the main chamber and the prechamber as a function of crank angle. Of particular influence on the combustion process are the air-fuel ratio and the residual gas fraction in the prechamber at the time of ignition. For example, at part-load opera-

tion with a volumetric efficiency of 0.66 and an engine speed of 2000 rpm, the average relative air-fuel ratio is 0.86 in the prechamber and 1.76 in the main chamber, if 25% of the total fuel per working cycle is injected into the prechamber. The total relative air-fuel ratio is 1.32. The maximum velocity in the connecting port is 75 m/sec shortly after top dead center. The differential pressure between the two combustion chambers is small because of the relatively wide port crosssection (1.5% of piston area). At the time of ignition the residual gas fraction in the prechamber is 15%. A universal combustion bomb apparatus was employed to conduct preliminary tests under ideal laboratory conditions and, in particular, to investigate the mixture formation phase and subsequent combustion. The use of high-speed photography, including application of shadow and schlieren techniques, has already proved extremely useful in general investigations concerning mixture formation and combustion [8, 9]. A section through the combustion bomb together with an insert representing the condition in a divided combustion chamber is shown in Fig. 2.1 The prechamber, which is cylindrically shaped contains the injection nozzle and the spark plug, and is connected to the main chamber by a large passage. The geometrical shape of the intake valve produces a high swirl in the main chamber. Three selected frames from a Schlieren f'dm are presented in Fig. 3. The first frame shows fuel injection into the prechamber and mixing with incoming air. The calculated average relative air-fuel ratio in the prechamber is 0.7. Ignition and flame propagation are shown in frames 2 and 3 of Fig. 3. Soot formation occurs in some areas of the prechamber. Combustion is completed in the main chamber, which contains pure air at the time of ignition. Films of the combustion process were made during tests with the single-cylinder engine using various combustion chamber patterns. In contrast to the combustion bomb, construction of the single-cylinder test engine, modified for high speed

1The apparatus was produced and the investigation conducted by U. Renger.

STRATIFIED CHARGE COMBUSTION

17

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Fig. 2. Section through combustion bomb with insert representing a divided combustion chamber. photography with the advantage of reproducing the actual flow pattern, permits observation of only self-illuminating processes in the main combustion chamber. A transparent piston crown and a fixed mirror inclined in relation to the cylinder axis were used with a window in the piston body and cylinder. Experimental Results from the Single-Cylinder Stratified-Charge Engine The tests currently being conducted on multicylinder engines were preceeded by comprehensive tests of various combustion chamber configurations. A classification by only four design characteristics (e.g., divided or open combustion chamber, arrangement of air and fuel supply, etc.) results in 25 configurations. Figure 4 shows versions of combustion chambers as used for preliminary testing. The investigations were carried out on a single-cylinder engine with a slightly-modified combustion chamber from a VW 1600 cm 3 engine. First an ignition cavity was provided in the cylinder head, into which fuel could be injected shortly before ignition in order to produce a relatively rich mixture. The remainder of the combustion chamber was filled with a weak mixture, depending on load and engine speed. The leanest possible operation using this process only increases the relative airfuel ratio slightly at approximately 1.6. Considerably better results were obtained with a divided unscavenged chamber concept. The

Fig. 3. Fuel injection and combustion in the divided combustion chamber (Schlieren photographs).

volume of the prechamber was only 5% of the compression volume. Enrichment was again carried out by fuel injection, and it was found that injection timing could be further advanced and thus fuel preparation in the prechamber improved. For the appropriate operating conditions of the engine, the main combustion chamber was filled with a weak mixture. The operational limit using this process could be extended still further in the weak mixture direction. These tests proved that further development of the stratified charge process with a divided combustion chamber was worthwhile since advantages were found to be present in comparison with the open combustion chamber in terms of both emissions and combustion stability.

18

No

W. R. BRANDSTETTER and G. DECKER the compression volume and is connected to the main combustion chamber by a relatively large flow transfer passage. The main combustion chamber is disc-shaped and contains no squish surfaces. This divided combustion chamber, which was developed from the standard version, has a surface to volume ratio of 4.13 cm "1 with a compression ratio of 8.5. The injection nozzle and spark plug are arranged in sequence in the flow direction, so that the spark plug receives a mixture produced by the incoming air with fuel dispersed in air. This avoids excessive richness at the spark plug. The amount of fuel injected into the prechamber is such that a relatively rich ignitable mixture (relative air-fuel ratio of 0.7 to 0.9) is present in the prechamber at the spark plug under all operating conditions. Mixture preparation is assisted by the shape of the prechamber, with the transfer passage entering almost tangentially. Operating at full throttle, load regulation is achieved by enriching the lean mixture introduced into the main combustion chamber. e ction Nozzle Prechamber /F-Insert

Fig. 4; Open combustion chamber with ignition cavity, unscavenged and scavengedprecombustion chamber design. Another version has also been investigated in which fuel injection into the prechafnber is replaced by an additional carburetor and in which part of the air/fuel mixture is fed directly to the prechamber by a small auxiliary valve, while the main combustion chamber is supplied with a homogeneous weak mixture depending on load and engine speed. The auxiliary valve is operated by a swing lever from the inlet valve rocker. The test results proved as good as those obtained during investigation of the small valveless prechamber. The operational limit was extended to relative air-fuel ratios in excess of 1.8 without misfiring or any noticable rise in HC emissions. Operation of a third valve on a OHV horizontally opposed engine also calls for considerable extra design expenditure. These preliminary studies resulted in the configuration shown in Fig. 5. The spherical prechamber comprises approximately 25% - 30% of

Fig. 5. Dividedcombustion chamber with spherical valveless prechamber. The start of fuel injection into the prechamber is preferably between 50 and 100 degrees crank , angle before top dead center. If the injection is too early, it seems that excess fuel collects on the wall of the prechamber; if too late, the time necessary for adequate mixing is not available, and misfiring results. Experiments show that the fuel injection timing does not need to be varied over the entire speed range. This is a considerable simplification in comparison with the mixture stratification process in an open combustion chamber,

STRATIFIED CHARGE COMBUSTION which requires the use of an injection governor [3], Figure 6 shows mean effective pressure (mep), specific fuel consumption and emissions in g/I-/Ph2 depending on relative air fuel ratio X3 . The operating range of the engine extends from X = 0.95 to approximately 3.2, whereas the lean operational limit in the conventional Otto engine is normally reached at X = 1.2 to 1.4. According to engine load, the amount of fuel injected into the intake ports is controlled, while the amount of fuel injected into the prechamber is kept constant. The CO and HC emissions under these operating conditions did not fall to the expected low levels. Evidently even in the excess air range certain zones remain in which complete combustion does not take place, most likely because of low combustion temperatures and slow flame propagation, and because of local rich mixture zones during the expansion stroke. Since combustion is influenced to a considerable extent by the geometry of the prechamber, tests were made with various injector and spark plug positions and the influence of passage patterns and sizes investigated. Figure 7 shows the measured HC and NO concentrations related to mean effective pressure with a specific injector and spark plug position and use of various transfer flow passages. Whereas the NO concentrations in the lower mep range varied only slightly among themselves, a distinct advantage was noted in favor of the larger cross-section passage at mean effective pressures above 5 bar. At low mean effective pressures, the HC concentrations are widely scattered since combustion chamber temperatures are low and the quality of mixture formation is not satisfactorly. However, with regard to HC emissions, the larger passage is less favorable, most probably because of the smaller passage velocity during the mixture preparation. Figure 8 shows, in similar fashion, the influence of injector and spark plug position when retaining the same

2Hydrocarbon emissions were measured as hexane with NDIR analyser; NO concentrations were measured with a NDIR ana|yser and emissions in g/HPh were calculated using the molecular weight of NO2. 3~kis the weight ratio of air to fuel divided by the same ratio for complete conversion of fuel to CO and H20,

19 pattern for the flow transfer passage. The variations are not very significant considering typical measuring accuracy.

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Fig. 6. Mean effective pressure, specific fuel consumption and emissions vs relative air fuel ratio (unthrottled, 2000 rpm). In addition to combustion chamber geometry, different fuel injector nozzles were investigated at various operating conditions. The results are shown in Fig. 9. Injector nozzles No. 1-3 are conventional gasoline injector types with varying delivery pressures and angles; injector nozzels No. 4-5 produce a highly concentrated jet pattern. Injector nozzles 1-3 differ only slightly in regard to the relative air-fuel ratio necessary to achieve a specific mep, whereas injector nozzles 4-5 require a highly enriched mixture. Fuel consumption is affected accordingly. The best HC emission resuits were obtained by injector nozzle No. 1, which had the lowest delivery pressure, especially

20

W.R. BRANDSTETTER and G. DECKER Crossection of the Passageway

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Fig. 7. Influenceof transfer passagegeometryon emissions (unthrottled, 2000 rpm). when operating in the part load range. The same is true for NO emission, in particular at high engine load. Injectors with low delivery pressures provided the best test results of all the injectors investigated, probably due to the avoidance to a large degree of gasoline deposits on the prechambet wall. For all tests the ignition timing was set for maximum output. Conventional coil ignition was used during the tests. The optimum firing point was always close to top dead center. Figure 10 shows NOx- and HC-emissions vs mep. The parameters are the portion of fuel in-

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jected into the prechamber, QVK/Q,and the ignition timing. All values apply to unthrottled intake conditions and an engine speed of 2000 rev/min. In conventional Otto engines, mixtures with a relative air-fuel ratio of more than 1.3 are difficult to ignite. In the stratified charge engine at low engine load (mep) as much as 90% of the total fuel per working cycle is injected into the prechamber, but only approximately 10% near full load. This guarantees that the relative air-fuel

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Fig. 10. HC and NOx emissions vs mean effective pressure for various fuel distributions and dependence on ignition timing (unthrottled, 2000 rpm). ratio near the spark plug is always between 0.7 and 0.9. Variation of the ignition timing indicates that, below half load, minimum emissions are achieved with ignition occurring shortly after top dead center, and at higher loads shortly before top dead center. Further improvements, in particular in the lower engine load range, can be obtained by intake air throttling. Figure 11 shows the dependence of specific fuel consumption and exhaust emissions on the degree of throttling-loss in volumetric efficiency compared with wide open throttle operation-for three different mean effective pressures at 2000 rev/min. Although no noticeable influence on fuel consumption is evident, intake air throttling is advantageous at low engine load because it produces a clear drop in emission levels. At high engine load excessive throttling leads to a renewed rise in emissions. From about half load the engine can be operated without throttling. A thermodynamic analysis of indicator diagrams provides important information concerning the combustion process and is a significant addition to the usual test evaluation material. Fig. 12 is

given as an example showing pressure and temperature development and the mass burning rate for a part load operating condition at 2000 rev/min. HC and CO oxidation are promoted by delayed combustion. Despite the expansion movement of the piston, an approximately constant value for the mean gas temperature over almost 120 degrees of crankshaft movement could be obtained. A corresponding development of the mass burning rate and the mass fraction burned is shown in the lower part of Fig. 12. The unusually small scatter of the peak pressure values for individual working cycles in comparison to the standard Otto engine with "homogeneous" mixture at the same air fuel ratio is also worthy of note (Fig. 13). This regularity in cyclic combustion is essential for low exhaust emissions.

Conclusions By means of mathematical simulation, a residual gas fraction in the prechamber of over 15% was calculated at the time of ignition. This leads, for the two-stage combustion process (with X prechamber = 0.86 and X main chamber = 1.76), to

22

W. Ro BRANDSTETTER and G. DECKER

a NOx-reduction of approximately 75% in comparison to a conventional Otto engine. Single cylinder engine measurements on the effect of passage geometry, injection nozzle and spark plug position (Figs. 7, 8 and 9) were supplemented by high speed photography of the fuel injection and combustion process in a combustion bomb, thereby reducing engine testing to the more promising combustion chamber configurations. 8SFC800. g/HPh

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In optimizing the combustion process for low exhaust emissions, a variation of the fuel distribution for the prechamber and the main combustion chamber, depending on engine load, is necessary (Fig. IO,QvK/Q= 10% - 90%) at part load operation. Exhaust emissions can be substantially reduced with the intake air throttled up to 30%. A thermodynamic analysis of the cylinder pressure development during the two-stage combustion, with an average relative air-fuel ratio of 1.2, shows a nearly constant gas temperature of

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STRATIFIED CHARGE COMBUSTION approximately 1500 K over 100 degrees crank angle (Fig. 12), and does not indicate the temperature peak, which is common in Otto engines, resuiting in NO formation. Experiments with four-cylinder engines indicate that the HC-emissions in particular are still not low enough to meet most stringent emission standards. Measures for further optimization of the combustion process such as prechamber inserts, which increase the prechamber wall temperature, are being investigated on single cylinder engines. This will affect, for example, the fuel distribution between prechamber and main chamber; ignition timing; intake air throttling, and other parameters that must be studied in further single cylinder engine testing.

The authors wish to express their thanks to their many colleagues at the Research Division who helped with this project, without whose cooperation and "know-how" this study wouM not have been possible.

23

References 1. Honda, Description of Honda CVCC Engine, Oct. 1972. 2. Report by the Committee of Motor Vehicles Emissions, National Academy of Sciences, Wash. DC, 15 Feb. 1973. 3. Simko, Q., Chorea, M., and Repko, L., Exhaust Emission Control by the Ford Programmed Combustion Process-PROCO, SAE-Paper 720052, January 1972. 4. Mitchell, E., Alperstein, M., Cobb, J., and Faist, C., A Stratified Charge Multifuel Military Engine-A Progress Report, SAE-Paper 720051, January 1972. 5. Vibe, 1., Brennverlauf und Kreisprozess yon Verbrennungsmotoren, VEB Verlag, Technik Berlin, 1970. 6: Woschni, G., Beitrag zum Problem des W~irme~bergangs im Verbrennungsmotor, MTT~ 26, 4 (1965). 7. Wrona, R., Rechenmodell zur elektronischen Berechnung der KenngrSssen eines mit Schichtladung betriebenen Ottomotors, MTZ 35, 1 (1974). 8. Huber, W., Stock, D., and Pischinger, F., Untersuchung der Gemischbildung und Verbrennung im Dieselmotor mit Hilfe der Schlierenmethode, MTZ 32, 9 (1971). 9. Pischinger, R., Bombenversuche mit schwer ziindenden Kraftstoffen bei SelbstziJndung, MTZ 24, 1 (1963). 10. Panduranga, V., Bombenversuche zur Ermittlung der Kohlenwasserstoff-Emission, MTZ 32, 9 (1971 ). Received 14 October 1974