Porsche stratified charae enaine This German engine uses less fuel and produces less exhaust emissions than the conventional four-stroke spark ignition engine does T. Ken Garrett Hemel Hempstead England
The stratified charge type of engine shows the greatest promise for solving both the problems of emissions and fuel economy over at least the next decade. Porsche has been engaged in basic research work in this field. The Porsche SKS engine is similar to the Honda CVCC unit: the essential feature of stratified charge, of course, is that it can render the engine tolerant of a wide range of fuels which, in the overall context of energy utilization, is a great advantage in getting the utmost out of each barrel of crude oil at the refinery is concerned. At the outset, Porsche had to chose between an open and a divided combustion chamber system. They chose the latter because of the difficulties of stratifying the charge consistently in the open-or single chamber-combustion system. These difficulties arise because of the effects of varying load and speed. Their studies were concentrated primarily on the four phases of the combustion process: ignition development of the combustion kernel the main combustion phase afterburning. For ultimate success, optimum conditions have to be established for each of these phases.
Phase 2-combustion kernel For the initial ignition and then the development of a stable kernel of combustion, it is necessary for the rate of generation of heat in a small volume of combustible mixture around the spark to be greater than the rate of loss through its boundaries. The velocity of turbulence also has to be relatively low to avoid a reduction in the rate of ionization-the essential process immediately prior to combustion. On the other hand, once the combustion kernel is firmly established, a high degree of turbulence is necessary for rapid and complete combustion. The Porsche SKS engine was designed with a main and small auxiliary combustion chamber, Figure 1, next to which is yet another, even smaller chamber, into which the sparking plug is screwed. The main combustion chamber is supplied with a very weak mixture through a conventional inlet valve. A rich, yet combustible, mixture is supplied to the auxiliary chamber either by direct injection of fuel or through a supplementary valve, according to whether injection or carburetion is used for the main mixture supply. 826
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FIGURE 1
Porsche SKS engine Combustion chamber layout Injector
Inlet valve
3%kk /
Main combustion chamber
/
Auxiliary chamber Ignitionchamber
The mixture flows freely from the auxiliary combustion chamber into the tiny ignition chamber, but the connection between the auxiliary and main combustion chambers comprises one or more nozzles, designed to generate swirl in the auxiliary chamber and thus to stratify the charge in it during the compression stroke and to direct jets of flaming gas out into the main chamber during the firing stroke. Accordingly, ignition is initiated in the relative stillness of the smallest chamber and, once the kernel of flame has developed, it spreads very rapidly throughout the turbulent auxiliary chamber, whence the flaming gases are forcibly ejected to spread the combustion throughout the weak mixture in the main chamber. Although the overall mixture strength can be considerably weaker than stoichiometric, combustion occurs regularly and consistently over a wide range of speed and load conditions. From the indicator diagram, Figure 2, it can be seen that, after an extremely short ignition delay, the burning spreads rapidly throughout both the auxiliary and main chambers. The relative rates of increase of pressure in these two chambers depend on the size and shape of the nozzles that interconnect them. Good results are obtained when the velocity of the
burning gases passing through the jets is high enough to increase the turbulence in the main combustion chamber. In these circumstances, the thorough intermixing causes combustion to originate in and spread rapidly from many centers in the very weak mixture in the main combustion chamber. Periodic fluctuations in gas pressure develop at a frequency determined by the relative volumes of the interconnected cavities-the main and auxiliary chambers. In contrast to the vibratory fluctuations experienced during knocking combustion, however, the fluctuations in gas pressure cannot be suppressed by increasing the octane number of the fuel or by retarding the ignition; moreover, variations in the physical and chemical properties of the fuel have little influence on the combustion process. On the other hand, the amplitudes are small, so there is no risk of mechanical damage. Similar vibrations are induced in the combustion chambers of gas turbines to encourage complete combustion. Phase 3-ignition Ignition will occur reliably in the auxiliary chamber at air:fuel ratios ranging from 0.4-1.2 times stoichiometric. As the mixture strength in this chamber becomes richer than 0.8, partly as well as completely burned products of combustion are ejected through the nozzles into the main chamber. The partly burned products include not only CO and C (partial products of combustion occurring instantly in the initial stages) but also H and OH radicals and many others. Because many of these products are highly active, they serve as a multiple of sources of ignition, setting up chain reactions throughout the main combustion chamber. As the richness of the mixture in the auxiliary chamber increases, the temperature of the wall of this chamber becomes high. But, with a reduction in the strength of the mixture in the auxiliary chamber, the quantities of products of incomplete combustion decrease and the temperature of the gases expelled into the main chamber increases. At the same time, ignition and combustion in the main chamber become irregular. Obviously, therefore, it is not the temperature of the expelled gases but the quantities of products of partial combustion that are important for the development of chain reactions in the main chamber. The regularity of ignition and combustion in the auxiliary chambers of multicylinder engines is much better than in the combustion chambers of a conventional engine, as can be seen from Figure 3. Volume 9, Number 9, September 1975 827
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Performance and emissions In Figure 4 is shown the variations in performance with changes in overall air:fuel ratio of a single cylinder research engine under part load. If the injection of fuel into the auxiliary chamber is regulated to give minimum fuel consumption, the SKS engine will run on overall air:fuel ratios as weak as 2.1 times stoichiometric. The limiting weakness is determined only by the ability of a wide-open throttle to pass sufficient air to maintain the required power output at that air:fuel ratio. At lower power outputs, overall air:fuel ratios of 0.8-2.2 times stoichiometric can be used. As with conventional engines, on the other hand, maximum power is obtainable only at an air:fuel ratio of about 0.9. In Figure 4, the engine with which the SKS unit is compared is of the most modern design. At their minimum values the brake specific fuel consumptions of the two do not differ much; however, the brake specific fuel consumption of a conventional engine increases as the mixture is weakened, in direct contrast to the characteristic of the SKS engine. Because of the better conditions for ignition and subsequent burning in the SKS engine, its optimum ignition timing is later than in a conventional one. Moreover, provided the relationship between the volume of the auxiliary chamber and the dimensions of the nozzle is optimized, variations in ignition timing have much less effect on the performance and emissions of an SKS engine as compared with a conventional one. Measurements of the exhaust concentrations, Figure 5, show that the constituents are qualitatively the same for both types of engine. However, quantitatively the NO, content of the SKS engine exhaust is 90% lower than the highest values obtained with a conventional engine and still 60% of that of a
conventional engine running on a very weak mixture. The CO emission is about the same as that of a conventional engine on weak mixture, while the HC output is lower. With another variant of the SKS engine, it is possible still further to reduce the HC emissions, and at the same time to lower the fuel consumption, but at the expense of increasing NO, output.
Control of the SKS engine Because of the wide range of mixture strengths acceptable in the Porsche SKS engine, qualitative control, by varying the strength of the mixture supplied to the main combustion chamber, becomes possible. This method has decisive advantages over quantitative control, by means of a throttle valve, so long as the conditions remain favorable for combustion. As the mixture strength is reduced beyond a certain value, however, there is a risk of the rate of progress of combustion becoming too slow. At this point, conventional throttling of the ingoing mixture becomes necessary. According to Lange and Gruden (see additional reading) it has been proved that, with the SKS engine, qualitative control give best results when the mean effective pressure exceeds half the maximum attainable value. Below the half-way point, quantitative control should be introduced to reduce specific fuel consumption and exhaust emissions. This latter method of control maintains the exhaust gas temperature at a level conducive to afterburning of the CO and HC emissions. It also reduces the NO, omissions. With the variant of this engine the NO, output is only IO-20% of that of a conventional engine, while the CO and HC emissions are about the same for these two types of engines. Research has shown that the performances obtained with methanol and gasoline as a fuel do not differ so widely in an Volume 9, Number 9, September 1975
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FIGURE
7
Relationship between exhaust gas emissions and load
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SKS engine as in a conventional one. This is because of the favorable method of propagation of combustion in the SKS unit. in Figures 6 and 7 the characteristics over the whole range of power output at a constant speed of 2,000 revlmin are compared. The greatest differences were in exhaust gas emissions, and the SKS engine run on methanol had approximately double the specific fuel consumption of either engine run on gasoline. The reason for the high fuel consumption was the lower calorific value of methanol. However, the thermal efficiency obtained with methanol was higher. Since the optimum ignition timing for methanol and gasoline in the SKS engine is approximately the same, obviously the rates of combustion are, similarly, virtually equal. With methanol, the CO emission of the SKS engine is 5070% lower than with gasoline. It was not possible to come to any valid conclusions regarding the actual reduction in HC emission-though it was obviously substantial-because the methods of measuring it in the exhaust gases are not directly comparable. Over a wide operating range, however, the NO, emission with methanol was almost zero. Additional reading Second Symposium on Low Pollution Power Systems Development, Dusseldorf, No". 1974. Paper by K. Lange and D. Gruden.
r.
Ken Garrett began his career in aeronautical engineering. In 1951, he was invited to the editoriaistaff of Automobile Engineer; by 1958 he was Editor of that publication where he remained until 1972. Since then he has been writing atiicles and broadcasting on technical and scientific subjects.
