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Lesson FiveTEXTOPEN CHAMBER STRATIFIED CHARGE ENGINESResearch and development efforts over several decades have resulted in a number of stratified charge SI piston engine concepts which appear to have potential for reduced exhaust emissions. As the name implies, the fuel is not distributed uniformly throughout the charge. The design goal was originally to improve part load fuel economy by operating the engine at or close to full throttle, and control the power by variation in fuel supply alone. To achieve satisfactory combustion, an easily ignitable mixture (generally fuel-rich) is created in the vicinity of the spark plug with air filling the remainder of the combustion chamber. As load increases, the volume fraction filled with ignitable mixture is increases. The improvement in efficiency comes from lower gas temperatures during combustion and expansion reducing dissociation, from reduced pumping losses, and from increased compression ratio permitted by the less severe detonation characteristics.A number of these stratified charge engine concepts, originally developed for improved fuel economy and multifuel capability, permit operation with overall fuel-air mixtures significant leaner than chemically correct, and with significant charge dilution with recycled exhaust gas. As a result, HC, CO and NO emissions can be reduced below equivalent conventional engine levels. Potential additional emissions problems with some of these stratified charge engines are soot and aldehydes, however.The concepts can be divided into open-chamber and divided-chamber stratified charge engines, depending on whether the aerodynamics of the fuel-air mixing process, or the geometry of a precombustion chamber, is the primary means of achieving the desired nonuniformity in fuel distribution. Here we will only describe one of these concepts, i.e., open-chamber stratified charge enginesFigure 1 shows schematics of two open chamber concepts now under extensive development. Figure 1 (a) is the Ford Programmed Combustion Process (PROCO) engine;Figure.1 (b) is the Texaco Controlled Combustion System (TCCS) engine. Both these engines employ a combination of inlet port swirl, cup-in-piston geometry, and high-pressure timed combustion-chamber fuel injection to achieve a local fuel-rich easily ignitable mixture near the spark plug, while the overall fuel-air ratio supplied to the engine is fuel lean.Despite the obvious similarities, the differences between these concepts are significant. The important differences are the fuel injection timing and the characteristics of the fuel spray. The PROCO concept employs a relatively low pressure fuel injector mounted on the cylinder axis; the injector tip is designed to produce a wide angle (about 90°) hollow cone fuel spray, with large droplets ( ~100μm), which have low initial velocities (~50m/s) relative to conventional diesel engine practice. Fuel injection starts early, about half way through the compression stroke. As a result, substantial vaporization of the fuel and mixing with air occurs before combustion, though large fuel-air ratio gradients persist throughout the combustion process.In the TCCS engine, fuel injection more closely resembles that in a diesel engine. The injectorand fuel pump are standard diesel engine components, the injector is angled as shown in Fig. 1(b) and the fuel spray directed into the cup to move with air swirl towards the spark plug location. Fuel injection starts just before combustion commences. Fuel droplet sizes are typical of diesel practice (~ 10μm) and initial droplet velocities are high ( ~ 200 m/s).The details of the combustion and emissions formation processes in these engines are not well understood. However, the emissions data which are available provide useful clues as to the events which occur inside the combustion chamber.Lavoie and Blumberg have analyzed the relationship between NO emissions and mixture stratification in the PROCO engine. Figure 2 shows the variation in NO exhaust concentration with overall fuel-air equivalence ratio for normal stratified engine operation, and for the same engine at the same load and speed, but with fuel and air premixed in a vapor tank outside the engine. For close to stoichiometric operation, stratification reduces NO emissions; for very lean operation premixed operation gives lower emissions. Since burning occupied roughly the same crank angle interval in each case, mixture stratification rather than different charge burning rates is the primary cause of this difference. The theoretical curves were developed from the work of Blumberg and Kummer by adding charge stratification to the analysis. Best agreement with the data trends was obtained by burning from richer-than-average to leaner-than-average mixture over the combustion duration interval, and by superposing small scale mixture stratification in the form of a gaussian fuel-air equivalence ratio distribution at each point in this mean combustion trend. 50% richer to 50% leaner than average with a local equivalence ratio standard deviation of 0.35 gave the best fit to measured data; we thus can conclude that substantial stratification is present. It should be noted that similar models of equivalence ratio nonuniformities have been found necessary to match observed NO and CO emission characteristics in steady flow liquid fueled combustors.A similar analysis of the TCCS engine concept is not yet available. However, work in progress at M.I.T. shows that the combustion process occurs in two distinct phases. The spark plug, with its long duration discharge, apparently acts as a flame holder and a continuous ignition source. Fuel-air mixture burns in the wake of the plug at about the rate the mixture reaches the spark plug location in the developing fuel-air spray. This rapid combustion occurs while the mixture is fuel-rich (just how rich depends on engine load), and lasts for the duration of injection. It is followed by a longer phase where fuel-rich products mix with excess air and burn to completion and lean overall equivalence ratios. These combustion characteristics give the TCCS concept a multifuel capability since apparently the development of the fuel spray between injector and spark plug location is not much different for gasoline, kerosene and diesel fuels.The use of inlet air swirl and cup-in-piston geometry results in high velocities and turbulence intensities in the charge at the end of the compression stroke. These engine concepts are characterized, therefore, by a fast burn. Because of rich mixture conditions at the spark plug and the fast burn, they are tolerant to substantial charge dilution. For example, the PROCO concept can be operated with up to about 40% exhaust gas recycle (EGR). A substantial reduction in NO emissions can therefore be achieved in a practical engine with about 30% EGR. There is, however, a performance penalty. As EGR is increased so the duration of the combustion process is increased.and the specific fuel consumption increases.The HC emissions from these engine concepts are comparable to those of a conventional engine. This is at first surprising since the fuel is injected primarily into the bowl in the piston and should therefore be kept from the piston crown cylinder wall crevice, a major source of HC in the conventional engine. However, the larger surface to volume ratio of these concepts may make quench layer HC more important. Also the higher compression ratio, faster burn and lean overall mixture ratio all reduce the exhaust temperature and make after burning more difficult. To achieve greater HC control, throttling at light load and combustion retard have been used. With an oxidation catalyst in the exhaust system, further HC and CO reductions can he achieved.The CO emissions from these engines are generally low due to the lean overall fuel-air ratio. However, one consequence of stratification is that CO emissions from these engines are higher with stratified than with premixed operation at the same lean equivalence ratio.In these fuel-injected stratified charge engines, the effect of variations in engine design and operating parameters on emissions is exceedingly complex. In addition to the parameters which are important in conventional SI engines, the fuel injection schedule, and the fuel distribution within the combustion chamber are important additional variables. A better understanding of the basic processes involved would provide a valuable framework for data interpretation and would assist in optimum concept development. Currently these engine concepts offer good control of NO within the engine cylinder with attractive fuel economy, and can achieve engine HC and CO emission levels comparable to or better than current conventional SI engine levels.Open-chamber stratified charge Wankel engines have been developed and tested, where fuel was injected into the combustion chamber close to the spark plug location. However, much more development will be required before the potential of this stratified charge concept can be adequately evaluated.(Adapted from the paper entitled “Pollutant formation and control in spark-ignition engines”)Fig. 1. Schematics of two open chamber stratified charge engines: (a) Ford PROCO; (b) Texaco TCCS.Fig. 2. Exhaust NO concentrations for Ford PROCO stratified charge engine as function of overall fuel-air equivalence ratio for normal and for premixed engine operation.NOTES TO THE TEXT1. Research and development efforts over several decades have resulted in a number of stratified charge SI piston engine concepts which appear to have potential for reduced exhaust emissions. As the name implies, the fuel is not distributed uniformly throughout the charge. 经过几十年坚持不懈的研究开发，已产生了多种分层充气火花点燃发动机的设计构想，它们在降低汽油机排污方面具有发展前途。