Particulate Matter Formation Mechanisms in a Direct-Injection Gasoline Engine
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Experiments were conducted to determine the particulate formation mechanisms of a single-cylinder direct-injection two-stroke gasoline engine. The engine was tested at four operating points; an idle condition with a highly stratified mixture, a 2000 RPM low load condition operated with an A/F of both 30:1 and 15:1, and a 2800 RPM moderate load 15:1 A/F condition. The engine utilized an air-assist injector that was also used as a N2-assist injector to provide a slightly richer local mixture. Propane fuel was also used with the injector to isolate the effects of spray impingement and fuel films. The oil-to-fuel ratio was externally controlled to determine the lube oil effect on particulate matter (PM). A venturi-type mini-dilution tunnel was designed and integrated to sample particulates. It utilized a critical orifice supply that allowed the dilution ratio (DR) to be changed and a case heater to maintain a sampling temperature at the instruments. A tapered element oscillating microbalance (TEOM) was used to measure particulate mass and a scanning mobility particle sizer (SMPS) was used to measure the number-based size distribution. The total particulate mass measured from the TEOM was compared to traditional gravimetric methods utilizing a Teflon filter and found to agree very well. NOx and CO2 concentration measurements were made in the dilution tunnel to be used as a tracer to determine DR. Lubrication oil consumption (LOC) was found to have a large effect on the PM for this two-stroke engine utilizing a lost oil system. Not only was the lube oil the dominant effect on PM with the normal fuel-to-oil ratio, it was found to have a complex interaction that changed with engine parameters and not just increase the particulate mass by some offset at all conditions. It was therefore determined that the engine would be operated at a low oil-to-fuel ratio comparable to the LOC rates of four-stroke engines (oil sump systems). An interesting trend in particulate mass with injection timing was observed at both stoichiometric operating conditions. A local minimum in particulate mass was found for a fairly retarded injection timing. The size distributions near this local minimum showed that the particulates appeared to change mode. Generally propane injection resulted in a significantly lower particle mass. It was determined, however, that this is not entirely due to spray impingement. The fuel composition was believed to be a significant effect when using propane and accounts for some of the particulate mass difference. Temperature was found to have a significant effect on particle mass. The observed greater particulate mass for air-assist injection over N2-assist was likely due to higher incylinder temperatures. Analysis of the size distribution curves suggests that temperature strongly affects the small mode of particles. This corresponds with advanced timings, closer to a homogeneous condition. The local burning zone A/F had a large effect on particulate mass for very rich mixtures (retarded timings). It also has a significant effect on the large particle mode seen in the size distributions, believed to be elemental carbon particles.