Particulate Matter Emissions in Gasoline Direct-Injection Spark-Ignition Engines: Sources, Fuel Dependency, and Quantities

Abstract

This study investigates sources of particulate matter (PM) from a direct-injected spark-ignition (DISI) engine, all of which are likely the result of poor mixture formation, although through varying mechanisms. Nine unique gasoline formulations, including both oxygenated and non-oxygenated fuels, with varying distillation curves and concentrations of different hydrocarbon classes were employed to investigate the effect of fuel on soot formation. The engine experiments were further supported by reacting and non-reacting experiments in a spray-chamber.The metal-engine experimental results indicated that engine-out PM emissions of the nine fuels generally conforms to their sooting tendency, here quantified by Particulate Matter Index (PMI). Discrepancies with the engine-out PM emissions and PMI were observed under transient operating condition, which is not reflective of the test conditions used to formulate the PMI. A diisobutylene surrogate blend and a high-olefin full-boiling range fuel were investigated further in the optically-accessible engine, showing soot generation from different sources. These differences came from the variation in their spray characteristics, as confirmed in the non-reactive spray-chamber experiments.While the earlier tests were conducted with a fouled injector, the injector was later cleaned and fuels were tested in the optical engine. High olefin fuel and diisobutylene blend showed significantly lower soot emissions with clean injector. E30 fuel exhibited spray impingement and subsequent diffusive combustion, known as pool fire, on the piston top characterized by a yellow flame due to soot luminosity. A final round of experiments in the spray-chamber mimic this pool fire mechanism. A stoichiometric flame was initiated after a spray has impinged on a wall and formed a film. Results showed a large time delay between the flame passing over the fuel film and the formation of soot mass. The slow soot formation via pyrolysis in spray-chamber suggest that pyrolysis may not be a dominating soot-production pathway in SI engines, even for conditions with significant wall wetting. The findings of this study highlight the many considerations which influence the effective design of a combustion system when the minimization of PM emissions is a goal, and the ways in which fuel formulation can change soot-production pathways in DISI engines.