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Large reductions in low-load NOx emissions can be obtained by replacing conventional Diesel or spark ignited combustion by HCCI combustion in reciprocating engines. Currently, HCCI combustion is limited to operating conditions with lean air/fuel ratios or large amounts of EGR. However, a numerical model shows that, even if high equivalence ratio HCCI operation were satisfactorily attained, the NOx reduction potential vs. DI-Diesel combustion would be much smaller. Thus, high-load HCCI operation may best be obtained through highly boosted fuel-lean operation. Alternatively, HCCI combustion may be suited well for “dual mode” engine applications, in which spark ignition or conventional Diesel combustion is used to obtain full load. Avoiding wall impingement with heavy fuels is critical for achieving good emissions and fuel consumption, and it appears that a large degree of mixture inhomogeneity can be tolerated from a NOx benefit standpoint.

A recently developed in-cylinder fuel injection probe was used to deposit a small amount of liquid fuel on various surfaces within the combustion chamber of a 4-valve engine that was operating predominately on liquefied petroleum gas (LPG). A fast flame ionization detector (FFID) was used to examine the engine-out emissions of unburned and partially-burned hydrocarbons (HCs). Injector shut-off was used to examine the rate of liquid fuel evaporation. The purpose of these experiments was to provide insights into the HC formation mechanism due to in-cylinder wall wetting. The variables investigated were the effects of engine operating conditions, coolant temperature, in-cylinder wetting location, and the amount of liquid wall wetting. The results of the steady state tests show that in-cylinder wall wetting is an important source of HC emissions both at idle and at a part load, cruise-type condition. The effects of wetting location present the same trend for idle and part load conditions.