Effects of In-Cylinder Fuel Spray Formation on Emissions and Cyclic Variability in a Lean-Burn Engine. Part 2: Results

The effects of injection system design, air/fuel ratio, coolant temperature and fuel volatility on engine-out hydrocarbon emissions, NO_x emissions and cyclic variability have been studied in a prototype 1.8l lean-burn SI engine. The results have been compared with ILIDS measurements of in-cylinder spray characteristics made under similar conditions, to establish the degree to which variations in fuel spray formation affect engine performance. The engine was found to exhibit a different behaviour when running lean compared to stoichiometric. In lean-burn operation, there was a clear tendency for the best performance to occur under operating conditions which produced charge stratification in the cylinder, whereas when the engine was run with a stoichiometric air/fuel ratio, the best performance was achieved under engine operating conditions which gave the most homogeneous charge in-cylinder. The main effects of increasing mid-range or back-end fuel volatility were to increase evaporation from the inlet port wall film, thereby reducing the contribution made by strip atomisation to the overall in-cylinder droplet population early in the intake stroke. The optimum fuel system for lean-burn operation was the combination of an air-assisted injector used with back-flow injection timing.