Effects of In-Cylinder Fuel Spray Formation on Emissions and Cyclic Variability in a Lean-Burn Engine. Part 1: Background and Methodology

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 correlate with engine performance. The lean-burn engine and the methodology of the combined study are described. The engine was found to exhibit a different behaviour when running lean compared to stoichiometric. When running lean, there was a negative correlation between steady-state engine-out NO_x emissions and cyclic variability (COV of IMEP), and a positive correlation between steady-state engine-out hydrocarbon emissions and cyclic variability. Engine-out hydrocarbon emissions and cyclic variability in lean-burn operation were both positively correlated with in-cylinder droplet size (Sauter Mean Diameter) and the volume of fuel in the cylinder in the droplet phase early in the intake stroke. The correlation was not strong, but indicated that enhanced fuel atomisation was generally advantageous.