Previous experiments using an air-assisted spray in a two-stroke direct-injected engine demonstrated a significant improvement in combustion stability at part-load conditions when a wide injection spray was used. It was hypothesized that the decrease in variability was due to the spray following the combustion chamber wall, making it less affected by variations in the in-cylinder gas flows.
For this study, experiments were conducted to investigate engine spray combustion for cases where engine performance was not dominated by cyclic variation. Combustion and emission performance data was collected for a wide range of injection timings at several speed/load conditions. Experimental data for combustion shows that combustion stability is relatively unaffected by injection timing changes over a 40 to 100 degree window, and tolerant to spark gap projections over a range of 0.7 to 5.2 mm, depending on operating conditions. However, exhaust emissions are much more sensitive to injection timing. Spray characterization data shows that the air/fuel mixture exiting the injector tends to follow the hemispherical chamber walls, concentrating the fuel mass on the perimeter. The range of injection timings used resulted in air/fuel mixtures which should have had significant differences in the state of the mixture at the time of combustion.