Fuel Economy and Engine-Out Emissions from a Single-Cylinder Two-Valve Direct-Injection S.I. Engine Operating in the Stratified-Combustion Regime

This study is an experimental and computational investigation of the influence of injection timing, fuel spray orientation, and in-cylinder air motion on the combustion, fuel economy, and engine-out emissions of a single-cylinder, 2-valve, spark-ignition direct-injection (SIDI) engine, operating under stratified-charged conditions. For the best compromise between fuel consumption, combustion stability, engine-out hydrocarbon emissions and smoke, the engine required relatively retarded injection timings (in comparison to other charge- or wall-controlled DI engines), high swirl levels, and a spray orientation that is directed towards the intake-valve side and targets the ridge wall of the piston. The stratified combustion was thus found to be a combination of wall and charge control, as indicated by the indirect spray targeting relative to the spark location coupled with the high swirl requirements of the engine, as well as the crucial role of the piston ridge in deflecting the fuel vapor towards the spark gap. The CFD computations, which showed close agreement with the experimental results, suggest that minimizing the amount of fuel deposited on the piston surface is a requirement for low exhaust smoke. It was also concluded from the combined results of the computations and the experiments that the time-averaged air-fuel mixture in the immediate vicinity of the spark gap at the time of ignition should be biased towards a rich mixture to minimize misfires and cyclic combustion variability.