Effect of Compression Ratio on Stratified-Charge Direct- Injection Gasoline Combustion

Charge cooling due to fuel evaporation in a direct-injection spark-ignition (DISI) engine typically allows for an increased compression ratio relative to port fuel injection (PFI) engines. It is clear that this results in a thermal efficiency improvement at part load for homogenous-charge DISI engines. However, very little is known regarding the effect of compression ratio on stratified charge operation. In this investigation, DISI combustion data have been collected on a single cylinder engine equipped with a variable compression ratio feature.

The results of experiments performed in stratified-charge direct injection (SCDI) mode show that despite its over-advanced phasing, thermal conversion efficiency improves with higher compression ratios. This benefit is quantified and dissected through an efficiency analysis. Furthermore, since the engine was equipped with both wall-guided DI and PFI systems, direct comparisons are made at part load for fuel consumption and emissions. Interestingly, combustion efficiency deteriorates in SCDI mode as compression ratio increases, albeit not due to crevice loading as is the case in PFI operation. Mechanisms for the observed hydrocarbon emissions behavior are suggested for change in load and compression ratio. The conclusions reached in this investigation provide an experimental basis for adequately selecting a compression ratio in SCDI engines.