Stratified-charge combustion: modeling and imaging of a spray-guided direct-injection spark-ignition engine

摘要

Spray-guided spark-ignited direct-injection (SG-SIDI) engines are likely to be the next generation of gasoline engines. Compared to current wall-guided (WG-SIDI) engines, they offer improved fuel economy and substantially reduced hydrocarbon and soot emissions. Two stages of stratified combustion (partially premixed flame propagation and mixing-controlled combustion of rich products left from partial combustion of initially fuel-rich mixtures) are investigated here in an experimental SG-SIDI engine using detailed CFD modeling, high-speed spectrally resolved combustion luminosity imaging, and cylinder pressure analysis. The CFD model reproduces the spatial and temporal distribution of both combustion stages. Correlation of CFD calculations and experimental combustion measurements demonstrates that optimum ignition timing occurs for somewhat rich equivalence ratios (0.9-1.6) and relatively low flow velocities ( < 15 m/s) at the spark location. Both model and experiment show: (1) for fixed injection time, increased rich burning occurs for earlier ignition times and (2) higher in-cylinder swirl leads to asymmetric flame propagation and increased rich burning. Calculated and measured overall heat-release rates agree quite well over a wide range of engine operating conditions. These experiment-model comparisons provide a detailed picture of combustion processes in spray-guided SIDI engines and a sound scientific basis for engine design optimization.