In the effort to improve combustion of a Light-load Stratified-Charge Direct-Injection (LSCDI) combustion system, CFD modeling, together with optical engine diagnostics and single cylinder engine testing, was applied to resolve some key technical issues. The issues associated with stratified-charge (SC) operation are combustion stability, smoke emission, and NOx emission. The challenges at homogeneous-charge operation include fuel-air mixing homogeneity at partial load operation, smoke emission and mixing homogeneity at low speed WOT, and engine knock tendency reduction at medium speed WOT operations.
In SC operation, the fuel consumption is constrained with the acceptable smoke emission level and stability limit. With the optimization of piston design and injector specification, the smoke emission can be reduced. Concurrently, the combustion stability window and fuel consumption can be also significantly improved. The optimized piston also helps to reduce NOx emission with local mixture enrichment around the spark-plug gap and improved internal residual amount tolerance.
The study shows that one of the root causes of smoke emission at low speed WOT is liquid fuel impingement on the valve surface. The smoke emission level can be reduced with injector specification optimization. The mechanism by which split injection improves WOT performance is studied in detail. It is shown that at low speed WOT operation, the split injection improves the liquid spray distribution, thus improves the fuel-air mixing homogeneity and the engine output. At medium speed WOT operation, split injection does not have much effect on the mixing homogeneity, instead it improves the charge temperature distribution, thus reducing the knocking tendency.