CAI engine is well known to be advantageous over conventional SI engines because it facilitates higher engine efficiency and lower emission (NOx and smoke). However, its limited operation range, large cyclic variation, and difficulty in heat release control are still unresolved obstacles. Previous studies showed that a high load range of the CAI engine is limited mainly by the combustion noise caused by a stiff pressure rise (knock), and that a low load range is also limited by the combustion instability caused by the high dilution of residual gas.
In this study, the characteristics of each cycle were analyzed to find the cause of the cycle variation at the high load limit of CAI operation. Moreover, to improve combustion stability, we tested the in-cylinder fuel stratification by applying nonsymmetrical fuel injection to the intake port. Experiments were performed on a PFI single cylinder research engine equipped with dual CVVT and low lift (2 mm) cam shaft with NVO strategy. The in-house RGF prediction model was used to predict the cycle by cycle differences of in-cylinder trapped mass, RGF and bulk temperature.
The result of cyclic analysis shows that cyclic variation is caused by the cycle transition between normal combustion and knock when the in-cylinder charge condition changed. Non-uniformity of in-cylinder fuel concentration reduces the frequency at which the knock occurred by about 20 %. Due to this reduction of the number of knock cycle, NIMEP is increased about 5 %, COVIMEP improved about 1 %, Rmax, which represents the combustion noise, reduced about 20 % and NOx reduced about 15 %.