Effect of Octane Rating and Charge Stratification on Combustion and Operating Range with DI PCCI Operation

A single cylinder engine has been run with direct-injection premixed charge compression ignition (PCCI) operation. The operation is fueled with primary reference fuels for a wide variety of injection timing and equivalence ratio to investigate the effect of charge stratification and octane rating on PCCI combustion. The test results showed that although the change of the injection timing can gain the high combustion efficiency for a wide range of equivalence ratio, the combustion phasing where the high combustion efficiency is accomplished is not varied only by changing the injection timings. Therefore, the only change of injection timings does not improve the thermal efficiency which is influenced by the combustion phasing. On the other hand, at the fixed compression ratio, inlet air temperature and so on, the octane rating is useful in altering the combustion phasing. The experiments in which the octane rating was changed demonstrated that for the lean air-fuel mixture which produces low NO_x emission without EGR, the combustion phasing must be advanced to gain the better combustion efficiency at low load. By contrast, higher equivalence ratio allows the combustion phasing to be retarded with keeping high combustion efficiency.