Mechanism study of scavenging process and its effect on combustion characteristics in a boosted GDI engine

The scavenging process is helpful to suppress super-knock in a 4-stroke turbocharged gasoline direct injection engine because it can reduce in-cylinder thermal load and clear the suspending residual particles from previous cycles. This paper investigated the effects of dual variable valve timing on scavenging and combustion process by simulation and experimental study. The simulation results showed that the scavenging process improved with the advance of intake valve timing and the retard of exhaust valve timing. When the valve overlap duration was the same, the early occurrence of valve overlap was helpful to the scavenging process. The longer the valve overlap duration was the better the scavenging process would be. When exhaust valve timing was fixed, spark timing retarded with the advance of intake valve timing, nevertheless, the mixture mass, basic specific fuel consumption rate, combustion duration, and exhaust temperature increased with the advance of intake valve timing. In two early intake valve timing conditions, when exhaust valve timing was retarded, the mixture mass, basic specific fuel consumption, exhaust temperature raised, combustion duration and the length of ignition delay shortened. In three late intake valve timing conditions, the combustion characteristics were unchanged with the postponement of exhaust valve timing. The effect of dual variable valve timing on scavenging and combustion process was similar. Intake valve timing played a determinate role in the combustion performance of gasoline direct injection engine. The optimized scavenging process could lower the super-knock frequency and improve combustion characteristics.