Numerical investigation on the effects of valve timing on in-cylinder flow, combustion and emission performance of a diesel ignition natural gas engine through computational fluid dynamics

In the present study, a diesel engine was modified to a diesel pilot ignited natural gas engine and the influences of intake valve closing timing on in-cylinder flow, combustion and emission performance of engine were investigated by three-dimensional computational fluid dynamics simulation. Based on the geometric model and basic parameters of this engine, the simulation model was built under three operating conditions and then validated by experimental data. On this basis, the validated model was applied to investigate the effects of the intake valve closing timing strategy. The simulation results indicated that, the volumetric efficiency decreases with the retarding of intake valve closing timing in three cases while it increases as the intake valve closing timing is advanced by 10°CA at 1200 rpm. The in-cylinder turbulence kinetic energy decreases with the retarding of intake valve closing timing. The peak in-cylinder pressure decreases when the intake valve closing timing is either advanced or retarded at 50% load. Nevertheless, the maximum peak in-cylinder pressure occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load, which rises by 4.5 bar compared with that at the original intake valve closing timing. Additionally, the maximum heat release rate appears at advancing 10°CA intake valve closing timing at 100% load, which is 26 J/deg higher than that at the original intake valve closing timing. Simultaneously, the shortest combustion duration occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load. For the emissions, the NOx emissions decrease with the retarding of intake valve closing timing but the variation becomes unobvious with the advancing of the intake valve closing timing. Besides, the intake valve closing timing strategy has little effect on HC and CO emissions.