Numerical study of the effect of injection timing on the knock combustion in a direct-injection hydrogen engine

Abstract

Knock combustion is one of the main barriers to the use of hydrogen as an engine fuel. This paper focuses on the effect of injection timing on knock characteristics in a direct-injection hydrogen engine by numerical simulation. Firstly, the mixture distribution and process of flame propagation were described under different injection timings. We found that both the uniformity of mixture and the flame speed increase with the advance of injection timing. Secondly, for single injection, delaying injection timing makes the knock intensity decrease first and then increase. Besides, autoignition spot can appear either near the cylinder wall or at the bottom of piston pit when the combustion chamber is ω-shaped. Finally, the results showed that a coupling between pressure wave and flame front causes heavy pressure oscillations. Thermophysical properties and the Unburned Mass Fraction (UBMF) are both critical factors that influence knock significantly and knock intensity increases with UBMF.