Experimental and numerical study of the effect of injection strategy and intake valve lift on super-knock and engine performance in a boosted GDI engine

For four-stroke turbocharged gasoline direct injection engines, the improvement of pre-mixture homogeneity and the reduction of wall wetting are beneficial to improve combustion performance and emission characteristics, and specifically effective to suppress super-knock because of the decrease of oil dilution and soot formation. This paper investigated the effect of injection strategy and intake valve lift (IVL) on super-knock frequency and engine performance through experimental study. The validated numerical models revealed the mechanisms behind experimental results. The results showed that using late second injection under high-IVL condition and early single-injection under low-IVL condition reduced wall wetting and improved mixture homogeneity. This restrained the formation of pre-ignition origins (oil dilution and soot formation were decreased). The tendency of hot-spot self-ignition (local high-temperature region was narrowed) and end-gas detonation combustion (end-gas reactivity was reduced) was suppressed as well. These factors contributed to the reduction of the actual super-knock frequency in experiment. Further, late second injection under high-IVL condition and early single-injection under low-IVL condition also accelerated the flame propagation and promoted the complete combustion of fuel. Thus the dynamic and economic performance improved. CO and soot emissions also reduced, only NOx emission increased. Two-stage injection played the opposite role in engine performance respectively under high-IVL and low-IVL condition. In general, the combination of low-IVL and early single-injection showed the lowest super-knock frequency and best engine performance.