Simulation of the Effect of Intake Pressure and Split Injection on Lean Combustion Characteristics of a Poppet-Valve Two-Stroke Direct Injection Gasoline Engine at High Loads

摘要

Poppet-valve two-stroke gasoline engines can increase the specific power of their four-stroke counterparts with the same displacement and hence decrease fuel consumption. However, knock may occur at high loads. Therefore, the combustion with stratified lean mixture was proposed to decrease knock tendency and improve combustion stability in a poppet-valve two-stroke direct injection gasoline engine. The effect of intake pressure and split injection on fuel distribution, combustion and knock intensity in lean mixture conditions at high loads was simulated with a three-dimensional computational fluid dynamic software. Simulation results show that with the increase of intake pressure, the average fuel-air equivalent ratio in the cylinder decreases when the second injection ratio was fixed at 70% at a given amount of fuel in a cycle. With the increase of intake pressure, ignition timing advances, combustion duration slightly decreases first and then increases while the maximum pressure rise rate first increases and then drops. High intake pressure can prevent the occurrence of knock through decreased fuel-air equivalent ratio around the cylinder. The second injection timing can also influence combustion processes. The improved indicated thermal efficiency occurs when the second injection timing is 90°CA after top dead center. Delayed second injection timing is beneficial for the development of initial flame kernel. But when the second injection timing is 60°CA after top dead center, the fuel is only consumed by flame propagation. When auto-ignition occurs in the end mixture, the heat released by auto-ignition gradually increases with the delay of the second injection timing. Meanwhile, indicated thermal efficiency is increased when little amount of end mixture auto-ignites without knock.