Abstract: The effects of different direct injection methods of compressed natural gas(CNG) fuel into the cylinder on the microscopic physical fields, the flame propagation mechanisms and the NO formation mechanism were simulated and analyzed using software FIRE. The results show that, after injection the formation mechanism of CNG fuel mixture depends on its separation speed relative to the air in the cylinder, and the separation speed of CNG is controlled effectively by the injection timing according to the kinetic energy provided by gas fuel during the injection, which determines the distribution of concentration fields. During the compression stroke, the concentration distribution rises from the bottom to the top if the fuel was injected early and vice versa. The NO formation rate depends on the concentration and temperature of local mixture. The NO formation rate is almost 0 regardless of equivalence ratio when temperature is lower than 2000 K. The amount of NO production depends on the NO reaction rate, the size of production area and the reaction duration.

Key words: power mechinery and engineering, compressed natural gas direct injection(CNG-DI) engine, injection methods, mixture formation mechanism, combustion characteristic