Simulation of the NOX Emissions for a CNG Direct-Injection Engine

The distribution of temperature and NOX concentration field under different crank angle of a CNG direct-injection engine converted from a four-valve gasoline vehicle engine are calculated by the computation model of the CNG direct-injection engine work process using CFD software. The results show that the temperature field spreads around the center of the spark plug in initial combustion period. The position of maximum temperature in combustion chamber moves from the spark plug to the wall and temperature tends to uniform distribution as the piston goes down along with the spread of flame front and the completion reaction after the flame. Temperature distribution is vital to the generation of NOX which changes along with high-temperature zone. The change principle of quantity of NOX in the cylinder is that at first, the quantity continues increasing, then declines, and finally remains stable. Besides this, the generation amount of NOX increase along with the speed.