A study of the combustion and emission characteristics of compressed-natural-gas direct-injection stratified combustion using a rapid-compression-machine

The objective of the present study is to determine the characteristics of combustion and emissions of compressed-natural-gas (CNG) direct-injection combustion using a rapid-compression-machine which has a compression ratio of 10 and a disc-shaped combustion chamber. Combustion and emission characteristics are compared for three types of fuel injection (single side, parallel side and opposed side injection) and a homogeneous mixture. The results show that with fuel injection, the fuel could be burned up to an equivalence ratio φ of 0.2 with sufficiently high combustion efficiency except for the case of φ = 1.0, while with a homogeneous mixture, the lean burn limit was only φ = 0.6 with poor combustion producing higher unburned CH⁴ By adjusting the location of the spark plug and fuel injectors, the combustion limit was extended to φ = 0.02. The Combustion efficiency of the injection modes is over 0.95 except for φ = 1.0 and φ < 0.06 which gave a lower combustion efficiency. Incomplete combustion in the stratified rich zone reduced the combustion efficiency at large values of φ, and possible occurrence of bulk quenching resulted in the lower combustion efficiency for very lean mixtures. Combustion efficiency for the homogeneous mixture decreases greatly with leaner mixtures, which is probably due to the thicker quenching layer near the wall. Combustion duration with fuel injection was insensitive to φ and was much shorter than for the homogeneous mixture. It was also shown that the number and location of the injectors and the injection rate had little influence on the combustion and the exhaust emissions including NOx. The pressure rise due to combustion in the case of fuel injection is higher compared to that of homogeneous mixture combustion due to the lower heat loss to the combustion chamber walls resulting from a short combustion duration. Thus it is shown that stratified-combustion with extremely lean burn capability can be realized with CNG direct injection.