A basic behavior of CNG DI combustion in a spark-ignited rapid compression machine

A basic characteristics of compressed natural gas direct-injection (CNG DI) combustion was studied by using a rapid compression machine. Results show. that comparing with homogeneous mixture, CNG DI has short combustion duration, high pressure rise due to combustion, and high rate of heat release, which are considered to come from the charge stratification and the gas flow generated by the fuel injection. CNG DI can realize extremely lean combustion which reaches 0.03 equivalence ratio, phi. Combustion duration, maximum pressure rise due to combustion and combustion efficiency are found to be insensitive to the injection modes. Unburned methane showed almost the same level as that of homogeneous mixture combustion. CO increased steeply with the increase in phi when phi was greater than 0.8 due to the excessive stratification, and NOx peak value shifted to the region of lower phi. Combustion inefficiency maintains less than 0.08 in the range of phi from 0.1 to 0.9 and increases at very low phi due to bulk quenching and at higher phi due to excessive stratification. The combustion efficiency estimated from combustion products shows good agreement with that of heat release analysis.