Characteristics of CNG Direct Injection with Auto-Ignition

Characteristics of compressed natural gas (CNG) direct injection auto-ignition were investigated experimentally. A rapid compression machine (RCM) with the compression ratio of 10 was used. The diameter and thickness of the combustion chamber are 80 mm and 20 mm, respectively. After the compression start, fuel was directly injected with a single hole injector at the injection pressure of 7.0 MPa, and auto-ignition takes place. The fuel injection timing was varied from 50 ms to 60 ms from the compression start. Two kinds of natural gasses were tested; 12A (CH4: 99.1 %) and 13A (CH4: 86.3 %, C2H6: 5.2 %, C3H8: 1.9 % and others). A glow plug was installed in the cylinder in order to assist the ignition, which was set at 30 mm downstream from the fuel injector nozzle exit. Two kinds of auto-ignition processes were observed. For CNG 12A, auto-ignition always takes place after the end of the fuel injection. The ignition delay is relatively long (40 to 80 ms) and the fluctuation is large. For CNG 13A, under some conditions, auto-ignition takes place just after the start of fuel injection (designated as ‘early ignition’), and the combustion continues during the fuel injection duration. The ignition delay is much shorter when compared to the case of 12A, and the fluctuation is small. This suggests that the components of C2H6 and C3H8 in 13A are quite important in the auto-ignition process of natural gas.