Effect of compression ratio on combustion and emission characteristics of a direct-injection natural gas engine

An experimental study on combustion and emission characteristics of a direct-injection spark-ignition engine fueled with natural gas under various compression ratios was investigated. The results show that the compression ratio has a significant influence on the engine performance, combustion and emissions. When compression ratio is increased, the volumetric efficiency is increased, giving a fast burning rate and thus high thermal efficiency and power output. The maximum rate of heat release increases with the increase of compression ratio while the flame development duration decreases with the increase of compression ratio, and these behavior become more obviously with the increase of compression ratio at the small load or lean mixture combustion. This indicates that compression ratio has a significant influence on combustion duration for lean mixture combustion. Exhaust HC and CO emissions decrease with the increase of compression ratio, while exhaust NO_x increases with the increase of compression ratio. However, the exhaust HC emission tends to increase at higher compression ratio. Light knock is observed at high load at compression of 14, leading to the increase of NO_x. Compression ratio of 12 is a reasonable value for the compressed natural gas direct-injection engine based on the experimental study.