An experimental study on the combustion and emissions of a natural-gas direct-injection spark ignition engine under different compression ratios was carried out. The results show that the compression ratio has a large influence on the engine performance, combustion, and emissions. The penetration distance of the natural-gas jet is decreased and relatively strong mixture stratification is formed as the compression ratio is increased, giving a fast burning rate and a high thermal efficiency, especially at low and medium engine loads. However, the brake thermal efficiency is increased with a compression ratio up to a limit of 12 at high engine loads. The maximum cylinder gas pressure is increased with increase in the compression ratio. The flame development duration is decreased with increase in the compression ratio and this behaviour becomes more obvious with increase in the compression ratio at low loads or for lean mixture combustion. This indicates that the compression ratio has a significant influence on the combustion duration at lean combustion. The exhaust hydrocarbon (HC) and carbon monoxide emissions decreased with increase in the compression ratio, while the exhaust nitrogen oxide emission is increased with increase in the compression ratio. The exhaust HC emission tends to increase at high compression ratios. Experiments showed that a compression ratio of 12 is a reasonable value for a compressed-natural-gas direct-injection engine to obtain a better thermal efficiency without a large penalty of emissions.
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