Effect of compression ratio on cycle-by-cycle variations in a direct injection natural gas engine

Cycle-by-cycle variations of a direct-injection spark ignition natural gas engine under different compression ratios were investigated. The results show that there exist the partial burn cycles at low compression ratio, and the indicated mean effective pressure (p_mi) gives a low value and the coefficient of variation of p_mi(CoV_pmi) gives a high value. CoV_pmi decreases with the increase of compression ratio. However, when compression ratio is larger than 12, slight knock will occur at high engine load, leading to an increase in CoV_pmi. Better interdependency is demonstrated between p_mi and maximum cylinder pressure, while CoV_pmi gives a larger value than that of maximum cylinder pressure. In addition, better interdependency also demonstrates between flame development duration and main combustion duration. Cyclic variations occur at both flame development duration and main combustion duration. Shortening in flame development duration and later combustion duration leads to the decrease of CoV_pmi. Better interdependency presents between p_mi and flame development duration and/or total combustion duration. Cyclic variations of direct-injection natural gas engine are resulted from the cyclic variations in flame development duration and main combustion duration. They are corresponding to the mixture around the spark plug and mixture stratification in the cylinder.