Effect of Compression Ratio on Cycle-by-Cycle Variations in a Natural Gas Direct Injection Engine

Cycle-by-cycle variations of a natural gas direct-injection spark ignition engine at different compression ratios were investigated. The results show that the lean burn limit of the natural-gas direct injection engine can be extended to a larger overall excess air ratio compared with that of the homogeneous charge natural gas engine. The coefficient of variations (CoV) of indicated mean effective pressure decreases with the increase of compression ratio. However, CoV of indicated mean effective pressure is increased at high engine load when compression ratio is larger than 12. The cycle-by-cycle variations are more clearly demonstrated in CoV of indicated mean effective pressure rather than in CoV of cylinder peak pressure. Average values of flame development duration, main combustion duration, and total combustion duration are decreased and combustion is improved with increasing compression ratio. This is the reason for decreasing cycle-by-cycle variations in the natural gas direct-injection engine. Better interdependence exists between the indicated mean effective pressure and the flame development duration, as well as between the indicated mean effective pressure and late combustion duration. Cycle-by-cycle variations of the natural gas direct-injection engine are resulted from cycle-by-cycle variations in flame development duration and late combustion duration. This shows some difference to that of homogeneous charge natural gas engine, where cycle-by-cycle variations are mainly influenced by the variations in early flame development stage.