Effects of natural gas composition and compression ratio on the thermodynamic and combustion characteristics of a heavy-duty lean-burn SI engine fueled with liquefied natural gas

In this study, the effects of natural gas composition and compression ratio on thermodynamic and combustion characteristics were comprehensively investigated using a spark ignition liquefied natural gas engine modified from a heavy-duty compression ignition engine. LNG fuel samples with two different methane compositions and a series of compression ratios were tested and analyzed with the same boundary conditions at a lean burn condition of 1.38 excess air/fuel ratio. The results indicated that the in-cylinder pressures of the heavy-duty spark ignition engine increased with increasing compression ratio (CR), while the magnitude of the increase of the peak combustion pressure firstly increased, reached a peak value at the compression ratio of 14 and then decreased. However, the heat release phasing advanced with increasing CR, the ignition delay and the combustion duration shortened with the 50% combustion location advancing toward the top dead center with increasing CR. Apart from that, the cycle-to-cycle variations of the combustion duration and peak combustion pressure were decreased with increasing CR. The peak combustion pressure declined with the prolonged the combustion duration. In addition, the location of the maximum pressure rise rate advanced and the coefficient of variation of the indicated mean effective pressure decreased with increasing CR. Consequentially, the brake thermal efficiency increased and the brake specific fuel consumption decreased with the increase of the CR. The laminar flame speed of LNG 1-air was slightly higher than that LNG 2-air due to the higher ethane concentration and its higher laminar flame speed.