Effect of high compression ratio on cycle-to-cycle variations of a natural gas engine under low load conditions

High compression ratio (CR) design achieves better energy efficiency and carbon emission reduction in natural gas engines. However, it poses a major challenge to the operational stability of spark-ignition natural gas engines because of cycle-to-cycle variations (CCV). The mechanisms of CCV in high CR natural gas engines are not yet fully understood. To this end, the effect of high CR (11.6, 14.0, and 16.0) on CCV of a heavy-duty spark-ignition natural gas engine is investigated under low load conditions via experimental analysis and numerical simulations. The results demonstrate that the CCV does not simply decrease with increasing CR. In contrast, the CCV of CR 11.6 is lower than that of CR 14.0 and CR 16.0 at relatively low loads. Specifically, such as at 1000 rpm and 0.2 MPa BMEP, the coefficient of CCV in IMEP increased from 4.1% to 5.1% and 4.7%, respectively, after increasing the CR from 11.6 to 14.0 and 16.0. As the load increases, CCV decreases, and the CR has less influence on CCV. In addition, the ignition delay period of CR 11.6 is longer than that of CR 14.0 and CR 16.0, but its combustion duration is shorter than those of the latter two. The three-dimensional simulation results indicate that the combustion chamber of CR 11.6 can maintain a higher tumble ratio and turbulence kinetic energy in the cylinder than that of CR 14.0 and CR 16.0, which helps to reduce CCV under low load conditions.