To improve the performance of a liquid methane engine (LME) under universal characteristics, the in-cylinder combustion process and thermal power conversion under different compression ratios were studied by experiment. The experimental results show that increasing the compression ratio results in a linear increase in EEE (effective expansion efficiency) and EER (effective expansion ratio). Moreover, the thermal efficiency varies significantly with the compression ratio only at low and medium loads. The effective thermal efficiency can reach up to 48.69%. For high-speed conditions, it is recommended to reduce friction losses and pump gas losses. Furthermore, a 1-D simulation model was developed to simulate the in-cylinder energy flow with a compression ratio of 12.6. The simulation results show that exhaust timing has a more pronounced effect on the in-cylinder energy flow (thermal balance) than intake timing. When the engine speed is 800 rpm, advancing the exhaust timing by 20°CA can increase the effective thermal efficiency by about 1%.

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