Experimental study on combustion, emissions and thermal balance of high compression ratio engine fueled with liquefied methane gas

•High pressure cycle efficiency was disintegrated and various energy losses were quantitatively studied.•The maximum high pressure cycle efficiency reaches 49.6% at high speed and high load.•The variation trend of exhaust energy proportion is similar as high-pressure cycle efficiency and the maximum reaches 41.9%.•At low speed, the effective thermal efficiency can be further improved by optimizing the thermodynamic process.•At high speed, it is better to reduce the friction loss and pumping loss to improve the effective thermal efficiency. To improve the fuel efficiency of liquefied natural gas engine, an approach was proposed in our previous study, in which the liquefied natural gas is first purified into liquefied methane gas and then fueled to high compression ratio engine. To further evaluate the fuel efficiency of liquefied methane gas engine, in this study the thermal balance test was carried out, and then the thermodynamic performance of liquefied methane gas engine was analyzed and various kinds of energy losses were disintegrated. On this basis, the influence factors of various forms of energy flow in liquefied methane gas engine were discussed by combining with combustion and thermodynamics processes. Results show that, the proportions of indicated work in high-pressure cycle, exhaust energy and heat transfer loss are mainly influenced by engine speed. The high-pressure cycle efficiency increases with engine speed and the maximum value reaches 49.6% due to the higher compression ratio. The variation trend of exhaust energy proportion is similar as that of high-pressure cycle efficiency, while the proportion of heat transfer loss decreases with the engine speed increasing. Although the maximum high-pressure cycle efficiency appears in high speed conditions, the maximum effective thermal efficiency still appears at low speed due to the effect of friction loss and pumping loss (especially the former). Therefore, for the low speed conditions, the effective thermal efficiency of LMG engine can be further improved by optimizing the thermodynamic process (or combustion parameters); while for the high speed conditions, it is better to reduce the friction loss and pumping loss.