Effect of the Miller cycle strategy on methanol and ethanol engines under stoichiometric combustion and lean burn

To achieve carbon neutrality goals, there is an urgent need to improve the thermal efficiency of engines and the application of carbon neutral fuels. Alcohols are considered potential alternative fuels due to its low emissions and renewability. The Miller cycle is considered as an effective means of improving thermal efficiency and emissions. To enhance the effective and clean utilization of alcohol, this paper investigated the effect of Miller cycle on the high compression ratio methanol and ethanol engines under stoichiometric combustion and lean burn. It was demonstrated that the Miller cycle with early opening of intake valve timing (EIVO) can prolong flame development and combustion duration under stoichiometric combustion, regardless of whether methanol or ethanol was used. The Miller cycle for both intake and exhaust valve timing can effectively improve indicated thermal efficiency and reduce NOx emissions. The Miller cycle with EIVO reduced the NOx emissions of methanol and ethanol by 52.49 % and 71.92 %. The combination of lean burn and Miller cycle further improve the ITE and reduce emissions. Compared with the optimal Miller valve timing under stoichiometric combustion, the ITE of methanol and ethanol increased by 2.39 % and 1.84 %, the NOx emissions were reduced to 72.17 % and 65.09 %, respectively.