Theoretical and experimental investigations on energy balance on DMDF engine

•Model to analyze energy balance of DMDF mode was built.•Reduction in cooling loss of DMDF mode is the most effective factor at high loads.•Incomplete combustion loss in DMDF mode is the dominant factor for high SR at low load.•Cooling loss of DMDF mode is lower than that of D mode and even lower as SP rises.•High methanol temperature makes SR and incomplete combustion loss reduced.