Thermal efficiency definition
The percentage of the heat developed in the burning fuel charge that is actually used to develop power determines thermal efficiency. When fuel is combusted in an engine not all the fuel is converted into power. Some is lost in overcoming friction and some escapes down the tail pipe in the hot exhaust gases so this percentage is always less than 100%. Efficiency will vary according to engine design, use, etc. If an engine uses a great deal of the heat to produce power, its thermal efficiency would be high.