The thermal efficiency of a heat engine is given by the formula :
Thermal efficiency of an engine = Heat converted into work / Total heat taken in.
The efficiency of a heat engine can be 100% only if all the heat energy gets converted into work (or mechanical energy). The impossibility of converting heat energy completely into mechanical energy, limits the efficiency of heat engines. Since heat energy can never be transformed completely into mechanical energy, so the efficiency of a heat engine can never be 100 per cent. Early heat engines could convert only about 10% of the heat energy into mechanical energy, so they had 10% efficiency. The modern engines built with improved designs can convert about 40% of heat energy into mechanical energy, so they have 40% efficiency. Even if the design of heat engines is improved further, it will never be possible to convert all the heat supplied to an engine into mechanical energy or work. In other words, it will never be possible to design a heat engine having 100 per cent efficiency.