What kind of conversion must it perform to a machine to be a heat engine?

A heat engine must convert thermal energy (heat) into mechanical energy (work) by exploiting temperature differences between a heat source and a heat sink. It absorbs heat from the high-temperature source, performs work, and then expels some residual heat to the lower-temperature sink. This process often follows a cyclic operation, such as in the case of the Carnot cycle, to maximize efficiency.