What is the main function of a heat engine?

The main function of a heat engine is to convert thermal energy into mechanical work. This process is central to how heat engines operate, as they take in thermal energy from a high-temperature source (such as fuel combustion or steam from a boiler) and then perform work as a result of the temperature difference between the heat source and the environment. In essence, the engine harnesses the energy provided by heat and expels some of it to a lower temperature reservoir (the cold sink), allowing the remaining energy to be converted into work that can be used to power machines and vehicles.

This conversion is often demonstrated by the efficiency of the engine, which is determined by the temperatures of the hot and cold reservoirs, as described by the second law of thermodynamics. The objective of most heat engines is to maximize the work output while minimizing waste heat, which inherently requires an input of thermal energy that is transformed into useful work.