What type of device converts heat into mechanical work?

A heat engine is a device that converts heat into mechanical work.

A heat engine operates by taking energy from a hot reservoir, converting part of that energy into useful work, and rejecting the remainder to a colder reservoir. The working substance—often a gas, vapor, or liquid—undergoes a sequence of compression, expansion, heating, and cooling steps. In a repeating cycle, the net result is work output driven by a temperature difference.

Heat engines became practically important during the Industrial Revolution, especially through steam engines. Modern examples include internal-combustion engines, steam turbines, and gas turbines. A heat engine is not the same as a heat pump: a heat pump uses supplied work to move heat from a colder region to a warmer one, effectively running the process in reverse. No heat engine can convert all incoming heat into work in a cycle; the second law limits efficiency, with the ideal Carnot limit depending on the temperatures of the hot and cold reservoirs.