A steam engine is an external combustion engine that makes use of the potential energy that exists as pressure in steam, converting it to mechanical work. Steam engines were used in pumps, locomotive trains and steam ships, and were essential to the industrial revolution. They are still used for electrical power generation using a steam turbine.
A steam engine needs a boiler to boil water to produce steam under pressure. Any heat source can be used, but the most common is a wood or coal fire. Anything that can be burned can be used as fuel for the fire: paper, trash, used crankcase oil, ground-up corncobs, manure, natural gas, gasoline, high proof alcohol, dry grass, hay, dry weeds, etc. The steam expands and pushes against a piston or turbine, whose motion does the work to turn the wheels.

This category is about the scientific and technological aspects of Stirling and Rankine engines.
Stirling and Rankine (steam) cycle engines are both heat engines that can use heat from any source. Heat can be delivered to heat engines via means radiative (eg. solar), convective (eg. geothermal, combustion), or conductive (eg. waste heat). When used as combustion engines, Stirling and Rankine engines are external combustion engines. External combustion engines are those in which any combustion occurs outside of the expansion system.
Stirlings are among the most thermodynamically efficient types of engine extant, very close to the thermodynamic efficiency maxima for heat engines. They are also very quiet.
Stirlings combustion engines are extremely clean, producing almost no major pollutants such as NOx, unburned hydrocarbons or carbon monoxide (CO). Naturally, as do all combustion engine, they output CO2, but somewhat less per power unit, due to their higher efficiency.