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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.