Other meanings of External combustion engine
Engineering
An external combustion engine is a heat engine in which the working fluid is heated by combustion of fuel that occurs outside the engine, typically in a separate boiler or combustor. This contrasts with internal combustion engines, where fuel burns inside the engine cylinder. External combustion engines can operate on a wide variety of fuels, including biomass, coal, and solar thermal energy, and they often produce lower emissions of certain pollutants due to continuous combustion.
In an external combustion engine, the working fluid (gas or liquid) is heated in a separate heat exchanger, such as a boiler or a heater head, and then expanded to produce mechanical work. The combustion products do not come into contact with the moving parts of the engine, which reduces wear and allows for a wider range of fuels.1 External combustion engines are typically classified into two main types: those using a liquid working fluid that changes phase (e.g., steam engines) and those using a gaseous working fluid that remains in the gas phase (e.g., Stirling engines). The latter are often referred to as hot-air engines.
The first practical external combustion engine was the atmospheric steam engine developed by Thomas Newcomen in 1712, which was used to pump water out of coal mines.2 James Watt's improvements in the 1760s, including a separate condenser, greatly increased efficiency and led to the widespread adoption of steam power during the Industrial Revolution. In 1816, Robert Stirling patented the Stirling engine, a closed-cycle external combustion engine that operates on a fixed amount of gas and is known for its quiet operation and high efficiency potential.3 Although steam engines dominated the 19th century, Stirling engines found niche applications in the 20th century, such as in submarines and small generators.
Steam engines are the most common type, using water as the working fluid. They can be reciprocating or turbine-based, with steam turbines being the primary method of electricity generation in thermal power plants worldwide. Stirling engines are closed-cycle regenerative engines that can operate on any external heat source, including solar, geothermal, and waste heat. They are known for their high efficiency in small-scale applications and are used in some modern solar power installations. Other types include the steam turbine, which is a rotary external combustion engine, and the hot-air engine, which is an early form of the Stirling engine.
External combustion engines are used in a variety of applications today. Steam turbines generate the majority of the world's electricity in coal, nuclear, and concentrated solar power plants. Stirling engines are used in some solar thermal systems, such as the dish-Stirling systems in the Mojave Desert, and in niche applications like submarine propulsion (e.g., the Swedish Gotland-class submarines) and combined heat and power units.4 They are also used in cryocoolers for cooling infrared sensors and in some space probes, such as the Stirling radioisotope generator.
One lesser-known fact is that the first steam engine was not invented by Watt but by Hero of Alexandria in the 1st century AD, who created the aeolipile, a simple reaction turbine.5 Another is that Stirling engines were used in the 19th century to power fans in churches and small machinery, and they were also used in the early 20th century to power radios in remote areas. The term "external combustion engine" is sometimes used to include engines that use nuclear or solar heat, as the combustion is not the only heat source. Additionally, some modern research focuses on using external combustion engines for waste heat recovery in vehicles and industrial processes, which could improve overall energy efficiency.
External combustion engines are distinct from internal combustion engines in that the heat source is external to the working fluid, allowing for fuel flexibility and lower emissions.
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