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[[File:heat_engine.png|heat engine|thumb]] [[File:Carnot_Efficiency.svg|Carnot Efficiency|thumb|left]] [[File:Carnot_Efficiency2.svg|Carnot Efficiency2|thumb|left]] '''Heat engine''' is a [[thermodynamics|thermodynamic]] system that converts [[heat energy]] into [[mechanical work]]. Heat engines operate by transferring energy from a warm source to a cooler sink, thereby generating work in the process. This operation is based on the principles of the [[second law of thermodynamics]], which states that heat energy can only flow spontaneously from a hotter to a cooler body. The efficiency of a heat engine is determined by how well it converts the input heat into useful work.

==Types of Heat Engines==
Heat engines can be broadly classified into two categories: [[external combustion engine|external combustion engines]] and [[internal combustion engine|internal combustion engines]].

===External Combustion Engine===
In an external combustion engine, the combustion of fuel occurs outside the engine. The [[steam engine]] is a classic example, where water is heated in a boiler to produce steam, which then expands to do work, such as turning a turbine or moving a piston. Other examples include the [[Stirling engine]] and the [[steam turbine]].

===Internal Combustion Engine===
Internal combustion engines combust fuel within the engine itself. The most common examples are the [[petrol engine]] and the [[diesel engine]], which power most of the world's cars, trucks, and motorcycles. These engines operate on cycles such as the [[Otto cycle]], [[Diesel cycle]], and [[Atkinson cycle]], converting the energy released from fuel directly into mechanical work.

==Thermodynamic Cycles==
The operation of heat engines is often described in terms of thermodynamic cycles, which are sequences of processes that involve heat transfer, work, and changes in the state of a working fluid. Some of the most important cycles include:

* [[Carnot cycle]]: The most efficient cycle possible for a heat engine operating between two temperatures, serving as an ideal benchmark.
* [[Otto cycle]]: The ideal cycle for spark-ignition internal combustion engines.
* [[Diesel cycle]]: The ideal cycle for compression-ignition engines.
* [[Rankine cycle]]: A cycle used by steam turbines in power plants.
* [[Brayton cycle]]: The cycle on which all jet engines and gas turbines operate.

==Efficiency==
The [[efficiency]] of a heat engine is a measure of how much of the input heat is converted into useful work. The maximum efficiency a heat engine can achieve is given by the [[Carnot efficiency]], which depends only on the temperatures of the heat source and the heat sink. In reality, all practical heat engines operate at efficiencies below the Carnot limit due to various losses, including friction, heat loss, and irreversibilities in the process.

==Applications==
Heat engines find applications in a wide range of fields, from powering vehicles and generating electricity to providing propulsion for ships and airplanes. The design and optimization of heat engines are crucial for improving energy efficiency and reducing the environmental impact of energy use.

==Environmental Impact==
The operation of heat engines, especially those that burn fossil fuels, contributes to air pollution and the emission of greenhouse gases, which are major contributors to [[global warming]] and [[climate change]]. Efforts to reduce these impacts include the development of more efficient engines, the use of cleaner fuels, and the exploration of alternative energy sources.

[[Category:Thermodynamics]]
[[Category:Energy conversion]]
[[Category:Engines]]

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Heat engine - Revision history

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