heat engine

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In
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by ...

and
engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

, a heat engine is a system that converts heat to
mechanical energy In physical sciences, mechanical energy is the sum of potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other fact ...
, which can then be used to do
mechanical work In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throug ...

. It does this by bringing a
working substance For fluid power Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water Water is an Inorganic compound, inorgani ...
from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the working body of the engine while to the colder
sink A sink – also known by other names including sinker, washbowl, hand basin, wash basin, and simply basin – is a bowl-shaped plumbing fixture used for washing hands, dishwashing, and other purposes. Sinks have taps (faucets) that supply hot ...
until it reaches a low temperature state. During this process some of the thermal energy is converted into
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking * Work (physics), the product of ...

by exploiting the properties of the working substance. The working substance can be any system with a non-zero
heat capacity Heat capacity or thermal capacity is a physical property A physical property is any property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or with something, whether as an attribute or as a compon ...
, but it usually is a gas or liquid. During this process, some heat is normally lost to the surroundings and is not converted to work. Also, some energy is unusable because of friction and drag. In general, an
engine File:Jet engine.svg, 450px, Jet engine uses heat of combustion to generate a high-velocity exhaust as a form of reaction engine. Mechanical energy to power the aircraft's electrical and hydraulic systems can be taken from the turbine shaft, but th ...

converts energy to mechanical
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking * Work (physics), the product of ...

. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by Carnot's theorem. Although this efficiency limitation can be a drawback, an advantage of heat engines is that most forms of energy can be easily converted to heat by processes like
exothermic reaction reaction is famously exothermic. The reduction of iron(III) oxide by aluminium Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the Uni ...
s (such as combustion),
nuclear fission Nuclear fission is a reaction Reaction may refer to a response (disambiguation), response to an action, event, or exposure. Examples: *Adverse drug reaction *Allergy, Allergic reaction *Chemical reaction *Chain reaction (disambiguation) *Comment ...

,
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
of light or energetic particles,
friction Friction is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related en ...

,
dissipation In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed ...
and
resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''Th ...
. Since the heat source that supplies thermal energy to the engine can thus be powered by virtually any kind of energy, heat engines cover a wide range of applications. Heat engines are often confused with the cycles they attempt to implement. Typically, the term "engine" is used for a physical device and "cycle" for the models.

Overview

In
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by ...

, heat engines are often modeled using a standard engineering model such as the
Otto cycle An Otto cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and oth ...
. The theoretical model can be refined and augmented with actual data from an operating engine, using tools such as an
indicator diagram Richard's indicator instrument of 1875 An indicator diagram is a chart used to measure the thermal, or cylinder, performance of reciprocating steam and internal combustion engines and compressors. An indicator chart records the pressure Pr ...

. Since very few actual implementations of heat engines exactly match their underlying thermodynamic cycles, one could say that a thermodynamic cycle is an ideal case of a mechanical engine. In any case, fully understanding an engine and its efficiency requires a good understanding of the (possibly simplified or idealised) theoretical model, the practical nuances of an actual mechanical engine and the discrepancies between the two. In general terms, the larger the difference in temperature between the hot source and the cold sink, the larger is the potential
thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a Dimensionless quantity, dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, Furnace (house heatin ...
of the cycle. On Earth, the cold side of any heat engine is limited to being close to the ambient temperature of the environment, or not much lower than 300
kelvin The kelvin is the base unit of temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, ...

, so most efforts to improve the thermodynamic efficiencies of various heat engines focus on increasing the temperature of the source, within material limits. The maximum theoretical efficiency of a heat engine (which no engine ever attains) is equal to the temperature difference between the hot and cold ends divided by the temperature at the hot end, each expressed in
absolute temperature Thermodynamic temperature is the measure of ''absolute temperature'' and is one of the principal parameters of thermodynamics. A thermodynamic temperature reading of zero denotes the point at which the fundamental physical property that imbues mat ...
. The efficiency of various heat engines proposed or used today has a large range: *3% (97 percent waste heat using low quality heat) for the
ocean thermal energy conversion Ocean Thermal Energy Conversion (OTEC) uses the ocean thermal gradient between cooler deep and warmer shallow or surface seawaters to run a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work ( ...
(OTEC) ocean power proposal *25% for most automotive gasoline engines *49% for a supercritical
coal-fired power station A coal-fired power station or coal power plant is a thermal power station which burns coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with varia ...
such as the Avedøre Power Station *60% for a
combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbi ...
gas turbine A gas turbine, also called a combustion turbine, is a type of Internal combustion engine#Continuous combustion, continuous and internal combustion engine. The main elements common to all gas turbine engines are: * an upstream rotating gas compress ...
The efficiency of these processes is roughly proportional to the temperature drop across them. Significant energy may be consumed by auxiliary equipment, such as pumps, which effectively reduces efficiency.

Examples

It is important to note that although some cycles have a typical combustion location (internal or external), they often can be implemented with the other. For example,
John Ericsson John Ericsson (born Johan Ericsson; July 31, 1803 – March 8, 1889) was a Swedish-American inventor. He was active in England and the United States. Ericsson collaborated on the design of the railroad steam locomotive ''Novelty'', which co ...

developed an external heated engine running on a cycle very much like the earlier
Diesel cycle The Diesel cycle is a combustion process of a reciprocating internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that i ...
. In addition, externally heated engines can often be implemented in open or closed cycles. In a closed cycle the working fluid is retained within the engine at the completion of the cycle whereas is an open cycle the working fluid is either exchanged with the environment together with the products of combustion in the case of the internal combustion engine or simply vented to the environment in the case of external combustion engines like steam engines and turbines.

Everyday examples

Everyday examples of heat engines include the
thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam turbine connected to an electric ...

,
internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, t ...

,
firearms A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see #Legal_definitions, Legal definitions). The first firearms originated in 10th-century Histor ...

,
refrigerators A refrigerator (colloquially fridge) is a home appliance consisting of a thermal insulation, thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to its external environment s ...

and
heat pumps A heat pump is a device used to warm and sometimes also cool buildings by transferring thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the ...
. Power stations are examples of heat engines run in a forward direction in which heat flows from a hot reservoir and flows into a cool reservoir to produce work as the desired product. Refrigerators, air conditioners and heat pumps are examples of heat engines that are run in reverse, i.e. they use work to take heat energy at a low temperature and raise its temperature in a more efficient way than the simple conversion of work into heat (either through friction or electrical resistance). Refrigerators remove heat from within a thermally sealed chamber at low temperature and vent waste heat at a higher temperature to the environment and heat pumps take heat from the low temperature environment and 'vent' it into a thermally sealed chamber (a house) at higher temperature. In general heat engines exploit the thermal properties associated with the expansion and compression of gases according to the
gas laws The gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between pressure, volume Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space ...
or the properties associated with phase changes between gas and liquid states.

Earth's heat engine

Earth's atmosphere and hydrosphere—Earth's heat engine—are coupled processes that constantly even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds and ocean circulation, when distributing heat around the globe. A
Hadley cell#REDIRECT Hadley cell The Hadley cell, named after George Hadley, is a global scale tropical atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with ocean circulation is the mean ...
is an example of a heat engine. It involves the rising of warm and moist air in the earth's equatorial region and the descent of colder air in the subtropics creating a thermally driven direct circulation, with consequent net production of kinetic energy.

Phase-change cycles

In these cycles and engines, the
working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force ...
s are gases and liquids. The engine converts the working fluid from a gas to a liquid, from liquid to gas, or both, generating work from the fluid expansion or compression. *
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and othe ...
(classical
steam engine from Stott Park Bobbin Mill, Cumbria, England A steam engine is a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energ ...

) * Regenerative cycle (
steam engine from Stott Park Bobbin Mill, Cumbria, England A steam engine is a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energ ...

more efficient than
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and othe ...
) *
Organic Rankine cycle The Organic Rankine Cycle (ORC) is named for its use of an Solvent, organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point The boiling point of a substance is the temperature at which the vapor pressure of a li ...
(Coolant changing phase in temperature ranges of ice and hot liquid water) *Vapor to liquid cycle (
Drinking bird Drinking birds, also known as insatiable birdies, dunking birds, drinky birds, water birds or dipping birds,
,
Injector An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is Entrainment (hydrodynamics), entrained in the jet and carried through a duct to a region of higher pres ...

, Minto wheel) *Liquid to solid cycle (
Frost heaving Frost heaving (or a frost heave) is an upwards swelling of soil Soil (often stylized as SOiL) is an American rock band that was formed in Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interact ...

— water changing from ice to liquid and back again can lift rock up to 60 cm.) *Solid to gas cycle (
firearms A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see #Legal_definitions, Legal definitions). The first firearms originated in 10th-century Histor ...

— solid propellants combust to hot gases.)

Gas-only cycles

In these cycles and engines the working fluid is always a gas (i.e., there is no phase change): * (
Carnot heat engine A Carnot heat engine is a theoretical engine that operates on the Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded by Benoît Paul Émile Clape ...
) * Ericsson cycle (Caloric Ship John Ericsson) * Stirling cycle (
Stirling engine File:Stirling Animation.gif, 136px, upBeta-type Stirling engine, with only one cylinder, hot at one end and cold at the other. A loose-fitting displacer shunts the air between the hot and cold ends of the cylinder. A power piston at the open end ...

, thermoacoustic devices) *Internal combustion engine (ICE): **
Otto cycle An Otto cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and oth ...
(e.g. Gasoline/Petrol engine) **
Diesel cycle The Diesel cycle is a combustion process of a reciprocating internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that i ...
(e.g. Diesel engine) **Atkinson cycle (Atkinson engine) **Brayton cycle or Joule cycle originally Ericsson cycle (
gas turbine A gas turbine, also called a combustion turbine, is a type of Internal combustion engine#Continuous combustion, continuous and internal combustion engine. The main elements common to all gas turbine engines are: * an upstream rotating gas compress ...
) **Lenoir cycle (e.g., pulse jet engine) **Miller cycle (Miller engine)

Liquid-only cycles

In these cycles and engines the working fluid are always like liquid: * Stirling cycle (Malone engine) *Heat Regenerative Cyclone

Electron cycles

*Johnson thermoelectric energy converter *Thermoelectric (Peltier–Seebeck effect) *Thermogalvanic cell *Thermionic emission *Thermotunnel cooling

Magnetic cycles

*Thermo-magnetic motor (Tesla)

Cycles used for refrigeration

A domestic refrigerator is an example of a heat pump: a heat engine in reverse. Work is used to create a heat differential. Many cycles can run in reverse to move heat from the cold side to the hot side, making the cold side cooler and the hot side hotter. Internal combustion engine versions of these cycles are, by their nature, not reversible. Refrigeration cycles include: *Air cycle machine *Gas-absorption refrigerator *Magnetic refrigeration *Stirling engine#Stirling cryocoolers, Stirling cryocooler *Vapor-compression refrigeration *Vuilleumier cycle

Evaporative heat engines

The Barton evaporation engine is a heat engine based on a cycle producing power and cooled moist air from the evaporation of water into hot dry air.

Mesoscopic heat engines

Mesoscopic heat engines are nanoscale devices that may serve the goal of processing heat fluxes and perform useful work at small scales. Potential applications include e.g. electric cooling devices. In such mesoscopic heat engines, work per cycle of operation fluctuates due to thermal noise. There is exact equality that relates average of exponents of work performed by any heat engine and the heat transfer from the hotter heat bath. This relation transforms the Carnot's inequality into exact equality. This relation is also a Carnot cycle equality

Efficiency

The efficiency of a heat engine relates how much useful work is output for a given amount of heat energy input. From the laws of
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by ...

, after a completed cycle:. :$W + Q = \Delta_U = 0$ :and therefore :$W = -Q = - \left(Q_c + Q_h\right)$ :where :$W = -\oint PdV$ is the net work extracted from the engine in one cycle. (It is negative, in the Work (thermodynamics), IUPAC convention, since work is ''done by'' the engine.) :$Q_h > 0$ is the heat energy taken from the high temperature heat source in the surroundings in one cycle. (It is positive since heat energy is ''added'' to the engine.) :$Q_c = -, Q_c, <0$ is the waste heat given off by the engine to the cold temperature heat sink. (It is negative since heat is ''lost'' by the engine to the sink.) In other words, a heat engine absorbs heat energy from the high temperature heat source, converting part of it to useful work and giving off the rest as waste heat to the cold temperature heat sink. In general, the efficiency of a given heat transfer process is defined by the ratio of "what is taken out" to "what is put in". (For a refrigerator or heat pump, which can be considered as a heat engine run in reverse, this is the coefficient of performance and it is ≥ 1.) In the case of an engine, one desires to extract work and has to put in heat $Q_h$, for instance from combustion of a fuel, so the engine efficiency is reasonably defined as :$\eta = \frac = \frac = 1 + \frac = 1 - \frac$ The efficiency is less than 100% because of the waste heat $Q_c<0$ unavoidably lost to the cold sink (and corresponding compression work put in) during the required recompression at the cold temperature before the Power stroke (engine), power stroke of the engine can occur again. The ''theoretical'' maximum efficiency of any heat engine depends only on the temperatures it operates between. This efficiency is usually derived using an ideal imaginary heat engine such as the
Carnot heat engine A Carnot heat engine is a theoretical engine that operates on the Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded by Benoît Paul Émile Clape ...
, although other engines using different cycles can also attain maximum efficiency. Mathematically, after a full cycle, the overall change of entropy is zero: $\ \ \ \Delta S_h + \Delta S_c = \Delta_ S = 0$ Note that $\Delta S_h$ is positive because isothermal expansion in the power stroke increases the Multiplicity (statistical mechanics), multiplicity of the working fluid while $\Delta S_c$ is negative since recompression decreases the multiplicity. If the engine is ideal and runs Reversible process (thermodynamics), reversibly, $Q_h = T_h\Delta S_h$ and $Q_c = T_c\Delta S_c$, and thus. $Q_h / T_h + Q_c / T_c = 0$, which gives $Q_c /Q_h = -T_c / T_h$ and thus the Carnot limit for heat-engine efficiency, :$\eta_\text = 1 - \frac$ where $T_h$ is the
absolute temperature Thermodynamic temperature is the measure of ''absolute temperature'' and is one of the principal parameters of thermodynamics. A thermodynamic temperature reading of zero denotes the point at which the fundamental physical property that imbues mat ...
of the hot source and $T_c$ that of the cold sink, usually measured in
kelvin The kelvin is the base unit of temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, ...

s. The reasoning behind this being the maximal efficiency goes as follows. It is first assumed that if a more efficient heat engine than a Carnot engine is possible, then it could be driven in reverse as a heat pump. Mathematical analysis can be used to show that this assumed combination would result in a net decrease in entropy. Since, by the second law of thermodynamics, this is statistically improbable to the point of exclusion, the Carnot efficiency is a theoretical upper bound on the reliable efficiency of ''any'' thermodynamic cycle. Empirically, no heat engine has ever been shown to run at a greater efficiency than a Carnot cycle heat engine. Figure 2 and Figure 3 show variations on Carnot cycle efficiency with temperature. Figure 2 indicates how efficiency changes with an increase in the heat addition temperature for a constant compressor inlet temperature. Figure 3 indicates how the efficiency changes with an increase in the heat rejection temperature for a constant turbine inlet temperature.

Endo-reversible heat-engines

By its nature, any maximally efficient Carnot cycle must operate at an infinitesimal temperature gradient; this is because any transfer of heat between two bodies of differing temperatures is irreversible, therefore the Carnot efficiency expression applies only to the infinitesimal limit. The major problem is that the objective of most heat-engines is to output power, and infinitesimal power is seldom desired. A different measure of ideal heat-engine efficiency is given by considerations of endoreversible thermodynamics, where the system is broken into reversible subsystems, but with non reversible interactions between them. A classical example is the Curzon–Ahlborn engine,F. L. Curzon, B. Ahlborn (1975). "Efficiency of a Carnot Engine at Maximum Power Output". ''Am. J. Phys.'', Vol. 43, pp. 24. very similar to a Carnot engine, but where the thermal reservoirs at temperature $T_h$ and $T_c$ are allowed to be different from the temperatures of the substance going through the reversible Carnot cycle: $T\text{'}_h$ and $T\text{'}_c$. The heat transfers between the reservoirs and the substance are considered as conductive (and irreversible) in the form $dQ_/dt = \alpha \left(T_-T\text{'}_\right)$. In this case, a tradeof has to be made between power output and efficiency. If the engine is operated very slowly, the heat flux is low, $T\approx T\text{'}$ and the classical Carnot result is found :$\eta = 1 - \frac$, but at the price of a vanishing power output. If instead one choses to operate the engine at its maximum output power, the efficiency becomes :$\eta = 1 - \sqrt$ (Note: ''T'' in units of kelvin, K or Rankine scale, °R) This model does a better job of predicting how well real-world heat-engines can do (Callen 1985, see also endoreversible thermodynamics): As shown, the Curzon–Ahlborn efficiency much more closely models that observed.

History

Heat engines have been known since antiquity but were only made into useful devices at the time of the industrial revolution in the 18th century. They continue to be developed today.

Enhancements

Engineers have studied the various heat-engine cycles to improve the amount of usable work they could extract from a given power source. The Carnot cycle limit cannot be reached with any gas-based cycle, but engineers have found at least two ways to bypass that limit and one way to get better efficiency without bending any rules: #Increase the temperature difference in the heat engine. The simplest way to do this is to increase the hot side temperature, which is the approach used in modern combined-cycle
gas turbine A gas turbine, also called a combustion turbine, is a type of Internal combustion engine#Continuous combustion, continuous and internal combustion engine. The main elements common to all gas turbine engines are: * an upstream rotating gas compress ...
s. Unfortunately, physical limits (such as the melting point of the materials used to build the engine) and environmental concerns regarding NOx, NOx production restrict the maximum temperature on workable heat-engines. Modern gas turbines run at temperatures as high as possible within the range of temperatures necessary to maintain acceptable NOx output . Another way of increasing efficiency is to lower the output temperature. One new method of doing so is to use mixed chemical working fluids, then exploit the changing behavior of the mixtures. One of the most famous is the so-called Kalina cycle, which uses a 70/30 mix of ammonia and water as its working fluid. This mixture allows the cycle to generate useful power at considerably lower temperatures than most other processes. #Exploit the physical property, physical properties of the working fluid. The most common such exploitation is the use of water above the critical point, or supercritical steam. The behavior of fluids above their critical point changes radically, and with materials such as water and carbon dioxide it is possible to exploit those changes in behavior to extract greater thermodynamic efficiency from the heat engine, even if it is using a fairly conventional Brayton or Rankine cycle. A newer and very promising material for such applications is carbon dioxide, CO2. Sulfur dioxide, SO2 and xenon have also been considered for such applications, although SO2 is toxic. #Exploit the chemical property, chemical properties of the working fluid. A fairly new and novel exploit is to use exotic working fluids with advantageous chemical properties. One such is nitrogen dioxide (NO2), a toxic component of smog, which has a natural dimer (chemistry), dimer as di-nitrogen tetraoxide (N2O4). At low temperature, the N2O4 is compressed and then heated. The increasing temperature causes each N2O4 to break apart into two NO2 molecules. This lowers the molecular weight of the working fluid, which drastically increases the efficiency of the cycle. Once the NO2 has expanded through the turbine, it is cooled by the Thermal reservoir, heat sink, which makes it recombine into N2O4. This is then fed back by the compressor for another cycle. Such species as aluminium bromide (Al2Br6), NOCl, and Ga2I6 have all been investigated for such uses. To date, their drawbacks have not warranted their use, despite the efficiency gains that can be realized.

Heat engine processes

Each process is one of the following: *isothermal process, isothermal (at constant temperature, maintained with heat added or removed from a heat source or sink) *isobaric process, isobaric (at constant pressure) *isochoric process, isometric/isochoric (at constant volume), also referred to as iso-volumetric *adiabatic process, adiabatic (no heat is added or removed from the system during adiabatic process) *isentropic process, isentropic (reversible adiabatic process, no heat is added or removed during isentropic process)

*Einstein refrigerator *Heat pump *Reciprocating engine for a general description of the mechanics of piston engines *Thermosynthesis *Timeline of heat engine technology

References

* * * {{Authority control Energy conversion Engine technology Engines Heating, ventilation, and air conditioning Thermodynamics