In
thermodynamics
Thermodynamics is a branch of physics that deals with heat, 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 the four laws o ...
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
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
to
mechanical energy
In physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical energy is ...
, which can then be used to do
mechanical work
In physics, work is the energy transferred to or from an object via the application of force along a displacement. In its simplest form, for a constant force aligned with the direction of motion, the work equals the product of the force stre ...
. It does this by bringing a
working substance
For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinder ...
from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the
working body of the engine while
transferring heat to the colder
sink
A sink is a bowl-shaped plumbing fixture for washing hands, dishwashing, and other purposes. Sinks have a tap (faucet) that supply hot and cold water and may include a spray feature to be used for faster rinsing. They also include a drain ...
until it reaches a lower temperature state. During this process some of the thermal energy is converted into
work 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 of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat capacity ...
, 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
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...
is any
machine that converts
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
to mechanical
work. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by
Carnot's theorem Carnot's theorem or Carnot's principle may refer to:
In geometry:
*Carnot's theorem (inradius, circumradius), describing a property of the incircle and the circumcircle of a triangle
*Carnot's theorem (conics), describing a relation between triangl ...
. 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
In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...
s (such as combustion),
nuclear fission
Nuclear fission is a nuclear reaction, reaction in which the atomic nucleus, nucleus of an atom splits into two or more smaller atomic nucleus, nuclei. The fission process often produces gamma ray, gamma photons, and releases a very large ...
,
absorption of light or energetic particles,
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding (motion), sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative la ...
,
dissipation
In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form ...
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
** ''T ...
. 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, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
, heat engines are often modeled using a standard engineering model such as the
Otto cycle
An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines.
The Otto cycle is a description of what ha ...
. The theoretical model can be refined and augmented with actual data from an operating engine, using tools such as an
indicator diagram
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 in the cylinder versus the volume swept by the ...
. 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 performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
For a ...
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, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ph ...
, 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 a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
.
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 and produce useful work, usually in the form of electricity. OTEC can operate with a very h ...
(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 to generate electricity. Worldwide, there are about 8,500 coal-fired power stations totaling over 2,000 gigawatts capacity. They generate about a th ...
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 tu ...
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
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. 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 type of power station in which heat energy is converted to electrical energy. In a steam-generating cycle heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a st ...
,
internal combustion engine
An internal combustion engine (ICE or IC engine) 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 co ...
,
firearms ,
refrigerators and
heat pump
A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
s. 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 and temperature of a sample of gas could be obtained which would hold to approximation for all gases.
Boyl ...
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 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 fluids 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 describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sou ...
(classical
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
)
*
Regenerative cycle (
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
more efficient than
Rankine cycle
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sou ...
)
*
Organic Rankine cycle
In thermal engineering, the Organic Rankine Cycle (ORC) is a type of thermodynamic cycle. It is a variation of the Rankine cycle named for its use of an organic, high- molecular-mass fluid whose vaporization temperature is lower than that of w ...
(Coolant changing phase in temperature ranges of ice and hot liquid water)
*Vapor to liquid cycle (
Drinking bird,
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 entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...
,
Minto wheel)
*Liquid to solid cycle (
Frost heaving – water changing from ice to liquid and back again can lift rock up to 60 cm.)
*Solid to gas cycle (
firearms – 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 cycle
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
(
Carnot heat engine)
*
Ericsson cycle (Caloric Ship John Ericsson)
*
Stirling cycle (
Stirling engine
A Stirling engine is a heat engine that is operated by the cyclic compression and expansion of air or other gas (the '' working fluid'') between different temperatures, resulting in a net conversion of heat energy to mechanical work.
More spe ...
,
thermoacoustic devices)
*
Internal combustion engine
An internal combustion engine (ICE or IC engine) 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 co ...
(ICE):
**
Otto cycle
An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines.
The Otto cycle is a description of what ha ...
(e.g.
Gasoline/Petrol engine)
**
Diesel cycle (e.g.
Diesel engine
The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-ca ...
)
**
Atkinson cycle
The Atkinson-cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power density.
A variation of this approach is used in some modern au ...
(Atkinson engine)
**
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas ...
or
Joule cycle originally
Ericsson cycle (
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
)
**
Lenoir cycle (e.g.,
pulse jet engine
300px, Diagram of a pulsejet
A pulsejet engine (or pulse jet) is a type of jet engine in which combustion occurs in pulses. A pulsejet engine can be made with few or no moving parts, and is capable of running statically (i.e. it does not need ...
)
**
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 A Johnson thermoelectric energy converter or JTEC is a type of solid-state heat engine that uses the electrochemical oxidation and reduction of hydrogen in a two-cell, thermal cycle that approximates the Ericsson cycle. It is under investigation ...
*Thermoelectric (
Peltier–Seebeck effect)
*
Thermogalvanic cell
*
Thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
*
Thermotunnel cooling
Magnetic cycles
*
Thermo-magnetic motor (Tesla)
Cycles used for refrigeration
A domestic
refrigerator is an example of a
heat pump
A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
: 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 cryocooler
*
Vapor-compression refrigeration
*
Vuilleumier cycle
The Vuilleumier cycle was patented by a Swiss-American engineer named Rudolph Vuilleumier in 1918. The purpose of Vuilleumier's machine was to create a heat pump that would use heat at high temperature as energy input. The Vuilleumier cycle...util ...
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, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
, after a completed cycle:
[.]
:
:and therefore
:
:where
:
is the net work extracted from the engine in one cycle. (It is negative, in the
IUPAC convention, since work is ''done by'' the engine.)
:
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.)
:
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
, for instance from
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combust ...
of a fuel, so the engine efficiency is reasonably defined as
:
The efficiency is less than 100% because of the waste heat
unavoidably lost to the cold sink (and corresponding compression work put in) during the required recompression at the cold temperature before the
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, although other engines using different cycles can also attain maximum efficiency. Mathematically, after a full cycle, the overall change of entropy is zero:
Note that
is positive because isothermal expansion in the power stroke increases the
multiplicity of the working fluid while
is negative since recompression decreases the multiplicity. If the engine is ideal and runs
reversibly,
and
, and thus
[.]
,
which gives
and thus the Carnot limit for heat-engine efficiency,
:
where
is the
absolute temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
of the hot source and
that of the cold sink, usually measured in
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ph ...
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
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
. Since, by the
second law of thermodynamics
The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unles ...
, 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
and
are allowed to be different from the temperatures of the substance going through the reversible Carnot cycle:
and
. The heat transfers between the reservoirs and the substance are considered as conductive (and irreversible) in the form
. In this case, a tradeoff has to be made between power output and efficiency. If the engine is operated very slowly, the heat flux is low,
and the classical Carnot result is found
:
,
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
:
(Note: ''T'' in units of
K or
°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 continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
s. Unfortunately, physical limits (such as the melting point of the materials used to build the engine) and environmental concerns regarding
NOx production (if the heat source is combustion with ambient air) 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 NO
x 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
The Kalina cycle, developed by Alexander Kalina, is a thermodynamic process for converting thermal energy into usable mechanical power.
It uses a solution of 2 fluids with different boiling points for its working fluid. Since the solution boils ...
, which uses a 70/30 mix of
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
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 properties
A physical property is any property that is measurable, whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its changes between momentary states. Physical properties are ...
of the working fluid. The most common such exploitation is the use of water above the critical point (
supercritical water
Supercritical water oxidation (SCWO) is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantag ...
). The behavior of fluids above their
critical point changes radically, and with materials such as water and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
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
supercritical CO2.
SO2 and
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
have also been considered for such applications. Downsides include issues of corrosion and erosion, the different chemical behavior above and below the critical point, the needed high pressures and – in the case of sulfur dioxide and to a lesser extent carbon dioxide – toxicity. Among the mentioned compounds xenon is least suitable for use in a nuclear reactor due to the high
neutron absorption
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons ...
cross section of almost all
isotopes of xenon, whereas carbon dioxide and water can also double as a
neutron moderator for a thermal spectrum reactor.
#Exploit the
chemical properties
A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity.William L. Masterton, Cecile N. Hurley, ...
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
Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...
(NO
2), a toxic component of smog, which has a natural
dimer as di-nitrogen tetraoxide (N
2O
4). At low temperature, the N
2O
4 is compressed and then heated. The increasing temperature causes each N
2O
4 to break apart into two NO
2 molecules. This lowers the molecular weight of the working fluid, which drastically increases the efficiency of the cycle. Once the NO
2 has expanded through the turbine, it is cooled by the
heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, t ...
, which makes it recombine into N
2O
4. This is then fed back by the compressor for another cycle. Such species as
aluminium bromide (Al
2Br
6), NOCl, and Ga
2I
6 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 (at constant temperature, maintained with heat added or removed from a heat source or sink)
*
isobaric (at constant pressure)
*
isometric/isochoric (at constant volume), also referred to as iso-volumetric
*
adiabatic (no heat is added or removed from the system during adiabatic process)
*
isentropic (reversible adiabatic process, no heat is added or removed during isentropic process)
See also
*
Einstein refrigerator
*
Heat pump
A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
*
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...
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