The name given to a device that transforms disordered heat energy into ordered, useful, mechanical work. This is achieved by taking a working fluid at high temperature and high heat energy, and subjecting it to a thermodynamic cycle involving compression and expansion, during which time heat is expelled at a lower temperature. The differences in heat energy of the working fluid between input and output appear as work. For physical and theoretical understanding, the thermodynamic cycle is usually idealized, and does not take into account such factors as frictional losses. A large number of idealized thermodynamic cycles exist, of which the Carnot, Diesel, Otto and Stirling cycles are particular examples.
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In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". On Earth, the cold side of any heat engine is limited to close to the ambient temperature of the environment, or not much lower than 300 kelvins, so most efforts to improve the thermodynamic efficiencies of various heat engines focus on increasing the temperature of the source, within material limits.
The efficiency of various heat engines proposed or used today ranges from 3 percent (97 percent waste heat) for the OTEC ocean power proposal through 25 percent for most automotive engines, to 35 percent for a supercritical coal plant, to about 60 percent for a steam-cooled combined cycle gas turbine.
Everyday examples
Examples of everyday heat engines include: the steam engine, the diesel engine, and the gasoline (petrol) engine in an automobile.
Gas only cycles
In these cycles and engines the working fluid are always like gas:
Carnot cycle (Carnot heat engine) Ericsson Cycle Stirling cycle (Stirling engine, thermoacoustic devices) Internal combustion engine (ICE): Otto cycle (eg. low-speed diesel engine) Atkinson Cycle Brayton cycle or Joule cycle (gas turbine) Lenoir cycle (e.g., pulse jet engine) Miller cycleElectron cycles
Thermoelectric (Peltier-Seebeck effect) thermionic emission Thermotunnel coolingMagnetic cycles
Thermo-magnetic motor (Tesla)Cycles used for refrigeration
A refrigerator is a heat pump: a heat engine in reverse.
Vapor-compression refrigeration Stirling cryocooler Gas-absorption refrigerator Air cycle machine Vuilleumier refrigerationEfficiency
The efficiency of a heat engine relates how much useful power is output for a given amount of heat energy input. (It is negative since work is done by the engine.) dQh = ThdSh is the heat energy taken from the high temperature system .(It is negative since heat is extracted from the source, hence ( − dQh) is positive.) dQc = TcdSc is the heat energy delivered to the cold temperature system. (It is positive since heat is added 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 delivering the rest to the cold temperature heat sink. 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. 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.
Other criteria of heat engine performance
One problem with the ideal Carnot efficiency as a criterion of heat engine performance is the fact that by its nature, any maximally-efficient Carnot cycle must operate at an infinitesimal temperature gradient.
A much more accurate measure of heat engine efficiency is given by the endoreversible process, which is identical to the Carnot cycle except in that the two processes of heat transfer are not treated as reversible. As derived in Callen (1985), the efficiency for such a process is given by:
The accuracy of this model can be seen in the following table (Callen):
Efficiencies of Power Plants| Power Plant | Tc (°C) | Th (°C) | η (Carnot) | η (Endoreversible) | η (Observed) |
|---|---|---|---|---|---|
| West Thurrock (UK) coal-fired power plant | 25 | 565 | 0.64 | 0.40 | 0.36 |
| CANDU (Canada) nuclear power plant | 25 | 300 | 0.48 | 0.28 | 0.30 |
| Larderello (Italy) geothermal power plant | 80 | 250 | 0.33 | 0.178 | 0.16 |
As shown, the endoreversible efficiency much more closely models the observed data.
Heat engine enhancements
Engineers have studied the various heat engine cycles extensively in an effort to improve the amount of usable work they could extract from a given power source.
Heat engine processes
| Cycle/Process | Compression | Heat Addition | Expansion | Heat Rejection |
|---|---|---|---|---|
| Carnot | adiabatic | isothermal | adiabatic | isothermal |
| Otto (Petrol) | adiabatic | isometric | adiabatic | isometric |
| Diesel | adiabatic | isobaric | adiabatic | isometric |
| Brayton (Jet) | adiabatic | isobaric | adiabatic | isobaric |
| Stirling | isothermal | isometric | isothermal | isometric |
| Ericsson | isothermal | isobaric | isothermal | isobaric |
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) adiabatic (no heat is added or removed from the system during adiabatic process)
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