Heat Engine

 In thermodynamics and engineering, a heat engine is a system that converts heat or thermal energy to mechanical energy, which can then be used to do mechanical work.^[1][2] It does this by bringing a working substance 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 transferring heat to the colder sink until it reaches a low 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, 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.

Figure 1: Heat engine diagram

In general, an engine converts energy 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.^[3] 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 reactions (such as combustion), nuclear fission, absorption of light or energetic particles, friction, dissipation and resistance. 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.

EXAMPLE

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[7] 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.

Everyday examples Edit

Everyday examples of heat engines include the thermal power station, internal combustion engine and firearms. All of these heat engines are powered by the expansion of heated gases.

Earth's heat engine Edit

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.[8]

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.[9]

Phase-change cycles Edit

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 (classical steam engine)

Regenerative cycle (steam engine more efficient than Rankine cycle)

Organic Rankine cycle (Coolant changing phase in temperature ranges of ice and hot liquid water)

Vapor to liquid cycle (Drinking bird, Injector, 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 Edit

In these cycles and engines the working fluid is always a gas (i.e., there is no phase change):

Carnot cycle (Carnot heat engine)

Ericsson cycle (Caloric Ship John Ericsson)

Stirling cycle (Stirling engine,[10] thermoacoustic devices)

Internal combustion engine (ICE):

Otto cycle (e.g. Gasoline/Petrol engine)

Diesel cycle (e.g. Diesel engine)

Atkinson cycle (Atkinson engine)

Brayton cycle or Joule cycle originally Ericsson cycle (gas turbine)

Lenoir cycle (e.g., pulse jet engine)

Miller cycle (Miller engine)

Liquid-only cycles Edit

In these cycles and engines the working fluid are always like liquid:

Stirling cycle (Malone engine)

Heat Regenerative Cyclone[11]

Electron cycles Edit

Johnson thermoelectric energy converter

Thermoelectric (Peltier–Seebeck effect)

Thermogalvanic cell

Thermionic emission

Thermotunnel cooling

Magnetic cycles Edit

Thermo-magnetic motor (Tesla)

Cycles used for refrigeration Edit

Main article: 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 cryocooler

Vapor-compression refrigeration

Vuilleumier cycle

Evaporative heat engines Edit

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 Edit

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.[12] This relation transforms the Carnot's inequality into exact equality. This relation is also a Carnot cycle equality.

Heat Engine process

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).