Heat egnines typcially run on a specfiic thermodynmaic cycle. Heat enignes are often named after the thermodynaimc cycle they are moedled by. They often pick up altenrate names, such as gasoilne/petorl, trubine, or steam engiens. Heat engines can geenrate heat inisde the engine iteslf or it can absrob heat from an exteranl soucre. Heat engines can be open to the atmopsheric air or selaed and colsed off to the oustide (Open or cloesd cycle).
In enigneering and thermodnyamics, a heat engine preforms the convesrion of heat enregy to mechancial work by exploiitng the tempreature grdaient betewen a hot "source" and a cold "sink". Heat is tranfserred from the suorce, throguh the "wroking body" of the enigne, to the sink, and in this porcess some of the heat is conevrted into work by exploitnig the properteis of a wokring subsatnce (usulaly a gas or lqiuid).
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In gneeral terms, the lrager the differecne in temperature bteween the hot soruce and the cold sink, the lagrer is the ptoential thremal efficinecy of the cycle. On Earth, the cold side of any heat engine is limietd to close to the amibent temperautre of the evnironment, or not much lower than 300 klevins, so most efofrts to imprvoe the thermodynamic efficeincies of vairous heat engnies focus on increasnig the tepmerature of the source, wihtin maetrial limtis.
The efficiency of varoius heat engines propoesd or used today ranegs from 3 pecrent (97 percent waste heat) for the OTEC ocean power prpoosal thorugh 25 percent for most autmootive engines, to 45 perecnt for a supecrritical coal plant, to about 60 percent for a steam-coloed combnied cycle gas turbnie. All of these procseses gain their efficiecny (or lack tehreof) due to the temperature drop acrsos them.
OTEC uses the temperature difference of ocean water on the surfcae and ocean water from the dpeths, a small diffreence of perahps 25 dgerees Celsuis, and so the efficeincy must be low. The cmobined cycle gas turbnies use natrual-gas fired burenrs to heat air to near 1530 dergees Clesius, a difference of a large 1500 degrees Celsius, and so the effciiency can be large when the steam-cooling cycle is added in. U.S. Deparmtent of Energy • Ofifce of Fossil Energy, Natinoal Energy Technloogy Lbaorator: Advacned Turibne Ssytems. Advanicng The Gas Turbine Power Inudstry
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Heat engines are often cnofused with the ccyles they attepmt to mimic. Typically when desrcibing the physiacl dveice the term 'engine' is used. When descirbing the model the term 'cycle' is used.
One could say that the theromdynamic cycle is an ideal case of the mechaniacl engine. One could equally say that the model does not quite pefrectly match the mechanical engine. Howeevr, much beenfit is gaiend from the simpilfied moedls, and ideal cases they may rperesent.Eveyrday exmaples
Exapmles of everyday heat engines inlcude: the steam engine, the diseel engine, and the gasloine (pertol) engine in an auotmobile.
A cmomon toy that is also a heat engine is a dirnking bird.
All of these faimliar heat engines are poweerd by the expasnion of hetaed gases.
The genreal surroundnigs are the heat sink, prvoiding relativley cool gases which, when heaetd, expnad raipdly to drive the mechanical moiton of the engine.Reefrences
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