A Heat Engine may be defined as a device that operates in a thermodynamic cycle and does a certain amount of net positive work through the transfer of heat from a high temperature body to a low temperature body. A system power plant is an example of a heat engine.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbK72JdVUt_HFV0wucst7JCVa39HCZEkMV2iU2UoYiYWDgon7RdMYPHgvVb0T1mo3ZHizORpx6wOjcP4CqQiGQC1SBJXdOX2evu_cnJEJHffJNJvJ09psivFGfiuiIuCMp2UOHUW-6L8k/s1600/Heat+Engine.jpg)
Thermal efficiency for a heat engine, ηHE is defined as:
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjToXrOH3pr4lFNGfhBvdIPB0ymnLZNfmD47mcj_gxi3eVhwiqYokexg4_aza9pMV5gisS1mCWLkDFB597f6Hlf6kP0ZVX2fsoO0PToq11Z3lrxWsMk9ZwT2zt-F3TCy0zIiKF0CCZMnpQ/s1600/Heat+Engine+efficiency.jpg)
Where,
Wcycle = Net work output
QC = Heat rejected to the low temperature reservoir
QH = Heat added (heat added) from the high temperature reservoir
It follows from Carnot's theorems that for a reversible cycle (QC/QH) is
a function of temperatures of the two reservoirs only. i.e. QC/QH
=φ (TC,TH).
If we choose the
absolute (Kelvin) temperature scale then:
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcJI7w52oaNECxuwVMHiDm6YKfZATKwUmXLfQ4MxVaxQHvsRA4-ha74asYTCYxW-Pb-cCq-22FOojeGCB7lyfAOirG3vmcT1EWRqoKYXoBxvgYNgNnHXC-WQ7QD6RnFNJ0WJnhQWOM0Ko/s1600/Heat+Engine+efficiency2.jpg)
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