An Ideal (Carnot) Engine
An engine achieves maximum efficiency when it uses reversible processes.
Carnot's Principle: The efficiency of an engine using irreversible processes can be no greater than the efficiency of an engine using reversible processes that is working between the same temperatures. This is named for Sadi Carnot, a French engineer.
Carnot showed that for an ideal (or Carnot) engine, operating between temperatures Th and Tc, the efficiency is:
| ec
| = 1 - |
| Tc
|  |
| Th
|
|
The efficiency is maximized when the lower temperature is as low as possible, and the higher temperature is as high as possible.
| Comparing this to our previous result: e
| = 1 - |
| |Qc|
| |
| |Qh|
|
|
| For an ideal (Carnot) engine:
| | |Qc|
| |
| |Qh|
|
|
= |
| Tc
| |
| Th
|
|