Carnot designed a theoretical engine which is free from all the defects of a practical engine. This engine cannot be realized in actual practice; however, this can be taken as a standard against which the performance of an actual engine can be judged.
It consists of the following parts:
- A cylinder with perfectly non-conducting walls and a perfectly conducting base containing a perfect gas as working substance and fitted with a non-conducting frictionless piston (Fig. 18).
- A source of infinite thermal capacity maintained at constant higher temperature T1.
- A sink of infinite thermal capacity maintained at constant lower temperature T2.
- A perfectly non-conducting stand for the cylinder.
Efficiency of Carnot cycle
The efficiency of engine is defined as the ratio of work done to the heat supplied, i.e.,
So, efficiency of Carnot engine,
- Efficiency of a heat engine depends only on temperatures of source and sink and is independent of all other factors.
- All reversible heat engines working between same temperatures are equally efficient and no heat engine can be more efficient than Carnot engine (as it is ideal).
The efficiency of Carnotâ€™s heat engine depends only on the temperature of source (T1) and temperature of sink (T2), i.e.,
Carnot stated that no heat engine working between two given temperatures of source and sink can be more efficient than a perfectly reversible engine (Carnot engine) working between the same two temperatures. Carnotâ€™s reversible engine working between two given temperatures is considered to be the most efficient engine.