The Carnot Cycle
An ideal cycle would be performed by a perfectly efficient heat engine—that is, all the heat would be converted to mechanical work. A 19th-century French scientist named Nicolas Carnot conceived a thermodynamic cycle that is the basic cycle of all heat engines. He showed that such an ideal engine cannot exist. Any heat engine must expend some fraction of its heat input as exhaust. The second law of thermodynamics places an upper limit on the efficiency of engines; that upper limit is less than 100 percent. The limiting case is now known as a Carnot cycle.

Carnot cycle diagram - RF Cafe

                                      The Carnot Cycle

(1) Isothermal expansion of gas in cylinder as heat is added from source. Piston moves from V1 to V2, doing work on the system.

Carnot cycle - isothermal expansion - RF Cafe     Isothermal expansion - RF Cafe
(2) Heat source is removed and isolated. Adiabatic expansion continues until volume V3 is reached, performing additional work, at temperature T2.

Carnot cycle - adiabatic expansion - RF Cafe    Adiabatic expansion - RF Cafe
(4) Heat source is removed and isolated. Adiabatic compression continues until volume V4 is reached, performing additional work, at temperature T2.

Carnot cycle - adibatic compression - RF Cafe    Adiabatic compression - RF Cafe
(3) Isothermal compression of gas in cylinder as work is done by the system on the piston. Heat is be transferred to T2.

Carnot cycle - isothermal compression - RF Cafe    Isothermal compression - RF Cafe
Isothermal: Process in which no temperature change occurs in a closed system. Adiabatic: Process in which no heat is transferred into or out of a closed system.