 Thermodynamic cycle

A thermodynamic cycle returns a working substance to its initial thermodynamic state. Every state function, including internal energy, has zero net change over the complete cycle. With heat entering and work done by the substance taken positive, the first law of thermodynamics gives $W_{\mathrm{net}}=Q_{\mathrm{net}}$.

For a quasistatic simple compressible system, $W_{\mathrm{net}}=\oint pdV$ is the signed area enclosed in its pressure-volume diagram. Clockwise traversal gives positive net expansion work in the usual axes, while reversed traversal changes the sign. A heat engine accepts heat at higher temperature and rejects some at lower temperature; a refrigerator uses work to reverse that transfer. Returning the working substance to its initial state does not require its reservoirs or surroundings to return to theirs. A cycle can therefore produce net entropy even though the substance's net entropy change is zero.

A thermodynamic cycle is a series of processes that involve the transfer of heat and work between a system and its surroundings in a way that the system returns to its initial state at the end of the cycle. In other words, a thermodynamic cycle is a closed loop in which a working substance—typically a gas—undergoes various transformations (such as expansion, compression, heating, and cooling) that cause it to absorb and release energy.