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A pressure-volume diagram plots a thermodynamic process in the plane of pressure $p$ and volume $V$. For quasistatic boundary work, work done by the system is $W=\int pdV$. Compression and expansion have opposite signs under this convention.

For a closed cycle, the oriented enclosed area equals the net boundary work. A clockwise loop gives positive net work by the system. If the process is strongly irreversible, the external pressure at the moving boundary, rather than an assumed uniform equilibrium system pressure, must be used to calculate the actual work.

A steam engine is a heat engine that converts part of the energy carried by steam into mechanical work. In a reciprocating design, expanding steam pushes a piston; valves admit and exhaust the working fluid so that the motion can repeat. A crank can convert the reciprocation into rotation. A steam turbine instead extracts work from a continuously flowing stream.

The boiler supplies heat to water, but the engine's useful output depends on pressure changes during expansion and exhaust. For an idealized quasi-static piston, boundary work is $W=\int pdV$. The net work over a cycle is the signed enclosed area on a pressure-volume diagram. Some input heat must be rejected: turning boiler heating into motion does not circumvent the Second law of thermodynamics.

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.