B.4.11 (HL)—Heat engine cycles

A cyclic engine

A heat engine uses a working substance that undergoes a sequence of thermodynamic processes and returns to its initial state. Over one complete cycle, $$\Delta U=0$$.

Net work from the PV loop

The net work done by the gas during a cycle is the signed area enclosed by the path on a P–V diagram. A clockwise cycle gives positive net work by the gas; a counterclockwise cycle gives net work on the gas.

Energy bookkeeping

The engine absorbs energy from a hot reservoir, converts part of it to useful work, and rejects the remainder to a colder reservoir. The first law applies to every stage and to the full cycle.

Common trap

Returning to the initial state means zero net change in internal energy, not zero work or zero thermal energy transfer during the cycle.