How does a heat engine turn heat into work?

A piston can move when a hot gas expands against it, transferring energy into mechanical work. To repeat that push, a heat engine must return its working material to a starting state. Operating between hotter and cooler surroundings makes the return process part of the explanation, alongside the power-producing expansion.

Consider a gas cycle that expands while hot and is later compressed while cooler. At the same volume, cooler gas generally exerts less pressure, so returning it can require less work than the hot expansion delivered. The difference can leave net work at the shaft. Actual engines use different cycles, but all must include their return processes when counting output.

After a complete cycle, the working material has its original internal state and therefore no net increase in stored internal energy. Conservation gives net work = heat entering − heat leaving. In the illustration, 100 joules enter, 60 leave for the cooler side, and 40 leave as net work. Efficiency is 40/100, or 40 percent.

Heat rejection is more than a defect that perfect engineering could remove entirely. A complete cycle restores not only the working material’s energy but also its entropy, a property related to how energy is distributed among microscopic arrangements. Heat input brings entropy into the engine; shaft work does not carry it away. Releasing heat to a cooler reservoir provides an outlet. The second law forbids cyclic conversion of heat from one reservoir entirely into work with no other effect.

The 100–60–40 energy account balances one complete cycle, but the reservoir temperatures also restrict achievable efficiency. Since those temperatures are unspecified, the account cannot prove that any chosen reservoirs could deliver it. The piston and shaft identify energy-transfer roles; the essential repeating engine includes the return journey as well as the outward push.