Heat Engine
Definition
Any device that turns heat flowing from hot to cold into useful work — from steam engines to car engines to power plants.
Symbol: · Unit: unitless (efficiency); work output in joules
Technical definition
A cyclic device operating between a hot reservoir (T_h) and a cold reservoir (T_c), converting heat input Q_h into work W with efficiency η = W/Q_h = 1 − Q_c/Q_h. Carnot proved no engine operating between the same reservoirs can beat 1 − T_c/T_h.
Example
A coal power plant burns fuel to make 800 K steam, dumps waste heat into 300 K cooling water, and converts about 40% of the heat into electricity — the rest is rejected as required by the second law, not by bad engineering.
No heat engine can be 100% efficient — ever. This is not an engineering challenge waiting for better materials; the second law forbids it. Some heat must always be dumped to the cold reservoir, which is why power plants need cooling towers and car engines need radiators. Anyone promising a 100%-efficient engine is selling perpetual motion.