This document summarizes key concepts about heat engines: 1. Heat engines receive heat from a high-temperature source, convert some of that heat to work (e.g. rotating shaft), and reject the remaining waste heat to a low-temperature sink. 2. The Carnot cycle represents the theoretical maximum efficiency possible for a heat engine. The Carnot efficiency depends only on the high and low temperatures. 3. Practical heat engines like the Rankine, Otto, and Diesel cycles have lower efficiencies than the Carnot cycle but can extract useful work. Combined cycle gas turbines approach Carnot efficiencies.