The efficiency of a Carnot engine is defined as the maximum possible efficiency, calculated as 1 minus the ratio of cold temperature to hot temperature. In contrast, real engines have efficiencies that are less than or equal to this Carnot limit. This distinction is central to Carnot's theorem, which states that no engine operating between two heat reservoirs can be more efficient than a Carnot engine. Since truly reversible heat engines do not exist, the Carnot engine serves as an ideal benchmark for efficiency. Understanding this concept is crucial for evaluating the performance of various heat engines.