What is the primary indicator of the effectiveness of a heat engine?

The primary indicator of the effectiveness of a heat engine is thermal efficiency, which measures how well the engine converts the energy input into useful work. Thermal efficiency is defined as the ratio of the work output of the engine to the heat input from the fuel source. The higher the thermal efficiency, the more effectively the engine is converting energy into work, which is a primary objective in engine design and operation.

In practical terms, thermal efficiency can be influenced by factors such as temperature differences between the heat source and sink, the design of the engine, and the properties of the working fluid. By focusing on thermal efficiency, engineers can evaluate and compare different engines and optimize them for better performance.

While total energy input can indicate how much energy is available, it does not reflect how much of that energy is successfully converted into usable work. Mechanical output force and the distance the engine travels could also be relevant metrics, but they do not provide a direct measure of how efficiently energy is being transformed from input (heat) to output (work). Thus, thermal efficiency remains the most comprehensive indicator of a heat engine's performance.