What does the term "engine thermal efficiency" refer to?

The term "engine thermal efficiency" specifically refers to the measure of an engine's ability to convert the energy stored in fuel into useful work. This efficiency is expressed as a ratio or percentage that compares the useful work output of the engine to the total energy input from the fuel.

This metric is crucial for understanding how effectively an engine operates, as it indicates how much of the fuel's potential energy is transformed into mechanical energy that can be used to power a vehicle, versus how much is wasted as heat. A high thermal efficiency means that more of the fuel's energy is being utilized for work instead of being lost, which is a critical factor in engine design and performance.

The other options touch on different aspects of engine performance or characteristics but do not define thermal efficiency. For example, total energy output relates to the overall output without considering how efficiently that energy is produced. Emissions produced per mile driven focus on environmental impact rather than energy conversion efficiency. Cooling capacity is important for preventing overheating but doesn't relate to how effectively fuel energy is utilized.