What does the term "thermal efficiency" measure?

Thermal efficiency is a critical measure in assessing how effectively a thermal system converts the heat energy from fuel into useful work. Specifically, it refers to the ratio of the useful work output produced by the system to the total energy input derived from fuel. This ratio is often expressed as a percentage, indicating how well an engine or power plant utilizes the energy it consumes.

For example, if an engine has a thermal efficiency of 30%, it means that 30% of the energy from the fuel is converted into usable mechanical work, while the remainder is lost, primarily as heat. This concept is essential in engineering and engineering applications, as it directly impacts fuel economy, the performance of engines, and overall operational cost-effectiveness.

Other options do not accurately define thermal efficiency. The output speed of an engine pertains more to its operational performance characteristics rather than how efficiently it converts energy. Fuel consumption rate is a measure of how much fuel is being used, but it does not directly inform about the efficiency of energy conversion. Similarly, temperature retention in a boiler relates to heat management but does not quantify energy output relative to input.