Define the term "mechanical efficiency" in an engine.

Mechanical efficiency in an engine refers to the relationship between the useful work produced by the engine and the total energy input. It quantitatively measures how effectively an engine converts the energy available from fuel into mechanical energy that can perform work. This efficiency is crucial because it directly impacts an engine's performance, fuel economy, and operational costs. For instance, if an engine has high mechanical efficiency, it means that most of the fuel energy is being converted into usable power, rather than being wasted as heat or other forms of energy loss.

The other options touch on various aspects of engine performance but do not accurately define mechanical efficiency. While work output to thermal energy is relevant, it does not encapsulate the entirety of mechanical efficiency. Fuel consumption rates and total energy used by the engine are related concepts but do not specifically measure the relationship of output work versus input energy, which is the essence of mechanical efficiency. Therefore, the correct definition focuses on the ratio of useful work output to total energy input.