A term mainly used to describe engines. It is the ratio (usually given as a percentage) of outputted work to total work produced by the engine. An engineer would describe mechanical efficiency as the ratio of brake horsepower to indicated horsepower.

In theory, mechanical efficiency would be 100%; all the work produced by the engine would be available for use. However, losses through engine friction and inertia of the reciprocating parts lower mechanical efficiency.

Load is the biggest factor affecting mechanical efficiency. The lower the load, the lower the mechanical efficiency. The reason for this is that the friction and inertia losses of an engine more or less stay the same, but the work produced by the engine is less as the load decreases. For example, if an engine produced 100 indicated hp, and friction and inertia created a 10 hp parasitic loss, the brake horsepower would be 90 hp; a 90% mechanical efficiency. If the engine is running at half load, that is, 50 indicated horsepower, the friction and inertia loss would still be about 10 hp, leaving 40 brake hp, an 80% mechanical efficiency.

A formula estimating the mechanical efficiency of a 4-stroke Otto cycle engine based on percentage load is shown below:

Efficiency = Load% / (Load% / F)

Where F is equal to 100 minus the mechanical efficiency% at full load (usually 75-80%). Note here that percentage load is the percentage load for that RPM level. So a certain amount of horsepower being produced at 6000 RPM would only have around half the load percentage of the same amount of horsepower being produced at 3000 RPM, assuming the torque curve is more or less, flat.