For thermal engines this is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work.
Brake Thermal Efficiency
The ratio Brake Power to Heat of Fuel.
The mechanical efficiency takes into account friction losses in the bearings, piston sliding losses and valvetrain losses. The ratio of Brake Power to Indicated Power is the mechanical efficiency.
Power Lost to Friction
This can be calculated from the difference between Indicated Power and Brake Power.
Subjects: Mechanical Engineering