The various types of engine efficiencies:

Mechanical Efficiency:

It is defined as the brake power to indicated power. Mechanical Efficiency,

Volumetric Efficiency:

It is the indicator of the measure of the degree to which the engine fills to its swept volume.

It is defined as the ratio of actual mass of charge inducted during suction stroke to the mass of the charge corresponding to swept volume of the engine at atmospheric pressure and temperature.

Accordingly,

Alternately,

Thermal Efficiency:

Thermal Efficiency of an engine is the indicator of conversion of heat supplied into work energy. It is either based on I.P. or on B.P. accordingly, we have two types of thermal efficiencies:

(a) Indicated Thermal Efficiency:

(b) Brake or overall Thermal Efficiency, $η_{b}$ or $η_{o}$

Let, = Fuel consumption in kg/s

C.V. = Calorific value of fuel, kJ/kg

Indicated Thermal Efficiency,

Brake or overall Thermal Efficiency,

Relative Efficiency:

It is defined as the ratio of indicated thermal efficiency to the air standard efficiency of the theoretical cycle at the same compression ratio. Therefore,

Relative Efficiency,

Sometimes, $η_{r}$ is also calculated based on brake thermal efficiency.