Engine efficiency

Mechanical efficiency: It is the ratio of Brake power to the indicated power.
Mechanical efficiency = η_mech. = BP / IP = BP / (BP + FP)
Where IP = Indicated power
BP = Brake power
FP = Friction power
Thermal efficiency: Indicated thermal efficiency: It is the ratio of energy of indicated power to the energy supplied by the fuel.
Indicated thermal efficiency = η_it = IP (watts) / m_f.CV
Brake thermal efficiency: It is the ratio of energy of BP to the energy supplied by the fuel.
Brake thermal efficiency = η_bt = BP (watts) / m_f.CV
Where m_f = mass of fuel supplied / sec.
CV = lower calorific value of fuel.
Volumetric efficiency: The volumetric efficiency of an engine is defined as the ratio of actual air capacity to the actual air capacity.
It is also defined as the ratio of actual volume of the charge entered the combustion chamber during the suction stroke to the swept volume of the piston.
Volumetric efficiency = η_vol. = m_actual / m_ideal
Relative efficiency: It is the ratio of brake thermal efficiency to the air-standard efficiency.
η_rel. = η_bt / η_air