Mechanical efficiency: It is the ratio of Brake power to the indicated power.
Mechanical efficiency = ηmech. = 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 = ηit = IP (watts) / mf.CV
Brake thermal efficiency: It is the ratio of energy of BP to the energy supplied by the fuel.
Brake thermal efficiency = ηbt = BP (watts) / mf.CV
Where mf = 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 = ηvol. = mactual / mideal
Relative efficiency: It is the ratio of brake thermal efficiency to the air-standard efficiency.
ηrel. = ηbt / ηair
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