Efficiency in IC engines- 

When using liquid fuels, only the air induced is considered. The interchange of heat through the cylinder walls affects the volumetric efficiency of the engine. Another factor that has a vital influence on the volumetric efficiency is the resistance offered by the inlet valve to the incoming charge. If the velocity at which the charge enters the cylinder is too high, there will be excessive friction at the inlet passage while if it is too low it may give insufficient turbulence. Therefore, this velocity should be kept within limits. It has been found that an admission velocity of 80 meters per second produces enough turbulence and at the same time gives a volumetric efficiency of about 73%. The average figure for the volumetric efficiency of a well-designed naturally aspirated engine is 75%.

Mechanical Efficiency- 

Indicated Thermal Efficiency- 

Break Thermal Efficiency-

It is the ratio of break or shaft work obtained to the energy supplied by the fuel-

Efficiency Ratio- 

It is the ratio of indicated thermal efficiency to the corresponding air standard efficiency.

Relative efficiency = Indicated thermal efficiency/Air standard efficiency

Sometimes, relative efficiency may also be expressed on the basis of brake thermal efficiency as :

Brake relative efficiency = Brake thermal efficiency/Air standard efficiency.

Volumetric Efficiency-

It is defined as the ratio of the actual volume reduced to the N.T.P. of the charge admitted during the suction stroke to the swept volume of the piston. In other words,

Volumetric efficiency =The volume of the charge reduced to N.T.P. admitted during suction stroke/Volume swept by the piston.