The internal combustion engine is a spark-ignition engine or heat engine where the combustion of a fuel occurs inside the cylinder with the air. The efficiency of an Internal Combustion engine is a ratio between work done and energy supplied to an engine.

The efficiency of an engine are:

Mechanical Efficiency. This efficiency is defined as the ratio of brake power to the indicated power that is obtained at the crankshaft.

Mechanical efficiency takes in the mechanical losses in an engine.

The brake power is the power which is already available in the crankshaft. Where the indicated power is considered as the total power.

This power is available from the expansion of the gases in the cylinder without taking any kind of friction loss in account.

Brake Power is always less than the Indicated power.

Overall Efficiency. It is the ratio of the work received during the same time to the energy supplied by the fuel in the crankshaft at a given time.

BP  is brake power in kW, mf is mass of fuel consumed in kg per hour.

And C is the calorific value of fuel in kJ per kg of fuel.

Indicated Thermal Efficiency. The indicated thermal efficiency provides a concept of the power generated inside the cylinder with relation to the heat equipped as fuel.

This is also known as fuel consumption per I.P hour

Brake Thermal Efficiency. Brake thermal efficiency is underlined as the handling of thermal input from fuel as the brake power of an engine, no matter it is used engine or new. It is used to check how good an engine converts heat from fuel to mechanical energy.

This is also known as fuel consumption per BP hour:

Air Standard Efficiency. Air Standard Efficiency is defined as the ratio between work done to the heat supplied during the otto cycle.

Volumetric Efficiency. This is the theoretical maximum amount of air that can be drawn inside compared to the actual amount of air-fuel mixture drawn in an engine.