Internal combustion engine efficiency

We expressing engine performance in terms of power, it is also essential to express in terms of efficiency. 
Various efficiency are listed below : 

• Air-standard efficiency
• Brake thermal efficiency
• Indicated thermal efficiency
• Mechanical efficiency
• Relative efficiency
• Volumetric efficiency
• Scavenging efficiency
• Charge efficiency
• Combustion efficiency

we can discuss various engine efficiency one by one.

• Air-standard Efficiency :

The air-standard efficiency is also known as thermodynamic efficiency. It is mainly a function of compression ratio and other parameters.

• Brake thermal Efficiency :

The brake thermal efficiency is based on brake power of the engine. These efficiency give an idea of the output generated by the engine with respect to heat supplied in form of fuel.

• Indicated thermal Efficiency :

The indicated thermal efficiency is based on indicated power of the engine.
In modern engine an it is almost 28% obtained with gas and gasoline spark-ignition engines having a moderate compression ratio. 

• Mechanical Efficiency :

Mechanical efficiency takes into account the mechanical losses in an engine.
There is various mechanical losses in the engine :
Friction losses as in case of pistons, bearing, gears, valve mechanisms etc.
Power is absorbed by engine such as fuel pump, lubricating oil pump, water circulating pump, radiator and distributor etc.
Ventilating action of the flywheel. 
Charging in cylinder with fresh charge and discharging the exhaust gases during the exhaust stroke.
In general, mechanical efficiency of engines varies from 65 to 85%.

• Relative Efficiency :

The relative efficiency is the ratio of actual efficiency obtained from an engine to the theoretical efficiency of the engine cycle.

Relative efficiency = Actual brake thermal efficiency / Air-standard efficiency

• Volumetric Efficiency :

Volumetric efficiency is defined as the ratio of the actual mass of air drawn into the engine during a given period of time to the theoretical mass which should have been drawn in during that same period of time.

n_v = ṁ_act / ṁ_th 

• Scavenging Efficiency :

In case of two-stroke engines scavenging efficiency is defined as the ratio of the amount of air or gas-air mixture, which remains in the cylinder, at the actual beginning of the compression to the product of the total volume and air density of the inlet.

• Charge Efficiency :

The charge efficiency shows how well the piston displacement of a four-stroke engine is utilized. Various factor affecting the charge efficiency.

• Combustion Efficiency :

Combustion efficiency is the ratio of heat liberated to the theoretical heat in the fuel.
For a well adjusted engine it is varies form 92% to 97%.