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The power output of an engine depends upon the amount of air inducted per unit time.
The amount of air inducted per unit time can be increased by increasing the engine speed
or by increasing the density of intake air.
To increase the engine speed, the engine construction is required to be robust. Therefore
it has a limit.
The second alternative is to increase the density of intake air, is used to increase the
power output.
Thus, increasing the engine power by increasing the density of intake air is called as
“supercharging’. This can be done by supplying the air at a higher pressure at which the
engine naturally aspirates air from atmosphere by using a pressure-boosting device called as


The objects of supercharging are as follows:

  1. To increase the power output for a given weight and bulk of the engine, which is
    important for the aircraft and marine engines where weight and space is of
  2. As the altitude increases, the density of air decreases and normal I. C. engine when.
    operated at high altitude gives less power output due to decrease in density of intake
    air. Therefore to compensate the loss of power due to altitude of aircraft
    supercharging is done. There is 1 % loss of power per 100 m of altitude.
  3. To generate the more power from existing engine.


The effects of supercharging are as follows :

  1. A supercharged engine has air supplied to it at a pressure and density higher than
    atmosphere. This increases the volumetric efficiency.
  2. The mechanical efficiency of supercharged engine is more than that of naturally
    aspirated engine when both are running at same speed.
  3. The specific fuel consumption of a supercharged engine is less due to proper
    combustion achieved by better turbulence.
  4. For supercharged engine, chances of detonation increases due to higher pressure.


About 30 % of heat input goes with exhaust gases.
The exact percentage depends upon the type of engine and its operating conditions. This
exhaust gas can be used to run a gas turbine.
turbine develops the sufficient power to drive centrifugal compressor, which is
used to supply the air to engine. This results in increased power output and better thermal
efficiency of engine. Thus, supercharging done by driving compressor with the help of gas
turbine utilizing exhaust of engine is called as ‘turbo charging’.
The turbochargers are nothing but centrifugal compressors driven by exhaust gas of
Now-a-days turbochargers are more popular as they utilize the exhaust energy of engine
which otherwise go waste.
In order to supply sufficient energy to the turbo charger, the exhaust valvé is opened
much before the BDC as compared to naturally aspirated engine. This allows the exhaust gas
to escape at higher pressure and temperature giving the turbocharger enough energy to drive
the compressor.


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