The power of the engine can be increased by increasing the quantity of air or mixture in the cylinder at the beginning of compression stroke because it leads to the burning of more quantity of fuel. The amount of air or mixture can be increased by increasing the speed of the engine or by increasing the air density during suction stroke. If we want to increase the engine speed, it requires robust and rigid engine which leads to the higher inertia forces. The volumetric efficiency also falls with an increase in engine speed. The friction and bearing loads also increase. Therefore increasing the speed of an engine beyond particular limit is not possible. The power output can also be increased by increasing the compression ratio but it increases the maximum cylinder pressure.
At high altitudes the atmospheric pressure decreases which results in lower output of the engine so it is necessary to increase the atmospheric pressure before entering into the engine to compensate the decreased power. In order to increase the power of an engine we induct the dense air during the suction stroke. It increases the suction pressure. The process is called supercharging and the equipment used for this purpose is known as supercharger.
Purpose of supercharging:
1. To increase the power output of an engine.
2. To overcome the effect of high altitudes,
3. To reduce the size of an engine.
Types of superchargers:
1. Reciprocating compressor: They are rarely used because they are bulky and expensive. They have low thermal efficiency and speed is limited.
2. Centrifugal compressor: In centrifugal compressor the air enters axially and comes out radially. The kinetic energy of air is converted into pressure energy passing through vanes and diffuser. They are simple, small in size and having good isentropic efficiency. It can handle small as well as large quantity of air efficiently.
3. Root blower: Root blowers can be of two lobes systems or three lobes systems. The air is trapped between the impeller and casing and is carried around the rotor to discharge port. Three lobes produces less noise as compare to two lobe system. They give high mechanical efficiency. It has high volumetric efficiency.
4. Vane blower: It is a positive displacement rotary type blower. The rotor rotates in a large cylindrical casing. The rotor has four slots out of which one remains in contact with the casing at all the time. The air is trapped in between two adjacent vanes. As the rotor rotates the air volume keeps on decreasing which results in increasing the pressure. It provides air at 1.5 bars. It can handle large air volume.