Supercharging

Introduction:

It is known fact that power output of engine increases with an increases in amount of air fuel mixture in the cylinder at the beginning of compression stroke because it allows the burning of more quantity of air fuel mixture. Many times, as in aero engines, the mass of air intake decreases with increasing altitude because atmospheric pressure decreases and the effect is lower power output of the engine. It is therefore necessary to increase the pressure of the atmospheric air before entering into the cylinder to compensate the decreased power.

The amount of air inducted per unit time can be increased by increasing the engine speed or by increasing the air density during the suction stroke. The increase in engine speed requires a rigid and robust engine as the inertial load increases rapidly with increase in engine speed. The engine friction and bearing load also increases and volumetric efficiency decreases with increasing speed of the engine. Therefore, increase in speed of engine for increasing power above a particular limit of speed is not practically possible. And another method which is used to increase the suction pressure is known as supercharging and the equipment used for this purpose is known as supercharger.

The power output can also be increased by increasing the compression ratio, but this process is also not desirable as it increases the maximum cylinder pressure. The rate of increase in maximum pressure in the cycle with increased compression ratio is less than the rate of increase in BMEP in case of supercharged engine. Therefore, more power can be obtained by supercharging compared with by increasing compression ratio for the given maximum cycle pressure. In addition to this, the rate of increase in maximum temperature is also low in supercharged engine and this result in lower thermal loads.

Purpose of Supercharging:

Supercharging is a term used for a process which helps to increase the suction pressure of an IC engines above the atmospheric pressure. The main objective of the supercharging is to increase the air charge per cycle and permit the burning of larger amount of fuel and thus increase the power output of the engine.

Supercharging is commonly used in two stroke and four stroke engine as for petrol and diesel engines. Supercharging is preferred to fulfill the following requirements:

·        To overcome the effects of high altitudes, such as aircrafts engine or in stationary installations in the mountains,

·        To reduce the weight of an engine per kW power developed such as in aircraft or racing car engines,

·        To reduce the size of the engine to fit into a limited space, such as in locomotive or marine engines,

·        To increase the power output of an existing engine when the greater power demand occurs.

The power of the aircraft engine or engines used in the mountains is considerably reduced with increasing altitudes as air is rarified. This reduction in power is mainly due to lack of oxygen owing to the lower pressure of the atmosphere. Although the volume of the air drawn in the engine remains the same, its mass is greatly reduced on account of its low pressure. This deficiency of air is corrected by supercharging.

At higher altitudes, the engine power is reduced because of the following reasons;

·        Irregularities of fuel evaporation,

·        Excessive low temperature of air which cause delay in the combustion process,

·        Reduction in mass of air owing to its lower pressure.

Types of Supercharger:

Supercharging may be applied in a variety of ways as follows:

·        By independently driven compressor by electric motor,

·        By engine driven compressor,

·        By under piston supercharging,

·        By exhaust gas turbo-charging.

The first three methods require a certain amount of work to be done in compressing the charging the air. This work is to be abstracted from the engine itself assuming that the electric power is supplied to the engine at the expense of net output available to derive the external load. The third method is used for marine applications.

Different types of superchargers used in field are listed below:

·        Reciprocating compressor,

·        Centrifugal blowers,

·        Root blowers,

·        Displacement type rotary blowers,

·        Turbo chargers

Limitations of Supercharging:

The question arises in the mind of the readers, what should be the maximum or optimum pressure of the supercharged air supplied to the engine. It is obvious that the BP and bsfc both are in favor of supercharging but one cannot go on increasing the supercharged pressure as there are other limitations which limit the maximum supercharged pressure.