SUPERCHARGING

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Supercharging:

When the piston of a normal oil engine is beginning the compression stroke, the cylinder is full of air at atmospheric pressure. If means are adopted to cause the pressure at this point of the cycle to be greater than that of the atmosphere, the engine is said to be supercharged or pressure-charged.

Thus supercharging is the process of putting a higher pressure of the air charge in the cylinder at the beginning of the compression stroke. The purpose of supercharging is to increase the power which an engine of given displacement and speed can develop.

In a diesel engine burning of fuel develops the power. If it is desired to increase the power, more fuel must be burnt and therefore more air must be available, since each kilogram of fuel requires a certain amount of air, according to air fuel ratio. Other conditions being same, a given volume or space will hold a greater weight of air if:

1. Air charge density is increased. Pressure charging and air charge cooling do this.
2. Air charge pressure is increased. Pressure charging does this.

As the power of engine gets a boost by pressure charging, it is also termed as “boosting”.

Difference in scavenging and supercharging:

The scavenging process in 2-stroke engines uses a swish of air to push the spent gases and replace them with fresh air at approximately atmospheric pressure. But, Supercharging, whether in two-cycle or four -cycle engines, goes a step further and packs the cylinder with still more fresh air.

Advantages of Supercharging:

1. Higher power output for the same cylinder size, due to higher break mean effective pressure.
2. Less specific fuel consumption due to better combustion efficiency obtained by increasing air turbulence.
3. Better Mechanical efficiency because output increase is more than mechanical losses.
4. Less weight/power ratio.
5. Less space/power.
6. Less exhaust temperatures for the same power

Disadvantages:

1. Higher initial cost.
2. Higher firing and compression pressures. Hence better and stronger material is required for engine components.
3. Higher maximum temperatures, which again require better material.

Superchargers:

Supercharge air is provided either by positive displacement rotary blowers or by centrifugal blowers or by reciprocating piston pumps (not used now a days). These may be driven by:

1. By the engine itself.
2. From a separate power source (electric motor).
3. From an exhaust-gas turbine.

Rotary Blower:

Commonly used is “Roots” type. It consists of an oblong housing with flat end plates, of which two accurately machined rotors called “Impellers”, rotate in opposite directions. The impellers are mounted on two parallel shafts and have extending knobs, called “lobes”, which mesh closely with each other (like gear teeth) when they rotate. The impellers carry two or three lobes each. The blower shown in the following figure has three lobes. As the rotors turn, the entering air is first trapped between the housing and the valley between the two adjacent lobes and then carried to the outlet port where it is forced out at sufficient pressure to overcome that in the engine air receiver.  The two-lobed impeller gives maximum capacity from a given size of housing and also consumes the least amount of power.  However, its delivery pulsates (throbs) and it tends to be noisy. For these reasons, three lobes are often used, and also lobes of spiral shape are more efficient and quiet.