SUPERCHARGER: TYPES, METHODS AND WORKING PRINCIPLE
Superchargers are basically compressors/blowers which takes air at normal ambient pressure & compresses it and forcefully pushes it into engine! Power to the compressor/ blower is transmitted from engine via the belt drive.
The addition of extra amount of air-fuel mixture into the cylinder increases the mean effective pressure of the engine. An increment in MEP makes the engine produce more power. In this way, adding a compressor to the engine makes it more efficient.
TYPES OF SUPERCHARGER
There are mainly two types of supercharger. The first one is known as positive displacement supercharger and other one is known as Dynamic supercharger. The basic difference between both of them is that the positive displacement supercharger maintains constant level of pressure at all engine speed whereas the dynamic supercharger delivers increasing pressure with increasing speed. This is basic fundamental difference between them. These superchargers further subdivided as given below.
1. POSITIVE DISPLACEMENT SUPERCHARGER:
As we discussed in early section that these superchargers deliver the same volume of charge at any engine speed or these superchargers are not depended on speed of the engine. The major types of positive displacement supercharger are root style and twin screw.
1. Root style
This design has two specially designed rotors which rotate in opposite direction (one is clockwise and other is anticlockwise) to compress the air. According to the rotor design this supercharger is further subdivided into two type: Two lob rotor, three lob, four lob rotors etc. As the rotor rotate, they trap the air by these lobs coming from suction side or inlet port and forced it towards discharge side or outlet port. The amount of air compressed is independent on the engine speed and each time this supercharger compresses the same amount of air.
Best suited with high speed engine
Pulsing airflow at low speed.
Heavy in weight.
Create lots of heat due to friction.
Back leakage at low speed.
Provide same amount of air at both low and high RPM.
2. Twin screw supercharger
As the name implies, this type of supercharger have two screws which rotate in different direction. One of the screw rotate clockwise and other one is anticlockwise direction. The working of this supercharger is same as root type. It also sucks air from one side and delivered it to outlet port. This device provide smother air flow comparatively root style.
No back leakage problem.
Provide smother air flow.
High heat generation due to friction.
Noisy in operation.
3. Vane type supercharger
A number of vanes are mounted on the drum of the supercharger. These vanes are pushed outwards via pre-compressed springs. This arrangement helps the vane to stay in contact with the inner surface of the body.
Now due to eccentric rotation, the space between two vanes is more at the inlet & less at the outlet. In this way, the quantity of air which enters at the inlet decreases it’s volume on its way to outlet. A decrease in volume results in increment of pressure of air. Thus, the mixture obtained at the outlet is at higher pressure than at the inlet.
2. DYNAMIC SUPERCHARGER:
As we discussed earlier, these type of supercharger gives increasing air pressure as increasing engine speed. The supercharging effect in this type is highly depended on the engine speed. It also subdivided into following types.
1. Centrifugal Type
As the name implies this type uses centrifugal force to compress the air. The design of this supercharger is same as the centrifugal compressor. It has a impeller which is connected with the crankshaft with the help of belt drive. When the engine rotates, it makes rotate the impeller which sucks the air from one side. The centrifugal action acts on this air which increase its kinetic energy and delivery it to a diffuser. The air enter into the diffusion have high velocity at low pressure. The diffuser converts this high speed low pressure air to low speed high pressurized air. This high pressurized air then sent to the engine.
It is small in size.
The amount of air is not fixed.
2. Pressure wave
3. Axial flow
METHODS OF SUPERCHARGING
There are various other ways to force the air which doesn’t need extra power unlike compressors. The 2 most widely applied are –
• Ram effect supercharging
Here, the inlet manifold is designed in such a way that the air gets automatically pushed into the cylinder. The air continuously tries into the cylinder but the intake valves open/close several times a second ! Every time the valve closes, the air just rams into it. This generates a pressure wave which travels in the opposite direction until it hits the plenum & gets reflected back.
Now if the resonant frequency of the plenum & engine matches, this pressure wave carries more air into the cylinder doing the work of a supercharger.
• Under piston supercharging –
This type of method is generally adopted in large marine engines. It utilizes the bottom side of the piston for compressing the air. With proper timing of valves, this system gives an adequate supply of compressed air, as there are 2 delivery strokes to each suction stroke of each stroke !
ADVANTAGES AND DISADVANTAGES OF SUPERCHARGER
Advantages of supercharging
1. Higher power output
2. Greater induction of charge mass
3. Better atomization of fuel
4. Better mixing of fuel and air
5. Better scavenging products
6. Better torque characteristics over whole range
7. Quick acceleration of vehicle
8. Complete and smooth combustion
9. Even fuel with poor ignition quality can be used
10. Improved cold starting
11. Reduced exhaust smoke
12. Reduced specific fuel consumption
13. Increased mechanical efficiency
14. Smooth operation and reduction in diesel knock tendency
Disadvantages of supercharging
1. Increased detonation tendency in SI engines
2. Increased thermal stress
3. Increased heat loss due to increased turbulence
4. Increased gas loading
5. Increased cooling requirements of the engine