What is a Centrifugal Supercharger?

Centrifugal Supercharger CutawayA centrifugal supercharger is an engine driven compressor that compresses air and expels it with a centrifugal force from a rotating wheel (impeller) with radial vane. It is used to increase the power of an internal combustion engine by increasing the amount of available oxygen. The supercharger compresses the air that is entering the engine. The compressor is driven from the crankshaft by a belt, gear or chain drive.

Like any centrifugal pump, the boost provided by the centrifugal supercharger increases with the square of the speed. This means that the centrifugal design provides less boost at low engine speeds, thus preserving fuel economy. On the other hand, the design is also the most efficient, besting designs like the Roots type supercharger and twin-screw type supercharger, which have the only minor advantage of producing boost at low engine speed.

What does this mean in a practical sense? The Positive Displacement (PD) blower will typically produce more boost at the low RPM range. This is great if you only want to do burn outs, but when it comes to high horsepower racing and or street use then a PD blower has three distinct disadvantages over the centrifugal blower design.

First lets look at the three types of efficiencies.

Volumetric efficiency. This is how efficient the supercharger creates pressure. If a supercharger has a displacement of 10 liters and 8.4 liters exits the supercharger then it has a 84% efficiency. Secondly we have Adiabatic efficiency. This is how efficient the supercharger utilized the energy that drives the supercharger. A higher adiabatic efficiency means the supercharger consumes less energy from the engine driving it. Lastly we have temperature efficiency. Temperature efficiency is the difference between the temperature entering the supercharger compared to the temperature of the air discharged from the supercharger.

If two superchargers had the same volumetric efficiency, adiabatic efficiency, and temperature efficiency then they would produce the same horsepower on a given engine. So this is where it comes down to learning about the different supercharger types and their ability to produce boost pressure.

The Advantages of a Centrifugal Supercharger.

The first advantage is that typically at the upper rpm range a centrifugal supercharger can produce more boost than a PD blower, thus at high RPM when your engine is able to suck more air into the combustion chamber it can be fed more boost than it did at low rpm.

The second advantage is that on the street when you are wanting to drive your car to get some groceries you are not constantly pushing a fixed amount of boost into your engine like a PD blower and thus requiring an increased amount of fuel to match that boost.

The third advantage is that a centrifugal supercharger unlike a PD blower does not sit on top of the engine itself thus creating a far lower temperature efficiency than with a centrifugal blower. A twin screw or roots blower will absorb a massive amount of heat from thermal transfer from the engine block and heads.

To combat the loss of temperature efficiency many PD blower manufacturers will incorporate a complicated air to water intercooler which are only good for a short amount of time before the water is heat soaked. Or many will utilize an air to air intercooler to cool the discharge air temperature. However when you start with much higher charged air temperatures then it is harder to cool it down. A centrifugal blower will have lower air discharge temperatures and cooler air will allow you to make more power.

How much horsepower does a supercharger add?

Well the answer is not so simple. Each engine is different due to the amount of additional air and fuel it can utilize. The amount of horsepower added will always be a percentage of the original. Typically you will see a 40-80% gain in horsepower with a supercharger.

An engine produces power by igniting fuel and air inside of the combustion chamber. When the fuel and air are ignited, the pressure in the combustion chamber rapidly increases, applying an expansion force onto the piston. This force is in turn applied to the crankshaft which is designed to rotate. How much power generated is mainly determined by how much fuel and air is ignited inside the chamber to produce the driving force.

There are basically two types of induction systems for internal combustion engines; naturally aspirated and forced induction engines. Naturally aspirated (N/A) engines draw in air for combustion under normal atmospheric pressure conditions. This will change with your relative altitude above sea level. An engine will receive 14.7 psi of air pressure at sea level. As the piston moves down, the intake valve opens allowing the piston to suck air into the chamber. How well that chamber is filled with air is calculated as its volumetric efficiency.

A supercharger is a pump that pumps more air into the engine than it normally could under standard air pressure. This extra air from the supercharger creates pressure commonly called boost. So if a standard engine pumps 500 cfm naturally aspirated and a supercharger pumps 1000 cfm then the resulting back pressure is boost.

The higher the boost pressure matched with the correct amount of fuel will increase the of horsepower that can be generated by the engine.

 

If you are looking to buy a Centrifugal Supercharger then contact MC Racing.

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