What is a Centrifugal Supercharger?
A centrifugal supercharger is a specialized type of supercharger that uses natural centrifugal energy to force additional oxygen into an engine. Increased airflow into the engine allows the engine to burn more fuel which results in increased engine power output.
At its most basic, a centrifugal supercharger is driven off the engine's crankshaft and feeds compressed air into the engine. The centrifugal supercharger is generally attached to the front of the engine with a rigid bracket and driven via a belt-drive or gear-drive from the engine’s crankshaft.
How Centrifugal Superchargers Work
The centrifugal supercharger contains an impeller which spins at high speed to draw air into a small compressor housing (volute). When air leaves the impeller, it is traveling at high speed while having a low pressure. This low-pressure, high-speed air is sent through a diffuser which converts the airflow so that it is high-pressure, and low-speed. The air is then fed into the engine, where the additional airflow (caused by increased pressure) gives the engine the ability to burn more fuel and have a higher level of combustion. This results in a faster, more responsive vehicle due to greater engine efficiency.
Centrifugal Supercharger Components
A centrifugal supercharger is made up of several key components, each working together to compress air and boost the engine’s performance by increasing airflow through the engine. The main components for any centrifugal supercharger include the impeller, compressor housing, diffuser and transmission.
The impeller in a centrifugal supercharger is the main rotating component. The impeller pushes air into the blower and builds the pressure that translates directly into positive manifold pressure, also known as boost. The impeller must be able to withstand high operating temperatures, and must be durable enough to continually perform at high RPMs.
Compressor housing (volute)
The snail-shaped design of the compressor housing is a trait unique to centrifugal superchargers. Technically known as a collector, the purpose of this compressor housing is to gather the airflow and deliver it to a downstream pipe. Although compressor housings can be made from a wide variety of metals or alloys, they are typically formed or cast from aluminum. Aluminum is typically used for supercharger housings/volutes due to the combination of strength, weight, and resistance to corrosion. After the housing is cast, it is then machined to match the impeller design. During assembly of the supercharger the housing is attached to the transmission with securing bolts or band clamps.
Located between the impeller and the volute is the diffuser. Downstream of the impeller in the flow path, it is the diffuser's responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity.
Along with the step up ratio obtained through the drive system, a transmission step up is also required to obtain the impeller speeds necessary to create the desired boost. Additionally, the transmission contains bearings to support the shafts that are attached to the internal gears. Bearings are used throughout the system to help parts move smoothly and to reduce friction and wear. All centrifugal supercharger bearings must be able to withstand high-speed movement on a constant basis.
Proper lubrication is essential for continued centrifugal supercharger performance. The high speed required for the supercharger to create boost demand adequate lubrication for all moving parts. There are several methods of lubrication used in centrifugal superchargers. Some designs utilize engine oil to provide lubrication for the supercharger. On enclosed (self-contained) systems, the lubrication is low weight synthetic oil which is specifically engineered for high-speed use. The lubricating oil is distributed throughout the transmission via an oil slinger/pump.
Centrifugal Supercharger Efficiencies
There are several considerations in choosing the type of supercharger or boosting system for any vehicle. Three of the more important factors to consider are efficiency, heat transfer, and ability to intercool.
Centrifugal superchargers use a small portion the engine’s power to drive the movement of the supercharger’s internal components. Efficiency has both mechanical (power consumption) and thermal (heating of the compressed air) factors. A higher efficiency means the supercharger consumes less energy from the engine powering it, and produces less heat.
There is substantial heat within an engine compartment, and some supercharger designs allow substantial heat transfer from the engine and other components to the supercharger. This in turn allows additional heat to be transferred to the air being compressed inside the supercharger, effectively decreasing efficiency. A good illustration of this is comparing a centrifugal supercharger mounted to the side of an engine or in front of the engine, relative to a positive displacement blower mounted on top of an engine. Comparatively, the centrifugal supercharger has minimal heat transfer from the engine, and is typically positioned in a fresh air stream. The positive displacement supercharger, on the other hand, can heat soak fairly quickly from engine top mounting . Thermal imaging has been used to illustrate this concept well.
For street use, it is well known that air-to-air intercooling offers superior performance relative to air-to-water intercoolers. The only exception is if a vehicle’s design does not allow placement of an air-to-air intercooler of adequate size, or with adequate airflow, or if it is a race vehicle which is operated for only a short period and is able to utilize ice. This is in part due to the fact that air-to-water intercooling for street use involves a secondary heat exchanger, and is actually air-to-water-to-air intercooling. This two-step design limits intercooler system effectiveness relative to air-to-air designs. Positive displacement superchargers, however, are not able to easily utilize air-to-air intercoolers due to their engine-top mounting, and typically utilize air-to-water-to-air intercoolers. Almost all turbochargers and centrifugal superchargers utilize air-to-air intercooler systems due to their higher effectiveness.
Centrifugal Superchargers: Performance
Because of their high-efficiency design that includes minimal heat transfer, centrifugal superchargers produce larger power gains than positive displacement blowers. The ability to utilize highly effective air-to-air intercooling is a further advantage. A subset of superchargers known as turbochargers are “exhaust gas driven superchargers”, and may also be called turbosuperchargers . Relative to turbochargers, centrifugal superchargers will produce larger power gains when running pump gas on the street. With racing fuel to combat detonation, however, turbochargers can produce large power gains on racing engines. This difference in power gain dates back to use in tractor pulling in the early 1960’s, to later use at Indianapolis and the Can-Am racing circuit . Due to this, for hardcore racing, the primary advantage of centrifugal superchargers in race cars becomes consistency and repeatability.
Overall, centrifugal superchargers are an excellent way to boost engine performance in a reliable and street-legal way for a variety of vehicles.