Centrifugal type supercharger
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The centrifugal-type supercharger is an engine-driven compressor used to increase the power output of an internal-combustion engine by increasing the amount of available oxygen by compressing air that is entering the engine. This type of supercharger is practically identical in operation to a turbocharger, with the exception that instead of exhaust gases driving the compressor via a turbine, the compressor is driven from the crankshaft via a belt-, gear- or chain-drive.
Like any centrifugal pump, the boost provided by the centrifugal supercharger increases with the square of the speed, measured in RPM. This means that the centrifugal design provides little boost at low engine speeds, in some cases allowing air to pass back through the supercharger, such as during decelleration. 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 advantage of producing boost at any RPM.
Many World War II piston aircraft engines such as the Rolls-Royce Merlin and the Daimler-Benz DB 601 utilized single-speed or multi-speed centrifugal superchargers. Because high-performance aircraft engines were typically mated to constant-speed propellers and did not see a great variation in engine speeds, the poor low-rpm performance of centrifugal superchargers was not an issue. Turbo-supercharged engines, like some models of the Allison V-1710, combined a centrifugal supercharger with a turbocharger for better performance across a broad range of altitudes, using the engine-driven portion to provide a constant boost for extra power, while the turbocharger was used primarily to offset the effects of lowered outside air pressure as the aircraft climbed. Superchargers have since fallen from use in the aviation world, replaced by turbochargers of ever-improving quality.
Due to its design and lack of low-RPM boost it is often employed on near-standard compression engines. This means that it can facilitate airflow at higher engine RPMs, when most motors tend to have poor volumetric efficiency, without substantially increasing cylinder pressures at low- to mid-RPM operation, causing knock. This principle makes this type of supercharger ideally fit for a "bolt-on" type power adder, with no modification of the pistons and/or compression ratio necessary. Since gasoline must mix with air in a fairly narrow ratio to achieve combustion, the fact that centrifugals do not add much air at low and mid-range RPM's means fuel mileage is near-stock in the cruise RPM range. They appear to be most popular with cars that have a sufficiently large enough engine to provide adequate acceleration from a standing start without boost, while at the same time avoiding wheelspin. Then, the engine encounters breathing limitations in the mid-RPM range, often because it may only use two valves per cylinder. Centrifugals are also popular in places where the power-adder must be removed for frequent government engine inspections, as the exhaust system is unaffected (as it would be with a turbocharger).
However, detractors of the centrifugal-type supercharger (at least in street-driven automobile applications) note that it combines what some feel are the worst qualities of a turbocharger and a supercharger, since it doesn't develop appreciable boost at low RPM (Boost Threshold), but still uses up prodigious amounts of engine power to operate. Since it is crankshaft-driven and cannot benefit from a device like a wastegate on an exhaust-driven turbocharger to control its rotational speed, its boost threshold is always within a thousand or so RPM of redline. As such, the horsepower rating of the engine is greatly increased, but in a small part of the upper RPM range.
All supercharger types benefit from the use of an intercooler to reduce heat produced during compression.
several popular makes of centrifugal type superchargers for automotive applications are: Paxton, Powerdyne, Procharger, and Vortech.
