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Supercharger

A supercharger is a device used to pump fuel/air mixture, the charge, into the cylinders of an internal combustion engine under pressure. It is similar in purpose to the closely related turbocharger, but differs in that the supercharger is powered by gearing (or belts) off the engine's crankshaft, while the turbocharger is powered by the pressure of exhaust gases driving a turbine.

The supercharger is used as a power boosting device in aircraft and cars, although the turbocharger is more commonly used in both roles. Another use is in the Miller cycle engine, which uses a supercharger to alter the normal four-stroke engine cycle to operate more efficiently.

Automobiles

In cars the device is used to increase the "effective displacement" of an engine, and is often referred to as a blower. By pushing the air into the cylinders, it's as if the engine had larger cylinders, resulting in a "larger" engine that weighs less. Turbochargers are more commonly used in this role because they use "wasted" energy instead of using up power from the crank, but the supercharger reacts more quickly to power application and thus outaccelerates a car with the same amount of boost being provided by a turbo. However in many cases the added complexity is not worth the effort, and most gearheads[?] will reject any such device out of hand.

In 1900 Gottlieb Daimler (of Daimler-Benz / Daimler-Chrysler fame) became the first person to patent a forced induction system for internal combustion engines. His first superchargers were based on a twin-rotor air pump design first patented by American Francis Roots[?] in 1860. This design is the basis for the modern Roots Type Supercharger.

It wasn't long after its invention before the supercharger was applied to custom racing cars, with the first supercharged production vehicles being built by Mercedes in the 1920's. Since then superchargers (as well as turbochargers) have been widely applied to racing and production cars, although their complexity and cost has largely relegated the supercharger to the world of pricey performance cars.

There are three commonly used types used in todays automotive world: Roots type supercharger, and Twin-screw type supercharger, and Centrifugal type supercharger.

Aircraft

A more natural use of the supercharger is with aircraft engines. As an aircraft climbs to higher altitudes, the pressure of the surrounding air quickly falls off. At 18,000ft the air is at 1/2 the pressure of sea level. Since the air in the cylinders is being pushed in by this air pressure, this means that the engine will normally produce 1/2 power at this altitude.

A supercharger remedies this problem by compressing the air back to sea level pressures. This can take some effort, on the Rolls Royce Merlin engine for instance, the supercharger uses up about 150 horsepower. Yet the benefits are huge, for that 150 horsepower lost, the engine is delivering 1500hp when it would otherwise deliver 750. And while the engine might be fooled into thinking it's at sea level, the airframe is quite aware of the 1/2 air pressure, and the plane thus has 1/2 drag. For this reason supercharged planes fly much faster at higher altitudes.

A supercharger is only able to supply so much pressure. The boost is typically measured as the altitude at which the supercharger can still supply sea level pressure (1 ATM), and is referred to as the critical altitude. Throughout WWII British superchargers generally had higher critical altitudes than their German counterparts, and when combined with higher octane fuels that allowed for higher boost levels, British engines were generally able to outperform German ones.

Below the critical altitude the supercharger is actually delivering too much boost, which must be bled off or risk damaging the engine. This means that at least some of that 150 horsepower driving the supercharger is being wasted. For the early years of the war this was simply how it was, and this led to the seemingly odd fact that many early-war engines actually delivered less power at lower altitudes, because the supercharger was still using up power to compress air that was not delivering any power back. As the war progressed several newer controllers were introduced, notably hydraulic clutches, that allowed the boost to be very finely controlled over a wide range fo altitudes. This generally "flattened out" the power below the critical altitude.

Another issue was that its difficult to make a compressor that works well at a wide variety of densities because the "output" of the compressor is a simple pipe. At low altitudes (high air density) the compressor can't shove the air though the output fast enough, so some of the power is wasted. At higher altitudes (lower densities) the same compressor can't gather enough air in though the intake to supply the output. The solution to this problem was to introduce a transmission with several gears, which allowed the compressor to be run slower when there was too much air, and faster when there wasn't enough.

A final addition was the use of two compressors in series. This was used because the compressor also raises the temperature of the air considerably, to the point where the fuel will ignite as soon as it is mixed (no spark needed). In order to avoid this the "two stage" design was used; after being compressed "half-way" in the low pressure stage, the air flowed through a radiator where it was cooled back down (to some degree), before being compressed the rest of the way in the high pressure stage. At low altitudes one stage could be turned off completely.

It's interesting to compare all of this complexity to the same system implemented with a turbocharger instead. Since the turbo is driven off of the exhaust gases, simply dumping some of the exhaust pressure is enough to drive the compressor at any speed you want. In addition the power in the exhaust would otherwise be wasted, except to the extent that the exhaust itself provided thrust, whereas in the supercharger that power is being taken right off the engine. Thus at low altitudes the turbo robs nothing, and as the altitude increases it can use just as much power as it needs, and no more. Better yet the amount of power in the gas is the difference between the exhaust pressure and air pressure, which actually increases with altitude, so turbos generally have much better altitude performance.

Yet the vast majority of WWII aircraft used superchargers. This was due entirely to their small size, lack of any piping, and no need for the high-heat metals needed in the turbine. The size of the piping alone is a serious issue, consider that the Vought F4U[?] and Republic P-47 used the same engine, but the huge barrel-like fuselage of the latter was needed to hold the piping to the turbo in the rear of the plane.


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