Originally Posted by JRinKY
You guys are close, but are still just a little short of the real story.
The supercharger is almost always a positive displacement pump. That means that a revolution of the input shaft results in the movement of a relatively fixed amount of air. The faster you turn it, the more air it pumps, and the difference is approximately linear. Turn it twice as fast, you get twice as much air. It isn't a perfect relationship due to the compressibility of air, but its close enough for this discussion. A supercharger is sized to the engine it will feed, and turns at a speed that is normally within 2x engine speed. This low speed makes bearing and shaft design easier, but requires the supercharger to be relatively large and heavy. Also, because it is a positive displacement pump, the components must be manufactured to very close tolerances, which is very expensive. Large, heavy, and expensive are not particularly desirable characteristics.
The turbo-supercharger, or turbocharger, is not positive displacement, but is rather a centrifugal fan. It doesn't displace much air at all at low speeds, and its "displacement" varies approximately with the square root of its speed, so if you want twice as much air, you have to spin it four times as fast. The extremely high speeds make shaft and bearing design critical, and the fact that it is driven by hot exhaust gases makes lubrication and material selection a bit tricky. Better materials, and better oils, primarily synthetics, make the reliable turbocharger possible. The turbocharger is relatively small, light, and cheap.
There was a hybrid device, a mechanically driven centrifugal blower, called the Paxton Supercharger. It was developed before precision machining made the supercharger possible, and before materials and lubrication technology made the turbocharger possible. It worked, but was not capable of very much boost, and the mechanical drive was very fragile, so as soon as better technology came along, it pretty much went away.
Both devices heat the air. If you compress air, it will get hotter. Both heat the air to about the same degree. Turbo design pretty well isolates intake air from the exhaust-heated components, and the air is not in the turbo for very long, anyway.
Both devices require power to run. The belt-driven supercharger takes it directly from the crankshaft, the exhaust-driven turbocharger takes it from the exhaust. This would seem to be "free energy" but it is not. The turbine causes backpressure, and to maximize the turbine's efficiency, the exhaust timing is advanced, and both of these remove some power from the engine's power stroke. Fortunately for us, the supercharging of the intake adds much more power than is consumed doing it. Both devices can be setup to deliver the same boost pressure, and both can be sized to feed any displacement engine, although typically multiple turbo's are use for large displacement performance engines, to minimize lag.
The phenomenon known as "turbo lag" occurs during rapid acceleration. It is caused by the inertia of the turbine/compressor wheel, and cannot be eliminated. Making the rotating components of the turbo very light helps to eliminate lag, another benefit of better material science. The belt-driven supercharger does not exhibit any lag, but it also cannot idle like a turbo can. As cruising speeds, the turbo idles along, not really pumping a lot of air, which makes it pretty efficient. The supercharger is always pumping the same volume of air, so its power losses are always high.
The turbo is the lighter, smaller, cheaper, and more efficient option. A little lag during acceleration isn't really that large a price to pay.
Is anyone still reading ? does anyone really care about all this ? Probably not. But, it is what it is.
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11-05-2006, 09:37 AM
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