JIM, Hi. Let's get one thing straight before we advance this thread too far, so we all understand what is involved. All methods of increasing induction air feed pressure to the cylinder of an internal combustion engine are methods of SUPERCHARGING. Even "nitrous" comes into this category. However, for reasons of convenience, they are subdivided into more colloquial terms and connotations of them.
Thus, what we call a "turbo" (as in the MPS) is in fact turbo-supercharging, using its exhaust to drive the turbine (gas pressure-driven) vane which is (usually) on a common shaft with the induction air compressor vanes which operate in separate adjacent chamber. Nitrous would be Nitrous oxide-supercharging, as it increases pressure, but also adds oxygen, and also improves charge density by air chilling. It has little common application as its effects are of short duration, a matter of minutes as it uses a bottle-stored source.
What we commonly term just "supercharging" is the common expression for a mechanically (including hydraulic and electric) driven vane, rotor or impeller system that, unlike a turbo supercharger [which has two chambers to keep (driving) exhaust gas and (driven) induction air separate], only has one chamber, as the vanes, on a shaft which is externally driven, only have to deal with induction air, acting as a pump, sucking air in and blowing it out under higher pressure.
There are many types of impellers. There are multiple (usually 2) interlocking lobe-style rotors, sliding vane single rotors, and fixed vanes systems that look not unlike one side of our turbochargers. They can be gear, belt, chain, or electrically driven. Some have clutches that are operated electrically or mechanically or even barometrically, via bellows, levers or cams, so that the operator can choose to have it on or not or it controls itself according to external atmospheric pressure, as in piston driven aircraft where many controls have been used to compensate for varying heights of operation and thus changing atmospheric pressure as the plane ascends or descends. Superchargers are less popular for production cars as they are often larger, heavier and more complex and require a separate drive system. Modern piston aircraft also lean towards turbocharging for the same reasons.
The VW system uses both with results of great efficiency. The supercharger is mechanically driven from the word go, and thus produces boost at low rpm. The exhaust driven turbocharger only comes on song at higher rpm, when there is enough exhaust pressure to operate it effectively. Thus the power delivery is quite linear and the power available from small capacity is relatively high.
It is theoretically possible to "supercharge" an MPS motor, but if you've seen the engine bay, you'd realize that there is little room to mount a blower and its drive system. If I was going to try it just for the sake of it, and as a quick off-the-cuff idea without thinking it through too hard, I'd try for an electrically-driven system which means it could be mounted remotely behind the grill, along with its intercooler, then I'd set up an electric clutch so it only came into play when I kicked down beyond a certain throttle opening. That would eliminate the question of full time power draw which would be considerable for they draw a few Kw. This means that it would also want to be relatively low pressure.
Another reason turbos are popular as you get a sort of "free lunch". However, the great thing about superchargers is the huge torque from low rpm, giving great tractability. Turbos are often associated with some "lag" as they need some rpm to generate enough induction pressure. The VW gets the best of both, with little compromise.
Hope this helps. Cheers.
Last edited by Doug_MPS6; 08-08-2010 at 03:29 PM.
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