I think that a Supercharger COULD help a TDI in the 1/4 mile here is why.
(brainstorm mode so disregard grammer
)
The turbo on the TDI is made to produce very low end boost and continue to produce that boost throughout most of the TDI's usable rpm band. However as rpm's increase the turbo is not able to continue to produce a high volume of air for a given pressure. Let me explain further. As the engine revs it (the turbo) can produce I high volume of air and maintain a high pressure 17-20 PSI. Because the engine rpm's are low the turbo can fill the low volume required per second i.e fill a given number of cylinders per second and still maintain a given PSI. As the revs increase's the volume of air required per pound of pressure increases proportional to the rpm's of the engine. The turbo becuase it's compressor is optimised and fixed can only produce air pressure and volume in a very limited amount i.e. more cylinders to fill per second so pressure starts to drop off due to the limits of the compressor wheel design.
What I am getting at is that if you could supplement the turbo's volume of air so that as rpm's increase you begin to feed in "volume" at high pressure to continue to feed the cylinders at a linear rate proportional to rpm. Am I clear???
What I envision is a dual stage or an axial flow compressor or a system that utilizes "bleed valves". This is a common problem in turbojet engines. At low speeds the engine can suck in and digest absolutley huge amounts of air. In order to compensate for the very high pressure on the ground relative to the conditions at high altitudes, the engines must "bleed" some of that compressed air overboard. In the caswe of an axial flow compressor utilizing a "VNT" or "VGV variable guide vane arangement to vary the flow of air through the compressor stage you can develope an extremely efficient unit that can produce not only the pressure at any given rpm but also fill the volume of the engine throughout and even at a surplus for any given RPM. The only problem with an axial flow design is that the centrifigul compressor is far more efficient thus the better solution.
What would happen is at low rpms you use a bleed system to vent off unneeded air and still keep the turbine on speed for low end boost, this really lends itself to the VNT design! since the compressor is also controlled by the ECU, as the rpms begin to increase the turbine begins to spool and the bleed valves are still venting air to limit volume. As the turbo begins approaching it's rpm limits, the bleed valves can begin to close to feed in the otherwise surplus volume of air into volume hungry revving motor. What this also does is utilize a bigger compressor wheel. This results in less heat (Mickey are you paying attention?). This set up would prevent possible surging and overrevving by trying to spin the turbo out of it's design range.
The compressor would not require a larger intercooler since it is compressing the air at a slower rate (fill in the formula skypup) it just is not using that volume until it is absolutly required. Since the air in the compressor is being bled off the air is not impeeding the spinning of the compressor since it under a no load type situation. As the rpms increase the bleed valves close and since the turbine is up to optimum speed the compressor can now start loading the turbine and feed all the volume you could ever need. Now the extra fuel and heat are sufficient to feed the turbine and there is enough air to feed the cylinders in my opinion best of both worlds!
Now all you need to do is get the extra fuel to feed the engine through new fuel mapping and youare in the ball game!
You could utilize a belt driven supercharger to supplement volume but this would require diverter valves or a type of clutch similar to what is on the Air conditioning system, but the size and space constraints are prohibitive. But the problem with this is it takes away a lot of HP to produce any amount of boost and volume. This is why I feel a turbo using some of my ideas above would be better since the turbo is recovering and converting the free energy or rather the recovered otherwise wasted engergy of the exhaut gasses.
I am sure this will get all you guys thinking about a possible 2nd generation of VNT turbo!
DB
[This message has been edited by Drivbiwire (edited February 09, 2001).]