Discuss twin screw vs centrifugal superchargers 2

[bob8907]

Have a few different applications but all will be higher rpm (6,000-11,000rpm) units. I really like the throttle response of screw but hear that centrifugal might have an edge for all our HP on top end. Can a properly designed and applied screw system be better than centrifugal?
Design goals are to maximize power potential on pump fuels so obviously inlet temp is a huge consideration. Either charger will mount directly on the engine so that form of heating is irrelevant. Very interested to see if there are better screw designs out there.

 

[TDIMeister]

Eaton's latest-generation TV-S Rootes (yes, you are not mistaken) blowers have peak efficiencies of 78%, up there with the best aerodynamic (centrifugal) devices.

Selection all comes down matching the maps of the available chargers out in the market to the boost pressures and mass flows in which your particular engine will operate. Thermodynamically, the engine does not distinguish the type of charger you choose, but rather the mass flow, pressure and temperature of the discharged air and the mechanical power used to drive it.

Only the unregulated torque curve will look markedly different between a centrifugally-supercharged engine compared to other devices because the flow/RPM characteristic is fundamentally different.

 

[CH5OH]

a centrifugal compressor has only a good efficiency at the design rpm, to operate an engine on different rpm you can use a few turbo's with different design characteristics and bypass valves to cover the full rpm range
a roots blower can deliver an amount of air, depending on the rpm, since it is mechanically coupled.Therefore, the efficiency of motor and supercharger will be in favour of the turbo.

 

[RomeoReject]

If you're asking from a dedicated performance perspective, I know that some of the drag teams I race run centrifugal superchargers capped at 40psi, but then over-driven several times so that they reach that boost sooner. Because centrifugals are more-or-less based upon turbocharger technology, the ideology seems to be that they should be safe at 40-50000RPM, although I find that a bit hard to believe (Otherwise companies such as Vortech would routinely advertise as such).

I'm a performance nut, and my own personal advice is this: Road-racing, drifting and street-driven vehicles benefit more from a positive displacement (Especially a twin-screw) supercharger. Drag-racing and oval-racing vehicles benefit more from a centrifugal supercharger (And again, I'd personally advise you to leave the ratio alone).

 

[TDIMeister]

Right, I forgot to mention that the choice of compressor type depends in part to the pressure ratio you want to run. Positive-displacement chargers (e.g. Rootes) lose a lot of efficiency at high PR. Centrifugal or screw-type may be better for this application...

 

[vic52]

In response to overreving a centrifugal supercharger to get pressure up quicker. Rather I have used an oversized Vortec V2 T trim, the efficiency curve of which tends to match the BMW 4 valve 3.6 litre M5 with tapered gently curved inlet tracts, big butterflys & considerably flared trumpets(on the principal of more square inches gives more pounds). The taper accelerates the intake gases to give better filling.
This resulted in 430 hp flow on flow bench, & indicated a 295degree cam with 10mm lift to flow 500hp into cylinders.

To give good midrange torque the standard compression of 10/1 was kept, with boost limited to 6 or 7 psi(max comes up quicker). Both water intercooling & Motec controlled water/methanol injection into the 6 throttle bodies of the 1980 engine design. Trying to maintain good mass at the low pressure & cope with the high compressions.

The danger is 26psi capability at 55,000 which has to be blown off in large amounts sucking drive hp & generating heat. The objective is to use an intake restrictor to reduce the excess pressure & max efficiency at max hp.
What to use as a guide?
Initially I scaled up the 35mm-380hp racing restrictors,plus a margin for the severe intake pressure drop prior to Mach 1. Gives at 44mm 610hp, hopfully giving 500hp for the motor, & avoiding blow off.

But then I couldnot remember any normally aspirated motor wth a 50mm throttle body developing over 380hp.
Vortec intake is 88mm diam & max air pump is 860hp.

Any ideas how to calculate this? Vic.

 

[BrianPetersen]

Not sure whether it applies in your case, but centrifugal fans can have their output regulated by a set of variable-angle vanes at the inlet that gives a variable amount of pre-swirl to the air being drawn in. If you pre-swirl the air going in, it reduces the angle of attach of the blades relative to the flow stream, that reduces the amount of "lift" and therefore pressure that the fan generates. It's more efficient than simply throttling the intake. For maximum output, the vanes are set straight so that there is no pre-swirl.

A centrifugal supercharger is a fancy centrifugal fan but with a much higher pressure ratio, but honestly I don't know what would happen if you try to use that trick on one with a high pressure ratio.

 

[bob8907]

Let me further add to the data and my questions here. A target engine is a twin cylinder 1.0L engine with a peak rpm around 8000. Problems are this is NOT a drag race only engine and more or less a closed course engine. It is important to get throttle response and part throttle performance as well as top end, top speed apps. I am trying not to give too many details because they will not be relevant to this discussion. I am trying to learn any major benefits and or deficiencies of each design.


Also, to add another stick in the fire, I am curious if a twin screw (not lobed roots) can be mounted further from the manifold without consequence? It is pretty typical to mount a centrifugal type well away from the intake but roots are typically mounted right to the engine. In my configuration, it may be required to mount the charger remotely and pipe compressed air back to the intake.

 

[Warpspeed]

"Throttle response, and part throttle performance".

To me that suggests placing individual throttle bodies as close to the intake valves as is possible, (and using EFI if the class rules allow).

The interesting thing about doing that, is that a very basic centrifugal supercharger installation will work pretty well, whereas a positive displacement supercharger will need a lot more thought, extra plumbing, and some type of air bypass system for down stream individual throttles to work.

Centrifugals also tend to be a lot smaller and lighter in weight than a positive displacement supercharger for a given application.

Now with a very small and responsive engine, and crisp throttle response, the way to get acceleration at low road speeds is via the gearbox, not by just building up massive supercharger boost at ultra low engine rpm.

What works really well in a road car, may not be the optimum choice for a racing bike.