2 speed superchargers

[merc-blue]

I have been looking at superchargers recently as i think they are under rated .
Is there any real reason why 2 speed supercharging systems havent been developed? yeah turboing is easy but can you get top boost at 1500RPM and keep it to 9000RPM?

the idea is simple,
downlow no torque so give a bigger ratio advantage, normaly you cant do it becasue you wont be able to rev to more than 5000RPM without killing the charger
once you start getting to the upperlimit of the first ratios revs it switches to a second ratio, via a second clutch with internal gearing or a second belt, or few other possibilities

Or even run a CVT like a golf buggy

Can any body give some insight into why it isnt being used as a option to supply FI power at both low and high revs?
does it really take THAT much effort to turn over a charger?

[MWP]

That idea would work for centrifugal superchargers (like Vortech), but has no use with roots/screw superchargers.

Roots & screw produce boost from idle and keep it constant (well close enough) the whole way through the rev range.

does it really take THAT much effort to turn over a charger?

Yes it does.

[merc-blue]

its not about turning a charger faster but more make it easyer to turn it and to allow a larger rev range without overrevving (the charger)

It more out of curiousity and practicality that i want to investigate 2speed chargers

[oldcorollas]

main issue is power required.
a twinscrew might take 20-25kw at full noise on a 4A
an SC12 is maybe 30-35kw? (pooma )

try and find a compact CVT than can handle say, 40kw at 12000rpm...
you would be lucky if current small car gearboxes will do that.

other issue is the SC is a positive displacement air pump. at high revs, you still want it to be turning over at X times faster than the engine, to supply enough air.

onyl reasons to have variable gearbox would be to a) have higher boost down low, or b) have lower boost down low.
reducing SC speed up high still means you will run out of air....

there was article abotu 2 speed box on a vortech style charger... but that will work only cos they suck at low rpm

[MWP]

I still cant see why you would want to do this...

If the point of this is so you can rev over a certain RPM that would normally be limited by the max allowable RPM of the SC, then get a bigger SC and run it at a lower ratio.

[celicapain]

I heard that the top fuellers use about 1000-1500 hp running their supercharges.
of course, power putput isnt the issue for them

Also their was an american nitro racer who ran an overdrive gear on his supercharger, but they banned it before it hit the track, so im sure its possible from an engineering point of view.

which bring us to why? if you wanna do something crazy go teh twincharger! itd do exactly what your looking for, plus boggle the minds of the masses!

[merc-blue]

Its not that i want to ddo one,
the concept intrested me, i read about 2 speed chargers on merlins and thats whay sparked my intrest

[Benno]

merlins used dual stage superchargers to keep up engine performance at high altitudes(low pressure/thin air)

it was used as a means of evening out performance, otherwise they would have a noticable power drop between high and low level flying.

speeds differed on the 2 stages but over all load wasn't changed that much cos the low air density/pressure on the high stage

There is alot of documentation on merlin's failing due to second stage being activated too low causing overstressing of the charger and engine.

[Ben Wilson]

It's an idea which I've been toying with conceptually for a while.

I'm thinking that you drive a variable displacement piston pump off the crankshaft, and use a gear motor on the front of the supercharger. You'd get the advantage of being able to select exactly how much boost you want all the way through the rev range, plus you can put the supercharger anywhere you want. I'm thinking you'd be able to mount the supercharger on the inlet tank of your intercooler.

[rms]

merlins used dual stage superchargers to keep up engine performance at high altitudes(low pressure/thin air)

Centrifugal not rootes type.

[mrshin]

I suppose electric supercharging could be looked into here too... Obviously, as has to be explained to people all the time, 12 volt systems are no good for this, but run a seperate alternator for a much higher voltage (hundreds of volts) and that would at least give a very readily controllable supercharger, allowing any amount of boost at any revs. The high voltage could also be used for other stuff too (finally... electric cooling fans that generate REAL breeze!)

[myne]

Hmmmm....

CVT springs to mind. I know it's constantly variable, but, it can be designed in a way to make it stepped.

http://cars.about.com/od/thingsyouneedtoknow/a/CVT.htm

[Norbie]

A centrifugal compressor with constantly variable speed? Sounds a lot like a turbocharger.

[Ben Wilson]

Except without the lag...

[wa5]

how about two seperate superchargers, activated in the same way as a 4AGZE. one geared for low revs and one for high, the same as a twin turbo setup?

or one super charger with two seperate AC style clutches (same as GZE) one geared for low speeds and one for high (with different sizes pullies) and switch from one to the other at a preset engine speed? (or with a cool range change override switch on the gear stick..... on a black XB coupe maybe )had I read the first post properly I would have seen the above idea already mentioned!

as to the amount of power used to turn a supercharger, BRM (British F1 outfit) built a supercharged 1.5 litre V16 in the 50s which made around 400 hp ( I have heard anywhere from 350 to 650 actually) I read somewhere a long time ago that the blower took around 3 times the HP that the engine made at the flywheel to make the 400 at the flywheel.

dont take these figures as gospel as I read this a long time ago.