I know these exist in a few places. What I want to know is, when they're turned off, are they a meaningful restriction? Do you need some way to circumvent them or can the motor breath right through the blower? I'm thinking centrifugal.
I know these exist in a few places. What I want to know is, when they're turned off, are they a meaningful restriction? Do you need some way to circumvent them or can the motor breath right through the blower? I'm thinking centrifugal.
my only thought is the air running thru the impeller tines will try to force the unclutched pulley to spin backwards...may not be fun for some or all moving parts if theyre not designed to ...ahem...go both ways (Im thinking of the reducer gearing etc)
the supercharger on my mr2 spins the proper direction when the clutch is disengaged. i have not done any testing though to see if there is a restriction. wouldn't matter much on my autocross car anyway.
but when it's not engaged, it doesn't have to bypass the blower in order to breath?
Nope. At least not at low engine speeds. At higher RPM, it would become a restriction, even if it freewheeled a bit.
If you're thinking of replicating the Mad Max solution, you'd need a clutch driven blower coupled with an intake bypass solution. Maybe one of those electrically operated exhaust cut-outs?
i should add that toyota did have a bypass valve, but i have never had it installed either.
I believe the Mercedes version on the 1.8 Kompressors freewheels when the clutch is disengaged. But that's a Roots type blower, not a centrifugal.
Judging by your avatar, you've been watching Mad Max again.
Any one know if their are any clutches that could be pulse width modulaided / consistently slipped
I'd think a blower with a cvt type transmission would be incredible.
turboswede said:Nope. At least not at low engine speeds. At higher RPM, it would become a restriction, even if it freewheeled a bit.
If you're thinking of replicating the Mad Max solution, you'd need a clutch driven blower coupled with an intake bypass solution. Maybe one of those electrically operated exhaust cut-outs?
On a Roots, you can only breathe through the bypass if the rotor isn't turning. You'll need a fairly hefty size bypass - I can tell you that an M45 on a 1.6 Miata engine is a sloooooow beast with a broken belt. Should be the same with a twin-screw.
I don't see why you'd clutch a centrifugal. Since they're a fan and not a pump, I don't think they'd have the same parasitic losses.
When superchargers do have bypass valves, it's usually to reduce their drag or pumping losses when they don't actually need to be pumping. Usually the valve just recirculates the output to the input so the supercharger is pumping air in a circle. If all that is behind the throttle plate then you are also pumping very low density air which also reduces losses.
If you think about clutched superchargers, think about the type of clutch they have, and the amount of power it takes to run the supercharger. It's sort of pointless to think of what kind of restriction the supercharger will be at high rpm/high load because you can't or shouldn't engage the clutch at that point anyway. The supercharger guts are so heavy, and the power it takes to drive it to pump air so much, that you shouldn't be engaging the clutch any time other than low rpm/load or your clutch will not last very long. So will your clutch EVER not be engaged at high rpm/load? It will not restrict much at low rpm/load, and it will be engaged before high rpm/load.
If you are trying to have a car that is supercharged some times and NA at other times and you don't already have the supercharger setup in place, it would probably be easier to do a turbo setup with an electronic wastegate. That can at least more or less be 'turned off' in a way where it doesn't cause much drag on the engine and can be reengaged at high rpm/load if you wanted to.
My english teacher mother would not be proud of my sentence structure. Let me fix this next one.
On a Roots supercharger, when the rotor is not turning, you can only breathe through the bypass. And it's tiny. You've got a very effective restrictor plate in place :)
The only technical reason I can think of for bypassing the supercharger is that the extra drag does cause a bit of fuel efficiency loss on the highway. We usually see a mpg or two less. A turbo, on the other hand, usually sees a mpg or two more because you now have an intake system that is completely free of restriction. That's a technical reason. If the real reason is because it's cool and Mad Max had one, well, that's reasonable :)
In reply to Teh E36 M3 :
It does exist, Procharger
Think about a roots blower being a positive displacement pump. Even at idle, even behind the throttle plate, it is still going to pump more air than the engine naturally breathes just like it does at every other rpm according to its drive pulley ratio. So even at idle your supercharger would basically be trying to take the 20"vac in front of it and turn it into 15" or something like that after. It's still going to try to do work. Leaving a bypass valve open reduces the restriction and the loss from this, and it also CAN be used to artificially deaden the response of the engine when it's in an rpm range where touchiness is unwanted. It has decent reasons for existing.
I totally understand why they have bypasses, I've been involved in some supercharger design work.
I'm looking at the clutched supercharger scenario. At that point, the supercharger is not turning, just like if you'd removed the belt. Since the rotors are well sealed, it does not pass ANY air in this situation. So you can only feed the engine through the bypass, and the bypass is very small so you have a very limited amount of power available. Like "can't really maintain 75 mph on the highway" sort of situation.
Thus my statement that if you're going to clutch the supercharger, you need a big bypass.
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