Postive Displacement Superchargers
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Postive Displacement Superchargers
Recently I've seen several people claim that positive displacement superchargers "in effect" add to the engine's actual displacement.
This claim seems absurd to me, so I did a little bit of reading:
http://en.wikipedia.org/wiki/Supercharger
From what I can tell, they do nothing of the sort, and this claim is ridiculous. Displacement on an Otto-cycle engine is (1/2 * bore)^2*pi*stroke*#cylinders, and it will always be, regardless of the type of induction.
I can see a twinkle of actual logic behind the claim, however. A positive displacement 8-71 blower delivers 71 cubic inches of air into each of eight cylinders at every intake stroke, so it would seem like you could add 8*71ci to your engine's displacement and claim that the supercharger makes the smaller engine make the power of the theoretical larger engine, since it's combusting the same amount of air and a similar amount of fuel.
That might be reasonable as an approximation, but it just doesn't make sense in a technical discussion (which is where I always see it used). There are so many other factors involved than the volume of air... this just seems nuts to me.
Am I missing something?
This claim seems absurd to me, so I did a little bit of reading:
http://en.wikipedia.org/wiki/Supercharger
From what I can tell, they do nothing of the sort, and this claim is ridiculous. Displacement on an Otto-cycle engine is (1/2 * bore)^2*pi*stroke*#cylinders, and it will always be, regardless of the type of induction.
I can see a twinkle of actual logic behind the claim, however. A positive displacement 8-71 blower delivers 71 cubic inches of air into each of eight cylinders at every intake stroke, so it would seem like you could add 8*71ci to your engine's displacement and claim that the supercharger makes the smaller engine make the power of the theoretical larger engine, since it's combusting the same amount of air and a similar amount of fuel.
That might be reasonable as an approximation, but it just doesn't make sense in a technical discussion (which is where I always see it used). There are so many other factors involved than the volume of air... this just seems nuts to me.
Am I missing something?
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From: Raleigh, NC
Not really. It doesn't actually make the displacement any larger, but it does make the motor act like it has more displacement. The same can be said for all forms of forced induction. I think the difference is in how "directly" the positive displacement blowers feed the air in. It's almost making boost at idle.
I'd call it more of an expression that has been "blown" out of proportion
(shameless pun, I know)
I'd call it more of an expression that has been "blown" out of proportion
(shameless pun, I know)
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From: Okemos, MI
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From: Western Ky
Just a couple of things....the displacement of a 8-71 is 430 CI per revolution but you would need a 1:1 drive ratio to obtain that....here is a quote that covers the application I think it is easy to see how people would draw the conclusion of added displacement:
"With a supercharger, the amount of air and fuel that can be
packed into the cylinders greatly exceeds the 100% volumetric
efficiency of a highly refined unblown engine. Since the
air is now being forced into the engine, you can put a substantially
denser fuel/air charge into the cylinders. On most
street type blown applications running 6 to 7 pounds of
boost, approximately 40 to 50% more fuel and air can be
packed into the cylinders than in a comparable unblown
engine. The reason that larger displacement engines make
more power and torque than smaller ones is that more fuel
and air are available for combustion. As a result of super-
charging, a small displacement supercharged engine can
produce similar horsepower and torque to a naturally aspirated
larger displacement engine."
"With a supercharger, the amount of air and fuel that can be
packed into the cylinders greatly exceeds the 100% volumetric
efficiency of a highly refined unblown engine. Since the
air is now being forced into the engine, you can put a substantially
denser fuel/air charge into the cylinders. On most
street type blown applications running 6 to 7 pounds of
boost, approximately 40 to 50% more fuel and air can be
packed into the cylinders than in a comparable unblown
engine. The reason that larger displacement engines make
more power and torque than smaller ones is that more fuel
and air are available for combustion. As a result of super-
charging, a small displacement supercharged engine can
produce similar horsepower and torque to a naturally aspirated
larger displacement engine."
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Posts: 1,374
From: Raleigh, NC
Excuse me if you already now all of this:
As mentioned, volumetric efficiency is increased because the mass of air filling the cylinder if increased, in comparison to the naturally aspirated engine's volumetric efficiency at the given RPM and atmospheric pressure (a specific mass). At 100% VE, the air pressure of the cylinder equalizes with atmospheric pressure. A VE higher than this indicates a relative, positive pressure differential between the cylinder and atmosphere. In effect, this increases the displacement of the engine because the only concern with displacement is to compare the mass of air possible at a given pressure. If we could use any pressure we wanted (so as to attain any mass we wanted), this would be of no concern. Obviously, there are many reasons we do not use high pressure, miniature V8s.
Though, positive displacement blowers do have a significant advantage. The volume of air (trapped between the corresponding rotors) is mostly constant. By increasing the rate of compression and exhaustion of the blower, the flow changes. Because the change in flow is accomplished with a rate, a measure of change (in this case cycle) per time, the output is near linear. Equally so is the internal combustion engine, for most parts. And, the result is evident in boost at the blip of the pedal.
As mentioned, volumetric efficiency is increased because the mass of air filling the cylinder if increased, in comparison to the naturally aspirated engine's volumetric efficiency at the given RPM and atmospheric pressure (a specific mass). At 100% VE, the air pressure of the cylinder equalizes with atmospheric pressure. A VE higher than this indicates a relative, positive pressure differential between the cylinder and atmosphere. In effect, this increases the displacement of the engine because the only concern with displacement is to compare the mass of air possible at a given pressure. If we could use any pressure we wanted (so as to attain any mass we wanted), this would be of no concern. Obviously, there are many reasons we do not use high pressure, miniature V8s.
Though, positive displacement blowers do have a significant advantage. The volume of air (trapped between the corresponding rotors) is mostly constant. By increasing the rate of compression and exhaustion of the blower, the flow changes. Because the change in flow is accomplished with a rate, a measure of change (in this case cycle) per time, the output is near linear. Equally so is the internal combustion engine, for most parts. And, the result is evident in boost at the blip of the pedal.
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From: Okemos, MI
mgray, if I understand your post correctly, you're saying that positive displacement superchargers push a constant (or approximately constant) amount of air into each cylinder at every intake stroke. This much I knew from the reading I did yesterday.
But you also seem to be saying that this gives them an inherent advantage over dynamic compression superchargers.
I'm not sure I follow that one. Is it because most dynamic compressors don't make boost at idle? And if so, does that mean that the advantage is gone as soon as the revs reach a certain point?
But you also seem to be saying that this gives them an inherent advantage over dynamic compression superchargers.
I'm not sure I follow that one. Is it because most dynamic compressors don't make boost at idle? And if so, does that mean that the advantage is gone as soon as the revs reach a certain point?
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From: Bridgewater, MA
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From: Western Ky
The main disadvantage of the roots is the heat factor.
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From: Okemos, MI
Okay, so in the end, a positive displacement supercharger is constantly maintaining the same manifold air pressure, regardless of RPM, whereas a dynamic compressor (super or turbo) builds up as RPMs rise, and then if the RPMs rise enough, it falls off again at the top.
Right?
Right?
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Joined: Oct 2001
Posts: 459
From: Debary, FL
Great info thus far...
I don't want to hijack or derail this thread, but I was talking to a few people about a similar subject yesterday. Hope everyone won't mind me adding my $0.10 worth
In a nutshell... I see roots blowers w/ a ci or L rating. Like the KB blowers for 03'-04' cobras, they have a 2.2L & 2.4L. They also offer a 1.7L for the 99'+ 2V GT's. I understand that fine... however, what about a centrifugal blower? Or a turbo car? How do I convert boost into displacement?
I grow tired of the, "But I have a smaller motor" ricer excuse. If the blower or turbo added no power, then why have it? We all know they add substantial power to pretty much anything.
I would like to find a forumla, based on compressor size, and pounds of boost used that would show me roughly how much displacemnt it makes a motor "think" it has. In example... Supras w/ a 2JZ 3.0L, 66mm turbo, and 30 psi.
I appreciate any help and insight anyone here can offer.
I don't want to hijack or derail this thread, but I was talking to a few people about a similar subject yesterday. Hope everyone won't mind me adding my $0.10 worth
In a nutshell... I see roots blowers w/ a ci or L rating. Like the KB blowers for 03'-04' cobras, they have a 2.2L & 2.4L. They also offer a 1.7L for the 99'+ 2V GT's. I understand that fine... however, what about a centrifugal blower? Or a turbo car? How do I convert boost into displacement?
I grow tired of the, "But I have a smaller motor" ricer excuse. If the blower or turbo added no power, then why have it? We all know they add substantial power to pretty much anything.
I would like to find a forumla, based on compressor size, and pounds of boost used that would show me roughly how much displacemnt it makes a motor "think" it has. In example... Supras w/ a 2JZ 3.0L, 66mm turbo, and 30 psi.
I appreciate any help and insight anyone here can offer.
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Joined: Nov 2004
Posts: 1,374
From: Raleigh, NC
Great info thus far...
I don't want to hijack or derail this thread, but I was talking to a few people about a similar subject yesterday. Hope everyone won't mind me adding my $0.10 worth
In a nutshell... I see roots blowers w/ a ci or L rating. Like the KB blowers for 03'-04' cobras, they have a 2.2L & 2.4L. They also offer a 1.7L for the 99'+ 2V GT's. I understand that fine... however, what about a centrifugal blower? Or a turbo car? How do I convert boost into displacement?
I grow tired of the, "But I have a smaller motor" ricer excuse. If the blower or turbo added no power, then why have it? We all know they add substantial power to pretty much anything.
I would like to find a forumla, based on compressor size, and pounds of boost used that would show me roughly how much displacemnt it makes a motor "think" it has. In example... Supras w/ a 2JZ 3.0L, 66mm turbo, and 30 psi.
I appreciate any help and insight anyone here can offer.
I don't want to hijack or derail this thread, but I was talking to a few people about a similar subject yesterday. Hope everyone won't mind me adding my $0.10 worth
In a nutshell... I see roots blowers w/ a ci or L rating. Like the KB blowers for 03'-04' cobras, they have a 2.2L & 2.4L. They also offer a 1.7L for the 99'+ 2V GT's. I understand that fine... however, what about a centrifugal blower? Or a turbo car? How do I convert boost into displacement?
I grow tired of the, "But I have a smaller motor" ricer excuse. If the blower or turbo added no power, then why have it? We all know they add substantial power to pretty much anything.
I would like to find a forumla, based on compressor size, and pounds of boost used that would show me roughly how much displacemnt it makes a motor "think" it has. In example... Supras w/ a 2JZ 3.0L, 66mm turbo, and 30 psi.
I appreciate any help and insight anyone here can offer.
So you could argue that the 1.8L with 1 bar of boost (measured in the cylinder at bdc) acts like a 3.6L. It's not exactly true but a good generalization.
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From: Okemos, MI
So if you want your 1.8L turbo to act like a 3.6L, you'll need to know your volumetric efficiency (VE) and incorporate that into the calculation. It will, of course, still be a generalization.
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