Technical Discussion on Compressor Wheel Efficency
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From: KC, KS
Technical Discussion on Compressor Wheel Efficency
I have spent literally many hours trying to figure out the answer to my question, any technical input is appreciated.
Simply stated, my question is why are compressor maps 2D?
A common misconception is that a larger turbo flows higher boost and/or more volume more efficently.
More to stimulate thought:
Lets say you are in the dead center of a compressor map with your setup, so you are running as efficently as possible. Now, there is two axies (4 directions) you can move which will make you less effiecent.
UP: This is the easiest to explain in my mind, to increasing boost the turbo is going to have to spin faster than it did previously, which somehow makes sense in my mind that with all that friction of a faster spinning turbo, the air will heat up.
DOWN: Why would a slower moving fan be less efficent than a faster moving fan? The turbo has to compress less air and therefor do less work, so wouldn't it be more effiecent?
LEFT: Again, the turbo has to move less air and therefor do less work, so wouldn't it be more effiecent?
RIGHT: The turbo will always have to spin somewhat faster in order to move more lbs/minute of air, which makes sense to heat up the air some, but wouldn't seem to make much of a difference.
A:
B:
B:
A:
Select:
Start: <-nevermind the little joke if you don't get it.
So anyway, why isn't compressor effiency linear with repect to airflow? Common sense tells me that an efficent turbo for a big V8 @ 4lbs of boost would be efficent for a honda @ 20lbs of boost. Any thoughtful input is appreciated! I hope this doesn't keep me up at night any more (i'm serious too)!
Simply stated, my question is why are compressor maps 2D?
A common misconception is that a larger turbo flows higher boost and/or more volume more efficently.
More to stimulate thought:
Lets say you are in the dead center of a compressor map with your setup, so you are running as efficently as possible. Now, there is two axies (4 directions) you can move which will make you less effiecent.
UP: This is the easiest to explain in my mind, to increasing boost the turbo is going to have to spin faster than it did previously, which somehow makes sense in my mind that with all that friction of a faster spinning turbo, the air will heat up.
DOWN: Why would a slower moving fan be less efficent than a faster moving fan? The turbo has to compress less air and therefor do less work, so wouldn't it be more effiecent?
LEFT: Again, the turbo has to move less air and therefor do less work, so wouldn't it be more effiecent?
RIGHT: The turbo will always have to spin somewhat faster in order to move more lbs/minute of air, which makes sense to heat up the air some, but wouldn't seem to make much of a difference.
A:
B:
B:
A:
Select:
Start: <-nevermind the little joke if you don't get it.
So anyway, why isn't compressor effiency linear with repect to airflow? Common sense tells me that an efficent turbo for a big V8 @ 4lbs of boost would be efficent for a honda @ 20lbs of boost. Any thoughtful input is appreciated! I hope this doesn't keep me up at night any more (i'm serious too)!
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From: Somewhere in California
Re: Technical Discussion on Compressor Wheel Efficency (Cyphear)
hmmm, it''s been a long time since I've talked turbos to anyone, but I'll give this one a shot.
My most basic interpretation of what a compressor map, is it is a chart that tells you how much work a turbo does. The inside part of the chart tells you how efficiently you are doing that work, and the left and lower edge tells you how much pressure and how much air it would take to work at XX% efficiency. There are only 2 main factors affecting turbo efficiency, pressure and amount of air, which is why it is 2D. A lot of the things in a turbo car that add or subtract from the turbos effective efficiency fall under these two factors. IE. an increase in backpressure affects the amount of pressure and airflow throughout the system, affecting the overall efficiency of the turbo. You boost less and because of that move les air.
If you look at fuel and timing maps, you'll see that they are 3D. That is because the amount of fuel is based off of 1) RPM 2) load and 3) timing. A change in any of those factors means an increase or decrease in fuel. Same thing with the 2D compressor map, an increase or decrease in pressure and/or CFM will increase or decrease the efficiency of the turbo.
...or something like that
My most basic interpretation of what a compressor map, is it is a chart that tells you how much work a turbo does. The inside part of the chart tells you how efficiently you are doing that work, and the left and lower edge tells you how much pressure and how much air it would take to work at XX% efficiency. There are only 2 main factors affecting turbo efficiency, pressure and amount of air, which is why it is 2D. A lot of the things in a turbo car that add or subtract from the turbos effective efficiency fall under these two factors. IE. an increase in backpressure affects the amount of pressure and airflow throughout the system, affecting the overall efficiency of the turbo. You boost less and because of that move les air.
If you look at fuel and timing maps, you'll see that they are 3D. That is because the amount of fuel is based off of 1) RPM 2) load and 3) timing. A change in any of those factors means an increase or decrease in fuel. Same thing with the 2D compressor map, an increase or decrease in pressure and/or CFM will increase or decrease the efficiency of the turbo.
...or something like that
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From: southwest, PA, USA
Re: Technical Discussion on Compressor Wheel Efficency (BlueShadow)
when talking about efficency the T3/T67 comes to mind(sorry bandwagoners) by looking at the exhaust and compressor wheels one has to think about wheels speeds. In my mind I would have to say that this turbo has to be bad for efficency. the T3 wheel on the exhaust side has a small diameter and takes very little to spin that side. on the compressor side you have this monster compressor wheel. now when the compressor is pushing out say 20psi how fast is that exhaust wheel having to spin to make the big wheel spin that fast. think of the forces on the T3 wheel at 20psi. centrifugal force would be amazing on that small wheel at higher boost levels. the point I am tring to make is that efficency of a mismatched turbo like this you would have to take into consideration more than just less boost= working less and more boost= working more. you have to look at the momentum, centrifugal force, CFM each wheel flows, etc. I am sure I am goin to get flamed for saying the T3/67 is a inefficent turbo but that is the way it is in my eyes
Joined: Sep 2003
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From: Vero Beach, Florida, USA
Re: Technical Discussion on Compressor Wheel Efficency (Cyphear)
UP: This is right.
DOWN: By putting less air through the turbine, you are putting less work to spin it. The vane position and location, as well as size of each wheel is specifically designed for a certian amount of flow energy. To give it less than that, the parasitic effects of the mass and vanes positioned for minimal flow restriction become a markable hinderance.
I won't go into detail about the harvesting of flow energy, but I can give an example. Anyone who was raised in central California (or has driven through there) has seen wind-powered electricity. These large fans have very few blades- usually 3 long slender ones- and their area is very small compared to the wind-face. These capitalize on the high flow energy of large wind masses moving ~20mph.
In the Utah mountains, there are wind-electic farms too. These fans are much different, as the air is much more sparse than in the central valley of California. The fans have a dozen broader and shorter blades, as the air here would go around the blades of a Californian fan. Utah fans need higher flow restriction to harvest the energy as efficently as possible.
LEFT: You are thinking of this in the wrong sense. It is not the turbo moving less air, it is less air moving the same turbo. Like guarding Shaq with Jason Kidd, the mismatch will create a larger inneficiency in work performed.
RIGHT: You got this right too...
A:
B:
B:
A:
Select:
Start: Just don't do this in the newer games! I had a ship explode on me!
DOWN: By putting less air through the turbine, you are putting less work to spin it. The vane position and location, as well as size of each wheel is specifically designed for a certian amount of flow energy. To give it less than that, the parasitic effects of the mass and vanes positioned for minimal flow restriction become a markable hinderance.
I won't go into detail about the harvesting of flow energy, but I can give an example. Anyone who was raised in central California (or has driven through there) has seen wind-powered electricity. These large fans have very few blades- usually 3 long slender ones- and their area is very small compared to the wind-face. These capitalize on the high flow energy of large wind masses moving ~20mph.
In the Utah mountains, there are wind-electic farms too. These fans are much different, as the air is much more sparse than in the central valley of California. The fans have a dozen broader and shorter blades, as the air here would go around the blades of a Californian fan. Utah fans need higher flow restriction to harvest the energy as efficently as possible.
LEFT: You are thinking of this in the wrong sense. It is not the turbo moving less air, it is less air moving the same turbo. Like guarding Shaq with Jason Kidd, the mismatch will create a larger inneficiency in work performed.
RIGHT: You got this right too...
A:
B:
B:
A:
Select:
Start: Just don't do this in the newer games! I had a ship explode on me!
Thread Starter
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From: KC, KS
Thanks for the replys!
Freemantle,
Tell me if i'm right on this.
Lets get back to my basic overall simulation of this question. Take a V8 @ 4psi or a 4Cylinder @ 20psi. Both are flowing 35lbs per minute at a given RPM. It seems to me the turbo is doing just about the same ammount of work in both situations, giving off 35lbs per minute of air. However, if one is efficent, the other will be far from efficent. I think the answer might be that while one wheel is designed to "push" against high boost, the other is designed to "push" against low boost. Hence the reason for the difference in inefficencys between the two situations.
Freemantle,
Tell me if i'm right on this.
Lets get back to my basic overall simulation of this question. Take a V8 @ 4psi or a 4Cylinder @ 20psi. Both are flowing 35lbs per minute at a given RPM. It seems to me the turbo is doing just about the same ammount of work in both situations, giving off 35lbs per minute of air. However, if one is efficent, the other will be far from efficent. I think the answer might be that while one wheel is designed to "push" against high boost, the other is designed to "push" against low boost. Hence the reason for the difference in inefficencys between the two situations.
Thread Starter
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From: KC, KS
Re: (Arturbo)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Arturbo »</TD></TR><TR><TD CLASS="quote">Up up, down down, left right left right abab select start.</TD></TR></TABLE>
someone needs to ban this guy!
you had me all excited for a post with cool high tech motor/turbo acronyms i hadn't heard of, but even better, now I can get unlimited weapons! (or was it invincible?)
someone needs to ban this guy!
you had me all excited for a post with cool high tech motor/turbo acronyms i hadn't heard of, but even better, now I can get unlimited weapons! (or was it invincible?)
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From: puyallup, or, usa
Flow through a centripital compressor varies at the square of shaft RPM. But mechanical internal friction varies directly with shaft RPM. The fluid friction varies directly with flow, which varies at an x^2 ratio with shaft RPM. The work energy of compressive force varies directly with the pressure difference.
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke
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Joined: May 2002
Posts: 15,814
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From: Dallas, TX, USA
Re: (liveforphysics)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by liveforphysics »</TD></TR><TR><TD CLASS="quote">Flow through a centripital compressor varies at the square of shaft RPM. But mechanical internal friction varies directly with shaft RPM. The fluid friction varies directly with flow, which varies at an x^2 ratio with shaft RPM. The work energy of compressive force varies directly with the pressure difference.
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke</TD></TR></TABLE>
You mispelled centrifugal...
lol, inside joke... ask Art about it..
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke</TD></TR></TABLE>
You mispelled centrifugal...
lol, inside joke...
ask Art about it..
Thread Starter
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From: KC, KS
Re: (liveforphysics)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by liveforphysics »</TD></TR><TR><TD CLASS="quote">Flow through a centripital compressor varies at the square of shaft RPM. But mechanical internal friction varies directly with shaft RPM. The fluid friction varies directly with flow, which varies at an x^2 ratio with shaft RPM. The work energy of compressive force varies directly with the pressure difference.
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke</TD></TR></TABLE>
Very informative. especially the first paragraph. Thanks!
When you combine these ratios, you will understand why its quite logical that compressor maps appear as they do.
Also, when looking at the pressure on a comp map, remember, dynamic pressure is just a resistance to flow. This is why the map will only have a pressure factor for trying to compensate for compression work.
Now, this being said, the map of a turbo is a very rough estimate on its best day. The choice and design of down pipe and down pipe mateing flange, the design of the inlet to the compressor, the thermal state and barametric pressure of the inlet charge, and the normal operating temp for the turbomachine all greatly effect the shape and numeric values of the map.
-Luke</TD></TR></TABLE>
Very informative. especially the first paragraph. Thanks!
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