Turbo Theory. What moves the turbine?
02-12-2004, 12:00 PM
#1
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Turbo Theory. What moves the turbine?
Many people like to think that turbos are a "free lunch" that it takes the "wasted energy" and trnasforms it into useful energy...
Everybody says that the differential in pressure from between the engine and the turbine and the turbine and the free flow exhaust is what drives the turbine... That exchange in heat is what "sucks" the air out of the turbine... No one seems to account that the piston is the one pushing those gases out against backpressure to move the turbine...
Can someone pleae elaborate and provide useful links about this?
On all the links i have found they only talk about the differential in presssure and that is it.. backpressure makes the pistons push out those gases on order to move the turbine...
Let me know what you guys think.
Everybody says that the differential in pressure from between the engine and the turbine and the turbine and the free flow exhaust is what drives the turbine... That exchange in heat is what "sucks" the air out of the turbine... No one seems to account that the piston is the one pushing those gases out against backpressure to move the turbine...
Can someone pleae elaborate and provide useful links about this?
On all the links i have found they only talk about the differential in presssure and that is it.. backpressure makes the pistons push out those gases on order to move the turbine...
Let me know what you guys think.
02-12-2004, 12:07 PM
#2
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Exhaust gas flow and heat energy is what moves the turbine.
You have losses until the turbo starts producing boost, and then the system is basically in 'closed loop'. More manifold pressure/flow equals more exhaust gas/flow.
You have losses until the turbo starts producing boost, and then the system is basically in 'closed loop'. More manifold pressure/flow equals more exhaust gas/flow.
02-12-2004, 01:16 PM
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That is a good question! The answer lies within the cam configuration. The amount of overlap should equalize the pressures between the exhaust and intake. In other words, when the intake lobe opens the valve, the pressure should be balanced between the intake and exhaust.
This is the major problem with people who generalize that the LCA for a turbo cam is 114-116... not true. If there is excessive back pressure, there are instances where there is 0-overlap... yes, ZERO overlap. You will have very short duration, you'll want high lift, and you LCA can be as much as 126 to get the lobe separated if the backpressure is extreme.
On a high flowing system with the turbo inches from the exhaust ports, the efficiency is higher and results in less back pressure. Less backpressure requires less time to 'balance' the pressures and more overlap can be implemented.
This is the major problem with people who generalize that the LCA for a turbo cam is 114-116... not true. If there is excessive back pressure, there are instances where there is 0-overlap... yes, ZERO overlap. You will have very short duration, you'll want high lift, and you LCA can be as much as 126 to get the lobe separated if the backpressure is extreme.
On a high flowing system with the turbo inches from the exhaust ports, the efficiency is higher and results in less back pressure. Less backpressure requires less time to 'balance' the pressures and more overlap can be implemented.
02-12-2004, 01:33 PM
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Yes... but what about that backpressure.. what pushes that backpressure through the turbine to move the turbine and create all that boost...
I do not think you can get to 1:1 boost vs backpressure.. in either case... that backpressure is pushed by the pistons to move the turbine. the pressure differential accounts to the ease of movement of the turbine regarding that same push.. but that push is done by the pistons hence the similar powerloss as the supercharger and its belt.
INTMD8.. all you are saying is to avoid contamination of the intake charge with exhaust waste... but what accounts for the turbine movement?
I do not think you can get to 1:1 boost vs backpressure.. in either case... that backpressure is pushed by the pistons to move the turbine. the pressure differential accounts to the ease of movement of the turbine regarding that same push.. but that push is done by the pistons hence the similar powerloss as the supercharger and its belt.
INTMD8.. all you are saying is to avoid contamination of the intake charge with exhaust waste... but what accounts for the turbine movement?
02-12-2004, 01:38 PM
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Originally posted by INTMD8
Exhaust gas flow and heat energy is what moves the turbine.
You have losses until the turbo starts producing boost, and then the system is basically in 'closed loop'. More manifold pressure/flow equals more exhaust gas/flow.
Exhaust gas flow and heat energy is what moves the turbine.
You have losses until the turbo starts producing boost, and then the system is basically in 'closed loop'. More manifold pressure/flow equals more exhaust gas/flow.
02-12-2004, 02:34 PM
#7
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its not FREE, but pretty close to it. THe exhaust gases WANT to go out of the exhaust valve, it doesnt take the piston pushing it out for it escape (remember, its an explosion in there!)
highlander-
it is a "closed loop" system at least compared to a supercharger. As hp goes up, exhaust energy goes up, which helps spin the turbo.
Turbos can actually get BETTER than 1:1 boost/exhaust back pressure ratio!
highlander-
it is a "closed loop" system at least compared to a supercharger. As hp goes up, exhaust energy goes up, which helps spin the turbo.
Turbos can actually get BETTER than 1:1 boost/exhaust back pressure ratio!
Last edited by JordonMusser; 02-12-2004 at 02:38 PM.
02-12-2004, 02:43 PM
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Then it should be called foward pressure instead of backpressure...
Also.. its the same thing of a closed loop in the S/C the more boost it makes the more power is at the crank to move the S/C so it doesn't apply perse...
If gases want to come out then the turbine is not a restriction and a turbine is a HUGE restriction in the exhaust flow, its just that it adds power due to the compressor.
If there was NO compressor at all, just the turbine on the exhaust do you think the car will make 98% of its power?
Have you guys tried to run your turbo engines w/o the inlet pipes on? you will see how much power it draws from the engine....
Also.. its the same thing of a closed loop in the S/C the more boost it makes the more power is at the crank to move the S/C so it doesn't apply perse...
If gases want to come out then the turbine is not a restriction and a turbine is a HUGE restriction in the exhaust flow, its just that it adds power due to the compressor.
If there was NO compressor at all, just the turbine on the exhaust do you think the car will make 98% of its power?
Have you guys tried to run your turbo engines w/o the inlet pipes on? you will see how much power it draws from the engine....
Last edited by Highlander; 02-12-2004 at 02:47 PM.
02-12-2004, 03:50 PM
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Backpressure is usually used in discussions on exhaust systems. So far as the exhaust cycle in the cylinder, the piston just scavenges the cylinder on the way up, the exhaust valve is open way before that, usually while the piston is still traveling down. So there is very little (if any) resistance on the piston traveling back up. This is also where you get reversion if the intake valve opens too soon, the piston will pump some of the spent fumes back into the intake.
A typical turbo impeller spins freely, so the restriction is the actual wheel and shaft, and the size of the turbine inlet. Most are pretty small, so you will induce some backpressure to the exhaust. But there is no hard fast number on how much HP it takes to turn a given turbo.
A typical turbo impeller spins freely, so the restriction is the actual wheel and shaft, and the size of the turbine inlet. Most are pretty small, so you will induce some backpressure to the exhaust. But there is no hard fast number on how much HP it takes to turn a given turbo.
02-12-2004, 04:36 PM
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EngineerMike explained it pretty well in another thread. For example, the STS system will cause moderate backpressure due to the less efficient exhaust flow/temps which requires a smaller turbine housing. The backpressure could be at 18 psi while the boost is 8-9 psi. In effect, the backpressure is twice the pressure of the intake and would restrict the exhaust gases from wanting to leave. The expanding gases naturally want to escape, but it is easier to push against 9 psi (on the intake) than 18 psi on the exhaust. You want little to no overlap on that system... you will want to balance the pressures as best as possible.
02-12-2004, 04:55 PM
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Re: Turbo Theory. What moves the turbine?
Originally posted by The Highlander
Many people like to think that turbos are a "free lunch" that it takes the "wasted energy" and trnasforms it into useful energy...
Everybody says that the differential in pressure from between the engine and the turbine and the turbine and the free flow exhaust is what drives the turbine... That exchange in heat is what "sucks" the air out of the turbine... No one seems to account that the piston is the one pushing those gases out against backpressure to move the turbine...
Can someone pleae elaborate and provide useful links about this?
On all the links i have found they only talk about the differential in presssure and that is it.. backpressure makes the pistons push out those gases on order to move the turbine...
Let me know what you guys think.
Many people like to think that turbos are a "free lunch" that it takes the "wasted energy" and trnasforms it into useful energy...
Everybody says that the differential in pressure from between the engine and the turbine and the turbine and the free flow exhaust is what drives the turbine... That exchange in heat is what "sucks" the air out of the turbine... No one seems to account that the piston is the one pushing those gases out against backpressure to move the turbine...
Can someone pleae elaborate and provide useful links about this?
On all the links i have found they only talk about the differential in presssure and that is it.. backpressure makes the pistons push out those gases on order to move the turbine...
Let me know what you guys think.
Bill
02-12-2004, 04:58 PM
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My calc's are about 3/4 the way down this page:
http://web.camaross.com/forums/showt...y&pagenumber=2
Exhaust energy is determined by its temperature, not its pressure. But, in order to extract energy from the hot gas, you must drop the pressure across the turbine housing.
Basically, no matter how efficient the turbo, it will add exhaust backpressure that the piston must work against during the exhaust stroke.
My calc's imply a 4 psi increase in backpressure for this combo, but remember that these calc's assume all of the exhaust goes through the turbine, but in actuality some of the exhaust bypasses the turbine through the wastegate, so backpressure will be higher than 4 psi.
Someone posted some datalogged exhaust pressures for a race turbo Mustang. He has measured around 16 psi backpressure on an 18 psi boost set-up, but his was an 8 second car with a very well-matched combo.
The general rule that I've read is that exhaust backpressure will be 2 X boost pressure in a typical street set-up, 1.5 X boost pressure in a well-matched street/strip turbo, and approaching 1 X boost pressure in a race turbo.
Mike
http://web.camaross.com/forums/showt...y&pagenumber=2
Exhaust energy is determined by its temperature, not its pressure. But, in order to extract energy from the hot gas, you must drop the pressure across the turbine housing.
Basically, no matter how efficient the turbo, it will add exhaust backpressure that the piston must work against during the exhaust stroke.
My calc's imply a 4 psi increase in backpressure for this combo, but remember that these calc's assume all of the exhaust goes through the turbine, but in actuality some of the exhaust bypasses the turbine through the wastegate, so backpressure will be higher than 4 psi.
Someone posted some datalogged exhaust pressures for a race turbo Mustang. He has measured around 16 psi backpressure on an 18 psi boost set-up, but his was an 8 second car with a very well-matched combo.
The general rule that I've read is that exhaust backpressure will be 2 X boost pressure in a typical street set-up, 1.5 X boost pressure in a well-matched street/strip turbo, and approaching 1 X boost pressure in a race turbo.
Mike
02-12-2004, 05:14 PM
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As far as the "what drives the turbo, pressure or heat?" question goes, from the ENGINEERING THERMODYNAMICS textbook:
"Thus Joule concluded that (du/dv)t=0; that is, the internal energy of the gas is a function of temperature only."
[It takes a while to get your mind around this fact.]
Therefore, the energy using to drive the turbo comes from the difference in temperature in exhaust gas before and after the turbine. That is why you can produce 18 psi boost using only 16 psi backpressure with the same mass flow rate, which at first glance seems to be a violation of the energy conservation laws of physics, but isn't.
The pressure drop is a byproduct of the temperature drop.
Mike
"Thus Joule concluded that (du/dv)t=0; that is, the internal energy of the gas is a function of temperature only."
[It takes a while to get your mind around this fact.]
Therefore, the energy using to drive the turbo comes from the difference in temperature in exhaust gas before and after the turbine. That is why you can produce 18 psi boost using only 16 psi backpressure with the same mass flow rate, which at first glance seems to be a violation of the energy conservation laws of physics, but isn't.
The pressure drop is a byproduct of the temperature drop.
Mike
02-12-2004, 05:24 PM
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Well.. from that standpoint it is free energy... then... why do superchargers still exist? i mean... there are many YS-trim guys running great times, similar to what turbo guys with similar matched turbine sizes are runing...
02-12-2004, 05:34 PM
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Ummm. . . No.
Turbo's add exhaust backpressure. It takes energy to push the piston up against this backpressure during the exhuast stroke.
Mike
Originally posted by engineermike
. . .My calc's imply a 4 psi increase in backpressure for this combo, but remember that these calc's assume all of the exhaust goes through the turbine, but in actuality some of the exhaust bypasses the turbine through the wastegate, so backpressure will be higher than 4 psi.
Someone posted some datalogged exhaust pressures for a race turbo Mustang. He has measured around 16 psi backpressure on an 18 psi boost set-up, but his was an 8 second car with a very well-matched combo.
The general rule that I've read is that exhaust backpressure will be 2 X boost pressure in a typical street set-up, 1.5 X boost pressure in a well-matched street/strip turbo, and approaching 1 X boost pressure in a race turbo.
Mike
. . .My calc's imply a 4 psi increase in backpressure for this combo, but remember that these calc's assume all of the exhaust goes through the turbine, but in actuality some of the exhaust bypasses the turbine through the wastegate, so backpressure will be higher than 4 psi.
Someone posted some datalogged exhaust pressures for a race turbo Mustang. He has measured around 16 psi backpressure on an 18 psi boost set-up, but his was an 8 second car with a very well-matched combo.
The general rule that I've read is that exhaust backpressure will be 2 X boost pressure in a typical street set-up, 1.5 X boost pressure in a well-matched street/strip turbo, and approaching 1 X boost pressure in a race turbo.
Mike
Mike