What spins a Turbine Wheel

[compression]

I believe what he's saying is that as the exhaust gases pass the turbine and enter the exhaust, they expand to fill the capacity of the exhaust piping, and since the volume has increased, pressure decreases, thus temperatures decrease. They're not expanding due to cooling as you suggest - rather the opposite. They cool because they've expanded.

so you are saying heat has absoluley nothing to do with spinning the turbine wheel?

 

[Denji]

With so many variables i dont really see how the number of moles in the gas could be constant, please explain again how this does not change, it seems like the only real constant here is the volume and of course R=.08206

The only times you can really account for moles being constant to my knowledge is when you are dealing with the same sample of gas, here however, its a constant flow, so it seems it would be impossible to say that, but you guys may have a better grasp of this than me, im no engineer, im a biology major that has taken a 2 semesters of physics and chemistry.

 

[huafist]

so you are saying heat has absoluley nothing to do with spinning the turbine wheel?

I'm saying there are factors that govern why the heat is what it is at certain points in the system. Heat is related to pressure in fluids. Heat has plenty to do with KE transfer because the cooler turbine wheel readily accepts the energy transfer from the hotter gases passing through the turbine housing. That's why when the turbo gets hotter, it's effeciency is decreased.

Saying heat has absolutely nothing to do with spinning the turbine wheel would be like saying "Heat is important, but pressure has nothing to do with it." You can't do it - they're all inter-dependant.

 

[brrrnntsshhhh]

ok I got a little lost while reading all of that , but I would have to disagree that the number of particles/atoms n would be constant, it would fluctiate a little bit, but nothing to figures of significance, that makes it practically constant. also i am unsure of how much pressure is in the turbo, but the ideal gas law doesnt apply to those at very high pressure, because when the particles are compacted so close together, the volume occupied by the molecules IS significant, though i doubt turbos to get to this high of a pressure, i'm curious to see if anyone has clarity on this aspect.

 

[DGajre777]

You got that backwards. Most axial flow engines use many stages of compressor since each stage only provides around a 1.4:1 pressure ratio, and a 1-3 stage turbine. There are exceptions, but most of the ones I've seen they are like that.

Yup, you are right. I got it backwards. I meant 10 stage compressor and 3 stage turbine. I should know this being a damn flight instructor...

 

[dsm-onster]

ok I got a little lost while reading all of that , but I would have to disagree that the number of particles/atoms n would be constant, it would fluctiate a little bit, but nothing to figures of significance, that makes it practically constant. also i am unsure of how much pressure is in the turbo, but the ideal gas law doesnt apply to those at very high pressure, because when the particles are compacted so close together, the volume occupied by the molecules IS significant, though i doubt turbos to get to this high of a pressure, i'm curious to see if anyone has clarity on this aspect.

It's understandable to suspect the number of particles fluctuate, but...

After the powerstroke, ideally all the air/fuel mix should be burned... Of course it's most often not. Yet, in the process no atoms are destroyed. Molecules are broken down and rearanged but the total (measurable) mass remains the same in a combustive proces. When you burn leaves, though all that is left is carbon, it is because the smoke has disipated into the atmoshere.

Does this make sense?

Esentially, in a basic chemical equation, both sides must balance. The total number of atoms in heach element must be accounted for.

 

[dsm-onster]

Heat has nothing to do directly with spinning because thermal energy is not converted to mechanical energy in a turbine.

However, heat affects spinning indirectly because it affects the air. More heat will allow more pressure and volume with the same number of particles of air. Volume and pressure is what spins the turbine.

Pressure describes the direct intereaction of one object onto another. Neither a change in volume, nor temperature, nor number of moles directly describe the intereaction of the molecules with the blades. So each variable other than pressure have as much of a direct or indirect affect as another. All three together produce pressure. You cannot describe pressure w/out volume, temurature, and moles:

P = nRT/V

 

[dsm-onster]

R = 8.315 joules/(mole x kelvin) for those wanting to know. Seems like a bunch of simple thermodynamics to me. The problem with the thermo in this realworld situation is that in thermo there is usually either an isobaric or isothermal, or isochoric process, meaning one out of the three components(volume, temperature, pressure) is held constant. Though the physics still hold, all three components are changing throughout the cycle of the motor, which creates a problem to an extent. I mean I can pump out equations and stick numbers into them all night to calculate values but it really does me no good because the physics aren't %100 applicable, that is, if you know physics. For instance, someone posted earlier that Work = the integral of Force x distance, which is also the same as pressure times the integral of volume, which leads to: W = nRT(ln(V2/V1). This equation will tell you the work done on a system with a change in volume which would be cool cause then you can calculate how much work occurs from the change in volume from the manifold to the turbine housing since the volumes are different but then again, this only applies to an isothermal process, where Temperature is held constant. The problem is the temperature changes throughout the cycle of the turbo, so the work equation is basically useless.

Absolutely! It is exceedingly difficult to describe this sutuation so that our heads don't explode. It certainly would require an understanding of multivariates. And still math equations only supply theoretical relationships. Not every mathematical law has been discovered.

My point here is to not think of it so much as equations, but apply laws to situations. I just got done reading a huge discussion about horsepower and torque on a university of VA car club forum. Those kids only know equations and how to put them in context, but they know nothing about how cars, much less turbo systems, really work.

Yup! I'm sure it would be difficult for them to calculate the reasons why a leaner mixture increases spool.

This is a much better way to observe real world experience. Equations allow you to speculate values. Applying Laws allows you to speculate reality. All we may know is the Law of Conservation of Energy and temperature increases pressure (the kenetic energy in a system of particles). But, with this and given actions, we can understand a real world occurance.

The volume of the system does not increase (it decreases).

The total mass of the system of particles does not increase or decrease.

 

[No_Skillz]

Pressure describes the direct intereaction of one object onto another. Neither a change in volume, nor temperature, nor number of moles directly describe the intereaction of the molecules with the blades. So each variable other than pressure have as much of a direct or indirect affect as another. All three together produce pressure. You cannot describe pressure w/out volume, temurature, and moles:

P = nRT/V

I have already addressed moles, volume and temperature. All things being constant, a rise in temp will also net a rise in pressure and/or volume. I think you're agreeing with me but I don't get what you're trying to say.

You quote PV=nRT all the time, but nothing in that says anything about thermal energy -> kinetic energy. My point is that heat does nothing directly to spin the turbine. It's all dependent on the flow of the air, which does depend a bit on heat. The affect of heat therefore is indirect.

 

[dsm-onster]

Heat/velocity play a large role in turbo spooling. That is the biggest criticism of the STS remote mount turbos. They have to run smaller hot sides to make up for some of the lost spool due to the heat/vleocity lost in the long piping leading to the turbo.

BTW, this is a great real world example. Thanx!

 

[dsm-onster]

I have already addressed moles, volume and temperature. All things being constant, a rise in temp will also net a rise in pressure and/or volume. I think you're agreeing with me but I don't get what you're trying to say.

You quote PV=nRT all the time, but nothing in that says anything about thermal energy -> kinetic energy. My point is that heat does nothing directly to spin the turbine. It's all dependent on the flow of the air, which does depend a bit on heat. The affect of heat therefore is indirect.

We're going in circles becasue we're agreeing w/ each other . . .

Thermal energy (heat, described in temperature), raises pressure. Pressure is kenetic energy. Thermal energy is the sum of the kenetic energy of the individual particles in the system. I think we're saying the same thing, right?

 

[No_Skillz]

We're going in circles becasue we're agreeing w/ each other . . .

Thermal energy (heat, described in temperature), raises pressure. Pressure is kenetic energy. Thermal energy is the sum of the kenetic energy of the individual particles in the system. I think we're saying the same thing, right?

Wtf are you trying to say? WTF

Just kidding. Yes we are.

So basically what we're both saying is that
Heat -> turbine is wrong.
Heat -> air -> turbine is right.

And so we've come to the conclusion that air spins the turbo.

Now correct me if I'm wrong, but I believe the reason that heat is bad for the efficiency of a turbo because it will eventually heat up the compressor side, thus making it blow hot air. Even though what's coming through the intake is the same pressure and volume, now there are less particles, and detonation in the engine is dependent on particles, not pressure or volume.

 

[dsm-onster]

Wtf are you trying to say? WTF

Just kidding. Yes we are.

So basically what we're both saying is that
Heat -> turbine is wrong.
Heat -> air -> turbine is right.

And so we've come to the conclusion that air spins the turbo.

Now correct me if I'm wrong, but I believe the reason that heat is bad for the efficiency of a turbo because it will eventually heat up the compressor side, thus making it blow hot air. Even though what's coming through the intake is the same pressure and volume, now there are less particles, and detonation in the engine is dependent on particles, not pressure or volume.

Heat is assisting in spool. I.e. look at some gains that have been seen when wrapping the exhaust mani with heat reflective and insulative materials. Heat causes more pressure. The pressure is what causes the higher spool. Air spins the blades but the amount of energy that the air has is dependent on the force of the piston AND the temperature of the gases. Letting the gases cool will slow the work done over time. nanokpsi brought out a very good point about the remote location turbo chargers and their slower spooling.

Too much heat will encourage coking in the bearing housing.

IMHO, the individual air particles does not contact the compressor blade long enough to have much affect on intake air temps. However, the adiabatic process certainly causes 99% of the heat rise in intake air temps.

 

[pneumo]

You guys have covered the math very well, so I thought I'd add some measurements to help you fill in the blanks.

TDO5 turbine inducer area; 177mm circumference x 9.5mm blade width= 1681mm^2 area.

turbine exducer area; 49mm exducer diameter (1885mm^2 area) minus 17.8mm shaft diameter (249mm^2area) =1636 mm^2 area.
So area decreases slightly as the exhaust goes through the turbine wheel.
These measurements do not account for the thickness of the blades, but generally the area occupied by the turbine blades is greater at the exducer, so the difference in area between the inducer and the exducer is even greater.

Backing up a bit, the area of the volute of a Mitsu 6cm housing where it first meets the turbine wheel is roughly 644mm^2, or the same size as a 1.1" pipe.

So it seems that exhaust gas velocity peaks in the volute at the entrance to the turbine wheel, decreases as it passes through the inducer, and rises slightly again as it leaves the turbine wheel in the exducer, and falling again after the turbine wheel. There are no mechanisms to increase temp or pressure in the turbine wheel/housing, so these changes in volume will lead to decreasing pressure where volume increases, increase in velocity where volume decreases and decreases in velocity where volume increases, and decreasing temp where volume increases.

 

[Ryan95tsiAWD]

IMHO, the individual air particles does not contact the compressor blade long enough to have much affect on intake air temps. However, the adiabatic process certainly causes 99% of the heat rise in intake air temps.

Just to add to what you are saying...

ANYTHING spinning at 150,000+ rpms (the turbo) is going to generate heat which would transfer to both hot and cold sides.

 

[1SloColt]

Heat is assisting in spool. I.e. look at some gains that have been seen when wrapping the exhaust mani with heat reflective and insulative materials. Heat causes more pressure. The pressure is what causes the higher spool. Air spins the blades but the amount of energy that the air has is dependent on the force of the piston AND the temperature of the gases. Letting the gases cool will slow the work done over time. nanokpsi brought out a very good point about the remote location turbo chargers and their slower spooling.

Too much heat will encourage coking in the bearing housing.

IMHO, the individual air particles does not contact the compressor blade long enough to have much affect on intake air temps.

Couldn't be more right

However, the adiabatic process certainly causes 99% of the heat rise in intake air temps.

I'm confused here. Q = transfer in heat, adiabatic = (Q=0). An adiabatic material will not transfer heat. Did I misinterpret the statement?

 

[DSM90AWD]

Just to add to what you are saying...

ANYTHING spinning at 150,000+ rpms (the turbo) is going to generate heat which would transfer to both hot and cold sides.

The direct heat transfer from the turbine side is negligible as the air is traveling ~.5 the speed of sound past the compressor blades. What does cause heat (as dsm-onster mentions) is the compression of the intake air.

I'm confused here. Q = transfer in heat, adiabatic = (Q=0). An adiabatic material will not transfer heat. Did I miss interpret the statement?

He's talking about the the compressor side of the turbo. Compressing air creates heat via the adiabatic process. Jeff Lucious wrote a nice primer on this subject

 

[1SloColt]

He's talking about the the compressor side of the turbo. Compressing air creates heat via the adiabatic process. Jeff Lucious wrote a nice primer on this subject

Right on, I knew I had to be misinterpreting the statement somehow, .

 

[suicidal2af]

I got into a discussion w/ someone about what propels the turbine wheel. He was saying the heat of the exhaust does not contribute to spool and that only the exhaust stroke forcing the gases over the blades causes the wheel to turn. Those equations and quotes prove that ALSO much of the energy propelling the wheel is from the hot gases expanding as they cool. They push the wheel along as they expand. I was proving that temperature differentials can yield work. This accounts for the tremendous difference in temperature between the exhaust gases in the manifold and the exhaust gases in the down pipe. All that heat energy just didn't disappear. It defies the Law of Conservation of Energy. It was converted to turbine wheel motion.

I never said that heat does not contribute to spool. I said that heat in and of itself does not spool a turbo; if you want to throw the ideal gas law around, PV is what spools a turbo.

I used the analogy of dropping EGT by 100c but adding two more cylinders of equal bore and stroke, and you argued that it will spool faster because it has more 'heat energy'. Adding cylinders would increase n, not T. In this situation, you've increased spool while decreasing the factor of the equation that you put all of the emphasis on.

n is not a constant. You can do lots of things to increase n. Building a stroker is one way. Adding an intake cam and upping the boost some will do it, too. So will running nitrous.

V is not necessarily a constant either. You can run a larger manifold, which will decrease V. You can run lower compression pistons, which will do the same thing, also slowing spool. Changing any of those will affect spool even with an identical EGT.

My point in the other thread is that velocity is the only thing that has a direct effect of how fast a turbine spins. Temperature can make a difference in velocity, and always will. But heat alone doesn't spool a turbo.

 

[dsm-onster]

That's ok. No one was arguing that T itself causes the turbine to rotate. A temperature differential (delta T) does. The heat energy does. Heat a child's baloon and it pops. Pressure increased even though volume did. No turbo operates in an atmosphere where the ambient temperature is 1200* F. Containing the heat is a method to increase spool. It increases pressure as directly as a decrease in volume increases pressure and as directly as increseing the number of molecules increases pressure.

Simply put, n is constant when the changes you describe are not made. I am sorry if my words lead you to believe that the number of particles cannot increase between different systems or different adjustments to the system. Setups are different... Different setups call for different ways to increase spool, different flows, different fuel and air consumption rates. However, in a particular system, n does not change during the exhaust stroke. Therefore n is constant when describing the work being done to the turbine wheel in that particular setup during the exhaust stroke.

PV. T has as much a relationship to P as V. P spools the turbine through delta V and delta T. There is no delta n. The only way to isolate P to one side of the equation is to Lump V and T together on the other. A change in T influences velocity as much as a change in V (volume) does.

This thing about direct influence or indirect influence is obviously a difference of opinion on what "direct" and "indirect" denotes. So I think this will just end up in a circular conversation. To me, if any one property can change the results of others then that one property has a direct influence on the system... Of course, nothing changes anything except change itself. So the only true direct influence over any state is change.

 

[No_Skillz]

What does cause heat (as dsm-onster mentions) is the compression of the intake air.

One of the things I hadn't thought of. I assumed the heat would be from the heat of the turbo, and let it go at that. Thanks Jon.