What spins a Turbine Wheel

[dsm-onster]

I've started a new thread becasue this discussion was getting way off topic in the original thread.

So what spins a turbine wheel?

Admitting that there are more than one variable in the ideal gas law means that more than one variable can change the energy state of the wheel. The temperature of the gases exiting the cylinder cannot remain the same over time. It continues to decline releasing energy. This raises the pressure in the exhaust manifold and contributes a large amount of the total energy propelling the turbine wheel. See Stirling Engine Info and this Stirling Engine.

The Ideal Gas Law: PV = nRT

P = Pressure
V = Volume
n = number of particles
R = the gas constant
T = Temperature

With a spinning turbocharger, all that is constant is n and R, the number of molecules and the gas constant:

PV/T = nR

T can have as much of a direct effect on PV as n. Lets let the R issues go for the sake of simplicity. Temps change pressure drastically! (Note: a steam engine) However, n is constant here of course. There cannot be more mass before the power stroke than after (although Einstein debates this, we will leave it that there is no measurable loss in mass).

Pressure is defined in part by temperature. Further, it cannot be defined w/ out temperature. Temperature affects pressure as much as the number of molecules or the volume:

P = (nR)/(VT)

The exhaust stroke develops mass flow that blows on the turbine blades turning the wheel, but much more than just the energy from the piston's upstroke is propelling the wheel.

The Law of Conservation of Energy applies here. Where there is an temperature drop, energy is lost. It must be recovered somewhere!

As the temperature drops, the gases expand driving the turbine wheel even harder than simply blowing on it w/ the gases. This directly increases the rotational motion of the blade because, if the temperature of the outside (in the downpite and out the tail pipe) were the same, then a lesser amount of energy would be lost by the hot exhaust gases. This would be like putting the turbo system, including the exhaust, in a 1300* oven.

Energy lost goes somewhere. It just can't become non-existant. So it becomes mechanical energy turning the wheel.

Of course, mass flow spins the turbine wheel. The force driving the mass flow is greatly increased by the expansion proccess of cooling molecules. At least 1/3 of the energy lost in the combustion process is through heat transfer. Much of that is recovered in a turbocharger. That is why a turbo can propel the driveshaft at over twice the horsepower normally realized in an n/a appliation.

These equations I provide stand firm. There is a temperature differential accross the turbo. Therefore, the energy lost (going from the hotter exhaust mani to the cooler downpipe) has to go somewhere.

A turbo functions through an isentropic expansion/compression process:

W = m (H2 - H1) (work output/input of an isentropic turbine)
(H2 - H1) = Cp ( T2 - T1) (change of enthalpy using constant specific heat values)
R ln (P2/P1) = Cp ln (T2/T1) (Constant specific heat, isentropic expansion/compression process)

Cp is the specific heat at a constant pressure of the gas.
R is the Universal gas constant
P2/P1 Is turbine outlet pressure divided by inlet pressure.
T2/T1 is turbine outlet temperature divided by turbine inlet temperature.
W = work
m = mass flow rate
H2 and H1 are Enthalpy values

Sources:

Maximum Boost by Corky Bell

The Garrett website:

A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor ​

Take this test from a Honeywell site :
http://www.honeywell.com/sites/ts/tt/turbofactsbenifits_IQtest.htm. Pay attention to the answer to question #2.

As well, here are some other reputable sites that mention this:
Boosting Your Knowledge of Turbocharging (Part 1 of a 2 part series)
http://www.mustang50magazine.com/techarticles/17739/

Here's a Google Search.

 

[tstkl]

for a second there I saw this and tought... some newb doesnt know that exhaust gases turn the turbine wheel, and hes even a PROVEN member, but then I it was you and thought... thats even worse!!!!

now I see that your actually writing something worth reading on this forum, unlike this post I just wrote.

 

[compression]

?? oh my lord , you make my head hurt with equations and mathematical numbers and . damn. exhaust gases spin a turbo and i know you know that what is the reason for the question?

 

[dsm-onster]

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.

 

[nanokpsi]

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.

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.

 

[KeltonDSMer]

I think you make this too complicated to present your idea.

First of all, pressure is only slightly related to volume. The definition of pressure is not P=nRT/V...it is Force/area. PV=nRT is a relationship most gases in most situations abide by.

Work equals the line integral of Force*dR where R is a length or distance over which the force is applied. The limits of integration are simple because I do not know the actaul distance from the exhaust valve to the turbine, so lets use d as that distance.

This gives us work=force*d

Work=KE1-KE2 (kinetic energy). So force*d=KE1-KE2

Temp is defined as the average kinetic energy of a system, so 1/2mv^2 is directly related to temp.

Force is what drives the turbine, so lets examine what will increase this force.

Not much: We have F*d=KE1-KE2 and F=Pressure* area (area is constant)
-Higher dm/dt (change in mass/time), meaning burning more fuel/air per rev or running a higher RPM
-Bigger change in velocity of the exhaust. This is why big exhuasts help spool; the larger pressure drop accross the turbine creates a temp drop, (thus slowing velocity of the gas meaning that KE is dropped at the turbine).
-More pressure. I'm not quite how to explain myself without going in a circle but you get the idea


Well, looks like I did not clear anything up. In fact, I am more confused now than when I started typing.

I think we both are on the same page, but I'd like to think about this again and get back to it.

 

[compression]

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.

ok i understand that , but what are you trying to do? find a better exhaust manifold design?
i dont understand your motive for asking this question?

sure heat plays a huge role in turbo spooling.


----------------\--------------------/---------------------
hot gas==== === turbine wheel=== warm gases===
----------------/..........................\---------------------


there were hot gases they go through the turbine housing and come out a much cooler gas then they were and the turbine wheel is spinning, what do you think happened?

but i am still confused on how this knowledge will benefit you in increasing performance

 

[DGajre777]

Heat does play a big role in spinning a turbine wheel. This is why aircraft jet engines have (for example) a 10 stage compressor and 3 stage turbine. The 3 or more stages of turbine are to make the turbine blades spin as the exhaust gases cool and exit the jet engine.

Your friend who said that heat has nothing to do exhaust on a turbine wheel is wrong.

EDIT: Mixed up compressor and turbine stages.

 

[napkinthief]

I am confused on your calculations, but understand the concept. What would be a benefit of having a higher egt then? faster spool or more holding psi to redline?

 

[KeltonDSMer]

Re-read my above post before reading this if you have not already.

I have worked out the algebra in the previous post and have concluded that the arguments are all in the same.

Sure, you can say that higher pre-turbine temp increases the force on the turbine...temp is simply avereage kinetic energy.

The other guy saying that an increase in exhuast velocity will do the trick...true. Doubling the gas velocity increases KE by a power of two.

Spool is all about force on the turbine, (F=ma). Total flow depends on maximum work per time unit (power).

If you want better spool, increase the pressure. To do so (since area is constant), pressure must be increased to do so (P=F/A). In order to increase the pressure, KE or the number of molecules must increase. Cams, boost, all the things we normally change do exactly this...move more air per rev. Now an increase in average KE (temp) also increases pressure. Again, mass and or velocity will accomplish this.

There is no "dumping" of heat energy into the turbine. KE is KE weather you think about it as heat, or velocity and mass. What is important is that the work done on the turbo equals the KE of the gas before the turbine minus the KE post turbine...period.

Again, spool is proportional to force on the turbo for the same reason torque and acceleration are directly proportional (F=ma). Maximum flow capabilities depend on the maximum work/time the turbo can put out (just like horse power through the motor).

 

[Booster2k]

The higher velocity over the convex turbine blades causes a negative pressure. The positive pressure on the concaved side of the wheels pushes the blade in the direction of travel. This is what spins the wheel! Bernoulli's Principle, which was founded based on Newtons second law. Ohh and I believe the temp reduction is due to a pressure drop, all gases cool as pressure decreases.

 

[compression]

The higher velocity over the convex turbine blades causes a negative pressure. The positive pressure on the concaved side of the wheels pushes the blade in the direction of travel. This is what spins the wheel! Bernoulli's Principle, which was founded based on Newtons second law. Ohh and I believe the temp reduction is due to a pressure drop, all gases cool as pressure decreases.

this makes perfect sense as well

 

[Ryan95tsiAWD]

Very good information guys.

Basically to increase pressure without the usual upgrades, good heat management helps IE: turbo/manifold wrap...

So...does this mean that by using cast manifolds (that retain heat better than tubular) you would have an advantage right out of the box?

 

[MILITISINVICTUS]

My degree was from a college of business, so the below is not written as if I'm even somewhat trained in engineering.. for the sake of the conversation though, help me out with these tidbits:

"P = Pressure
V = Volume
n = number of particles
R = the gas constant
T = Temperature

With a spinning turbocharger, all that is constant is n and R, the number of molecules and the gas constant:"

n is constant? It seems to me that V would be constant at the manifold, and n would be more a function of the fluctuating temperatures of gases exiting the cylinder head.

"Those equations and quotes prove that ALSO much of the energy propelling the wheel is from the hot gases expanding as they cool. "

From my limited science background, I believe substances contract as they cool, with water being the only natural substance to contradict that rule.

I want to keep with your discovery process here, not trying to disprove anything mentioned. These bits are distracting me, right or wrong..

 

[aero_sallee]

This is why aircraft jet engines have (for example) a 3 stage compressor and 10 stage turbine. The 10 or more stages of turbine are to make the turbine blades spin as the exhaust gases cool and exit the jet engine.

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.

 

[KeltonDSMer]

It's just the damn KE transfer. Look at my previous posts.

 

[dsm-onster]

My degree was from a college of business, so the below is not written as if I'm even somewhat trained in engineering.. for the sake of the conversation though, help me out with these tidbits:

"P = Pressure
V = Volume
n = number of particles
R = the gas constant
T = Temperature

With a spinning turbocharger, all that is constant is n and R, the number of molecules and the gas constant:"

n is constant? It seems to me that V would be constant at the manifold, and n would be more a function of the fluctuating temperatures of gases exiting the cylinder head.

n is constant. n is the number of particles in the gas. However, the system includes the volume decrease of the cylinder as it cycles through the exhaust stroke. The number of the particles increase in the exhaust mani and turbine housing as the piston travels up, but the number of particles in the cylinder go down. The cylinder is included in the total system volume.

"Those equations and quotes prove that ALSO much of the energy propelling the wheel is from the hot gases expanding as they cool. "

From my limited science background, I believe substances contract as they cool, with water being the only natural substance to contradict that rule.

I want to keep with your discovery process here, not trying to disprove anything mentioned. These bits are distracting me, right or wrong..

We're not cooling the particles. We are giving them an escape route. Over the turbine blades. They consequently cool because some of the kenetic energy of each particle was used (when they struck the blades). This makes them slower and more lathargic.

Hot particles have more kenetic energy. They are bouncing around at high velosity. If the "room" they are bouncing in is small, then they will bounce off the walls for a long time until all the kenetic energy is expended. If a door to the room is opened, then they will speed out and the kenetic energy is lost. Refrigerators and air conditions function based on this concept. As the compressor compresses the inert gas in the heat exchanger, the gas heats up and the heat radiates from the exchanger removing some of the energy from the system of particles. Then a valve is opened and the particles flow to an evaporator. The particles greatly cool in the process an dkeeps your meat cool.

Substances usually contract, in part, because the kenetic energy of the system of particles has gone down and the particles have an opportunity to settle into a more compact state. Water and some other substances are different becasue of their molecular shape and polarity.

 

[huafist]

"Those equations and quotes prove that ALSO much of the energy propelling the wheel is from the hot gases expanding as they cool. "

From my limited science background, I believe substances contract as they cool, with water being the only natural substance to contradict that rule.

I want to keep with your discovery process here, not trying to disprove anything mentioned. These bits are distracting me, right or wrong..

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.

 

[dsm-onster]

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.

The volume of what has increased? The volume of the gases go down becasue the piston travels up the cylinder bore during the exhaust stroke. The turbine housing does not expand. Nor does the exhaust manifold. How can the volume of the gas go up if the space the gas fills does not go up?I do not know the volume of the exhaust manifold. But I doubt it is much larger than 500cc.

 

[huafist]

The volume of what has increased? The volume of the gases go down becasue the piston travels up the cylinder bore during the exhaust stroke. The turbine housing does not expand. Nor does the exhaust manifold. How can the volume of the gas go up if the space the gas fills does not go up?I do not know the volume of the exhaust manifold. But I doubt it is much larger than 500cc.

The volume of the exhaust plumbing post-turbo is greater than that pre-turbo.

 

[dsm-onster]

The volume of the exhaust plumbing post-turbo is greater than that pre-turbo.

But the gases have to travel through the turbine wheel to get there.

 

[dsm-onster]

The volume of the exhaust plumbing post-turbo is greater than that pre-turbo.

But the gases have to travel through the turbine wheel to get there.

It makes sense that the gases cool after entering the down pipe becasue there is more volume this allws the gases to expand and cool. But i'm discussing pre turbo only. The temperature steadily goes down as it travels down the exhaust. Howver, there is a sharp decrease in temperature at the turbo.

Pre turbo the gases are pushed by the piston in hte exhaust stroke AND the heat of the gases is high which provides more kenetic energy. This heat povides more pressure onto the inside wall of the exhast mani, the turbine housing, the piston top, the cylinder wall, the exhaust runner wall, and the turbine blades. If there were less heat then there would be less pressure on these surfaces. P = nRT/V. Since there is pressure on the turbine blades from temperature and ,of course, the reduction of volume, work is done on the turbine wheel. If there were less heat, then there would be less work done to the wheel. The reduction in volume does not account for all of the work done on the wheel. A large amount of work is done by the hot gases wanting to expand. It's like leaving a baloon in your car in the middle of the summer. They pop, because pressure goes up. Pressure goes up becasue temperature goes up.

 

[1SloColt]

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.

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.

 

[huafist]

But the gases have to travel through the turbine wheel to get there.

It makes sense that the gases cool after entering the down pipe becasue there is more volume this allws the gases to expand and cool. But i'm discussing pre turbo only. The temperature steadily goes down as it travels down the exhaust. Howver, there is a sharp decrease in temperature at the turbo.

The reason I brought it up is precisely because it has to travel through the turbine wheel. I probably should've read a little closer because I I wasn't aware that the conversation was restricted to pre-turbo.

Pre turbo the gases are pushed by the piston in hte exhaust stroke AND the heat of the gases is high which provides more kenetic energy. This heat povides more pressure onto the inside wall of the exhast mani, the turbine housing, the piston top, the cylinder wall, the exhaust runner wall, and the turbine blades. If there were less heat then there would be less pressure on these surfaces. P = nRT/V. Since there is pressure on the turbine blades from temperature and ,of course, the reduction of volume, work is done on the turbine wheel. If there were less heat, then there would be less work done to the wheel. The reduction in volume does not account for all of the work done on the wheel. A large amount of work is done by the hot gases wanting to expand. It's like leaving a baloon in your car in the middle of the summer. They pop, because pressure goes up. Pressure goes up becasue temperature goes up.

Right. Fairly straightforward fluid dynamics.

I wanna say though, this is a GREAT thread. Thanks for bringing this up.

 

[ZmanIV]

After reading most of this post I have come to the conclusion that My head hurts