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Old 10-19-2004, 10:57 PM   #1
theformula175789
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Default Somebody Explain Turbine efficiency PLEASE!!

I have an issue ive been alittle lost on for awhile now. When I first got into turbo cars I assumed, like so many, that higher boost means higher airflow = higher power. To me that makes sense, granted no changes are made to the engine, by swapping turbo A with bigger turbo B, if turbo B creates more boost it must be flowing more air. Boost is a side affect created becasue the engine cant take in all the air that flows from the turbine. Right? So if more boost is created then more air must be flowing from the turbine, right?
But that does not add up becasue, taking my car as an example, with the stock turbo I had a max MAF V of around 4 at 18 psi. With my SR30 I have a 4.5 V at the same PSI. So, the SR30 must be flowing significantly more air while creating the saem pressure. Somebody please explain to me how this is possible.
I always thought that the HP difference between the two turbos was due to adiabatic effciency. But now Im thinking it must be a combination of increased adiabatic efficency and increased airflow, correct?
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Old 10-19-2004, 11:28 PM   #2
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Here's my hack:

Supposedly your answer lies in researching the Boyle and Charles' Ideal Gas Law, the definition of adiabetic, and Waals theory on thermodynamics.

The act of compressing air (increasing mass) causes the air to heat up. Based on the size of the compressor housing and the compressor turbine (and other factors such as different modifications to the turbine), the turbo will push out x volume of air that is y degrees at z rpm. When you try to increase the amount of volume, the temp will go up. If you try and increase the boost (volume of charge air) past what's commonly known as 'outside the efficiency range', the temp of the charge air may have reached a dangerous point, and the actual volume may not have increased by much.

That is what you call 'an idiot trying to describe something in layman's terms.'
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Old 10-19-2004, 11:56 PM   #3
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Turbine efficiency? Sounds like you're asking about compressor efficiency. Compressor efficiency is how much the compressor heats the air compared to an adiabetic process. (i.e., 100% efficiency would still result in a temperature rise with pressure increase at a rate of P1/P2 = T1/T2)

Here's a graph showing the relationship from Aquamist:



The important thing to remember is that the engine doesn't care much about boost pressure in regards to how much power it will produce. It cares about the mass of oxygen available to mix with fuel. The "extra" heat introduced by the compressor implies a lower density in the charge. Here's another graph from Aquamist that illustrates that:

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Old 10-20-2004, 01:03 PM   #4
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Ok, so your saying that the jump from 4MAF V to 4.5MAF V at the same boost and RPM is due the the adiabatic effciency of the turbine adding less heat to the air, thus the air is denser, thus the turbo can force a higher volume of air into the constant volume of the cylinder?

I thought that might be it, but that just seemed like to big an increase in airflow to be explained by air density change alone. SO thats it?
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Old 10-20-2004, 01:16 PM   #5
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The other side to this answer is in the turbine. In simple layman terms the larger and more efficient the turbine (how much gas it can flow) will also determine the amount of air that can be processed through the combustion process.

As explained, "boost" or excessive manifold pressure is a simple by product of combustion inefficiency. In other words, there's more air available then can be pumped through the engine. The more efficient the turbine housing, the more of that air doesn't "back up". Typically, the compressor and turbine are matched to the dynamics of the engine. In such, that the compressor can produce the airflow required of it efficiently, while having a turbine that allows it to move that air out.

Other considerations cause compromises in either effective expected output, or response (lag). But in the end, you try and find a balance that doesn't throw the BP/EP off.

In super layman terms, the more efficient the turbine, the more "airflow" you move with less of a "surplus" of air pre intake valve, since it's used and not stored in the manifold for the next intake valve to open.

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 10-20-2004, 02:59 PM   #6
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Quote:
Originally Posted by RiftsWRX
The other side to this answer is in the turbine. In simple layman terms the larger and more efficient the turbine (how much gas it can flow) will also determine the amount of air that can be processed through the combustion process.

As explained, "boost" or excessive manifold pressure is a simple by product of combustion inefficiency. In other words, there's more air available then can be pumped through the engine. The more efficient the turbine housing, the more of that air doesn't "back up". Typically, the compressor and turbine are matched to the dynamics of the engine. In such, that the compressor can produce the airflow required of it efficiently, while having a turbine that allows it to move that air out.

Other considerations cause compromises in either effective expected output, or response (lag). But in the end, you try and find a balance that doesn't throw the BP/EP off.

In super layman terms, the more efficient the turbine, the more "airflow" you move with less of a "surplus" of air pre intake valve, since it's used and not stored in the manifold for the next intake valve to open.

Jorge (RiftsWRX)
www.ProjectWRX.com
Or, in super-duper layman's terms, you've increased the VE (Volumetric Efficiency) of the engine by reducing exhaust backpressure.

Pressure is a sign of restriction, not flow. Turn on your hose with your thumb in the nozzle - you've got lots of pressure, but 0 flow. Let a little water through, and you've got flow, but less pressure acting on your thumb. Reduce the restriction (by pulling your thumb out), and you allow flow to increase, even with the same original pressure.
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Old 10-20-2004, 03:10 PM   #7
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This would be why clipping turbine wheels is a common way of getting more power at the cost of a little spool. More air has to go through the turbine to move it at the same rate as b4, and this lowers the pressure on the engine/uppipe side of the turbo. This is also why ya don't see many turbos with huge compressors and tiny turbines or the other way around. There has to be some kind of compramise between the two or there ends up being either too much flow from the compressor/poor boost control/a lot of pressure on the engine or slow spool/low flow. There's obviously a lot more that goes into it(which I don't know), but this is the ooberdoober laymans explination.

peace
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Old 10-20-2004, 04:55 PM   #8
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Are we talking turbine or compressor here. Compressor efficiency is the percentage of adiabatic heating. 100% adiabatic is the "ideal" compressors at best can achieive <80% of adiabatic.

Turbine efficiency is a measured value. At the best you can look at Garrett turbine flow numbers, for instance the GT25R turbine flows at best 18 lbs/min. That means anything above that is going through the wastegate. BIG PROBLEM, what temperature was this flow rate measured at, well, they dont say. Anecdotally, these turbine flow maps are done at "room temp" not exhaust temp.

I'm not sure what they are worth. Then there is the "black art" of turbine flow that is affected by housing size, design, and wheel size and design. There are some general rules, but most would agree that the rules are not very predictive. In fact they suck. At very high temperatures with the turbine turning at hear or post-sonic speeds the fluid dynamics don't follow normal rules. Strange things happen, like bigger wheels flow more exhaust, when you would think if the the wheel is obstructive it would flow less.

You also have pressure drop. To that degree that there is a pressure drop as exhaust passes through the turbine, work can be done. Why then does back pressure ultimately lead to less power, the PR is higher? Well, it's more complicated than just PR there is also turbulent flow that causes high back pressure behind the turbine, with no real work due to the drop.

In the end, it's empirical. Turbo makers spend little time telling customers how they have optimized the turbine side to our cars. In a word, they don't optimize, they mix and match and pray. Most or the time it does not work. Occasionally, the mix and match works wonderfully--FP Green is an example.

We need more measurement up and down stream of turbos to study efficiency and it ain't there. Even on the compressor side measurement is non-existent. So what if the SR55 has a wheel "capable" of flowing 55 lbs/min. The question is: can it flow 55 lbs/min on a WRX or STi. I doubt it, but no one tells you this.
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Old 10-20-2004, 06:27 PM   #9
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Quote:
Originally Posted by serendipity
Pressure is a sign of restriction and flow.
fixed that for ya.
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Old 10-20-2004, 07:06 PM   #10
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Quote:
Originally Posted by theformula175789
Ok, so your saying that the jump from 4MAF V to 4.5MAF V at the same boost and RPM is due the the adiabatic effciency of the turbine adding less heat to the air, thus the air is denser, thus the turbo can force a higher volume of air into the constant volume of the cylinder?

I thought that might be it, but that just seemed like to big an increase in airflow to be explained by air density change alone. SO thats it?
You have logs showing an increase from 4v to 4.5v MAFS at the same boost and RPM, around the same ambient temperature? With that large a jump I would start looking for boost leaks.
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Old 10-20-2004, 08:23 PM   #11
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he probably didn't take into account humidity and density altitude, but that would be a streatch to get a .5V jump, but I don't know much about SR30s.
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Old 10-20-2004, 08:47 PM   #12
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Or try lowering boost for more power, cooler charge temp, lean fuel for higher EGT to lower back pressure. The "power" is going somewhere, but not to the wheels.

If all you are doing is making heat, you boost will equate but not make power. P=nRT/V. Higher T, higher P. You don't care about P, you care about n. n = PV/RT. Optimize P, for highest n and lowest T.
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Old 10-21-2004, 12:34 AM   #13
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Quote:
Originally Posted by Macabre
You have logs showing an increase from 4v to 4.5v MAFS at the same boost and RPM, around the same ambient temperature? With that large a jump I would start looking for boost leaks.

More likely, his *peak* MAF went from 4v to 4.5v, (with the same peak boost) which is a different statement. Stock turbo peaks at 16 psi at 4krpm, for example, but only flows 4v MAF near redline (since boost drops as RPM increases, etc), while the bigger turbo maintains 16psi to redline, so better VE + higher boost at high RPM = .5v MAF change.

(I'm sure you already knew this, which is why you specified same boost and RPM, but what the hell )
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Old 10-21-2004, 09:35 AM   #14
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Quote:
Originally Posted by zaxrex
he probably didn't take into account humidity and density altitude, but that would be a streatch to get a .5V jump, but I don't know much about SR30s.
Seeing how I've been the only tuner on the car, it indeed did gain a .5v increase jumping to the SR30.

His car runs so well that he's run 12.3's on that SR30 on pump gas. Matt's car is one of those "wed. cars".

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Old 10-22-2004, 11:32 PM   #15
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Quote:
Originally Posted by bboy
We need more measurement up and down stream of turbos to study efficiency and it ain't there. Even on the compressor side measurement is non-existent. So what if the SR55 has a wheel "capable" of flowing 55 lbs/min. The question is: can it flow 55 lbs/min on a WRX or STi. I doubt it, but no one tells you this.
Gotta love having entire post erased... you'd think I'd learn at some point.

Anyhow, in summary, my post went something like this:
Are you referring to the average "local turbo shop" or a company like Garrett (owned by Honeywell, a $30 Billion company)? Although I haven't the slightest clue as to what kind of equipment Garrett has, I'm leaning towards "the best that money can buy." For a company of that size, anything less than that seems silly. Then again, Honeywell Transportation Power Systems (Honeywell Turbo Technologies (Garrett Worldwide Engineering HQ)) is about a block from StopTech HQ, so I might as well just go over and ask them. Shoot, I'll talk to those guys (and gals) at SEMA and see if I can weasel anything out of them.

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