Turbine Efficiency - Garrett Turbocharger 1

[NakedTestMonkey]

To my understanding turbine efficiency (in a turbocharger) is related between pressure ratio and mass flow. Wouldn't the efficiency also be related to the shaft RPM of the turbine as well as the angle of attack created by the housing's acceleration of the exhaust gas?

Ie. Moving from a .63 A/R housing to a .81 A/R housing would produce a smaller turbine pressure ratio at the same mass flow correct? Wouldn't it also deaccelerate the exhaust gas flow? How would this deacceleration affect the gasses interaction with the turbine?

And, if the turbine is turning at 150,000 RPM in a small turbocharger to produce a target airflow, and the compressor is enlarged to one that produces the same airflow at 100,000 RPM would this effective drop in operational RPM produce a more efficient turbine due to turbulance factors? or would it produce the same efficiency but with less overall work.

Reading up on this has left me somewhat confused, so hopefully I haven't confused you with the question lol.

 

[Warpspeed]

The trick with a turbo is to match the speed and energy required by the compressor, to the speed and energy delivered by the turbine.

Unless your application is something truly unusual or extreme, a standard modern off the shelf ball bearing turbo will likely offer the best overall performance.

The reason being, computer simulation, and very sophisticated testing by the turbo manufacturers these days. In most cases mixing parts will be a backward step unless there is a highly unusual reason for doing so.

 

[SMOKEY44211]

I believe the efficiency is determined by the delta T. of inlet vs. outlet temps. The greater the temp. difference indicates more heat is being converted to mechanical work. Your reasoning regarding A/R ratio of the inlet housing is correct but the are many variables that come into play that may make higher A/R housing more desirable. One that comes to mind if the operating parameters cause the exhaust flow through the housing to approach mach speed it may be desirable slow the exhaust gas down. Other reasons may include compressor map (trying to avoid surge regions), reynolds #'s, intercooled/not intercooled and a host of others.-------Phil

 

[NakedTestMonkey]

Warpspeed, I am running a Garrett GT3582R on my engine currently. It produces a little over 600whp @ 9200 RPM. Sizing the turbocharger in relation to the high flow requirements (10700 RPM Peak) and the small displacement (2.0L) was a bit of a task. Fortunately the surge housing was available for this turbocharger and it so far seems to work as planned.

However with the GT40R 82mm wheel being applied to the smaller T350 series turbine the turbo is giving me wonders as to it's turbine efficiency. I am pondering if I am losing spool-up due to the mismatch, and if I could get similar results moving up to the full GT40R large frame turbo.

From what I have seen the Garrett GT40R's 77mm turbine seems to be much more efficient if you can provide the sufficient exhaust gas flow. Problem is, my motor being so small doesn't even hit 30 lbs. / min. of flow until 6000 RPM. While I know it will provide excellent gains once my engine comes onto power, I am on the threashold for tolerable powerband.

I'm trying to gain a larger understanding of the fluid flow within the turbine before making the large decision to move to the T4.

 

[Warpspeed]

As you well know, there is going to be a tradeoff between power band and peak power.

A larger turbine will undoubtedly lower turbine inlet pressure and increase top end power, but at the expense of a higher boost threshold and a reduced power band.

Where turbine a/r (and compressor a/r) come into this mainly has to do with boost pressure required to achieve the required mass flow.

A two litre engine making 600Hp is going to run fairly high boost, and require a smaller a/r housings at both ends, with a correspondingly higher exhaust back pressure. The whole engine and turbo combination run much higher pressure ratios.

An eight litre V8 monster making the same 600 Hp will require less boost, less turbine inlet pressure, and larger a/r housings at both ends.

But 600 Hp is still roughly 900 CFM, and in both cases requiring a roughly similar sized turbo, but likely with radically different housing requirements.

 

[NakedTestMonkey]

Well within the turbo dynamics you sometimes can move up in sizing without alot of lag difference; which is essentially what I am trying to determine.

The engine makes ~630whp at 24 PSI, peak torque is at 6700 RPM, peak power at 9200 RPM.

I also have the benefit of running variable VE curves, as the engine I am running has a twin profile camshaft.

 

[nocutt]

To my understanding turbine efficiency (in a turbocharger) is related between pressure ratio and mass flow. Wouldn't the efficiency also be related to the shaft RPM of the turbine as well as the angle of attack created by the housing's acceleration of the exhaust gas?

I believe efficency in this context is only a thermal issue...on the other hand, effectivity or the ability to make the turbine more effective will be determined by such things like pitch, weight, speed etc of the turbine...

...Ie. Moving from a .63 A/R housing to a .81 A/R housing would produce a smaller turbine pressure ratio at the same mass flow correct? Wouldn't it also deaccelerate the exhaust gas flow? How would this deacceleration affect the gasses interaction with the turbine?And, if the turbine is turning at 150,000 RPM in a small turbocharger to produce a target airflow, and the compressor is enlarged to one that produces the same airflow at 100,000 RPM would this effective drop in operational RPM produce a more efficient turbine due to turbulance factors? or would it produce the same efficiency but with less overall work.

By gas flow I am assuming you are talking about velocity? ( I initially was think pulses)...Yes since the volute has increased then velocity also slows down...the effect is deceleration of the wheel as you pointed out. The turbine slows down...it is self evident as 'lag'. The only way I can think of to increase turbine efficiency is making the turbine do more work, the implication made here is increasing the pressure differential across the turbine (as already mentioned)...increase the inlet pressure, while reducing the outlet pressure...it still goes back to a thermal issue.
Paraphrasing Warpspeed...with the size of the current engine you will only narrow your powerband. The bigger volute is definately a potential, however as you well know..."nothing in thermodynamics is free"...

 

[womble]

I think you may need to also consider exhaust manifold and exhaust outlet design if stepping up to the larger frame turbo so you can spool it up and keep a decent power band. You could consider the following:

1. Going to a divided turbine housing (if offered) and use a pulse manifold, keeping it short as possible whilst not compromising flow.
2. Use a manifold material with a higher service temp and also some form of turbo support. This will allow you to use a thermal blanket or some other form of insulation on the header to increase exhaust temp to the turbine. You need to make sure the turbine housing and wheel can also cope with the higher temps.
3. Use a conical diffuser after the turbine. This is often used in turbomachinary to improve the pressure recovery after the turbine and improve its efficiency. Included angle is normally around 7-14 degrees. It should expand out to the diameter of your exhaust i.e. 3-4".

Hope this helps.

 

[NakedTestMonkey]

Womble,

First off thanks for the reply.

Secondly, I am planning to move up to the T4 divided housing with a new manifold design using 321 Stainless. The current manifold I have is 321 and it has held up to track usage and moderately aggressive anti-lag.

I am inquisitive about the usage of the diffuser. To a certain degree the v-band housing seems to incorporate a similar tactic. Albeit more for packaging reasoning, it might be easier to rework it to my current setup.

http://www.atpturbo.com/root/releases/images/release091905/3_TO_4INCH_TRANS_FEAT3.jpg