What does a Turbine Map show?

[PEI330Ci]

Garrett and BW show them for most models, but what IS it showing?

From my data, none of it corresponds with what is going on with my engine. I have mass flow that exceeds the turbine map by factors of greater than 2, and the pressure ratio across the turbine far exceeds the range of the turbine map.

Can any one enlighten me?

[Kevin325i]

At it's simplest form the turbine map will show you a choked flow condition for a given wheel and housing.

It is entirely possible you're operating way right on the map because you've hit a choked condition. This is useful to show how much exhaust will be bypassed at choke.


It's possible you're choking the wheel and actual mass flow across the turbine wheel is significantly lower because you're bypassing a lot of exhaust gas via the wastegate.

The next most possible thing is that because you've choked the exhaust wheel and flow is sonic you're into a little bit different set of equations. The math doesn't actually hold up to be those perfectly horizontal lines on the turbine map. You can waste a lot of energy to the environment (heat) and be at very low efficiency with a huge expansion ratio and drive up mass flow a little bit.

[PEI330Ci]

Kevin, thanks for taking the time to contribute.

I'll expand a little bit.

I have a situation where I have measured exhaust flow of 70+ Lb/min, with waste gate duty cycle of about 50% to hold 19 PSI of manifold pressure.

The turbine wheel measures 55mm on the exducer, and the waste gate is a Tail 44.

The Turbine Map shows a maximum of 26 Lb/min of flow.

How can an orifice that measures 44mm, at 50% of poppet valve lift, flow nearly twice as much as the 55mm turbine outlet?


On the bigger turbos, like the GTX5533-98mm, compressor flow is 220 Lb/hr and the turbine flow is a maximum of 70 Lb/hr. That's a BIG delta, where a 102mm turbine wheel is normally mated to a 60mm waste gate valve.


I'm not trying to poke an argument out of people, I legitimately want to learn how to use a turbine map for system design.

[someguy2800]

Here is something to think about. Most hot rod'd diesels do away with the wastegate entirely and they just make what they make. In that case if the compressor is moving 70 lbs of air then the turbine is flowing 75 lbs or so because it has no other place to go. According to the turbine maps that is impossible, but its done every day. I have no idea why the turbine maps they generate differ so much from the real world or how to use them for anything. Seams to me that if they don't actually mean anything I don't know why they even post them. Actually I asked a garrett rep at PRI about 10 years ago how to make any sense of them and he basically shook his head and said don't worry about it.

[Def]

The turbine maps are accurate, you're likely just looking at the graph wrong. The left axis on Garrett maps is *CORRECTED* exhaust flow.

https://www.grc.nasa.gov/www/k-12/airplane/wcora.html

Once the flow chokes (line is flat), you still get more flow by increasing the upstream pressure which increases the density of the exhaust. But the speed in the smallest area is sonic (Mach 1), and cannot go any faster.

You're also bypassing a lot of wastegate flow at this condition, sometimes more than the total turbine flow.

The BW turbine maps that plot your swallowing parameter (phi) on a given wheel/turbine housing curve is a more useful configuration of the data, because it can determine other parameters when you do the curve fitting on MatchBot.

[PEI330Ci]

Def,

For the most part, the properties of air going into a Compressor are relatively constant. The Compressor Map that Garrett provides assumes this.

For turbines though, the properties of the air going into the turbine housing change a lot. "A" given mass flow, could have widely varying velocity through the turbine wheel because of changes in density. (Driven by pressure and temperature)

What is Garrett correcting? How do we use the Turbine Map to calculate a real world choke point?


And for reference, I have started to delve far deeper than the Nasa link you have posted. (Thanks for that, it will be useful to many)

Last month I ordered this:

https://www.springer.com/gp/book/9783319176437

I just haven't been able to start reading it yet....

[Def]

Def,

For the most part, the properties of air going into a Compressor are relatively constant. The Compressor Map that Garrett provides assumes this.

For turbines though, the properties of the air going into the turbine housing change a lot. "A" given mass flow, could have widely varying velocity through the turbine wheel because of changes in density. (Driven by pressure and temperature)

What is Garrett correcting? How do we use the Turbine Map to calculate a real world choke point?


And for reference, I have started to delve far deeper than the Nasa link you have posted. (Thanks for that, it will be useful to many)

Last month I ordered this:

https://www.springer.com/gp/book/9783319176437

I just haven't been able to start reading it yet....

Corrected flow is a term that denotes standard temperature and pressure conditions:

https://en.wikipedia.org/wiki/Corrected_flow

It's sea level and 59 deg F.

So you take real gas properties in your turbine, use the turbine map to see where it goes flat (choked flow condition), and then transform your corrected flow at a given pressure ratio to actual mass flow.


I'm sure the rotordynamics book has some good stuff, but I'd start with some basic Fluid Dynamics books to understand the fluids side of things first before jumping into the rotordynamics book. That's typically like a 4th year elective type course in a BSME curriculum. It's likely going to gloss over some of the basics with the assumption you know them (Fluids and Thermodynamics are more 2nd/3rd year courses).

[Kevin325i]

I have a situation where I have measured exhaust flow of 70+ Lb/min, with waste gate duty cycle of about 50% to hold 19 PSI of manifold pressure.

The turbine wheel measures 55mm on the exducer, and the waste gate is a Tail 44.

The Turbine Map shows a maximum of 26 Lb/min of flow.

How can an orifice that measures 44mm, at 50% of poppet valve lift, flow nearly twice as much as the 55mm turbine outlet?


On the bigger turbos, like the GTX5533-98mm, compressor flow is 220 Lb/hr and the turbine flow is a maximum of 70 Lb/hr. That's a BIG delta, where a 102mm turbine wheel is normally mated to a 60mm waste gate valve.

Like Def stated you'll need to work that 70 lb/min back to a corrected flow at standard temperature and pressure. You're probably closer to ~20 lb/min doing some rough napkin math of 300 kPA in the manifold at 1000K. But this is really all just guessing numbers.

The higher flow rate across the wastegate valve is going to be because of lower losses across the wastegate when compared to the turbine. In this case the losses are actually work to turn the shaft in the turbo.

Here is something to think about. Most hot rod'd diesels do away with the wastegate entirely and they just make what they make. In that case if the compressor is moving 70 lbs of air then the turbine is flowing 75 lbs or so because it has no other place to go. According to the turbine maps that is impossible, but its done every day. I have no idea why the turbine maps they generate differ so much from the real world or how to use them for anything. Seams to me that if they don't actually mean anything I don't know why they even post them. Actually I asked a garrett rep at PRI about 10 years ago how to make any sense of them and he basically shook his head and said don't worry about it.

Exactly! But again that is at elevated temperature and pressure so the resulting point on the turbine map will be a lower corrected flow rate.

[Def]

Here is something to think about. Most hot rod'd diesels do away with the wastegate entirely and they just make what they make. In that case if the compressor is moving 70 lbs of air then the turbine is flowing 75 lbs or so because it has no other place to go. According to the turbine maps that is impossible, but its done every day. I have no idea why the turbine maps they generate differ so much from the real world or how to use them for anything. Seams to me that if they don't actually mean anything I don't know why they even post them. Actually I asked a garrett rep at PRI about 10 years ago how to make any sense of them and he basically shook his head and said don't worry about it.

The Garrett published turbine maps are basically useless because there's no description of the turbine wheel to housing interaction besides a gross flow behavior. Some wheel/housing combinations have horrid efficiency across much of the flow regime, some are much better, but they might both be rated to the same or very similar "peak efficiency" number that's thrown up in the corner. That's probably where he was getting at the "don't worry about it." That and he'd have to explain how to correct airflow to std temp and pressure conditions, which isn't something most people are exposed to.

[PEI330Ci]

Does anyone have an excel spreadsheet that they have already worked all this stuff out with? Or am I going to be the first one?

I don't mind doing the work; just trying to capitalize on the knowledge of others.

[PEI330Ci]

Like Def stated you'll need to work that 70 lb/min back to a corrected flow at standard temperature and pressure. You're probably closer to ~20 lb/min doing some rough napkin math of 300 kPA in the manifold at 1000K. But this is really all just guessing numbers.

The higher flow rate across the wastegate valve is going to be because of lower losses across the wastegate when compared to the turbine. In this case the losses are actually work to turn the shaft in the turbo.

Good explanation, I'm going to simplify this with numbers.

Air at sea level, with a temperature of 68 deg F and pressure of 14.50 PSIa, has a density of 0.07419 Lb/ft3

The exhaust gas is at an average of 1654 deg F, and at mean pressure of 3.348 BAR (48.5 PSIa). Total average mass flow (across the sample period of 3.0 seconds in 4th gear) is 72.57 Lb/min

The density of air at 14.50 PSIa, and 1654 deg F = 0.01852 Lb/ft3

The density of air at 48.5 PSIa, and 1654 deg F = 0.06194 Lb/ft3

So we can see that the exhaust pressure has brought the density back up to 81% of ambient.

The other issue is that the choke point is governed by the speed of sound, and that changes with temperature and pressure. So you wouldn't be able to simply say that the flow rate should be 81% X 26 Lb/min....

[Def]

The corrected turbine map should still be valid, so it chokes at a given *corrected* mass flow. Don't worry about local gas speed or anything else, the mass flow equations hold up.

Knowing where you choke the turbine is useful because now your pressure rise for more massflow is a linear increase, not the exponentially more flow with increased pressure you achieved pre-choked. There's more flow capacity after choking the turbine, just know the pressure is going to shoot up in a hurry trying to coax much more flow out of it. Same thing on WG sizing, if you're right on the ragged edge while choked, you will probably need to step up the WG size if you significantly change the upstream pressure.

[hsvturbo]

Good explanation, I'm going to simplify this with numbers.

Air at sea level, with a temperature of 68 deg F and pressure of 14.50 PSIa, has a density of 0.07419 Lb/ft3

The exhaust gas is at an average of 1654 deg F, and at mean pressure of 3.348 BAR (48.5 PSIa). Total average mass flow (across the sample period of 3.0 seconds in 4th gear) is 72.57 Lb/min

The density of air at 14.50 PSIa, and 1654 deg F = 0.01852 Lb/ft3

The density of air at 48.5 PSIa, and 1654 deg F = 0.06194 Lb/ft3

So we can see that the exhaust pressure has brought the density back up to 81% of ambient.

The other issue is that the choke point is governed by the speed of sound, and that changes with temperature and pressure. So you wouldn't be able to simply say that the flow rate should be 81% X 26 Lb/min....

Not sure if this helps, my calculator at a fixed flow orifice will flow 36.26 lb/min air at 1654F @ 48.5 psia inlet (34psid), as compared to 72.57 lb/min air at 68F.