What happens when a turbocharger hits it's stonewall point?

[Lith]

Hi all,

My first physics question here - hope it's worthy and someone has some experience here :)

Hypothetically speaking, say you have an engine capable of swallowing 85lb/min of air at an absolute pressure of ~280kpa at sea level running a turbocharger running the compressor which the attached compressor map describes, and then try and run the setup at that pressure - assuming that there is still wastegating happening when the turbine speed reaches the 128100rpm stonewall point... what would you expect to happen next?

I have always pictured that so long is there is wasted exhaust energy at that point then the boost control system will be able to shut the gate and increase turbo rpm in an effort to increase flow, which will fail as the inlet air velocity will already be sonic - so turbo rpm will be forced to rapidly increase in vain until there is no longer sufficient exhaust flow through the turbine to further gain turbo rpm and everything will level off.

Does this sound plausible, or are there any other explanations people may have? Any input would be appreciated!

Cheers :)

 

[JBA]

By Googling "turbocharger overspeed" you will find multiple references on the subject.

 

[Baluncore]

Welcome to PF.

when the turbine speed reaches the 128100rpm stonewall point... what would you expect to happen next?

I would question your assumptions about the data provided. Top speed is marked as 128.1 RPM, not 128100 RPM.
Are you sure that is turbo-compressor?

85 lbs air/minute requires about 5.7 lbs of fuel/minute. At a specific fuel consumption of 0.33 pounds per HP hour that suggests a 1000 HP engine. So I would suspect that is the RPM of a heavy marine diesel engine driving something like a roots compressor.

It is hard to tell without having a reference back to the original data.

 

[Lith]

By Googling "turbocharger overspeed" you will find multiple references on the subject.

I have done so before and the references talk about it as a magical thing but don't elaborate on what's happening - like if the speed actually literally exceeds stonewall or not.

Welcome to PF.

I would question your assumptions about the data provided. Top speed is marked as 128.1 RPM, not 128100 RPM.
Are you sure that is turbo-compressor?

85 lbs air/minute requires about 5.7 lbs of fuel/minute. At a specific fuel consumption of 0.33 pounds per HP hour that suggests a 1000 HP engine. So I would suspect that is the RPM of a heavy marine diesel engine driving something like a roots compressor.

It is hard to tell without having a reference back to the original data.

Good spotting, I am 100% certain - that should be krpm. Here is a reference: https://www.full-race.com/store/borg-warner-efr/turbos-efr-series/borgwarner-efr-8374-turbo-2/

 

[Baluncore]

The link provided gives a diagram that shows maximum velocity 560 m/s.
560 m/s = 1840 ft/s = 1252 mph.
That is the speed of sound in the compressed air, which will limit the airflow.

 

[Lith]

The link provided gives a diagram that shows maximum velocity 560 m/s.
560 m/s = 1840 ft/s = 1252 mph.
That is the speed of sound in the compressed air, which will limit the airflow.

Yes, I've acknowledged the airflow restriction in my original post - can you please read my original post to check the question I asked, or let me know what part I've not made clear enough?

I realize that the airflow cannot be increased after sonic velocity BUT I want to know if I'm right in suspecting that turbine rpm can continue to increase if there is sufficient excess exhaust energy available to accelerate the turbine further... which I'm guessing may result in a vacuum at the inducer or something as air can no longer "fill the gap" fast enough to keep up with requirements?!

 

[Baluncore]

I realize that the airflow cannot be increased after sonic velocity BUT I want to know if I'm right in suspecting that turbine rpm can continue to increase if there is sufficient excess exhaust energy available to accelerate the turbine further...

If the compressor cavitates the airflow will fall. There can be no more air, so no more RPM, so no more exhaust energy to accelerate the turbine. The wall is in a feedback loop.

 

[Lith]

If the compressor cavitates the airflow will fall. There can be no more air, so no more RPM, so no more exhaust energy to accelerate the turbine. The wall is in a feedback loop.

Cool thanks for the response, that is getting closer to what I was looking for - however I'm not sure if you read what I said about the fact that in my hypothetical situation it hit the stonewall limit with exhaust energy to spare due to the wastegate dumping gas at the time... your comment reads to me like you are assuming there is no wastegate and it's using all available exhaust energy to drive the turbine at that point, but my assumption is that there is a given amount being dumped - so while the airflow no longer increases, the gate closes and the exhaust energy directed at the turbine DOES increase as previously some of the gas was being dumped.

I mentioned all that in the original post, are you saying that one of my assumptions there are inaccurate or impossible or did you still not read the whole post? I realize that there is a possibility that I could be wrong about what some of the things I hypothesized about may be incorrect, that is what I am hoping to know - just so far you've just reaffirmed bits I've already addressed unless... unless I have missed something.

 

[Baluncore]

You have neglected to plot the speed of sound against air pressure.

If the waste gate is open then the air pressure is regulated. If you close the waste gate, compressor outlet pressure will increase, so the speed of sound will increase, and the air flow velocity can again increase to the new limit.

Don't forget that the air inlet duct will become the limiting factor at some point.

 

[JBA]

Since you didn't find the information you required on my first search reference, I think you will find better information searching "turbocharger stonewall point" where sonic flow choking in the unit is discussed.

 

[Lith]

OK to clarify here - I understand that you can't exceed the stonewall point in terms of flow, and I know that in flow in can't be exceeded then flow out can't either... I am asking about whether it's possible to drive the turbo speed past that point if there is more exhaust energy available than is required to hit the stonewall.

None of those searches I've done so far explore that, and none of the comments so far address it at all. I've learned all I've learned that way, asking is NOT my first normal line as I like thinking and researching but the thing that brought me here was that this particular point is one I've not seen tested or discussed specifically and I erroneously assumed that once I reach that point that a forum like this would be a good place to bring it up and discuss - but so far it seems the culture is to tell people to search themselves, or answer different questions to what I asked.

It's clearly possible to go past such the stonewall point in rpm (not flow) if you are using a belt/crank driven centrifugal supercharger and I feel that using an exhaust-driven supercharger that it could be too, I wasn't expecting the answer to be given on a platter but I guess I'll have to look elsewhere for a decent conversation on the topic - or perhaps even try and find a way to test it myself.

 

[Tom.G]

Not being very knowledgeable about turbos, I ran across this; sounds close to your query, maybe it will give a lead to something useful.

https://www.turbobygarrett.com/turbobygarrett/turbo_speed_lines

Many hits from:
https://www.google.com/search?source=hp&q=stonewall+point+of+turbochargers

Hope it helps! Sometimes we get too wrapped up in "giving an answer" for answers sake.

 

[Lith]

Not being very knowledgeable about turbos, I ran across this; sounds close to your query, maybe it will give a lead to something useful.

https://www.turbobygarrett.com/turbobygarrett/turbo_speed_lines
Hope it helps! Sometimes we get too wrapped up in "giving an answer" for answers sake.

OH! Legend! I've read most of Garrett's stuff on reading compressor maps and explanations of terms and realistically for the most part they don't plot it like an rpm stonewall in the way Borg Warner did in the compressor map - instead defining a cut off for compressor efficiency, though the way they have written that caption it basically paints a picture which suggests my theory is correct.

Thanks for the effort mate.

 

[jack action]

I am asking about whether it's possible to drive the turbo speed past that point if there is more exhaust energy available than is required to hit the stonewall.

What happens when a turbocharger hits it's stonewall point?

You get a normal shock.

This is why you see the 128k rpm drops vertically when at 79 lb/min. When you try to put more energy, the speed doesn't increase because the flow is choked. But the normal shock increases in intensity. The greater is the normal shock, the lower is total pressure after the normal shock. The energy is transformed into heat, i.e. an increase in temperature.

This is why it doesn't matter that your engine can pull 85 lb/min @ 280 kPa: The compressor cannot deliver that. So the energy in the exhaust will not increase as you think. It is very unlikely that the wastegate will be open at that point. And trying to increase the compressor speed will lower the engine inlet pressure, which will lower the exhaust energy level, which will lower the compressor energy input and you get into a descending circle that will lead to some stabilization point.

@boneh3ad can probably better explain the phenomena.

 

[Baluncore]

I am asking about whether it's possible to drive the turbo speed past that point if there is more exhaust energy available than is required to hit the stonewall.

What you seem to be arguing is that the stonewall is not a stonewall. It seems that you don't want there to be a stonewall. What would happen if an “irresistible force” acted on an “immovable object”? If the impossible occurred, what would happen? Your hypothesis suggests there might be “more exhaust energy available” when the definition of a stonewall makes that impossible. By definition, once you hit the ultimate airflow limit there can be no more exhaust energy available to increase airflow. The stonewall must be defined by airflow, not by compressor RPM. The airflow stonewall is curved because it is pressure dependent.

If you can do something that invalidates an earlier false limit, then the real or ultimate stonewall may again be approached. The waste gate was just a distraction. You must restrict the waste gate to the minimum waste flow permitted to find the real stonewall.

Turbo-chargers operate in a closed loop. Once inlet air is limited, combustion is limited and the exhaust flow is limited, so the compressor airflow is limited. That is the real stonewall. The RPM of the turbocharger is not really that important.

Superchargers can be driven by the engine shaft itself in which case, as with the turbocharger, the stonewall is in a self limiting closed loop. But a supercharger that is driven by a different motor may increase airflow until flow is limited by the geometry, dimensions and speed of sound in compressed air. That sets the air available and so the fuel consumption of the engine. Supercharger RPM may be increased by running the charger motor faster, but once airflow is limited at the stonewall, there can be no more air. The stonewall is an airflow stonewall not an RPM stonewall.

 

[HowlerMonkey]

Don't forget to include the specifics on the turbine that powers the compressor.

 

[Rx7man]

Don't forget about the mechanical limits of the turbo.. go too fast and you could quite possibly have a turbine or compressor explosion... especially at the peak pressure area where surge is possible and adds enormous stress to the compressor.
Also, when you start to operate a compressor significantly out of it's efficiency range, increasing the pressure doesn't make up for the increased heat.. IE, you have more pressure, but not more mass flow... Especially considering the turbine will have to work harder to get there, meaning higher exhaust pressure, meaning more remaining exhaust gas in the combustion chamber taking up space for the fresh air to come in.

I had a programming bug in my Holset 351VE controller on the dyno.. That turbo is good to about 40 PSI.. I accidentally got it to 58 PSI, my exhaust pressure was above 100PSI, and it made some very unhappy noises.. well.. it darned near blew up.. Turbo is rated to 125,000 RPM, datalogs show it up around 140,000... I got lucky!

 

[HowlerMonkey]

It's more diminishing returns than a brick wall.

 

[Rx7man]

It's more diminishing returns than a brick wall.

True..
Also, the compressor map ONLY deals in how the compressor behaves and is completely separate from the system as a whole.. Usually the compressor map is limited by the structural limits of the compressor wheel and the speed of sound.
Looking at the entire system is different.. you can see the surge limit on the top/left side of the compressor map, but it's not said the system as a whole can ever push it to those limits.
A very interesting tool is the Borgwarner "Matchbot", which is a more comprehensive look at the system as a whole
http://www.turbos.bwauto.com/aftermarket/matchbot/index.html#version=1.4