Turbo Compressor Surge – Sound & REAL Cause
There are two types of turbo compressor surge, one which happens when the throttle is closed after acceleration and the other which happens under full throttle. What causes surge and what can be done to stop it.
Key Takeaway
Contrary to consensus, I believe air velocity is the cause of compressor surge.
Lift Off Compressor Surge
This video gives an example of the sound of compressor surge/wastegate flutter/turbo flutter when lifting off. As it shows in the video the sound is caused by air moving back through the compressor because it has nowhere to go when the throttle is closed.
This isn’t ideal and causes extra wear on the turbo.
It happens because the recirculating valve/dump valve/blow off valve/BOV is not opening enough between gear changes. The valve is not letting the air pressure escape fast enough.
The solution is to soften off the BOV so it can open faster and more.
Full Throttle Compressor Surge
Full throttle compressor surge is much more damaging because the forces involved are greater. Compressor surge/turbo flutter happens when we are asking the turbo to operate in an area to the left of the surge line on the turbo’s compressor map.
Kyle Engineers has done an excellent video on full throttle compressor surge, this is the what the surge sounds like. Imagine this happening as the car is accelerating at full throttle. I think the R32 in this video also has some slight compressor surge.
Turbo Flutter/Compressor Surge Causes
The cause of lift off compressor surge we covered.
The cause of full throttle compressor surge is more difficult to explain and it might be said that we still do not know 100% why it happens.
Kyle in his video explains its the blades of the compressor wheel stalling, it’s an aerodynamic problem with the blades, they are no longer operating as intended. His explanation as to why this phenomenon happens is that the pressure differential between the compressor inlet and outlet is too large.
Kyle says something along the lines of “When we hear the sound, this is the pressure difference stabilising and when the pressure difference shrinks the stall stops. This is why we hear the sound coming and going.”
Doubts
Personally I am not sure I am convinced by this argument for two reasons.
First. As we get higher in the RPM range arguably the pressure difference between the inlet and the outlet of the compressor will get larger but we are less likely to get compressor surge in these circumstances. For this reason I do not think it is a pressure differential issue.
The second reason I am not convinced, is that as air flow increases the less likely we are to get compressor surge. More air moves us to the right on the compressor map, moving us away from surge, even though boost pressure ratio is the same.
Andre at HPA puts the cause of compressor surge down to the turbo trying to feed too much air into the engine, more air than the engine can handle and this pushes the turbo into surge.
where it cannot consume all of the air coming out of the engine, and this can push the turbocharger into surge.
https://www.hpacademy.com/previous-webinars/209-what-is-compressor-surge/
Personally I am not convinced by this argument either because we can run the same engine at a higher boost level and it won’t necessarily run into surge. There is not a direct relationship between boost level and surge.
We cannot even say that turbo RPM is a main driver of when surge happens because at low flow rates, a turbo RPM can be a problem yet at higher air flow rates the same turbo RPM is fine.
In short compressor surge appears unaffected by the pressure ratio (difference between inlet and outlet pressure of the compressor).
However compressor surge does appear to be affected by airflow.
It All Comes Down To Airflow Through The Turbo
I think Kyle & Andre are on to something when they say we are looking at an aerodynamic phenomenon but what is the real cause of this phenomenon?
At this moment in time, I think it’s down to airflow or more specifically, air speed.
The engine is physically the same in all conditions (RPM, load etc), same cylinder head port size, same exhaust size, same turbo size, same inlet size, you get the idea.
Therefore when we alter the airflow rate through an engine we are changing the speed the air is passing through the intake, the intercooler, the intercooler piping, throttle body and so on. But I think the key one is the turbo compressor.
At lower CFM rates, the slower the air is moving through the inlet. However the more boost the turbo is making, the faster the turbo is spinning.
And surge happens when we try to make too much boost at too low a flow rate.
Or to put it another way,
surge happens when we are spinning the turbo too fast versus the speed the air is travelling through the turbo.
And it is this difference in speed that causes the turbo compressor to surge/stall as described by Kyle and Andre.
Andre gives a real world example of a drag racer that is running into compressor surge and their answer is put an additional throttle on the intercooler piping to bleed off boost to avert compressor stall.
what they can do here is actually bleed off some of that excess air if the turbocharger runs into surge so that Colin or the driver can stay at wide open throttle.
https://www.hpacademy.com/previous-webinars/209-what-is-compressor-surge/
22:43 That’ll allow the turbocharger to come back out of surge, it can accelerate through into an RPM range where the engine can then accept all of the air flow again,
If I am understanding Andre correctly he is saying that bleeding off some boost is the reason why this method works at stopping the compressor from surging.
If I am right and airspeed is the real cause of surging, the reason this valve on the intercooler piping works is that is maintains airspeed through the compressor which stops the blades from stalling.
Wrapping Up
Bringing this article to a close I’ll summarise the key points.
- There are two types of compressor stall.
- Both are aerodynamic phenomenon.
- The actual cause of compressor stall, especially under full throttle is not understood completely
Good progress can be reported, but the paper will not be an unrestricted celebration of our successes because, after 75 years of research, we are still unable to predict the stalling behavior of a new compressor or to contribute much to the design of a more stall-resistant machine
https://asmedigitalcollection.asme.org/turbomachinery/article-abstract/138/1/011001/378558/Stall-Surge-and-75-Years-of-Research?redirectedFrom=fulltext
- but general consensus is that it caused by the engine being supplied with too much air.
- I disagree, my theory is that compressor surge is an aerodynamic phenomenon and it is caused because of the difference in speed between the rotation of the compressor and the speed of the air travelling through the compressor. The difference in speed is too large.
Solutions
Full Throttle Compressor Surge
To reduce the tendency of the compressor to surge at high boost/Pressure Ratios, we need to increase the speed of the air through the turbo. To increase the speed we need to increase the volume of flow. To increase the flow volume we can vent some of the air to atmosphere (like the throttle on the intercooler piping in drag racing example above) or we can electronically open our BOV/recirculating valve/dump valve at low air flow, high Pressure Ratio (PR) target conditions (and when compressor experiences surge).
Lift Off Turbo Flutter
The solution to turbo flutter when changing gear and/or closing the throttle is the same and full throttle. Use a BOV/Recirculating valve and open it when throttle is closed.
