Turbo Compressor Surge – Sound & REAL Cause
What is turbo compressor surge? What causes it? Is it bad? How do we stop it? In this article I’m going to dive in to the types of compressor surge, the theories of why they happen, I’m going to give my take on the subject using the most convincing arguments I’ve found and then give some methods that reduce or stop surge in my experience.
The Basics
There are two types of turbo compressor surge,
-One which happens when the throttle is closed after acceleration
-Two which happens under/during full throttle.
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/or 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.
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.
I think the R32 in this video also has some slight compressor surge.
Is Turbo Flutter Bad?
For lift off flutter, some say yes other no (Stav-Tech).
I think general consensus for on-throttle surge is it is bad for the life of the turbo. And it also reduces the performance of the engine because of the fast boost pressure fluctuations which make for uneven power delivery and can also effect the reliability of the engine because the ECU is having to keep up with rapidly fluctuating boost pressures/air flow volume through the engine.
Generally speaking, I don’t think it will shorten the life of a turbo if we don’t have any and it won’t hurt engine reliability if we don’t have any. And further, turbo surge does not improve engine performance or power.
YouTubers – Turbo Flutter/Compressor Surge Causes
Two of the best tech channels for cars I have found are Kyle Engineers and High Performance Academy.
Both of these channels have done videos on compressor surge so lets dig in to what they have said.
Kyle Engineers
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.
High Performance Academy
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.
I think this description is fair enough but it doesn’t explain why the turbo enters surge. Yes we are operating to the left of the surge line but what is the surge line? What creates the surge line? Why is the boost pressure unpredictable in surge?
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 is down to airflow and more specifically, air speed.
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,
I think Andre is almost right here. Yes bleeding off the excess air is stopping the surge. But this is not the root of the solution. Bleeding off the air is maintaining airspeed through the turbo compressor and this is what is pulling the turbo out of surge. Maintaining air speed is the solution and this is achieved by bleeding of air.
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
- General consensus is surge is caused by the engine being supplied with too much air. Yes this is true, by why is too much air a problem? What makes the air “too much”? Because air speed drops too far vs the speed of the compressor wheel.
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 solution as full throttle. Use a BOV/Recirculating valve and open it when throttle is closed.
