EvilBevel

Technical question: a chap I know lost his VF23 turbo (vane broken off on the inlet side) last week, and IHI examined it and thought the cause was "overspeed".

Could someone explain to me exactly what that means ? Pressure too high ? Too high at high revs ? Something completely different ?

Worth mentioning is that his dumpvalve was behaving irratically (not blowing off at times), but in my limited knowledge that would normally only have an effect on the bearing (if any, 360° bearing should be able to cope?), not on the vanes.

Thanks for any info on this.

Theo

Adam M

I know nothing about turbos, but have heard of over spooling them, ie forcing them to rev too high. If the waste gate doesnt open, then all the exhaust will be blowing through the impeller forcing it to spin faster and faster. It must be possible for the blade tips if over spooled to go super sonic, in which case, the shock wave may well damage the blades.

I was once told too that running without a dump valve was the equivalent force of hitting each blade with a hammer. This sounds like it could be the case!

Bob Rawle

No thats not right, over speed is when the "system" resistance of the engine/exhaust is such that the exhaust gas velocity is great enough to force the turbo up in speed, as the system increases the turbo will speed up as there is less load for it to work against (check out the fan laws). so overspeed is when you "ask" the turbo to produce too much boost at a small throttle opening. The dump valve not working will cause reverse pressure to try and stall the impeller (as opposed to stall as in overspeed)which will cause the fragile IHI impeller to lose blades.

In short if you run a turbo on the wrong side of the surge line as defined by its compressor map then you could cause it to overspeed and hence fragment blades.

Typically this happens under part throttle and relatively light engine loads.

Sith

Ok, I'm lost again! Going through one of my dense patches.

Bob - When you say 'system resistance' do you mean back pressure through exhaust system and down pipe off the Turbo? ie not sufficient back pressure pushing against the turbo, allowing it to over speed.

Cheers,

P.

BugEyed

Let's not get into a discussion on the merits of backpressure - that is for ur cousins over the water.

The basic problem is that the turbo runs too quickly. Causes can be the wastegate failing to open, or the inlet stalling due to the failure of the inlet side.

In the case you are describing, it sounds as if the problem is (was?) the dump valve. This appears to have caused the inlet to have stalled, hence the problem. If too little air reaches the impeller when the tubo is running at high speed, it will stall with the possible result of blade failure.

In short, make sure the dump valve works, or sort the turbo wastegate control such that it does not cause the the turbo to boost except when the throttle is open.

Duncan

pat

There is another possibility for overspooling the turbocharger.... a large leak in the intake system will prevent the turbocharger from producing sufficient pressure to open the wastegate, with the result that the blade tips could end up going supersonic, which is not a good thing. But it's all a bit involved.... if the leak is large enough then there will not be enough air getting in to produce enough exhaust gas to cause the turbine wheel to drive the impeller to supersonic speed...

There is also one more possibility, and that is severely retarded ignition timing, very little useful energy is extracted from the charge in the cylinder, and as a result the exhaust valves may get burnt, and the large amount of energy in the exhaust gas will cause the turbo to spool faster... but the wastegate should open, just a question whether it can bypass enough gas.

Finally, if you get a very severe misfire on high boost, it can cause the charge not to light until it hits the manifold. This is pretty disastrous, as there will be so much energy that even a fully open wastegate would not be able to flow enough gas bypassing the turbine, ie it would just keep building speed even if the wastegate was wide open. I think that even my wastegate might be struggling with this, even though it is rated to one thousand BHP.

All of these problems would cause the impeller to go supersonic, the shockwave from which can shatter the blades. Fans do not like going supersonic!

I always regarded shock loading / reversion due to lift off / crossing the surge line as a separate issue.... ie the impeller was generating a level of boost that was just not sustainable at that flow rate (basically it was pushing gas that had nowhere to go) and therefore reversion occured (the compressor stalls, air flows back). But you learn something new every day


Cheers,

Pat.

Adam M

Thanks for clearing that up Pat,

it makes sense.

I knew you had said something previously about the turbo going supersonic.

Bob,

Is what you are suggesting similar to putting your hand over the end of a vacuum cleaner and so reducing the volume of air that the fan has available to push, hence reducing drag, causing it to speed up?

Bob Rawle

Adam, yes that is what its about, a fan can produce pressure with "no flow" and the lack of load allows it to speed up, as you say, sticking your hand over the vac simulates the issue with the turbo except the speeds are much greater. In my book stall is when the turbo overspeeds due to restriction in the outlet (part throttle). A fully stalled fan means its not moving air but could be producing pressure, it is certainly not physically stopped though.

submannz

This is a very common problem with the Subaru twin turbo models after the cat converters are removed. The cat converters creates the required back pressure to stop the roller bearing turbo from over spooling. If anyone wants to put a big bore on the twin turbo models make sure it's only a cat back system.

Pete Croney

Another point to consider is cavitation.

The exhaust gas is accelerating the impeller at a rate that is faster than the intake air is accelerating to fill the vanes. The impeller reaches the point where the leading edge doesn't cut into enough air so it spins in a vacuum, effectively with no resistance.

Boom.

AJbaseBloke

Plenty of TTs have gone in this way here, quite a few STis too.

Speaking mostly of the TT, it is well known here that without very good boost control (and generally well balanced and thought out tuning), these engines will let go as fast and completely as Pete's last word/sentence/paragraph above.

Cautiously tune her...but never the less, enjoy The good tuning guys will look after you and your mate. Good luck.

[This message has been edited by AJbaseBloke (edited 07 August 2001).]

Bob Rawle

And that description Pete is called "stall" in the world of fans. Pressure but no air, if you could listen to the fan (turbo compressor) then you would hear it "chuffing" as the air cavitates, this is partly what happens when the turbo is run on the wrong side of the surge line.

AJbaseBloke

Bob,

Would you hear it/anything from the driver's seat (my guess is not...but I defer to your experience and knowledge)?

Is there anything that could warn an owner of impending doom - and maybe help them avoid Pete's very succinct "Boom" result?

I have an electronic boost guage with audible warning, but was wondering if something else could give some more "comfort zone"?

Cheers

EvilBevel

Thanks for all the answers guys. I'm starting to see a bit clearer now.

I myself have driven around with an "uprated" dumpvalve that did not behave, and sometimes the car was producing 0.2 bar boost at full throttle at 4000 rpm ...

Dumpvalve stuck in "open" position...

I guess I should be glad my VF23 is still happily spinning then ...

One thing still puzzles me ... on light throttle, high(ish) RPM, the engine load is quite low, so normally it shouldn't produce a lot of exhaust gasses... why would it still overspeed ?

And finally, a hypothetical question ( ): if one was using a bleedvalve to experiment (thus effectively deleting the solenoid), could that possibly alledgedly have played a part in the departed VF23 ?

Theo [just to be clear, I'm not talking about my own car]

Bob Rawle

If you go across the surge line then you normally hear a sort of whistle/chuffing sound. This can be also produced by the high velocity of air through the throttle body when on medium boost at part throttle. Its fair to say that this is far more audible when an induction kit is fitted. A standard car with the standard airbox could still be doing this but no audible noise may be heard.

Light throttle/ medium high revs at say 0.6/0.7 bar boost would generate the sort of conditions I am talking about given the throttle response is good enough. A standard car will not normally be sensitive enough. Link throttle response is far more so and can genmerate this condition.

Using a bleed valve could pull the boost too high too soon generating the overspeed condition for certain, trying to use too much boost at part throttle is what pushes the turbo across the surge line.

IHI wheels are pretty "fragile" and overspeed can cause the blades to stretch, touch the housing and snap off.