How to design a turbo for high altitudes?

GarretP, The trip you just did is not applicable to this situation. I think it is concensus that steady state driving like you just did is A-OK. It's the accelerating transients that are killing these turbos. According to mickey, a common factor is they blow when "pulling away from a stop." But thanks for the consideration.

Well, on to the good stuff, well really good stuff.

Originally posted by SkyPup:
Thanks for clearing that misunderstanding up here aaron.

Everyone drives their cars differently across the entire gamit from super econo to super high performance.

You are right that Mickey's conditions are the most severe TDI torture test that I have ever heard of, and he doesn't even have 50 cetane fuel to assist him either.

Running full boost uphill under load at low rpms is the most damaging range to drive your TDI in due to the temperatures, pressures, and loads involved in. I would not maintain a 10-12 psi boost load uphill for more than 40-50 seconds without backing off to keep the temps down, or downshift to take the load off the turbo.

The turbo is a direct link to the load you are experiencing, and as you mentioned, a HSDI passenger car engine rarely sees a load like a 60,000 pound semi-truck does. Short burst of full load are entirely normal as you cannot floor the throttle for very long without exceeding the max speed limit, wherein the boost drops off as you take off the load.

I have had the cruise control on through the mountains 4,000-5,000 feet and going uphill I have had to take it off since it was putting too much load on the engine for uphill climb on long 10-13% grades, operating the throttle manually to keep the boost levels at medium levels 8-10 psi is much less stress on the turbocompressor.

Your intercooler air at 8-10 psi boost is just about right, there is really no need for intercooling to lower the intake air temps at that pressure. Once you get to 10-12 psi of boost the turbocompressor is working very hard and the intercooler is needed to cool down the boosted air. At 14 psi and above, our two TDIs both max out at 19 psi steady, the compressor is working extremely hard and the air temps are very high, putting a major strain on the intercooler to maintain lower intake air temps and decreasing the efficency of the compressor wheel dramatically as the temps rise from the continued high boost demand.

Very few OEM turbocharged vehicles of any make or manufacture have turbocompressors that are designed to put out more than 8-12 psi of boost, since this requires the use of an intercooler and a much more efficent wheel and creates much more load, temps, pressures, etc. Operation at anything over 10-12 psi for any length of time heat saturates the entire system and is NOT going to lead to long term reliability no matter who makes it or designed it. Excess heat destroys engines.

The altitude adjustments maps OEM on the TDI keep the max turbo pressure down below max operational limits for temperatures and the MAP-IAT sensor also decreases turbo boost too based on an intake air temperature correction factor.

Neither of these negative feeback sensors cuts down on the FUELING though, so you can still continue to make major heat due to the amount of fuel, even though the turbocompressor has been backed down by your internal barometric pressure maps and the temp correction of the IAT-MAP sensor.

It is up to you to manually decrease the fueling via the throttle, or risk toasting your turbo machinery. It will not overspeed and it will not surge under these conditions.

There is little to no chance of any overspeed occuring unless your air filter is dirty and occluded.

There is little to no chance of surge occuring unless you are full loading the turbo at the torque peak.

There is little to no chance of choke occuring unless you are giving it full throttle and no throttle and then full throttle again and the engine and turbospeed are mismatched from run-on.

Pay attention to those parameters and you should experience no problems during the lifetime of your TDI.

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<font size="2" face="Verdana, Helvetica, sans-serif">SkyPup, SkyPup, SkyPup,....

You missed a key piece to the situation... the smoke map. It caps fuelling for certain combinations of low fuel mass and low engine RPM. Hence the "saturation" condition I explained earlier.

When a stock TDI gets fat injectors, the ECU doesn't know that. The maps are calibrated for stock injectors. The additional fuel from the fat injectors can overdrive the turbine and result in overspeed. Same deal with the chipping feature that mods fuelling, obviously.

This is the really interesting stuff from SP:

The altitude adjustments maps OEM on the TDI keep the max turbo pressure down below max operational limits for temperatures and the MAP-IAT sensor also decreases turbo boost too based on an intake air temperature correction factor.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">The boost, IAT & MAP sensors don't sense turboshaft speed. Shaft speed control is implied but certainly not assured. Once the system is modded for more boost and or fuel, all bets are off and turbo overspeed is sure to happen. Especially at altitude.

Neither of these negative feeback sensors cuts down on the FUELING though, so you can still continue to make major heat due to the amount of fuel, even though the turbocompressor has been backed down by your internal barometric pressure maps and the temp correction of the IAT-MAP sensor.

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<font size="2" face="Verdana, Helvetica, sans-serif">No, but the smoke map does. It can and will prevent overfuelling, if not tricked by chipping or fat injectors that will in turn overdrive the turbo and lead to overspeed. The main influencing factors in fuel regulation are the MAF and smoke map, after the pedal position. Look at the smoke map.... low air mass and low RPM results in fuel mass being capped off. You missed that SkyPup. Wait, maybe the chippers don't use the smoke map.... If they did, the smoke map would override the increased fuelling and what good would that do? Certainly wouldn't increase torque over stock.

Smoke is an accepted part of modding a TDI. It means "real man power" above and beyond the power at where the smoke map caps injected fuel mass. Doing anything to disable or diminish the smoke map puts the turbo at risk of overspeed due to additional fuelling beyond what the smoke map would allow.

It is up to you to manually decrease the fueling via the throttle, or risk toasting your turbo machinery. It will not overspeed and it will not surge under these conditions.

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<font size="2" face="Verdana, Helvetica, sans-serif">This wouldn't have to be done manually if the ECU or other system components weren't modified. The smoke map will cap the fuel mass dependent on the air mass and engine RPM, if the system has not been disturbed by mods such as fat injectors and chips that will lead to overdriving the turbine.

There is little to no chance of any overspeed occuring unless your air filter is dirty and occluded.

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<font size="2" face="Verdana, Helvetica, sans-serif">A stock A3 paper filter has 22 mbars of pressure drop at 3500 RPM. Absolute diddly squat. No problem. One squished mosquito on the windshield causes 387 mbars of intake pressure drop. No problem. High boost (=high PR) at low engine RPM where the flow through the compressor is low can easily cause overspeed or surge. Hey, if the system is not corrupted, the smoke map can also help prevent turbo overspeed..... Hmmm..... Did ya ever think of that???

There is little to no chance of surge occuring unless you are full loading the turbo at the torque peak.

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<font size="2" face="Verdana, Helvetica, sans-serif">This can happen anytime in normal vehicle operation. Duh!!!

Altitude just makes all the above much worse.

Hey SkyPup, that saturation that I was talking about, that's the smoke map capping off the max fuel mass delivery rate for the low RPM & air mass condition. That's why cars smoke when they get fat injectors. The ECU still thinks the injectors are stock and applies the stock maps. It don't work. Just like the stock maps won't work to provide implicit turbo overspeed protection when the ECU is chipped, or fat injectors are added, other things to add boost or fuel that the ECU does not "know" will result in turbo overspeed. All these types of mods lead to turbo overspeed since the turbo speed is not sensed, but implied by stock parameters.

You are absolutely galactically oblivious to the smoke map and its function. If the smoke map is side stepped when chipped, you should expect turbo overspeed. If fat injectors are added, turbo overspeed is to be expected due to the additional energy available from the additional fuelling to drive the turbine.

Ted_G, I don't think adjusting the max. boost nozzle position is a bad thing or should be avoided. Making the minimum nozzles bigger sort of "derates" the turbo, reducing turbo related load, speed and torque. This will help the turbo survive those transients.

You don't even need to go to higher altitudes to blow your VNT turbo up, all you need to do is wait until winter and some cold air.

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<font size="2" face="Verdana, Helvetica, sans-serif">Are you implying that as the temperature drops, the less dense the air becomes?

didn't Boundless tell you that denser air is the primary cause of surge yet?

Boundless: The ECU doesn't need to directly measure shaft speed. All it needs to do is take pressure, temperature and airflow data and CALCULATE the shaft speed. You cannot "overspeed" the turbo, as long as the sensors are all working. You simply cannot. It's impossible. I tried, believe me. All you get is a Warp Field Collapse. If the ECU cannot physically keep turbo speed within predetermined limits, it simply cuts fuelling.

YOU CANNOT OVERSPEED THE TURBO! No how, no way. Not ever.

Aaron's turbo failure happened at low RPMs, while accelerating MODERATELY away from a red light in FIRST GEAR.

It's one hell of an impressive failure, too. We pulled what's left of the turbo out of the car today. The compressor wheel and HOUSING were sheared completely off! Torn off. Gone. Nothing but shreds and tatters.

Frankly, I'm not altogether convinced that some sort of weird road debris didn't leap up and hit the unit. Like an axe handle or something. I've never seen any failure that looks like this one, and I'm at a loss to imagine how it could happen. Think about it. If the wheel "froze" to the housing while he was accelerating "normally" away from a light, would there be enough kinetic energy to literally rip the housing clean off the turbo? The torque that would require is unimaginable.

Unlike every other VNT-15 failure I've heard of, the shaft didn't fail inside the center section. It failed right at the base of the compressor wheel. It was clearly TWISTED off. The engine began to run out of control on the motor oil, but Aaron quickly cut it off when he noticed it was revving at 2000 rpms at "idle." It didn't hydrolock.

The bearings are, to all appearanced, in perfect condition. The shaft journals are as well. The parts rotated perfectly. (What was left of them.)

The VNT mechanism operated smoothly and freely. No problems there.

I can tell you all one thing, right now: You can argue about this one until you get lightheaded, but you'll NEVER diagnose it. Ever. Forget about it. It's the weirdest damn thing I've ever seen. I don't even feel comfortable calling it a "typical high altitude failure" anymore. There is NOTHING typical about this one.

-mickey

Thanks for the nice report Mickey, sounds like aarons compressor wheel ingested a piece of debris and blew.

I know you took the turbomachinery apart yourself and inspected it to come up with your report here, but Boundless has spent countless hours proving you wrong, that it cannot be what you have said it is here.

Are you absolutely certain that this indeed is aaron's blown turbo? And are you sure that the compressor wheel was toast and the VNT mechanism was fine? Boundless may think that you were confused between the two different things on the differing ends of the same shaft, so maybe the truth could be that the VNT was toast from overspeed and the compressor wheel was really the perfect one?

Boundless, could you please provide us a list of all the automotive turbochargers in the world that have an rpm sensor inherent in the unit that monitors turbine-compressor wheel speed and adjusts the unit accordingly?

Be sure to let us know if your list is longer than ZERO.

And don't forget, the VNT turbine mechanism turns at the same speed as the compressor wheel. Also, the VNT15 we have does not have the simple nozzle orifice adjustment like you said it does, instead it has adjustable vanes.

[ March 31, 2002, 07:47: Message edited by: SkyPup ]

Originally posted by mickey:
Boundless: The ECU doesn't need to directly measure shaft speed. All it needs to do is take pressure, temperature and airflow data and CALCULATE the shaft speed. You cannot "overspeed" the turbo, as long as the sensors are all working. You simply cannot. It's impossible. I tried, believe me. All you get is a Warp Field Collapse. If the ECU cannot physically keep turbo speed within predetermined limits, it simply cuts fuelling.

YOU CANNOT OVERSPEED THE TURBO! No how, no way. Not ever.

Aaron's turbo failure happened at low RPMs, while accelerating MODERATELY away from a red light in FIRST GEAR.

It's one hell of an impressive failure, too. We pulled what's left of the turbo out of the car today. The compressor wheel and HOUSING were sheared completely off! Torn off. Gone. Nothing but shreds and tatters.

Frankly, I'm not altogether convinced that some sort of weird road debris didn't leap up and hit the unit. Like an axe handle or something. I've never seen any failure that looks like this one, and I'm at a loss to imagine how it could happen. Think about it. If the wheel "froze" to the housing while he was accelerating "normally" away from a light, would there be enough kinetic energy to literally rip the housing clean off the turbo? The torque that would require is unimaginable.

Unlike every other VNT-15 failure I've heard of, the shaft didn't fail inside the center section. It failed right at the base of the compressor wheel. It was clearly TWISTED off. The engine began to run out of control on the motor oil, but Aaron quickly cut it off when he noticed it was revving at 2000 rpms at "idle." It didn't hydrolock.

The bearings are, to all appearanced, in perfect condition. The shaft journals are as well. The parts rotated perfectly. (What was left of them.)

The VNT mechanism operated smoothly and freely. No problems there.

I can tell you all one thing, right now: You can argue about this one until you get lightheaded, but you'll NEVER diagnose it. Ever. Forget about it. It's the weirdest damn thing I've ever seen. I don't even feel comfortable calling it a "typical high altitude failure" anymore. There is NOTHING typical about this one.

-mickey

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<font size="2" face="Verdana, Helvetica, sans-serif">mickey, what is "Warp Field Collapse"? I've seen it used before but was never interested enough to ask, until now that is.

Now this is getting good... I've been wondering what happened since it was reported that turbo parts fell out when the belly pan was removed for inspection, some time ago. You've answered that question.

mickey, was the belly pan punctured in any way that would indicate something could've hit the turbo? The usual suspect is a piece of broken leaf spring. They're all over the roads, out here anyways....

Also, Aaron had his snow screen in... If debris did it, it broke off from the car post snow screen. Was the air filter compromised? If not, the debris was post filter... anything missing from the MAF? Inspect upstream from the compressor.

But still, breaking off the compressor housing is quite sensational!

mickey, the shaft break, you say it was TWISTED off.... Is the break a helix shape? Is that what you mean by twisted? mickey, twisted can mean different things to different people, and it means something very specific to ME's. I'm not doubting you or challenging what you've seen, but what ME's use for these types of definitions and descriptions is very specific, I just wanna clarify the TWIST and break description as best as possible before we get to see pics and the parts. You are dealing with quite the sensational failure there. I agree that we'll probably never know for sure, too much post initial failure damage contamination.

Is the break kinda flat, like is was hacksawed?

If it looks flat, please do not touch the broken ends of the shaft(s). I would like to see these parts ASAP. Also, is there an outter ring on the fractured surface, as when looking into the end of the shaft, that is a little shiny, and the surface more towards the center is flat, as in not shiny or glossy?

mickey & Aaron, if you could give us a better description of the shaft break along the lines of these questions, and anything else you notice, the ME's would be very happy!!! Please, please, please....

Keep us posted, this is more interesting than a SIMPLE turbo overspeed failure!!!

If you Fed-X the turbo parts to Boundless right now, be sure to include the customary legal chain of evidence documents to prove that the turbo pieces you sent him are actually from aaron's blown tubo and not from some other turbo that was not at high altitude. Probably best if you include the reading from his altimeter, boost gauge, VAG-COM, speedometer, etc. at the exact time the turbo blew too so we can be absolutely certain that the this evidence is not some kind of sham designed to disprove his extensive turbo theories.

Oh, and BTW, I just spoke at great lengths with a senior engineer at a major turbomachinery manufacturer with twenty-five years of turbo experience, he ran the entire series of Boundless theories and hypothesis on turbo meltdown through his Cray SuperComptuer last night. He told me that Boundless's theories are worthless. I thanked him for his help with this difficult dilemma and told him I didn't need a multimillion dollar supercomputer to reach the same conclusions myself.

Hey Boundless,

I have the original parts that fell out in a plastic bag waiting to send to you, but I figured I'd wait until the rest of the turbo came out so I could send it all together.

The original parts are not molested, except for having fallen on my face when I pulled the engine cover off.

The shaft hasn't been mistreated or anything in the meantime, and I've kept all of the parts. I'd take pictures, but you'll have all the parts soon enough anyway. *I* am interested in hearing about your findings.

I did have the snow screen in place, and the pipercross filter is fine too (clean and sturdy). I actually removed all the intake when I returned the EGR and CCV to stock before wasting my time with the dealer. I too thought about ingesting something. But I could not find any evidence of parts getting sucked in through the intake.

The cover on the bottom is fine. It has not been damaged, and I don't suspect anything of hitting the turbo housing from under the car.

Now this is a long shot - but I'm trying REALLY HARD to think about how this might have happened. When I connected the epsilonian device, I stripped the #5 wire on the MAF while the airbox was open. Then I thought to myself: "Hey - don't let any plastic wire parts get on the INSIDE-side of the air filter!", so I made sure I found the parts that I stripped (they on the ground), and I covered the intake up again.

And this was several months ago too! But is it remotely possible that something could have been injested, and sat around waiting to get sucked into the turbo? I really doubt it, but I'm trying to be very objective.

Bottom line is - we'll probably never know what happened. I think we all agree on that. I'd like to keep it from happening again, and I'll probably keep my revs up anyhow, even though I don't REALLY think that caused the failure. But better safe than sorry, and there are folks here that know turbos better than me that feel it'd be beneficial to let it rev more. OK, I will.

Now, to Skypup: You are a wealth of knowledge. I've seen several posts from you that are full of wonderful information. I have no doubt that you are quite knowledgable about the TDI and turbos.

But between the occasional excellent posts you give us, we all have to deal with the rantings of an arrogant prick. I can smell your conceit seething from here, and I detest it. I'm sure I speak for a silent majority on this list.

I sincerely appreciate your helpful posts, but please don't ruin it here for everybody else. I want to keep this thread educational. So: if you don't have anything nice to say, don't say anything at all. Didn't you ever watch Bambi?

Boundless: thanks for posting in spite of opposition. There are some that just want to fight, and others that enjoy a good discussion. Even though we'll never figure it out, at least we'll have some fun thinking about it.

I have spoken.

Aaron

Amen to that, aaron

Originally posted by aaron:
...I can smell your conceit seething from here, and I detest it. I'm sure I speak for a silent majority on this list.

I have spoken.
Aaron

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<font size="2" face="Verdana, Helvetica, sans-serif">You don't speak for me.

Originally posted by aaron:
...... I'm sure I speak for a silent majority on this list.
....

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<font size="2" face="Verdana, Helvetica, sans-serif">I wouldn't be so sure, and no, you don't speak for me, either!

- Richard

With all due respect, I think overspeed IS important, esp. at high altitude. Think about this: At idle, the VNT is calling for max boost. Put it in gear, let the clutch out, and hammer the pedal. The turbo starts spooling up to make boost. By the time the ECU figures out it's got enough boost and starts opening the VNT, the turbo is already starting to overspeed due partly to the thinner air. The other reason is the LAG TIME of the vacuum actuator and VNT mechanism. There is no way a pokey vacuum actuator system can react fast enough to control a turbo with the response time of ours, especially in lower gears where the engine can accelerate quickly.

CONTIUOUS overspeed is likely not possible due to the ECU mapping. However, TRANSIENT overspeeds (overshoots) are quite likely the cause of some of these problems. Anyway, until someone puts an optical tachometer on their turbine and records this data, everyone is merely giving opinions.

Chris

BTW, are ANY of you posting on this thread engineers of any sort? I'm an aerospace engineer with a fair bit of thermodynamics and fluid mechanics background (but I'm no turbomachinery expert). I detect a lot of self appointed "guru-ism" here - "experts" with no real engineering background. I haven't worked the math on any of this, but I may have to dig out my texts and get to work. I think that there is more going on here than any of you think. What about materials defects? Vibrations? Imbalance? Resonant frequencies? You all just can't read a few papers and graphs and say "we've seen the light".

Originally posted by mickey:
The "overspeed" question is moot. You cannot "overspeed" a stock turbo, no matter how you chip it or fool with the VNT. The ECU simply won't allow it. It knows your altitude. It knows your manifold pressure. The turbo is "mapped", just like the fuelling. It simply cannot be made to "overspeed."

-mickey

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<font size="2" face="Verdana, Helvetica, sans-serif">

[ April 01, 2002, 00:14: Message edited by: Chris B ]

Aaron, you are in good hands with Boundless, who cares what anyone else has to say about it, it is your blown turbo, you blew it up, and you need Boundless to tell you why you blew it up, I wouldn't listen to anyone else about it besides him if I were you, but I am not so I don't.

Good luck on your next turbo, be sure to adjust the VNT mechanism just like Boundless told you to.

OVERSPEED?????

Sure..............after four years of this!



SURGE???????

Sure.........after four years of this!



Aaron, maybe you can get Boundless to tell you why both my TDI VNTs work so well after four solid years of constant abuse?????

Then again, maybe you can't.

Originally posted by Boundless:
Here is a pic of the screw to open up the minimum nozzle size...

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<font size="2" face="Verdana, Helvetica, sans-serif">Boundless, don't you think you should mention that you stole this VNT pic from here, or did you actually take the exact same photograph yourself of this VNT15?

http://www.technologie-entwicklung.de/Gasturbines/VNT15-Turbo/vnt15-turbo.html

[ April 01, 2002, 05:32: Message edited by: SkyPup ]

Chris B,

Excellent point on the overshoot overspeed condition. Davin posted an excellent graph that shows this overshoot.

Also, if you are familair with fluid calcs, backing into compressor speed, when it is sharing the same shaft with the expander, is going to be a very gross. Since the expander is involved, engine RPM and fuelling rate are also necessary. In my opinion, I don't see calculating the compressor speed as feasible, but I'm not going to go into it any more. Folks have made their minds up and that's fine. I have not seen that the compressor speed is calculated in any documentation I have seen, and it would take Bosch authored documentation for me to believe it, or a much better presentation than has been posted so far.

There are plenty of calcs going on by real engineers. You are reading the results of those calcs. There are, or is, a self appointed guru that says he has papers. But as an engineer, you know that buying the books is not the same as the years of study and practice required to understand what is in the books. These are the same folks that use fire extinguishers to make it look like their car is doing a burnout.

Chris B, check out these other related threads:

Surge, overspeed, ....

and

Some Turbo Loading....

and

High Altitude = Bye bye Turbo?

Some folks have looked at the resonance route. I think it has already been posted on one of threads above, or it will be soon. Resonance is probably not an issue, although there is still some vibration/imbalance/resonance consideration going on yet. Well, there are several modes of resonance and most have been eliminated as causes. Mostly because most of the natural frequencies are way far away from the operating range of the turbo.

There's still one or two modes that are still being discussed.

Since the housing is broken, something else happened here, I believe, and as Aaron said, we'll probably never know for sure. But, it is definitely fun and educational discussing it. If folks are open-minded about it, there has been some very good info posted about surge & overspeed,....

I do know that if I were designing a turbo, I would make sure all parts are contained in the event of a failure, especially overspeed failures.

Oh Skypup - I don't know why mine works and yours doesn't. And I know that, after flaming you, I'm going to have another failure in six months and I'll be begging you to help me.

So anyway . . . I'm just saying that, I know you're intelligent, the list needs your input, and we would never be the same here without skypup. And I'm not just saying it because I know I'll need your help.

But I wish you'd cool down a bit, that's all. I'm feeling more bold because this is "my" thread

Aaron

Boundless: The break has a slight twist to it. I've never seen one like that. These are hardened shafts, and they generally snap cleanly. Mine looked "hacksawed."

Chris B: The ECU is NOT calling for "max boost" at idle. And if you've ever observed boost readings on a gauge, you'd know that "overspeed" simply doesn't happen. At sea level, a chip plus a 1-turn VNT adjustment gets you a 20 psi peak and 18 psi sustained. Up here, you can chip the car and crank the VNT all the way to the mechanical limit and never see a 20 psi peak...and your max sustained boost cannot go higher than 16 psi. I've seen the map for the stock compressor, and this is well within the "safety" zone. Again, the ECU knows your altitude. It will not allow the turbo to overspeed. It'll cut the fuelling in order to prevent that, if all else fails.

Besides, Aarons failure happened at low speed, during moderate acceleration.

-mickey

Not a "helix" break. The shaft is too hard for that. (Unless heat softens it...I see where you're going with this.) Just a slight twist.

By "road debris" I am talking about something hitting the outside of the housing, though there is always the possibility of something going through the intake and mangling the turbo internally. Aaron's intake is intact, including the screens, so that doesn't seem likely. But looking at the damage, I can envision something smacking the compressor housing really hard from the outside, snapping it off and breaking the shaft. A failure from "fatigue" would seem likely to occur in the middle of the shaft, where the "twisting" is greatest...and that's exactly where we usually see them. In this case, it really looks like the wheel damage and snapped shaft were secondary to what broke off the compressor.

No damage to the belly pan, though...but it's not inconceivable that something got jammed up there without hitting the pan. There are ways and means. I want to look at the OUTSIDE of the housing to see if there is any scraping or gouging.

-mickey

"Warp Field Collapse" was first coined to describe the limp-home mode you get when you crank up a tuning box too much. In that case, the pump is unable to supply as much fuel as it's being asked to do, and the ECU responds with a Warp Field Collapse...it cuts fuelling way down.

The same thing happens with a bad MAF sensor. Briefly shutting off the key will reset the WFC, but it'll return again as soon as you call for high airflows and the MAF doesn't respond with the appropriate data.

I generally use the term to describe any situation in which the ECU cuts fuelling and/or boost, in response to abnormal sensor data. The effect is the same: The ECU dumps ALL the vacuum from the VNT controls, and cuts fuelling. You get ZERO boost, and very little power.

-mickey

Another thought: In most cases, the blades of the compressor are roughly "machined", or "shaved", when the shaft snaps and the spinning blades hit the compressor housing.

In Aaron's case, however, the compressor wheel just looks like somebody smashed it to pieces with a hammer.

It looks as if the housing hit the wheel, rather than vice-versa. Hit it HARD, too, and stopped it instantly.

-mickey

"Hey - don't let any plastic wire parts get on the INSIDE-side of the air filter!", so I made sure I found the parts that I stripped (they on the ground), and I covered the intake up again.

And this was several months ago too! But is it remotely possible that something could have been injested, and sat around waiting to get sucked into the turbo? I really doubt it, but I'm trying to be very objective.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">It's possible. What we saw seems to be consistent with an external blow...but I can imagine similar results from something getting stuck in the compressor and physically jamming it, just as the shaft speeds are zooming up. That still seems like it wouldn't produce enough torque to shear off the housing...but SHOCK, combined with the torque, might do it.

Either way, we're talking about a problem that started outside the turbo. I don't believe this was a "high altitude failure." Something physically BROKE the turbo.

-mickey

Ok, maybe not max boost at idle, but aren't the vanes fully closed until the ECU starts to see the desired boost and then it opens them up to reduce/maintain boost level? In addition, your boost gauge may be too slow in response to the overspeed transients that can fatigue the shaft and/or wheel. Finally, to maintain boost as altitude climbs, the compressor will have to spin faster - we all know that. Boost PSI is rather meaningless when trying to describe compressor RPM's in a direct relationship. It depends on several variables.

Altitude alone is not a turbo-killer. Aircraft engines regularly fly to 25,000 feet with turbocharged engines with no problems. They rely on variable wastegates to keep boost under control at low altitudes. As the aircraft climbs, the wastegate closes to maintain boost until it's fully closed - the 'critical altitude' where it can no longer maintain full boost.

Now, a/c engines are WAY different than our little TDI. They are fixed load, constant RPM engines. But, the density change from 0 feet to 25,000 feet is rather huge compared to 0 to 5000, and those turbos don't blow up.

I'm not trying to shoot anyone down, but I'm just trying to say that I think there's a lot more going on with these turbos than anyone thinks, as as of yet, nobody has presented ANY facts on metallurgical analysis of turbo shafts, compressor wheels, or hard test data on pressure vs. compressor rpm. There are some nuts out there building jet engines out of truck turbochargers. They've found optical tachs that read compressor rpm directly. Rig one up and use a data-logger PC laptop to see what a turbo really is doing.

Chris

Originally posted by mickey:
Chris B: The ECU is NOT calling for "max boost" at idle. And if you've ever observed boost readings on a gauge, you'd know that "overspeed" simply doesn't happen. At sea level, a chip plus a 1-turn VNT adjustment gets you a 20 psi peak and 18 psi sustained. Up here, you can chip the car and crank the VNT all the way to the mechanical limit and never see a 20 psi peak...and your max sustained boost cannot go higher than 16 psi. I've seen the map for the stock compressor, and this is well within the "safety" zone. Again, the ECU knows your altitude. It will not allow the turbo to overspeed. It'll cut the fuelling in order to prevent that, if all else fails.

Besides, Aarons failure happened at low speed, during moderate acceleration.

-mickey

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I've been thinking of taking this high altitude turbo business underwater for a real test dive.

In order to best understand the fluid dynamic state of the compressor wheel and the torque on the shaft between the compressor and turbine, it would be better to model it underwater where the fluid densities could be carefully controlled and optimized.

I have a 125HP V-6 two stroke outboard engine on my Boston Whaler that I could easily hook up the compressor wheel off my spare VNT-15 and take it for a run. I could test it in both fresh and salt water to compare the efficency of the wheel based in different densities of water, similar to the sea level vs high altitude argument. Then I could simply put it in a garbage can filled with water and turn it on wide open and shine my halogen timing light on the compressor wheel to time the RPMs exactly and wait for it to surge, choke, overspeed, and explode.

I'll try taking the compressor wheel offshore deep sea fishing on a king mackeral run and then take it out on one of the nearby fresh water lakes for some big bass fishing. I'll let you know the results.

LOL , then my estate could sue Panasoinc for millions and millions for failure to warn against jumping into a pool with their turbcharged hair dryer on!

I'm looking forward to the huge roostertail that VNT compressor wheel puts out behind the boat!

Ok, maybe not max boost at idle, but aren't the vanes fully closed until the ECU starts to see the desired boost and then it opens them up to reduce/maintain boost level?

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<font size="2" face="Verdana, Helvetica, sans-serif">No, they're not. I've forced them closed at idle, and the engine note changes significantly.

Boost gauges respond VERY quickly. Not instantaneously, of course...but they can react more quickly than the boost can change. It takes time to spin a wheel up to 150,000 rpms.

As for the high altitude question: The VNT-15s have a relatively high failure rate above 4000 feet, and nearly zero below that level. Stock or otherwise. Obviously altitude is involved.

According to my Turbo Expert (Robert at Forced Performance) the VNT-15 used in the TDI has a pathetically thin little shaft. Thinner than anything he's seen. He said it cannot stand a boost-to-backpressure ratio of more than 2:1, or anything even approaching that on a regular basis. He didn't seem at all surprised that they'd fail at high altitudes. In fact, he laughed at it.

Your airplane analogy isn't applicable. Aircraft use vasty over-engineered turbos that are specifically designed to operate at high altitudes. Automotive turbos have to work from sea level (or lower) all the way up to around 13,000 feet...and do so EFFICIENTLY. If they run with wide-open wastegates at low altitudes, then they loose too much efficiency. An airplane is only at "low altitudes" when it's taking off or landing. Most of the time, it's cruising along at a constant high altitude. The turbos low-altitude efficiency is irrelavent.

The approximate height of the highest pass in the Rockies is about 13,000 feet. I routinely hit 11,000 on my summer drives. That's a LOT more complicated to deal with than designing a turbo for an aircraft application! Much of the country, out West, is above 4000 feet. Yet one turbo has to do the job for all engines sold in the whole country (the whole WORLD, actually) and do so efficiently. The airplane's turbo just needs to be strong, understressed and reasonably efficient at its CRUISING altitude. Below that, the wastegate just "wastes" everything. The more exhaust heat is "wasted", the less efficient the engine. The overall system is very reliable, as it must be, but is nowhere near as efficient as a modern passenger car setup except when at cruising altitude.

I think the answer is simple: As Robert has theorized, VW cheapened up the VNT-15 design. The bean counters ruined them. No ball bearings. Thinner shafts. The result is a unit that doesn't tolerate high altitudes. Sure, the GT-series compressor wheel is happy enough up here, and the compressor mapping is what the electronic control is based on. Your ECU knows what the wheel will tolerate, and when it's at its most efficient. But it doesn't know anything about the SHAFT. And neither, apparently, does Volkswagen. These are two separate issues.

The A3 turbos are the same basic design, but with a wastegate. They rarely fail, no matter what the altitude. They are built better, that's all.

-mickey

mickey,

I'd like to see the remnants of your turbo shaft. If you could add them to the box with Aaron's parts, that would be fine.

Thanks.

Mickey the only difference between an aircraft turbo and an automotive one is the $3,000 yellow tag

There is no magic in pistion aircraft turbos. Most that I have seen were either Rayjay or Garrett. The one thing that is slightly different is the wastegate is not integral to the turbo in case it needs to be replaced. I think its safe to say the turbo is simply matched to provide continous power of 80-90% of rated power indefinately.

I personally think the automotive applications are more severe due to fuel ratios. On TSIO aircraft engines short for (Turbo Supercharged Injected Opposed) you adjust mixture to maintain at least 50-75F degrees cool of peak EGT (TIT actually). The turbo is never overheated or left unmonitored for any period of time. Another factor is that the air inlet is always being rammed from air behind the prop in the extremely cool temperatures at high altitude.

Automotive engines never monitor EGT and it would not suprise me to find them running much hotter due to emissions reasons.

DB

[ April 01, 2002, 15:26: Message edited by: Drivbiwire ]

Not only does the barometric pressure drop with altitude, the temperature also does.

From the surface on up to the troposhere, the temp drops 3.5 degrees F. every 1,000 feet.

Aircraft engines are simple, sturdy, rugged designs that are intended to run at almost constant HIGH load all day long without any significant risk of problems. It has to be this way for the FAA to certify them. Same goes for the turbochargers. Simple, BIG, understressed, heavy duty. Lag time is of little consequence. They are designed to be BULLETPROOF above all else. No fancy VNT mechanism here, they use a simple, rugged external wastegate. No emission controls. Almost nothing in the way of electronics, and what electronics there is, is all doubled up to protect against failure.

Automotive applications are totally unlike this. You cannot even compare an automotive turbocharger to an aircraft one except to state that they operate using the same laws of physics.

The average light aircraft engine has the same architecture and level of sophistication as the original VW Beetle flat-four, except for having redundant ignition, electrical, and fuel systems. Same design philosophy is used for the external separate turbocharger used with that engine.

My mechanical-engineer opinion: in the interest of reducing lag time, they lightened the rotating parts of the VNT15 too far, and then pushed the result too far (perhaps un-knowingly) to get better performance out of it, and perhaps did not do component testing or simulation adequate to reveal the high altitude / low rpm / full load operational reliability problem. Wouldn't be the first time factors of safety got trimmed down a little too far in the interest of some noble objective. Been there done that myself, in fact. It happens all the time ... sometimes resulting in failures far more spectacular and devastating than these.

As for the thoroughness of VW's component testing under adverse conditions ... ask cold-climate B4 Passat owners about door handles, and instrument panel power supplies.

Okay overspeed does not mean overboost! Two different things entirely! Overspeed after surge would not show up on a boost gauge!

Re: turbo quality. I think we have a chicken v egg problem here. Does the VNT turbo fail because of its cheap build, or does it only spool up fast enough in transient to fail because of the VNT?

I don't have engineering facts. But how many turbos have failed, and what are the common traits, aside from them all being in VWs?