Taking the T out of my TDi

[gldgti]

Last question before I try this on the weekend, do any of you guys think my EGTs may get excessive due to the reduced boost at low rpm?



I would have thought that this would "choke" the turbine so the engine would just flatten out even earlier than it does now. At any rate, it should be easy to tell as my off idle response wouldn't change at all if the vanes stay closed. But I won't try to rev it out if this is the case.

No - the low boost/default setting will basically open up the turbine as much as possible. The idea is to reduce the pressure across the turbine to almost zero (so it can't harvest any energy from the exhaust gasses). In this configuration. the exhaust will flow more easily than any other setting.

The air/fuel ratio will tend to the rich, soot making side of things for sure, but in my experience with n/a diesels this wont be a problem. Indeed, it is very hard to create dangerous EGT's without a turbocharger in operation - because for one, the total mass flow of fluid is much lower (no boost).

As for a real world example - dont know if you've seem my thread in the mk1 section about the cabriolet - but I have a switch to turn the turbo on and off. OK, so its a wastegated turbo and not a VNT, however, the effect on exhaust manifold pressure with turbo boosting and turbo not boosting is the same - wastegate open = lower manifold pressure -> vanes open = lower manifold pressure.

Anyway, with the wastegate open, the engine revs much more freely, but ofcourse, theres little/no boost, so overall performance is much lower.


PS - my theorising that you quoted above was just incase the actuator was set up the other way around, which does not seem to be the case. If it was the case however, I am sure that the turbocharger would overspeed/overboost rather quickly. On another (old diesels) forum I am a member of, guys in hte states have been playing around with mechanical linkages and crude ways to control VNT turbo's on old vw diesels for ages, and some of them have had great success blowing headgaskets and turbo's by mucking up the linkage and thinking open was closed.

[kaanage]

Hmmm. I would have thought from this site's explanation that with the vanes closed, the exhaust would struggle to get through to the turbine at higher rpm hence my "choking" reference and that the vanes needed to open in order to allow the mass of gas to be able to pass through.
Are you saying that at higher rpm when closed, there is still enough flow to continue to accelerate the turbine beyond the open configuration, leading to over boosting?

If so, then it's logical that they would design the default position to be fully open to protect against the most likely actuator failures.

Since the vane positioning has the same effect as the wastegate, why do they bother since a wastegate is far simpler? Does the VNT allow for a wider boost map?

No idea how to solve your problem but why in the world would you want to drive slower?

Whats the issue mate? Maybe we can help fix the issue so you dont need to do something dodgy?

Struggling to maintain traction ?

Why doyou need to do that? Just curious that's all.

I'm trying to simulate the boost coming on later

[gldgti]

There's a rather ambiguous sentence on that otherwise great site you linked to, which is confusing. The site shows a picture with the vanes closed and explains rather simplistically that this is the low rpm pre boost setting. What should have been said is that this is the setting for optimal turbine acceleration. (it almost conveys that but its not well put).

I'll try to explain better:

The idea behind the VNT is yes, to operate over a wider range of conditions optimally. By adjusting the vanes in the turbine casing, you can direct the exhaust flow either tangentially towards the outside of the turbine blades (imparting lots of momentum, good for low gas flow rates) or you can open the vanes up and direct the gasses radially, imparting minimal momentum.

By doing this, you can achieve much greater turbine power at comparatively low gas flow rates. This is great when you are trying to eliminate turbo lag, for instance, which is a big problem for modern engine design where emissions are concerned.

So, if you want the turbine to accelerate, you "close" the vanes (direct the gasses tangentially towards the turbine blades).

Then, as the turbocharger produces the desired boost and gas flow is increasing (with engine revs and boost) you can adjust the vanes to impart less energy to the turbine, to stop it from accelerating.

In an old WG turbocharger, you might design the turbine housing to constrict the flow and impart maximum momentum to the turbine, to achieve similar performance to the VNT. But if you do that, its high flow rate efficiency suffers greatly.

With the VNT, you can keep a more open turbine geometry, allowing a more efficient high flow rate operation, whilst still being able to achieve good turbine acceleration at low gas flow rates by adjusting the vanes to direct the exhaust gasses directly at the blades.

Its important to convey, though, that the mechanism does not attempt to reduce boost/engine performance as a result of choking the exhaust flow.

I probably shouldnt have written about similarities in manifold pressure before, since its very easy to appreciate why a wastegate effects manifold pressure, but not easy at all to see how the vanes directly affect turbine acceleration - even if there is a concurrent effect on manifold pressure.

I hope that is a clear explanation.

[gldgti]

If so, then it's logical that they would design the default position to be fully open to protect against the most likely actuator failures.

precisely - which is what jazd's data about malfunctioning operation says.

[kaanage]

Well, I finally tried it and it made surprisingly little difference in the way the car felt except from about 1400rpm to 2300rpm where the engin felt very flat compared to usual.

It made the engine throw a warning light after I revved it out for a bit (no other effect, though) but that went away after I reconnected the vacuum line and drove about for a while.

[gldgti]

Yep, this is not surprising.

The '15' size VNT is still quite small and even with the vanes fully open should spool up with some decent flow through it. Its that low rpm range where the variable geometry is really used to generate some boost (and hence torque).

Thanks for the feedback.