VNT control idea????

[regcheeseman]

Stripping the EGR from my AAZ and I notice the wastegate controller has two ports either side of it's diaphragm.

boost opens the wastegate
vacuum from vac pump also opens the wastegate - controlled via a 12v solenoid.

Now I'm thinking VNT controller.....

Boost pressure opens up the VNT boost vanes against a light spring and a micro switch on the pump could apply vacuum to shut the vanes down at 0 ish throttle opening?

I'm thinking that the spring would be matched carefully to allow full opening at maybe 10 psi? and it would control the vanes up through the boost - damped with a flow limiter.

And when you back of the throttle, the added negative pressure from the vac pump would ensure the vanes shut down again.

Anyone used this system or believe it has any merits?

[rabbitman]

It would be best if the vanes were opened up all the way untill the throttle is pressed.

[regcheeseman]

really? why? Or is there confusion here?

[Turbinepowered]

really? why? Or is there confusion here?

It's a kind of "fail safe" mode, to keep your controller from failing in the fully closed, full boost, higher EGT position. It also gives you more control over the variability of the vanes, and ties it more closely with your load demand. More load requires more pedal which provides more boost.

[regcheeseman]

Correct me if I'm wrong but what you are saying is...

At idle - vanes wide open
As soon as throttle pressed - vanes close
As revs/boost build vanes open proportionally

Is this right?

Why the importance in having the vanes open at idle (say 900rpm)

But vanes shut at light throttle (say 1000rpm)

Or have I got this all wrong?

[blackdogvan]

Andrew what do you think about adjusting the vane stop setscrew to limit boost to less than around 10 psi under 2k RPM? Whats the RPM/boost profile under hard accelleration on a rolling start from idle on you 1.6?

[blackdogvan]

I personally wouldn't want to reduce or limit low-end boost under full pedal acceleration.  I guess to me the point to having the VNT is having the option of getting the boost early.  As far as trying to accurately depict the boost profile, that is a difficult task indeed.  In hard acceleration from a rolling stop at idle, both rpms and boost move up so rapidly in first gear that it's impossible to give accurate numbers between boost and rpm.  They both shoot up very quickly.  The boost lag is also the amount of time it takes to pump up the intercooler and piping.  Very difficult to give numbers such as at X rpms with vanes fully closed it will produce Y psi of boost.  In my next install I should have both tach and boost gauge visible in a video camera frame and be able to take snap-shots.  Even still it's hard tell if the current boost pressure is the max for a specific rpm or if it is building due to pumping up the intake volume.

Andrew  

Aren't you worried about too much boost at low RPM & the possibility of bent rods?

[regcheeseman]

That's kinda sorta but not really what I'm saying.

The vanes are not an ON/OFF switch, but rather a progressive adjustment.  Ideally the vanes should close to the point of delivering the ideal air to fuel ratio for the power requested.  I don't think that vanes should be fully shut at light acceleration, probably not even partially shut.  At light acceleration, no boost may still deliver plenty of air for a thorough burn and so the vanes should still be open.  Moderate acceleration should close the vanes further, heavy acceleration further still, etc...  Closing the vanes further than necessary to deliver adequate air (to burn the fuel that is required to generate the power requested) will just hurt engine efficiency.

Now I'm really confused...

from what you write above I'm getting that the further the vanes close the more boost is achievable.

i.e in v simple terms... vanes open = small turbo, vanes shut = large turbo

Or have I got hold of the wrong end of the stick?

If this is correct than my original theory of operation holds true except for a confusion around vane operation. Just to clarify, when the turbo is removed and all connections removed and the only thing acting on the vanes is the spring in the actuator - what do you call the state of the vanes. open or closed?


In operation then, vanes open at idle and close progressively with load/boost/rev increase?

[Rabbit on Roids]

the vanes at idle are wide ass open.
the vanes are also wide open when just cruising.
when you give the engine throttle, or it senses load, it closes the vanes until boost comes up to where it needs it.
so yes, at idle, and with no load, you essentially have a big turbo.
and when you get on the smoke pedal, it starts closing the vanes, essentially making your turbo "smaller" and quicker spooling. so, instead of having lag, it closes the vanes, spools the turbo like crazy, then when boost numbers come up, it starts opening the vanes again, making your turbo "bigger". then you let off the pedal, and the vanes stay in the same position, because now there is no load on the engine. when the vanes are closed, it builds a S*** TON of drive pressure. the vanes angle the exhaust, and kinda create orifices in a way. they speed up and direct the exhaust gas towards the turbine before it hits it. A VGT turbo in essence, has a variable size exhaust housing. need any more explaining? or did i just make you even more confused?

[regcheeseman]

need any more explaining? or did i just make you even more confused?

not at all - absolutely bang-on. Thanks very much.

I'll have to re-think my control mechanism to make it demand driven.

Cheers mate.

[Rabbit on Roids]

no problem. keep me updated on your refinements of your vnt control. i still have no controller for the vanes on my turbo.

[regcheeseman]

Thanks guys, I need to get a handle on the requirements for a controller

I'm currently looking to either a mechanical or electronic soloution but need to know what is required before deciding which path to take.

Despite being an electronics engineer I was hoping to remain with a pneumatic/mechanical setup - which is more the ethos of the IDI engine.

But it's looking like electronics has the edge in performance.

An interesting solution I have been shown over here was using a valve on the throttle arm to vary the vac pressure delivered to the actuator - the person who had done it was convinced that he had a 1/4 turn pressure regulator but I've never seen such a device. Supposed to be the pedal valve from a lorry's air brakes.

The motor build will come - I'm gathering parts and knowledge first.

[regcheeseman]

Ok just to confirm - with the turbo in it's current state, i.e. in a box, on a shelf waiting for it's time...

Are the vanes open or shut?

[Rabbit on Roids]

hot damn...
you guys are gonna have a ***in VNT controller if you keep going at this rate.
i might just have to build me one when i get home from alaska.

[regcheeseman]

With a view to electronic control I was looking at YPVS or EXUP servo motors from a yamaha.

5 connectors - 2 for power and the other 3 give a potentiometer type feedback

Ideal for actuation with no springs/diapragms required and decent feedback.

Even does that cool whirr whirr whirr on startup as the motor parks itself.