yashicamat

No, I'm not talking about fitting one. More to the point, is there one fitted as standard?! I noticed with the windows down just now, accelerating relatively quickly and changing at 2.5krpm resulted in that very distinctive noise! It's not audible from within the cabin with the windows up, but with them down and a wall or something nearby to amplify it, it's undoubtably present!

I was always under the impression that dump valves per se didn't work on diesels and the aftermarket ones actually generated the noise themselves rather than being engine produced.

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Rob

Nissan Primera GT in Silver
'56 Octy 2 2.0TDi Elegance . . . also gone but not forgotten
2002 Fabia 1.9TDi . . . . gone but not forgotten

http://uk.youtube.com/yashicamatonline

ScoobyChris

Isn't the noise you hear the wastegate on the turbo?

Chris

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Holding up the traffic in a little Italian..........

mannyo

There is no dump valve, the tubo is of the variable vane type. Maybe what you can hear is the system working.

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Manny
2005 Octavia 2.0TDi Elegance combi
my skoda website http://www.octaviars.co.uk

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yashicamat

It's not massively loud and for some reason isn't audible above 3000 rpm, I don't know if that's just the engine noise drowning it out though. My Fabia never made this noise and I was under the impression the PD100 had a variable vane turbo as well?

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Rob

Nissan Primera GT in Silver
'56 Octy 2 2.0TDi Elegance . . . also gone but not forgotten
2002 Fabia 1.9TDi . . . . gone but not forgotten

http://uk.youtube.com/yashicamatonline

Emily-Elizabeth

Has my fabia got the same thing, because I know what Rob means with that sound. I have wondered the same thing.

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freemansteve

You might all be interested in this:

Blowoff valve - Wikipedia, the free encyclopedia

but it is more applicable to throttled (petrol) engines rather than diesels.

Given that the 2.0 TDi (or similar) engine is not throttled, it doen't seem necessary to me to require a dump or a bypass valve; the compressed air would surely continue into the cylinders, the air flow metering would remain correct and the turbo would not stall.

I would however think that any turbo engine would always need a controlled valve to protect it from excessive boost if nothing else.

Are there any diesel engineers out there that can offer a definitive description of how the turbo and possible valving of it actually works on these TDi engines (i.e not just guessing; someone who designs or repairs these actual engines) ?

I think there are a lot of folk spending money on pointless add-on kit out there! Even worse, the dangerous children in my town with their dreaful Imprezas ought to have them MOT failed and sent straight to the crusher, because they are risiculously noisy when they race the streets at night. The are specifically modified to simply cause offense. Or maybe, keep the cars and put the small-****ed psychopaths in the crusher instead.

Cheers
Steve

mannyo

Yes, basically the same turbo although a different model.

__________________
Manny
2005 Octavia 2.0TDi Elegance combi
my skoda website http://www.octaviars.co.uk

Join the briskoda.net folding team click Here for more info

mannyo

Not an engineer, but basically what happens is that when you apply throttle the vanes in the turbo open blocking the airflow, thus the turbo spins. When you lift off the vanes close again allowing the air to pass through unhindered, so the turbo slows down gradually rather then stall. Thats wht no DV is needed.

__________________
Manny
2005 Octavia 2.0TDi Elegance combi
my skoda website http://www.octaviars.co.uk

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freemansteve

Sorry if this wandering off the thread a little, but:

I think the overall induction/exhaust system is a bit more involved from a control perspective than just relying on variable vanes.

The variable geometry applies only to the exhaust turbine of the turbo unit in an attempt to get relatively constant gas flow (trying to get faster spin up like with a smaller turbo, but better high-speed torque like a bigger turbo).

When you "apply throttle", in a diesel you are really just supplying more fuel, hence more exhaust gas is produced, but you only need to supply more air into the engine as the revs rise (it pumps faster). I think that for a given engine speed, the volume of air supplied is constant, irrespective of throttle position (not true on a petrol engine of course, which maintains a fuel/air ratio). By adding a turbo, you to supply air at a higher pressure compared with normal aspiration, meaning that there is more oxygen in each cylinder on a given induction cycle, hence more energy released (it’s as if the engine was bigger). If you pushed the throttle pedal in order to accelerate, you will get more torque by increasing the boost pressure right away, if possible, as well as adding more fuel, so it’s as if you are apparently expanding the size of the engine as well as burning more fuel.

Now, I would think you'd still need a wastegate as I said before, in case the exhaust pressure got too high, to avoid stifling the engine with back pressure, or wasting energy in providing unrequired boost. When the throttle is released, however, the exhaust turbine is no longer being driven very much by exhaust gases, irrespective of the state of variable vanes, but the input air impeller is hit with atmospheric pressure on one side, and boost pressure on the other, so it will slow down until the pressure is equal on either side of the impeller. On a diesel the slowdown is nowhere near as dramatically as in a throttled (petrol) engine. This means I'm thinking there could theoretically still be value in having a bypass valve (even though the turbo will probably end up being spun up from the input side, as a consequence of the engine simply sucking in air), but I don’t think it’s related to variable geometry of the exhaust side. So, the big question is, “do VAG turbo-diesels use bypass valves on the induction side”?

Nevertheless, I agree that a dump or blow-off valve would not serve a useful purpose, and indeed may be an entirely negative thing on a diesel.

Steve

justdoit

The only 'dump valves' I have heard of for diesels ain't actually dump valves, they are an electronic unit with a switch that connects somewhere that makes the whoosh noise when you lift of the pedal to change gear boy racers, ya gotta love 'em