VNT turbo. What are the advantages.

**Dougal** · 1st August 2011, 02:47 PM

I'm pretty sure Bush65 has a good handle on VNT turbo operation.

But there is a downside to VNT's, their turbines aren't as efficient as a fixed geometry turbo. They've traded off peak efficiency for a much wider operating range.
For automotive applications that's perfectly fine, but stationary, industrial and marine can be a different story.

**PAT303** · 1st August 2011, 02:59 PM

I have both and the VNT is better by a country mile. Pat

**rovercare** · 1st August 2011, 03:14 PM

Originally Posted by Blknight.aus

correct.... BUT...

if the normal turbo only produces say 22psi of boost and the VNT only produces 22 psi of boost.....

the advantage of the VNT is that its pretty much on full boost from the bottom of the rev range.

But, if, long spiel.......

Higher boost, broader range, simple

**B92 8NW** · 1st August 2011, 03:50 PM

I guess the advantages depend on which engine you have in mind.

I've been toying with the idea for a while of one of the 2256V kits for my 300Tdi but I really don't think it's worth the $2500 off the shelf price. If I was assured you could see 400 Nm and 130kW comfortably I'd do it, and I think you could on a Td5, but I just can't see it happening on a Tdi.

**Blknight.aus** · 1st August 2011, 04:09 PM

the post isnt so much for bush65 but for others who dont know how or what is being discussed.

the original post (that I didnt complete sending before the computers powersupply cooked itself and shut the pc down) didnt have the first 2 paragraphs in its entirety I put those in because of the mention of "opening" in regards to the VNT, a common misconception from people who dont know and are trying to wrap their heads around turbo chargers is that it "opens" like the wastegate on a normal turbo and is controlled in a similar manner.

**robbotd5** · 1st August 2011, 06:55 PM

"what happens to the EGT in a diesel IF you apply a restriction to the exhaust while its trying to make power?"

They go up?
Regards
Robbo

**Bush65** · 2nd August 2011, 11:34 AM

Originally Posted by Blknight.aus

...

in very specific situations you will get better performance from a "normal" type turbo charger than you will from a VNT type turbo. Typically a normal turbo is setup to be most effecient at a fairly specific set of gas flows, engine rpms and yada yada yada...

But if by normal type turbo you are referring to a waste gate turbo (smaller turbine with waste gate to spill exhaust gasses when exh manifold pressure, and turbine speed become excessive, then VNT is most likely superior.

If by normal you are referring to a turbo without waste gate (tubine large enough so EMP and speed are not excessive, then low speed performance is sacrificed - in that respect then the VNT is again superior.

Originally Posted by Blknight.aus

the post isnt so much for bush65 but for others who dont know how or what is being discussed.

the original post (that I didnt complete sending before the computers powersupply cooked itself and shut the pc down) didnt have the first 2 paragraphs in its entirety I put those in because of the mention of "opening" in regards to the VNT, a common misconception from people who dont know and are trying to wrap their heads around turbo chargers is that it "opens" like the wastegate on a normal turbo and is controlled in a similar manner.

When a waste gate opens, a significant amount of energy in the exhaust gasses is not available - a VNT can utilise this wasted energy.

**Bush65** · 2nd August 2011, 11:56 AM

VNT is technology that replaced waste gates in diesels some little while after the TD5 engine. But VNT technology has in turn been superseded for modern diesel engines, e.g. 3 litre V6 in disco 4.

Puma owners claim better performance than the TD5. IMHO, this improvement is more to do with the VNT turbo on the puma, vs the tiny waste gate turbo on the TD5.

**Blknight.aus** · 2nd August 2011, 05:04 PM

Originally Posted by Bush65

But if by normal type turbo you are referring to a waste gate turbo (smaller turbine with waste gate to spill exhaust gasses when exh manifold pressure, and turbine speed become excessive, then VNT is most likely superior.

If by normal you are referring to a turbo without waste gate (tubine large enough so EMP and speed are not excessive, then low speed performance is sacrificed - in that respect then the VNT is again superior.

When a waste gate opens, a significant amount of energy in the exhaust gasses is not available - a VNT can utilise this wasted energy.

good point, my bad, had my big boys turbo hat on for that one....

if youve got an unwastegated turbo OR the wastegate is only there for the prevention of overspeeding/overboost then the VNT wipes you off at the bottom of the range.

if youve got a wastegated turbo that relies on the wastegate to control the max boost pressure by dumping to get the advantage of an earlier and better spool at the mid range then the VNT beats you on effeciency at the bottom and the top of the curve

in both styles of normal turbo (assuming good fuel mapping and turbo setup) there is a very small window where the normal turbos are marginally more effecient than a VNT and this is only due to the vanes in the VNT being in the way of the exhaust gasses.

SheldonA · 2nd August 2011, 09:53 PM

Originally Posted by Blknight.aus

<SNIP>

now the killer question... (and this relates to the only REAL downside to VNT turbos and its a failure mode condition, points for the correct answer to the question below, bonus points if you can work out the next question and head of the class if you can then answer it)

what happens to the EGT in a diesel IF you apply a restriction to the exhaust while its trying to make power?

oh oh sir sir me me!

If exhaust is restricted, then max. air flow is reached. Boost can't go up as the turbine has max flow over it already. Your trying to make power which means your throwing more fuel into the cylinder, but your limited as you've already reached max air flow. So any extra fuel doesn't go through the complete dispersion, diffusion, atomisation, oxidisation process. ie. it makes soot and due to the energy this process absorbs in the form of heat (whilst not adding any through actaully burning), hence reducing you EGT and taking up valuable volume in the cylinder.

This soot does bad things to your turbo and the enviroment.

Next question?

How do you fix this??

Remove the restriction to the exhaust. And if the restriction is in fact the turbo itself;

1. Fit a bigger turbo - cost money and possibly noticable lag low down.

2. Reduce air flow by taking away any air that isn't being used in any actaul combustion but contributes to exhaust volume through expansion due to heat within the cylinder. Do this by reducing boost pressure. Reducing boost pressure means you also get better injector spray penetration = putting fuel into areas where there was unused air for combustion previously. Obvisouly only works if boost pressure was too high to begin with, and there will be a limit somewhere.

3. Fit an external wastegate - that relieves flow around the turbo. Would need to be more than just a set pressure relief. If 30psi was your setting for max boost, then there would be no differentiation in relieving flow at max flow (where restriction becomes a problem) and at say half flow or any other flow - which means you loose critical energy for building and maintaing boost. Ultimately you are wasting exhaust potential energy.

How did I go sir?

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