Thread: VNT turbo. What are the advantages.

Ok thanks. Understand where you were going with the initial question.

Can you tell us why EGT will rise with a restriction to exhaust gases?

Understand that diesel run lean on an overall cylinder air mass to fuel mass ratio, but in the injector spray/burn region it is never 'lean' (will always be in a certain range to promote a burn). Is it not total fuel being burnt that causes high EGT as opposed to total fuel being injected?

ie.

you get soot from excess unburnt fuel - which can only occur from lack of air (not the case as full boost is still possible from sticking vanes), or lack of heat (either in the lean areas of the cylinder where is is cooler near the wall, or the outside of droplets of fuel)

and seperately,

you get high EGT from amount of fuel burnt


So three scenerios are possible with soot/EGT problems;

1. Soot is formed but EGT ok - due to excess fuel being dumped into cylinder, but not enough is being burnt to cause excessive heat. Typical when you plant your foot from idle.

2. No soot, but high EGT - lots of fuel going in and it is all being burnt. Typical of nicely breathing and tuned engine at full load.

3. Soot and high EGT - even more fuel going in. Lots being burnt for high EGT and lots not being burnt in the droplet region which isn't being full atomised. Typical of madman/backyard mechanic trying to get the most out of his rig at full load.

Ah you changed some stuff since my reply...

Originally Posted by Blknight.aus

"if you put a restriction in the flow of exhaust gasses while the engine is making power the EGT's will rise"

Some terminology helps a bit here. Do you mean making, increasing or maintaining power? Think I can answer my own question about why EGT will rise due to restriction now.

Will assume you didn't mean making power.

With the knowledge of where you were heading with the original question...

If you meant increasing power - then yes EGT will always rise with or with out a restriction.

If you meant maintaining power - then EGT will rise with the restriction due to the need for extra power to overcome the new external losses (so really we are increasing power)

Originally Posted by Blknight.aus

if you have the restriction AFTER the turbo there is less pressure drop across the turbo so the turbine looses power and boost drops.

Yes, get where your coming from now. Assumed we were already at the equalibruim state.

I think my clarification between the seperation of soot formation to EGT is a valid extention on the scenerio you put with booth high EGT and soot (which would be scenerio 3). EGT is formed from the overall heat generated from combustion in the cylinder and soot is formed from localised low temperatures/localised lack of air.

But what was I thinking with reduced boost pressure and spray penetration - the boost pressure are such an order of magnitude lower than injector pressure

Thanks for the refresher

Originally Posted by Blknight.aus

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

Here's where my brain was working from;

Certain amount of fuel from injector pump (trying to make power) at given vane position on turbo -> flow is restricted (be it from turbo itself or further downstream) but max./high boost is achieved.

Given the same amount of fuel is being injected (trying to make power), due to restriction, boost is reduced -> leads to lower air mass. Eventually low enough boost will prevent complete combustion of this certain amount of fuel. Leads to;

1. soot due to incomplete burn of some fuel

2. reduced EGT. Fuel burnt now equals certain amount of fuel injected minus fuel turned into soot



In terms of the solution proposed at no. 2 to reduce air flow....

Following scenerio;

Upgrading from non-VNT or N/A motor. Injector pump inadequate at supplying max. amount of fuel now possible to be burnt. And turbo smaller than what could be matched to motor.

Therefore, at max. fuel delivery of this injector pump: full open vanes: flow restricted due to turbo sizing: boost still high (but more than required for a complete burn of fuel given the limitations of the injector pump) -> so we are getting a complete burn.

Therefore,

1. We have reached max EGT as max fuel is being delivered. No more fuel possible from injector pump to add for increased EGT.

2. Charge air temp higher than need be due to compressing air further than necessary.

3. Reduced efficiency due to pumping losses of boosting higher than need be.


Solution;

Reduce vane opening -> restricts flow further (less flow over turbine) -> lowers boost -> lower charge air temp -> still a complete burn (matching air requirements to fuel delivery now) -> EGT slightly lower (minimal? same amount of max fuel minus slightly lower charge air temp.) -> same (slightly lower) EGT X less charge air = less exhaust gas volume.

Therefore, less exhaust gas volume negates restriction due to vanes -> less loss of energy to overcome restriction + reduced pumping losses = more available power to use.

Originally Posted by Bush65

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

Here is an interesting question.

What is more efficient overall?
1. A fixed geometry turbine which has a higher efficiency but bypasses (i.e. completely wastes) a portion of the flow.
2. A variable geometry turbine which has lower efficiency by still extracts energy from the whole flow.

At very high flows the variable geometry has to win, simply because the drive pressure in the wastegated turbine is higher due to it being in the "really sucky" part of it's operating curve. (i.e. higher drive pressure or EMP).
At very low flows the variable geometry again will win because the wastegated turbine simply won't be able to extract enough energy from the low flow to work properly. (i.e. no boost).

But in the middle around maximum torque I'd expect the wastegated turbo to win. This is the point where it's wastegate is still closed and flows are as close as they'll get to the best operating points of the turbine.
At this operating point the VNT vanes are added extra drag and flow restriction but not providing any benefit.


So, how can you know how efficient your turbo is? There are two measurements which are possible for anyone to do.
The first is compressor discharge temperature. The more efficient your turbo compressor is, the lower the output temperature.
The second is the ratio of backpressure (drive pressure or EMP) to boost.

Who cares about it?,a VNT makes the engine more tractable right across the driving range from low speed outside woolies to high speed on the highway and everything in between so I don't give a toss about the technical mumbo jumbo,I just drive it and am thankfull clever people develope such things. Pat

Originally Posted by PAT303

I don't give a toss about the technical mumbo jumbo,I just drive it and am thankfull clever people develope such things. Pat

Umm, Pat. This forum is "technical chatter".