How does using a VTG help in reducing turbo lag

[monty37]

In what way does the VTG help reduce turbo lag at low speeds? and can the lag be eliminated completely?

 

[justsomeguy]

http://en.wikipedia.org/wiki/Variable-geometry_turbocharger

Lag cannot be eliminated entirely. Spinning up a turbine takes time.

 

[monty37]

So one can still feel the difference when driving at low speeds ...and when shifting to high speeds..but they are really effective only at high speeds na?

 

[justsomeguy]

You might not be able to "feel" it. If the geometry control is fine grained enough, the point when the turbo starts adding power might not be noticeable, but it would be measurable. I'm not sure what you mean by the latter half of your post.

 

[monty37]

The link says "VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds" so that's what I asked. And may I know what you mean by geometry control being fine grained?

 

[justsomeguy]

The link says "VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds" so that's what I asked. And may I know what you mean by geometry control being fine grained?

Low boost threshold : this means they start making boost at low RPM.

Minimal lag: Same thing.

Very efficient at high engine speed: Traditional (non-variable) turbos are trade-offs and only work well at one end of the RPM range or the other. Variable turbos work well at both ends.

By fine grained I mean the degree with which the geometry can vary in relation to engine RPM. The purely membrane/mechanical ones should are infinitely variable in theory. Electric and hydraulically operated vanes will be subject to the granularity provided by the control mechanism.

An electrically operated VGT that only has two states (open and closed) would function exactly like a non-variable turbocharger. I'm not familiar enough with the systems to say if the electrically operated ones are digitially controlled, but I would bet they are, and this means there are a finite number of "steps" the vanes can be put through between fully open and (nearly) fully closed.

The more steps you have, the more fine grained the control, and the better the turbo is going to operate over the full RPM range.

 

[monty37]

The control has mainly got to do with the ECU then right?

 

[justsomeguy]

The control has mainly got to do with the ECU then right?

I would assume so on the electrically operated ones. I believe most are purely mechanical though, operated via intake manifold vacuum.