[unni.ak]

How does Variable Turbine Geometry work?

The link gives a beginners insight into VGTs.

[tsk1979]

Note from mod, New thread created, continue discussion of VGT tun-ability on this thread

[dadu]

Got some more info to substantiate that they are tuned to work at varied engine speed (RPM's) also. Key points being:

• At low engine speed and low gas flow, the vanes of the VNT™ close, reducing the inlet area to the turbine. This results in an increase in turbine inlet pressure, which enhances turbine power and drives higher engine boost pressure.
• Garrett® VNT™ turbos are highly complex units, engineered and calibrated to meet the exacting performance parameters of automotive manufacturers.
• The first requirement is to set the turbo's critical minimum flow vane position. This intricate process takes account of the large number of parts and extremely tight tolerances within the Garrett® VNT™ and involves using a highly accurate turbine flow bench to measure and compensate for any variation in gas flow through the vanes of the turbo.
• Highly skilled technicians then set and lock the minimum flow position to suit the requirements of each individual application. It is only when the ‘minimum vane open' position is set that it is possible to calibrate the unit (to three decimal places), using four checkpoints to ensure correct operation.

Safari 2.2l uses a Garrett 1749v Turbo, it's VNT.

Source

[DerAlte]

That is straight out of Garrett's marketing collateral. But, it still doesn't explain the relationship with actual engine speed that the turbo is allowed to cut in, other than saying 'low'. As Mathur-sab was trying to explain, the turbo - variable or otherwise - starts turning with whatever exhaust gas flow, which is as soon as the engine idles, but unless the output pressure at air outlet is significant enough, it won't make a diff to the engine no matter how much the turbine huffs and puffs.

How is anyone to attribute improvement in driveability to early cut in of the turbo? The tuning you are referring to is pre-production tuning for a specific vehicle (Garrett application engineer's job), not automatic on-the road tuning at 'varied engine speeds' (ECU's job).

AFAIK VGT (as opposed to non-VGT) operation goes beyond just power boost by stuffing more air. The region between idling and old turbo cut-in (set somewhere between 1700 and 2200rpm for different diesel vehicles) is prone to higher emissions (gaseous and particulate) since the burning is comparatively 'cooler'. Pushing more air alleviates this problem. Normally, the vane position is kept at a certain constant position based on load/demand, but when the emission control function in the ECU senses anomaly (via lambda, etc.), one 'closes' the vanes a bit to instantly jack up O2 pushed in. That was not possible in non-VGT. Also, when the demand is not much (deceleration, engine braking, selective cyinder shutdown, ...) one opens the vanes to prevent the turbine from self-destructing. Ditto for turbine error conditions such as over-temp etc.

VGT really does not make a racing car out a plodding Safari, instead it actually makes a 2.2l Safari far more cleaner burning than the old TCIC. Had Tata's not used a VGT and stuck to TCIC, we wouldn't have had a BS3 compliant Safari. 2.2l is just an incremental boon, it is not a revolutionary improvement as far as diesel engines are concerned.

[Thomas8700]

what all is going on over here??

VGT for Euro 3 in the safari?

then how is an FGT unit in the punto or ritz able to achieve Euro 4 ?

[athalaga]

Originally Posted by Thomas8700

what all is going on over here??

VGT for Euro 3 in the safari?

then how is an FGT unit in the punto or ritz able to achieve Euro 4 ?

My 2 cents
1] The BASE engines differ here.

2] VGTs tend to improve low speed torque and overall fuel economy.

3] VGT usage at high engine output conditions versus a well-matched fixed-geometry turbocharger may not be justified.

4] VGTs w/o EGR lead to increased NOX emissions. VGTs can drive EGR and counter the increase in NOx emissions with relatively minor penalty in particulate matter (PM) emissions.

[athalaga]

Originally Posted by anupmathur

Point well understood!
However, please note that WORK needs to be done to get useful output from the turbocharger.
A certain mass flow and temperature of exhaust gases is required before any useful work can be done by the turbo.
It is not as simple as setting the vanes to a different angle and start getting useful boost at very low rpms!

Else there would never have been anything known as turbo lag!

+1 to that a minimum flow rate is required at the turbo end for the compressor to BOOST the intake. If that is not available no amount of changing the vane direction is going to help. however at the lower flow rates and temperatures VGT will produce more a little BOOST than a fixed vane geometry turbo. So tuning the ECU to control the VGT may not be possible if the exhaust flow rates and temperatures are below the threshold level.

[dadu]

Originally Posted by athalaga

+1 to that a minimum flow rate is required at the turbo end for the compressor to BOOST the intake. If that is not available no amount of changing the vane direction is going to help. however at the lower flow rates and temperatures VGT will produce more a little BOOST than a fixed vane geometry turbo. So tuning the ECU to control the VGT may not be possible if the exhaust flow rates and temperatures are below the threshold level.

Why is it so difficult to understand that this technology is present and works, thats the key selling point for VNT, no doubt some may be overhyped but then this technology wouldn't have survived for long and no auto manufacturer is a fool.

How VNT™ Works

A Garrett® VNT™ turbocharger works by adjusting the gas throat section at the inlet of the turbine wheel in order to optimize turbine power with the required flow velocity.

At low engine speed and small gas flow, the turbocharger reduces the throat section, increasing turbine power and boost pressure. At full engine speed and high gas flow, the VNT™ turbocharger increases the throat section, avoiding turbocharger overspeed and maintaining the booster pressure required by the engine.

The throat section modulation can be controlled directly by the compressor pressure through the use of a pressure actuator, or by the engine management system using a vacuum actuator. To modify the throat section, VNT™ Multivane™ models use a mobile multivane system composed of a number of vanes which rotate relative to the turbine wheel axis.

Source

[anupmathur]

Literature from vested interests is ALWAYS trying to mis-represent/gloss over the faults/shortcomings!
How about reading neutral journals?
There is no free lunch.

[dadu]

Originally Posted by anupmathur

Literature from vested interests is ALWAYS trying to mis-represent/gloss over the faults/shortcomings!
How about reading neutral journals?
There is no free lunch.

Agreed, I have read a study from "National Maritime Academy" and Ford on VGT's and their deduction is no different from what i posted.

Can you point to some references please.

[anupmathur]

Originally Posted by dadu

Agreed, I have read a study from "National Maritime Academy" and Ford on VGT's and their deduction is no different from what i posted.

Can you point to some references please.

OK, I'll give you a brief explanation.
Small little VGTs will spool up very fast and give usable boost from virtually idling rpm onwards. Then, even with the vanes FULL open, they'll fall woefully short of boost at higher BHP. Needless to say an exhaust gate will be MANDATORY to prevent absolute chaos due to the impossibly small total area available.
A larger VGT will fail to provide useful boost at low revs due to the inertia it possesses.

In short, the requirements are contradictory for boost at early rpms and for full bhp operation. A COMPROMISE is the only way out. Give some, gain some. Much like with suspensions and a lot of things in life! This is where art and engineering are merged, with experience.
There is no magic! There is no cure-all turbo available yet. And it will not be till someone re-writes the laws of Physics!

As for the kind request for relevant articles, I'd rather you Google than I! Sorry.

[dadu]

Originally Posted by anupmathur

OK, I'll give you a brief explanation.
Small little VGTs will spool up very fast and give usable boost from virtually idling rpm onwards. Then, even with the vanes FULL open, they'll fall woefully short of boost at higher BHP. Needless to say an exhaust gate will be MANDATORY to prevent absolute chaos due to the impossibly small total area available.
A larger VGT will fail to provide useful boost at low revs due to the inertia it possesses.

In short, the requirements are contradictory for boost at early rpms and for full bhp operation. A COMPROMISE is the only way out. Give some, gain some. Much like with suspensions and a lot of things in life! This is where art and engineering are merged, with experience.
There is no magic! There is no cure-all turbo available yet. And it will not be till someone re-writes the laws of Physics!

As for the kind request for relevant articles, I'd rather you Google than I! Sorry.

I agree that during engine operation at low engine speeds and load, little energy is available from the exhaust to generate boost but in order to maximize the use of the energy that is available, the vanes are closed. In doing so, the exhaust gas is accelerated between the vanes and across the turbine wheel increasing turbocharger wheel speed and “boost”. In general, this allows the turbocharger to behave as a smaller turbocharger.

Closing the vanes also increases the back pressure in the exhaust manifold which is used to drive the exhaust gas through the EGR cooler and EGR valve into the intake manifold. This is also the position for cold ambient warm up.

During Engine operation at moderate engine speeds and load, the vanes are commanded partially open.

The vanes are set to this intermediate position to supply the correct amount of boost to the engine for optimal combustion as well as providing the necessary back pressure for EGR operation.

During engine operation at high engine speeds and load, there is a great deal of energy available in the exhaust.

Excessive boost under high speed, high load conditions can negatively affect component durability, therefore the vanes are commanded open preventing turbocharger overspeed.

Essentially, this allows the turbocharger to act as a large turbocharger, not creating excessive back pressure.

Hope this clarifies, googling did not lead to any negative feedback by neutral journals or others. Its a technology adopted by many manufacturers and renowned vehicle brands.

[anupmathur]

Originally Posted by dadu

Its a technology adopted by many manufacturers and renowned vehicle brands.

None of the above counters what I have already said.

[dadu]

Originally Posted by anupmathur

None of the above counters what I have already said.

You are entitled to an Opinion, no doubt.

[anupmathur]

Originally Posted by dadu

You are entitled to an Opinion, no doubt.

Thank you!
I'm not making this into an ego tussle. After all I invented none of it!
Someday you'll understand there is no such universal turbo as you seem to believe!