VGT - FGT (Fiat Grande Punto)

Hi,

  I read an article somewhere on the internet regarding the Punto's specifications and found that it is going to come with "1.3 MJet FGT instead of the VGT" and also realized that VGT is better than FGT however only to cut down on the costs Fiat is going ahead with the FGT rather than VGT in Punto.

Now my question is what is this "VGT & FGT" ?? Would appreciate if some of the technical experts out there can brief about these terms. Thanks in advance.

Turbochargers are generally used to feed the voracious appetite of the engine for air, to be able to produce optimum power at higher output levels. Injecting fuel at a higher rate is not much of a problem, but feeding the engine enough air to be able to combust all that fuel is a greater design challenge. An air pump (turbine / turbocharger) that runs off the same engine is used to compress the air before being fed to the engine, so that the engine continues to get enough of air for combustion. The trouble with the turbines is they are inefficient at low speeds, and the airflow requirements of the engine keep changing depending on the engine load and power expected. So a fixed turbine is not ok at extremes of operating conditions.

To improve, the inlet area of the turbo is modified using a camera aperture like adjustable arrangement, and the inlet area is changed as per engines air requirement so that the turbo is optimally efficient at most speeds.

One recognizable benefit of VGT is low 'lag'. Lag is the term used for very little benefit from the turbocharger below engine speeds or ~2000 RPM, after which the turbo kicks in, and a sudden change in the engine response is felt. Verna was on of the early introductions in the Indian car scene with this technology. Today it is commonplace, in vehicles introduced after that.

FGT I guess would be Fixed Geometry Turbocharger... Not heard the term though..

This link can give more info about VGT.

Hey Thanks Krishna,

But seems to be too technical for me

Thing of an FGT as a fan with fixed blades, and a VGT as a fan with blades that can become bigger or smaller depending on wind, the wind being the exhaust from the engine.
So if there is less wind , the VGT blades are small, and it starts rotating with less speed. FGT blades are big, and do not rotate at all
If there is more wind, VGT blades become bigger, and rotate more. n FGT will, on the other hand , have to wait until the wind is big enough to start rotating.
Now imagine that this fan is connected to another fan which blows air into the engine.
thats essentially what happens in a turbo
And thats why they are called FIXED and VARIABLE geometry turbos

@Greenhorn,

  Thanks for a very simple explaination with great example.

so as a result, in a car with an FGT, there will be no power due to the turbo at lower rpm's as there is less air pressure at the exhaust, and the turbo has not started spinning yet. it remains this way, until at a certain rpm, the speed becomes enough and the fan starts spinning, and the car suddenly starts moving like what happens in NFS when you push the nitrous button  

in a VGT, the turbo starts spinning gradually at lesser rpm, untill it reaches full speed, so th power comes more linearly, and there is no SUDDEN surge

Your example is simple to understand, but its not the FAN that becomes bigger or smaller, but the nozzle that drives the fan using the exhaust gases.

By varying the size of the nozzle, it is able to extract some amount of thrust even when engine rpm is low - thus making the turbo (turbine) usable to some extent before the exhaust gases generate enough momentum to drive it like a FGT.

Keep those wheels turning!

@greenhorn & Steeroid: Thanks for the simple explanation.

Pleasure, anytime.

Some explanation with pictures that will illustrate how the nozzle/vane opens up is available here:

http://paultan.org/archives/2006/08/16/how-does-variable-turbine-geometry-work/ 

Keep those wheels turning!

 I used to think it was the angle of attack of the turbine blades which changed. Guess its that of the inlet !