Variable Geometry Turbocharger (VGT)

The main drawback to a turbocharger, besides cost, is its fixed geometry. The Aspect

Ratio (A/R) of a turbo, which is based on its geometry, has a direct relation to both the power

increase generated and the motor speed at which the power increase is generated. A smaller

A/R will produce boost pressure at a lower engine speed, but will be unable to provide a high

enough flow rate at higher engine speeds. This leads to higher exhaust manifold pressures,

lower pumping efficiencies , and lower power output. A larger A/R will create boost at higher

engine speeds, and thus create more power, but it will be unable to produce boost at lower

engine speeds. So an A/R must be picked to either; produce power at lower engine speeds for

quicker acceleration, or for higher engine speeds to produce a greater total power.

The time it takes for the engine to produce boost between transients is called lag. A

large A/R turbo will have a longer lag time than a smaller A/R turbo due its larger requirement

of energy from the engine to produce boost.

Variable Geometry Turbochargers are turbochargers whose geometry and thus

effective A/R can be altered as needed while in use. The most common design includes several

adjustable vanes around a central turbine. As the angle of the vanes change, the angle of air

flow onto the turbine blades changes, which changes the effective area of the turbine, and thus

the aspect ratio (A/R) changes.

The area between the adjustable vanes works as nozzles. These nozzles are thus varied

in size as a function of engine operating conditions. By opening the nozzles at high engine speed

or closing them at low speed, effectively changing the A/R with engine speed or demands, the

turbo can produce boost from a low speed without restricting flow at higher speed. Since they

can produce boost at lower engine speed Lag time is decreased.

Also since the vanes are remotely controlled the boost pressure can be altered without

changing engine speed. By adjusting the vanes you can increase exhaust manifold pressure

during transients (gear changes). Coming out of a transient with a higher exhaust manifold

pressure allows this stored energy, in the form of pressure, to be used to drive the turbo to a

higher boost level faster. By increasing the boost level faster Lag is once again reduced.

*Increasing Efficiency;

Turbochargers in general are a very good way to improve the efficiency of an engine.

By pressurizing the intake manifold, more air, and thus more fuel, is brought into the cylinder

every time the intake valve opens. This creates a volumetric efficiency of greater than 1. A

volumetric efficiency of even 1 is impossible in any real engine without some kind of forced

induction due to friction losses. This improves the overall efficiency of the engine by allowing it

to burn more air and fuel on every cycle. The high positive pressure generated also helps to

overcome any casting defects in the manifold, such as surface roughness (major losses) or tight

corners (minor losses), by providing a larger driving force, or pump head.

Fixed geometry turbochargers (FGT) work as any other centrifugal pump and thus have

a limited optimal operating range. VGTs have the advantage that many different pressure

ratios can be produced at a single engine speed due to the variable vanes changing the

effective area and A/R. The vanes can be manipulated to create an optimal boost pressure at

any speed. By producing an optimal boost through a larger engine speed range the overall

efficiency is increased.