Effects of Variable Geometry Turbochargers in Increasing Efficiency and Reducing Lag

What is a Turbocharger?

A turbocharger is a turbine that is driven by exhaust gases that compresses incoming air
into the engine. The “hot” side of the turbo receives its energy from the heat and flow energy
of the exhaust system. The “cold” side of the turbocharger pressurizes fresh air and forces it
into the engine. The pressure generated by the “cold” side is called the boost. The “cold” side is
driven by a shaft that is connected to the “hot” side.

What are the drawbacks to turbochargers?

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.

What is Lag?

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.

What is a Variable Geometry Turbocharger (VGT)?

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.


What are the benefits of Variable Geometry Turbochargers (VGT)?

*Reducing Lag Time

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.


What are the disadvantages of Variable Geometry Turbochargers (VGT)?

*Cost and Reliability
VGTs are very complex and require complicated control systems. The small moving
parts, sensors, and controllers make them more expensive to produce than FGT. All the parts
are exposed to extreme temperatures of over 1000oF making them wear quickly. Also due to
the extreme temperatures they need to be made from exotic materials which increase the cost
even more.

*Availability for Gasoline Engines
Typically VGTs are only available for diesel engines. Diesel engines produce much lower
exhaust temperatures than gasoline engines. These lower temperatures allow for the use of
more common materials and higher reliability. Through the use of newer materials borrowed
from the aerospace industry a few VGTs will be hitting the gasoline market soon. The 2007
Porche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbochargers used are
BorgWarner's Variable Turbine Geometry (VTGs).

Written By
Ted Rader, Sung Jung, and Ryan Wolfred 12/5/07


Variable geometry turbocharging for lower emissions and improved torque characteristics, J G
Hawley, F J Wallace, A Cox, R W Horrocks and G L Bird; Department of Mechanical
Engineering, University of Bath, UK; Proc Instn Mech Engrs Vol 213 Part D IMechE 1999





Effect of Variable Geometry Turbine (VGT) on Diesel Engine and Vehicle System Transient Response,
Zoran Filipi , Yongsheng Wang and Dennis Assanis; Automotive Research Center, University of Michigan, 2001

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