STEYR MOTORS Marine Engine 2012 Series, Service Manual

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Fuel System
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Variable Geometry Turbocharger (VGT)
Variable Geometry Turbochargers are turbochargers whose geometry and thus
effective aspect ratio (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 benefit of Variable Geometry Turbochargers lies in 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 a VGT can produce boost at lower engine speed lag time
is decreased. Coming out of a transient driving situation with a higher exhaust
manifold
pressure allows this stored energy, to form in a pressure, to be used to drive the
compressor into a higher boost level faster. VGT’s 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.