Forced Air Induction
The Garret Aviation VNT-25 The idea of forced
air induction by turbine, or
turbo, is not new and has it's mass production
roots in WWII fighter planes.
What is new, however, is its application to
passenger automobiles. Unlike a near
constant high RPM fighter engine, an
automobile requires wide-open throttle (WOT)
power availability throughout
its entire operating range. Previous automotive
turbo applications acted like
an on-off power switch with a five second delay,
decreasing drivability,
rather than providing the smooth linear powerband of a
normally aspirated
engine. Because the turbine is in a fixed position in the
exhaust stream, it
was plagued with sometimes uncontrolled production from the
compressor at
high engine speeds, commonly referred to as boost creep, and a
significant
decrease fuel economy versus a similar, but naturally aspirated
engine. The
Garret Aviation produced VNT-25 solved all of these problems with
its
innovative Variable Nozzle Turbine. Hands down it is the most advanced
turbo
ever mass-produced and it was the first of its kind on production cars.
One of
the most talked about problems with turbo charged engines is the
lengthy time it
takes for the turbo itself to accelerate to operational
speeds. This is commonly
referred to as turbo lag or turbo spool up time.
Under WOT, turbo lag results in
a seemingly underpowered engine that suddenly
comes to life as a delayed tire
melting rush of acceleration. Previously,
turbo lag was limited by decreasing
the size of the turbo itself. This
resulted in lower rotating mass and more
importantly, a smaller cross
sectional area, which accelerated exhaust gasses at
lower engine speeds.
Although the turbo is able to spool quicker due to its
size, for the same
reason its ability to move and compress large amounts of air
efficiently is
significantly reduced. Inherently a smaller turbo will produce
less maximum
horsepower than if it were replaced by larger turbo on the same
engine.
Previous turbochargers also used a fixed position turbine that powered
the
centrifugal compressor directly. Because the turbine is located directly
in
the exhaust stream, the turbine is a huge exhaust restriction. This
restriction
creates a constant exhaust backpressure that decreases fuel
economy even when
the turbo is not in use. At high engine speeds, the
restriction creates enough
pressure in front of the turbine (back pressure)
that the wastegate can no
longer limit turbine power by bypassing the exhaust
around the turbine. The
result is that turbo compresses more air into the
engine than is wanted. For
example, a turbo was set to produce a maximum
12psi boost pressure, but during a
period of sustained wide open throttle
high engine speeds the turbo is now
producing 14.5psi of boost and still
rising. This unwanted phenomenon is called
boost creep. The VNT-25 solves all
of these problems with an innovative turbine
called a Variable Nozzle
Turbine. Rather than a fixed turbine the VNT-25 uses a
ring of 12 moveable
paddles aligned around a central, but very small turbine
wheel. The entire
exhaust charge is then directed to the small turbine by the
paddles. Moving
the paddles varied the crossectional area that the exhaust must
pass through.
When the paddles are nearly closed the exhaust is accelerated
towards the
turbine wheel to increase power. Decreasing the crossectional area
of flow
accelerates normally slow, low engine speed, gasses and nearly
eliminates
turbo lag while allowing a large and efficient compressor wheel for
excellent
maximum engine power. Opening the paddles allowed the exhaust to flow
slower
and bypass the turbine to limit power. This unique arrangement
significantly
reduced backpressure, greatly improved fuel economy, and allows
excellent
control turbine power at sustained high engine speed, without the use
of a
bulky external wastegate. The Garret VNT isn't without its drawbacks. In
high
performance applications it is a turbo that has little to be desired.
The
engineers of this turbo, in their effort to reduce turbo lag as much
as
possible, kept the compressor and turbine as small as possible. The
smaller size
of the turbine and the compressor decreases the size and
therefore the weight of
the turbo internals. Keeping the weight as light as
possible reduces rotational
inertia to an absolute minimum, which results in
a much more responsive turbo.
Because the exducer, that is the
compressor, is of a compressor type,
operational speeds are very high. It is
not unlikely for a VNT to reach maximum
operational speeds of 173 thousand
revolutions per minute even though resting or
"cruise" speed of the turbine
is only 2000-6000 RPMs. It is this
latency of the turbo to accelerate to
operating speeds that is referred to as
turbo lag. Although the small size of
the turbine is ideal for a moderate
performance car, its size is a handicap
in racing situations. Inherent with a
small compressor is its ability to
quickly reach operating boost pressure. This
does not come with out a
penalty. Effectively this small compressor trades
efficiency for speed. As
any gas is compressed the temperature of it rises.
Smaller compressors
will tend to heat the compressed air more than would a
larger turbo for a
given pressure. Bernoulli's principal states that as a gas is
compressed the
temperature increases as the volume decreases. The inefficiency
of the VNT at
pressures over 15 pounds per square inch increases the temperature
of the gas
more than it is possible for it to compress, or decrease the volume.
The
result is that the increase in boost pressure is inversely proportional
to
the volume of air moved. As the compressor works to decrease the volume of
air,
the rise in temperature works to increase the volume. Eventually the
volume of
air is expanded by heat more than it can be compressed. The point
at which this
happens is referred to as the stall speed. Because a larger
turbo, although slow
to respond, is much more efficient at higher pressures
it will result in a much
cooler charge at a given pressure. A smaller
compressor also cannot move large
quantities of air at high pressures as
would a larger turbo be able to. The size
of the VNT, although ideal for
12psi as it was intended for, suffers greatly in
high performance
applications from stall speed of psi. The turbine also suffers
from a small
and compact A/R ratio. The A/R is the ratio at which the turbine
or
compressor housing is cast. The A/R is the ratio at which the volume of
the
housing as gasses enter the housing to the volume it exits. For instance,
the
size of connection on the intake side of the compressor is two and one
quarter
inches inside diameter and has a volume of 323 cubic centimeters
until it
reaches the compressor. The exit side is also two and one quarter
inches inside
diameter and contains a volume of 155cc's. The volume of each
path to the
compressor is misleading and cannot be determined from the
diameter of the exit
or intrance alone. The intake passage is a direct and
simple path to the
compressor cartridge. The exit, however, is fluted from
the from a very wide and
narrow, almost rectangular, passage at the side of
the compressor to a standard
2 ¼ inch inside diameter round pipe fitting.
This fluted shape insures that the
speed of the compressed charge is kept
relatively high. The high speed maintains
that the compressed charge is kept
away from the compressor. If it were allowed
to back up near the compressor,
the compressor would have to work much harder to
move the already dense air.
The result would be that the clready compressed air
would be further
compressed and heated. Although the small inlet and outlet
sizes contribute
to increased velocity With the introduction of the Garret
VNT-25 it is
now possible for a small displacement turbo charged engine to
operate and
perform nearly identical to a much larger engine. The ON/OFF switch
of turbo
power is gone and is now replaced by the safer, smoother, and much
more
linear acceleration comparable to naturally aspirated engines of much
larger
displacement. A VNT-25 equipped engine also has the potential to, and
usually
does, produce much more power than engines twice its size. However,
with
disciplined drivers, it does not loose the fuel economy characteristics
inherent
with small, normally aspirated engines when the turbo is not in use.
The VNT-25
combines the responsiveness of a small turbo with the efficiency
and performance
of a much larger turbocharger. Simply stated, the VNT-25 is
the ideal
turbocharger. It allows great power almost no turbo lag, great
responsiveness,
retains engine and compressor efficiency, and allows
excellent turbine control
from boost creep.
Bibliography
Ralph
C. Bohn, Angus MacDonald. Energy Technology. Fourth Edition, Peoria,
IL:
Macmillan/McGraw Publishing, 1992. Chrysler Passenger Cars Factory
Service
Manual vol.1; Engine and Chassis 1990.
www.alliedsignal.com/business/turbo/about_cas.html
http://idt.net/~vnt4/vntrpt.html
http://idt.net/~vnt4