Essay, Research Paper: Forced Air Induction


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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 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. BibliographyRalph 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.
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