vgt
TRANSCRIPT
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INTRODUCTION
Variable geometry turbochargers (VGTs) are a family of turbochargers, usually designed to allow the effective aspect ratio (sometimes called A/R Ratio) of the turbo to be altered as conditions change. This is done because optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds.
In many configurations, VGTs do not even require a wastegate; however, this depends on whether the fully open position is sufficiently open to allow boost to be controlled to the desired level at all times. Some VGT implementations have been known to over-boost if a wastegate is not fitted.
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TURBOCHARGING
Basic Theory
The advantage of turbo charging is obvious - instead of wasting thermal
energy through exhaust, we can make use of such energy to increase engine power.
By directing exhaust gas to rotate a turbine, which drives another turbine to pump
fresh air into the combustion chambers at a pressure higher than normal
atmosphere, a small capacity engine can deliver power comparable with much
bigger opponents. For example, if a 2.0-litre turbocharged engine works at 1.5 bar
boost pressure, it actually equals to a 3.0-litre naturally aspirated engine. As a
result, engine size and weight can be much reduced, thus leads to better
acceleration, handling and braking, though fuel consumption is not necessarily
better.
Turbochargers are a type of forced induction system. They compress
the air flowing into the engine. The advantage of compressing the air is that it lets
the engine squeeze more air into a cylinder, and more air means that more fuel can
be added. Therefore, you get more power from each explosion in each cylinder. A
turbocharged engine produces more power overall than the same engine without
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the charging. This can significantly improve the power-to-weight ratio for the
engine.
In order to achieve this boost, the turbocharger uses the exhaust flow
from the engine to spin a turbine, which in turn spins an air pump. The turbine in
the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm) --
that's about 30 times faster than most car engines can go. And since it is hooked up
to the exhaust, the temperatures in the turbine are also very high.
Turbochargers allow an engine to burn more fuel and air by packing
more into the existing cylinders. The typical boost provided by a turbocharger is 6
to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at
sea level, you can see that you are getting about 50 percent more air into the
engine. Therefore, you would expect to get 50 percent more power. It's not
perfectly efficient, so you might get a 30- to 40-percent improvement instead.
Inside A Turbocharger
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The turbocharger is bolted to the exhaust manifold of the engine. The
exhaust from the cylinders spins the turbine, which works like a gas turbine engine.
The turbine is connected by a shaft to the compressor, which is located between the
air filter and the intake manifold. The compressor pressurizes the air going into the
pistons. The exhaust from the cylinders passes through the turbine blades, causing
the turbine to spin. The more exhaust that goes through the blades, the faster they
spin.
On the other end of the shaft that the turbine is attached to, the compressor
pumps air into the cylinders. The compressor is a type of centrifugal pump -- it
draws air in at the center of its blades and flings it outward as it spins. In order to
handle speeds of up to 150,000 rpm, the turbine shaft has to be supported very
carefully. Most bearings would explode at speeds like this, so most turbochargers
use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil
that is constantly pumped around the shaft. This serves two purposes: It cools the
shaft and some of the other turbocharger parts, and it allows the shaft to spin
without much friction.
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Fixed Geometry
The demands on modern engines for wide operating speed ranges, high
torque rise and high specific power / litre have outstripped the capability of fixed
turbine geometry turbocharging. This is particularly true for automotive
applications using mid-range and heavy-duty products. In addition, construction
equipment requiring enhanced low speed response is increasingly specifying
wastegated (turbine bypass) turbochargers.
VARIABLE GEOMETRY TURBINE
Variable Turbine Geometry technology is the next generation in
turbocharger technology where the turbo uses variable vanes to control exhaust
flow against the turbine blades. See, the problem with the turbocharger that we’ve
all come to know and love is that big turbos do not work well at slow engine
speeds, while small turbos are fast to spool but run out of steam pretty quick. So
how do VTG turbos solve this problem?
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A Variable Turbine Geometry turbocharger is also known as a
variable geometry turbocharger (VGT), or a Variable Nozzle Turbine (VNT). A
turbocharger equipped with Variable Turbine Geometry has little movable vanes
which can direct exhaust flow onto the turbine blades. The vane angles are
adjusted via an actuator. The angle of the vanes varies throughout the engine RPM
range to optimize turbine behaviour.
VANES IN CLOSED POSITION
In the 3D illustration above, you can see the vanes in an angle which is
almost closed. The variable vanes are highlighted so that you know which is
which. This position is optimized for low engine RPM speeds, pre-boost.
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In this cut-through diagram, you can see the direction of exhaust flow
when the variable vanes are in an almost closed angle. The narrow passage of
which the exhaust gas has to flow through accelerates the exhaust gas towards the
turbine blades, making them spin faster. The angle of the vanes also directs the gas
to hit the blades at the proper angle.
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VANES IN OPEN POSITION
Above are how the VGT vanes look like when they are open.
This cut-through diagram shows the exhaust gas flow when the variable
turbine vanes are fully open. The high exhaust flow at high engine speeds are fully
directed onto the turbine blades by the variable vanes.
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VGT COMPONENTS
Bearing housing
A grey cast iron bearing housing provides locations for a fully-floating bearing system for the shaft, turbine and compressor which can rotate at speeds up to 170,000 rev/min. Shell moulding is used to provide positional accuracy of critical features of the housing such as the shaft bearing and seal locations. CNC machinery mills, turns, drills and taps housing faces and connections. The bore is finish honed to meet stringent roundness, straightness and surface finish specifications.
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TURBINE WHEEL
The turbine wheel is made from a high nickel super alloy investment
casting. This method produces accurate turbine blade sections and forms. Larger
units are cast individually. For smaller sizes the foundry will cast multiple wheels
using a tree configuration.
SHAFT AND TURBINE WHEEL ASSEMBLY
The forged steel shaft is friction welded to the turbine wheel. The
turbine blade edges are machined for accurate trim within the turbine housing. The
shaft bearing journals are induction hardened and ground for dimensional
accuracy.
JOURNAL BEARINGS ARRANGEMENT
Journal bearings are manufactured from specially developed bronze or
brass bearing alloys. The manufacturing process is designed to create geometric
tolerances and surface finishes to suit very high speed operation.
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THRUST BEARING
Hardened steel thrust collars and oil slingers are manufactured to strict
tolerances using lapping. End thrust is absorbed in a bronze hydrodynamic thrust
bearing located at the compressor end of the shaft assembly. Careful sizing
provides adequate load bearing capacity without excessive losses.
COMPRESSOR COVER
Compressor housings are also made in cast aluminium (cast iron for
high-pressure applications). Various grades are used to suit the application. Both
gravity die and sand casting techniques are used. Profile machining to match the
developed compressor blade shape is important to achieve performance
consistency.
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COMPRESSOR IMPELLER AND FASTENER
Compressor impellers are produced using a variant of the aluminium
investment casting process. A rubber former is made to replicate the impeller
around which a casting mould is created. The rubber former can then be extracted
from the mould into which the metal is poured. Accurate blade sections and
profiles are important in achieving compressor performance. Back face profile
machining optimises impeller stress conditions. Boring to tight tolerance and
burnishing assist balancing and fatigue resistance. The impeller is located on the
shaft assembly using a threaded nut.
VGT IN DIESEL ENGINES
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Variable Turbine Geometry technology is commonly used in turbo
diesel engines in recent years. It is primarily used to reduce turbo lag at low engine
speed, but it is also used to introduce EGR (Exhaust Gas Recirculation) to reduce
emission in diesel engines. Ordinary turbochargers cannot escape from turbo lag
because at low engine rpm the exhaust gas flow is not strong enough to push the
turbine quickly. This problem is especially serious to modern diesel engines,
because they tend to use big turbo to compensate for their lack of efficiency. A
Variable Geometry Turbocharger is capable to alter the direction of exhaust flow
to optimize turbine response. It incorporates many movable vanes in the turbine
housing to guide the exhaust flow towards the turbine. An actuator can adjust the
angle of these vanes; in turn vary the angle of exhaust flow.
Look at the following illustration:
At low rpm :
The vanes are partially closed, reducing the area hence accelerating the
exhaust gas towards the turbine. Moreover, the exhaust flow hits the turbine blades
at right angle. Both makes the turbine spins faster.
At high rpm :
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At high rpm the exhaust flow is strong enough. The vanes are fully opened
to take advantage of the high exhaust flow. This also releases the exhaust pressure
in the turbocharger, saving the need of wastegate.
VTG ON GASOLINE ENGINES
Although VTG technology is extensively used in diesel engines, it is
very much ignored in gasoline engines. This is because the exhaust gas of gasoline
engines could reach up to 950°C, versus 700-800°C in diesel engines. Ordinary
materials and constructions are difficult to withstand such temperature reliably.
In 1989, Honda produced a handful of Legend Wing Turbo, which
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employed a variable geometry turbocharger developed by itself. Its variable vanes
("wings") were made of a special heat-resisting alloy, Inconel. Nevertheless, the
experimental production run was never followed by mass production. In the next
one and a half decade Honda simply gave up turbocharging in all its petrol cars.
In the same 1989, Garrett produced a VTG turbocharger for use in the
limited production Shelby CSX, a car derived from Dodge Shadow. However, only
500 cars were produced. Neither Chrysler group nor any other car makers would
follow its footprints.
As compression ratio increases, modern gasoline engines have exhaust
temperature higher and higher. Experts estimated it could exceed 1000°C in the
foreseeing future. Perhaps this is why VTG technology for gasoline engines never
went into mass production.
In 2006, BorgWarner finally developed a VTG turbocharger for use in
Porsche 911 (997) Turbo. Both firms refused to reveal the technical details, but
said it employed "temperature-resistant materials derived from aerospace
technology". Hopefully the technology breakthrough will finally bring VTG
turbochargers into mass production gasoline engines.
DIFFERENCE
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Turbo
A turbocharger consists of a turbine and a compressor linked by a shared
axle. The turbine inlet receives exhaust gases from the engine exhaust manifold
causing the turbine wheel to rotate. This rotation drives the compressor,
compressing ambient air and delivering it to the air intake manifold of the engine
at higher pressure, resulting in a greater amount of the air and fuel entering the
cylinder.
VGT
Variable geometry turbochargers (VGTs) are a family of turbochargers,
usually designed to allow the effective aspect ratio (sometimes called A/R Ratio)
of the turbo to be altered as conditions change. This is done because optimum
aspect ratio at low engine speeds is very different from that at high engine speeds.
If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if
the aspect ratio is too small, the turbo will choke the engine at high speeds, leading
to high exhaust manifold pressures, high pumping losses, and ultimately lower
power output. By altering the geometry of the turbine housing as the engine
accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of
this, VGTs have a minimal amount of lag, have a low boost threshold, and are very
efficient at higher engine speeds.
ADVANTAGES AND DISADVANTAGES OF VGT
ADVANTAGES
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Increases efficiency over a range of engine rpms
Prevents turbocharger lag
Improve turbine response without altering maximum boost pressure
Controlling the vane angle allows the exhaust flow gases, at low engine
speeds, to pass over narrow, almost closed vanes. Gases accelerate as they move
through the narrow passage towards the turbine blades, which in turn accelerates
the turbine blades. The VGT has the advantage of being able to operate more
efficiently at all engine speeds, including low engine speeds. It has a low boost
threshold -in some cases without a wastegate- and a minimal turbocharger lag.
DISADVANTAGES
Difficult to use with gasoline engines because of high exhaust temperatures
VGT vanes can become clogged with particulate matter over time
CONCLUSION
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Variable Geometry Turbochargers were originally developed for auto-motive gasoline engines in cars about 20 years ago. Their cost and com-plexity, coupled with improvements in more traditional turbo designs, has kept them on the shelf until now. However, they provide big improve-ments in diesel engine efficiency and emissions, and it looks like their time has come. We can expect to see them on many light- and medium-duty truck engines and a surprising num-ber of passenger car diesel Engines over the next three years.
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BIBLOGRAPHY
Turbo: Real-World High-Performance Turbocharger Systems by Jay K. Miller
Air & Space Magazine by Hill Climb
Maximum Boost by Corky Bell
www.howstuffworks.com
www.cummins.com
www.wikipedia.org