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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