VGT

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INTRODUCTIONVariable 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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TURBOCHARGINGBasic TheoryThe 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 the charging. This can significantly improve the power-to-weight ratio for the engine.

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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

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

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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.

Fixed Geometry

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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 TURBINEVariable 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 weve 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?

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

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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.

VANES IN OPEN POSITION

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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.

VGT COMPONENTS

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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.

TURBINE WHEEL

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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 ASSEMBLYThe 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 ARRANGEMENTJournal 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.

THRUST

BEARING

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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 COVERCompressor 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.

COMPRESSOR IMPELLER AND FASTENER

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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

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

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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