Variable Turbo Chargers Geometry (VTG)

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. VGTs do not require a waste gate.

Most common designs
The two most common implementations include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. Generally for light duty engines (passenger cars, race cars, and light commercial vehicles) the vanes rotate in unison to vary the gas swirl angle and the cross sectional area. Generally for heavy duty engines the vanes do not rotate, but instead the axial width of the inlet is selectively blocked by an axially sliding wall (either the vanes are selectively covered by a moving slotted shroud, or the vanes selectively move vs a stationary slotted shroud). Either way the area between the tips of the vanes changes, leading to a variable aspect ratio.

Actuation
Often the vanes are controlled by a membrane actuator identical to that of a waste gate, however increasingly electric servo actuation is used. Hydraulic actuators have also been used in some applications.

Main suppliers

Several companies supply the rotating vane type of variable geometry turbocharger, including Garrett (Honeywell), Borg Warner and MHI (Mitsubishi Heavy Industries). The rotating vane design is mostly limited to small engines and/or to light duty applications (passenger cars, race cars and light commercial vehicles). The only supplier of the sliding vane type of variable geometry turbocharger is Cummins Turbo Technologies (Holset), who are effectively the sole supplier of variable geometry turbochargers for applications involving large engines and heavy duty use (i.e. trucks and off highway applications).

Other common uses
In trucks, VG turbochargers are also used to control the ratio of exhaust re-circulated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure exceeds the inlet manifold pressure, which promotes exhaust gas recirculation (EGR)). Although excessive engine back pressure is detrimental to overall fuel economy, ensuring a sufficient EGR rate even during transient events (e.g. gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation (e.g. Euro 5 for Europe and EPA 10 for the USA).

Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression type), so that an extra exhaust throttle valve isn’t needed. Also the mechanism can be deliberately modified to reduce the turbine efficiency in a predefined position. This mode can be selected to sustain a raised exhaust temperature to promote “light-off” and “regeneration” of a diesel particulate filter (this involves heating the carbon particles stuck in the filter until they oxidize away in a semi-self sustaining reaction – rather like the self-cleaning process some ovens offer). Actuation of a VG turbocharger for EGR flow control or to implement braking or regeneration modes generally requires hydraulic or electric servo actuation.

AUTOMOBILE ENGINES

The working of an automobile engine follows the same principle as an internal combustion engine. Air, from outside, enters the engine through the air cleaner and reaches the throttle plate.
The pedal in your car is the control for the amount of air that you would want to be taken in, and you control it by pressing on this gas pedal.
The air is then distributed through the intake manifold of the cylinders.

At some point fuel is injected into the air stream, and the mixture vaporizes and is drawn into the cylinders as they start their intake stroke.

This way, when the cylinder has reached its bottom, it has drawn in sufficient mixture. As it moves up, compressing the mixture, the spark plug ignites the mixture, and as the powerful gas formed expands, it pushes the cylinder to the bottom with the cylinder once again drawing in the mixture.

In designing automobile engines, you need to be a specialist in automobile engineering.
The consideration that is taken while designing such an engine is whether it should be a carburetor or a diesel one. carburetor engines are most commonly found in passenger cars and low capacity trucks, while trucks with a capacity over two tons are fitted with diesel engines, including dump trucks, trailer tractors and bus.

Increasingly the medium and low-capacity vehicles are being fitted with diesel engines, since the fuel consumption of these engines are 30% to 50% lower than the carburetor engines.
Diesel engines not only cost more, but maintenance is much more expensive than the other type of engine. Diesels require more metal parts per kilowatt.
The critical parts of diesel engines are made of alloy steel, and the fuel injection system is much more expensive than carburetor engines.

However, the cost of manufacturing carburetor engines has increased with the use of higher mechanical grade components, considering the thermal loads of the material used. At the same time the use of high alloys and increase in production costs have contributed to the higher price of such engines.

There is a sharp rise in using aluminum alloys in design of carburetor engines in passenger cars, and with the use of high octane petrol, the cost of operation of these cars have come down extensively. Using alloy steel in constructing the engine body and other parts of the engine, makes the car lighter and hence fuel consumption goes down substantially.

The main parts that are made of high steel alloy are the main casting of the engine, the cylinder head, water and oil pumps, oil filter housing, end covers of the generator and starter, and the intake pipes. It has been observed that by using high steel alloys, the weight of the car is reduced by 35%.

The power per liter, per unit of piston area, and the brake effective pressure are 6% to 8% lower in air-cooled engines, compared to engines having liquid cooling mechanism. This is due to the fact that in engines with liquid cooling there are great losses in cylinder charging caused by the high temperature in pipes, ducts in the head, cylinder walls and head, etc.

The size of air cooled engines are much bigger than the engines with liquid cooling having the same capacity, and this is because the cylinder axes difference is larger in air-cooled engines. Taking account of the radiator dimensions, if both engines are compared, the air-cooled engine will vary slightly with its height a little longer than or approximately the same length as the water-cooled engine. As far as the width and the height is concerned both engines are about the same.

The auxiliary units of the feed and ignition, and generator and starter systems are a bit difficult to fit on the body of the air-cooled engines, because of the presence of hoods and having a danger of over-heating.