Реферат: Forced Air Induction Essay Research Paper The

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