VARIABLE GEOMETRIC TURBOCHARGER

A Variable Turbine Geometry turbocharger is also known as a variable geometry turbocharger (VGT) or a Variable Nozzle Turbine (VNT). 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 waste gate, 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.
The most common implementation is a set of several aerodynamically-shaped vanes in the turbine housing near the turbine inlet. As these vanes move, the area between the tips of them changes, thereby leading to a variable aspect ratio. Usually, the vanes are controlled by a membrane actuator identical to that of a wastegate, although electric servo actuated vanes are becoming more common.

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