Variable Turbine Geometry

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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.

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.

The first production car to use these turbos was the limited-production 1989 Shelby CSX-VNT, equipped with a 2.2L Chrysler K engine. The Shelby CSX-VNT utilised a turbo from Garrett, called the VNT-25 because it used the same compressor and shaft as the more common Garrett T-25. This type of turbine is called a Variable Nozzle Turbine (VNT). Turbocharger manufacturer Aerocharger uses the term 'Variable Area Turbine Nozzle' (VATN) to describe this type of turbine nozzle. Other common terms include Variable Turbine Geometry (VTG), Variable Geometry Turbo (VGT) and Variable Vane Turbine (VVT).

The Peugeot 405 T16, launched in 1992, used a Garrett VAT25 variable geometry turbo charger on its 2.0 16v turbocharged engine.

The 2007 Porsche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbos used are BorgWarner's Variable Turbine Geometry (VTGs). VGTs have been used on advanced turbo diesel engines for a few years and on the Shelby CSX-VNT.(only 500 Shelby CSX-VNTs were ever produced, and 1046 Peugeot 405 T16s.)

The Jeep Grand Cherokee WK has an option of 3.0 liter Mercedes-Benz OM642 CRD V6 engine with Honeywell VGT.