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.
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.
Often the vanes are controlled by a membrane actuator identical to that of a wastegate, however increasingly electric servo actuation is used. Hydraulic actuators have also been used in some applications.
For the Mercedes Benz and tdi diesel or custom application crowd - The Holset he341ve VGT is the top of the line but almost impossible to find- the only source other that a Cummins dealer is SuperturboVGT at yahue. They have low hr units from a engine testing facility in Excellent shape. 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, 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 recirculated 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 backpressure is detrimental to overall fuel economy, ensuring a sufficient EGR rate even during transient events (e.g. gear changes) can be sufficient to reduce particulate 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.
History and examples of use
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, like the 1.9L Volkswagen TDI Engine using the Garrett VNT-15 turbocharger, on the Shelby CSX-VNT.(only 500 Shelby CSX-VNTs were ever produced, and 1046 Peugeot 405 T16s.) or in Fiat's 1.9 JTD engines.