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.
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.
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.