Variable-geometry turbocharger

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, a low boost threshold, and high efficiency at higher engine speeds.

The turbine's vanes rotate in unison, relative to its hub, to vary its pitch and cross-sectional area.

The area between the edges of the vanes changes, leading to a variable-aspect-ratio system with fewer moving parts.

VGTs tend to be much more common on diesel engines, as lower exhaust temperatures mean they are less prone to failure.

[1] Typically, VGTs are only found in OEM applications due to the level of coordination required to keep the vanes in the most optimal position for whatever state the engine is in.

Although excessive engine backpressure is detrimental to overall fuel efficiency, ensuring a sufficient EGR rate even during transient events (such as 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).

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).

In situations where engine load is constant like in stationary generators, fixed geometry turbochargers can provide higher efficiency over VGTs.

Several companies manufacture and supply rotating-vane variable-geometry turbochargers, including Garrett, BorgWarner, and Mitsubishi Heavy Industries.