Variable geometry turbochargers (VGTs), also known as variable nozzle turbines (VNTs), are a type of turbocharger commonly used in diesel engines to improve performance and efficiency across a wide range of engine speeds. Unlike traditional fixed-geometry turbos, which have fixed turbine vanes, VGTs have movable vanes or mechanisms that adjust the flow of exhaust gases onto the turbine wheel.

Here's how they work:

Adjustable Vanes or Mechanisms: VGTs have either movable vanes or a

mechanism that alters the angle of the vanes. These vanes are located in the turbine

housing, directly in the path of the exhaust gases flowing from the engine.

Changing the Flow of Exhaust Gas: By adjusting the angle of the vanes, the VGT can

control the flow of exhaust gas onto the turbine wheel. When the vanes are in a narrow

position, the exhaust gases are directed onto the turbine wheel with higher velocity,

which increases turbocharger speed and boost pressure at lower engine speeds.

Conversely, when the vanes are in a wider position, the exhaust gases are directed onto

the turbine wheel with lower velocity, allowing the turbocharger to maintain boost

pressure at higher engine speeds without causing excessive turbo lag.

Optimizing Performance: The ability to vary the geometry of the turbine housing allows

VGTs to optimize performance across a wide range of engine speeds and loads. This

results in improved throttle response, better low-end torque, reduced turbo lag, and

increased efficiency compared to fixed-geometry turbos.

Control System: VGTs require a control system to adjust the position of the vanes or

mechanism based on inputs such as engine speed, load, and throttle position. Modern

engine management systems use sensors and actuators to precisely control the VGT

operation, ensuring optimal performance under various driving conditions.

Overall, variable geometry turbos are a sophisticated technology that helps diesel

engines achieve better performance, efficiency, and emissions control compared to

traditional fixed-geometry turbochargers.

A good explanation from Engineering Explained.