How does a variable geometry turbocharger (VGT) optimize engine performance at different engine speeds?

A variable geometry turbocharger (VGT) optimizes engine performance at different engine speeds by adjusting the angle of vanes that direct exhaust gas onto the turbine wheel. At low engine speeds, when exhaust gas flow is low, the vanes are angled to narrow the passage through which the exhaust gas flows. This increases the velocity of the exhaust gas striking the turbine, causing the turbocharger to spool up quickly and provide boost even at low RPMs. This reduces turbo lag, the delay between the driver demanding power and the turbocharger providing it. At higher engine speeds, when exhaust gas flow is high, the vanes are opened to widen the passage. This prevents the turbocharger from overspeeding and producing excessive boost pressure, which could damage the engine. By adjusting the vane angle, the VGT can maintain optimal boost pressure across a wide range of engine speeds. This results in improved low-end torque, better throttle response, and increased peak power. In contrast, a fixed geometry turbocharger has a fixed turbine housing, which is a compromise between low-end responsiveness and high-end power. VGTs are commonly used in diesel engines to improve fuel efficiency and reduce emissions by enabling optimal combustion across the engine's operating range. An example would be a modern tractor engine that utilizes a VGT to provide strong pulling power at low speeds while also maintaining good fuel economy during road transport at higher speeds.