Variable Geometry Turbocharger

Variable Geometry Turbocharger (VGT) is a device that can allow the turbine to vary its aspect ratio unlike a fixed geometry turbocharger.

What is Aspect Ratio?

Aspect ratio is the ratio of different sizes of a geometric shape in different dimensions. For a rectangle, the aspect ratio is the ratio of its longer side (width) to the shorter side (height).

Aspect Ratio of a Variable Geometry Turbocharger:

For a VGT, the aspect ratio is defined as the ratio of the area for the exhaust gases to enter the turbine to the radius of the turbine. In short, it is the ratio of area of a turbine to the radius of the turbine.

In a VGT, the radius is fixed and the area can be varied by varying the angle of the vanes provided in the turbine. Hence, by varying the area, we can vary the aspect ratio of the turbine.

Advantage of varying the aspect ratio:

In a conventional turbocharger, the aspect ratio is fixed. If the aspect ratio is small, then it will be effective in producing boost pressure for an engine running at low speed, but will be unable to provide the boost pressure at higher speed. This will result in high exhaust manifold pressure, and lower power output.

Conversely, if the aspect ratio is larger, then it will be effective in producing boost pressure for an engine running at high speed, but won’t supply sufficient boost pressure at lower engine speed. This will result in turbo lag.   

What is Turbo Lag?

Turbo lag is the delay in the power generated by an engine when we press on the accelerator pedal. In other words, turbo lag is the little hesitation we feel when we press on the accelerator pedal and the engine takes a while to respond.

Turbo lag is commonly seen in engines fitted with a larger aspect ratio turbocharger. A larger aspect ratio turbocharger will start supplying adequate boost pressure only when the engine reaches a higher rpm. The time in reaching the higher rpm is the delay in providing the boost which results in turbo lag.

A variable geometry turbo will provide the best of both worlds and overcome all the problems mentioned for both the small and large aspect ratio turbos.

Design of a variable geometry turbocharger:

VGT is pretty much similar to the conventional turbocharger. The only difference is that the turbine is provided with a set of vanes arranged in a circle around the turbine. These vanes are attached to pivot points, about which the vanes can vary its angle. The vane angles are adjusted with the help of an actuator. The aspect ratio can be varied throughout the entire speed range of the engine.

Working of a Variable Geometry Turbocharger:

At Low Engine RPM:

When the engine is running at a low rpm, the vane actuator adjusts the vane angles so that the vanes are almost closed and there is very little space for the exhaust to pass through the vanes and spin the turbine.

At low engine speeds, the exhaust flow is less and slow. By closing the vanes, the exhaust flow is squeezed through the little gaps between the vanes. This helps in increasing the velocity of the exhaust flow. The high velocity exhaust then hits the turbine and spins it faster, hence providing enough boost even at low engine speed. The driver can feel the quick engine response while accelerating the vehicle.

At High Engine Speed:

At high engine speed, the exhaust flow rate is high. If the aspect ratio is kept small at high speed, the vanes cannot squeeze the huge amount of exhaust through the narrow gap and it can saturate the turbo, hence leading to power loss. Thus the vane angles are adjusted to open the vanes so as to allow more exhaust flow to increase the boost pressure.