How Does Variable Turbine Geometry Work

Variable Turbine Geometry A National Maritime Academy presentation

 On

a conventional turbocharger, the exhaust flow drives a turbine that is connected to a compressor on the intake side. By compressing the incoming air, the amount of oxygen in a given volume is increased. Since compression also causes an increase in temperature, the air must be cooled in a device known as a “intercooler”. With more oxygen present in each cylinder charge, more fuel can be burnt yielding greater energy.

Since higher exhaust pressures generate greater loads on the intake side, the intake pressure must be carefully controlled in order to protect the engine. The ‘boost pressure’ is limited using ‘waste gate’ valves that bypass excess pressure around the twin exhaust turbines.

Since a smaller turbine has a lower mass, it responds more quickly to increasing pressure, spinning up easily to its optimum speed. The key disadvantage of using a smaller turbo is that the back-pressure generated at higher engine speeds causes a significant reduction in performance. Resistance is caused by the smaller cross-sectional area through which the exhaust is required to flow.

Larger turbo units, which create lower back-pressure at higher rpm, take considerably longer to spin up under power due to the large cross-sectional area and relative inertia of the heavier turbine. Generally, this type of turbo will only be effective in the medium rpm range. This phenomenon, known as ‘turbo lag’, means there is virtually no turbo charging effect at lower engine speeds.

Variable Turbine Geometry technology is the next generation in turbocharger technology where the turbo uses variable vanes to control exhaust flow against the turbine blades.

The problem with the turbocharger that we’ve all come to know and love is that big turbos do not work well at slow engine speeds, while small turbos are fast to spool but run out of steam pretty quick. So how do VTG turbos solve this problem?

A Variable Turbine Geometry turbocharger is also known as a variable geometry turbocharger (VGT), or a Variable Nozzle Turbine (VNT).

A turbocharger equipped with Variable Turbine Geometry has little movable vanes which can direct exhaust flow onto the turbine blades. The vane angles are adjusted via an actuator. The angle of the vanes vary throughout the engine RPM range to optimize turbine behaviour.

Below, you can see the vanes in a angle which is almost closed. Variable vanes are highlighted.

The VGT vanes look like when they are open.

In this diagram, you can see the direction of exhaust flow when the variable vanes are in an almost closed angle. The narrow passage of which the exhaust gas has to flow through accelerates the exhaust gas towards the turbine blades, making them spin faster. The angle of the vanes also directs the gas to hit the blades at the proper angle.

Diagram shows the exhaust gas flow when the variable turbine vanes are fully open. The high exhaust flow at high engine speeds are fully directed onto the turbine blades by the variable vanes.

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