Variable Turbine GeometryVariable Turbine Geometry

A National Maritime Academy A National Maritime Academy presentationpresentation

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

Since higher exhaust pressures Since higher exhaust pressures generate greater loads on the generate greater loads on the intake side, the intake pressure intake side, the intake pressure must be carefully controlled in must be carefully controlled in order to protect the engine. order to protect the engine.

The ‘boost pressure’ is limited The ‘boost pressure’ is limited using ‘waste gate’ valves that using ‘waste gate’ valves that bypass excess pressure around bypass excess pressure around the twin exhaust turbines.the twin exhaust turbines.

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

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

Variable Turbine Variable Turbine GeometryGeometry

technology is the next technology is the next generation in turbocharger generation in turbocharger technology where the turbo technology where the turbo

uses variable vanes to control uses variable vanes to control exhaust flow against the exhaust flow against the

turbine blades. turbine blades.

The problem with the The problem with the turbocharger that we’ve all come turbocharger that we’ve all come

to know and love is that big to know and love is that big turbos do not work well at slow turbos do not work well at slow

engine speeds, while small engine speeds, while small turbos are fast to spool but run turbos are fast to spool but run out of steam pretty quick. So out of steam pretty quick. So

how do how do VTG turbosVTG turbos solve this solve this problem? problem?

A A Variable Turbine GeometryVariable Turbine Geometry turbocharger is also known as a turbocharger is also known as a

variable geometry turbocharger variable geometry turbocharger (VGT)(VGT), ,

or a or a Variable Nozzle Turbine (VNT)Variable Nozzle Turbine (VNT). .

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

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

The VGT vanes look like when they are open. The VGT vanes look like when they are open.

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

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

Thank You !Thank You !