Principles Of Variable Vane Operation
The variable vanes in the Variable Geometry Turbine (VGT) are used to increase or decrease the exhaust gas power transfer to the turbine wheel. Adjusting the pitch angle of the vanes greatly assists the boost pressure increase in the VGT at low engine speeds. The operation of the variable vanes is controlled by the Powertrain Control Module (PCM).
As engine speed, and therefore the exhaust gas mass flow increase, the PCM increases the vane opening. This avoids the overspeed of the turbines and provide a smooth high speed operation.
The VGT actuator also provides the PCM with a feedback signal to determine the pitch angle of the vanes. This makes sure that the power transfer from the turbine wheel to the compressor wheel is within the VGT speed and boost pressure requirements.
Schematic Diagram - Variable Vane Operation
A = LOW ENGINE SPEED: B = INTERMEDIATE ENGINE SPEED: C = MAXIMUM ENGINE SPEED.
| ITEM | DESCRIPTION |
|---|---|
| 1 | Powertrain Control Module (PCM) |
| 2 | Variable Geometry Turbine (VGT) actuator |
| 3 | Vane control ring |
| 4 | Variable vanes |
| 5 | Turbine wheel |
Low Engine Speed. At low engine speeds the mass flow of exhaust gas is low so the vanes are moved towards the closed position to reduce the Variable Geometry Turbine (VGT) inlet area. This reduction causes an increase in the gas velocity into the turbine wheel thereby increasing wheel speed and charge air pressure.
Intermediate Engine Speed. As the engine speed and the mass flow of exhaust gas increases the vanes are moved towards the open position to modulate the boost pressure.
Maximum Engine Speed. At maximum engine speed the vanes are almost fully open to maintain the gas velocity into the turbine wheel. The maximum position of the vanes is also an emergency default position in the event of an electrical fault. This lowers the chance of engine damage due to excessive charge air pressure.
When the vehicle is driven at high altitudes, the ambient pressure reduces causing the compressor wheel to rotate faster to achieve the same charge air pressure. To prevent the turbine wheel from over-speeding under these conditions, the Powertrain Control Module (PCM) protects the VGT by opening the vanes further. This reduces the turbine wheel speed. This is known as the altitude margin of the VGT. The PCM uses an internal barometric pressure sensor to monitor altitude.