Principles Of Variable Vane Operation
The turbine wheel of each turbocharger uses the engine exhaust gases to drive the compressor wheel. The compressor wheel draws in clean air, which is supplied to the engine cylinders in a compressed form.
The primary, VGT turbocharger allows the optimum inlet geometry (inlet area and flow angle) to be used over a wide range of engine operating conditions. This allows a rapid speed of response and higher charge air pressures at low engine speeds. The variable vane angle determines both the inlet area as well as the flow angle, as controlled by the ECM via the VGT actuator. The variable vanes allow efficient use of the exhaust gas energy which in turn improves turbocharger and engine efficiency.
Schematic Diagram - Variable Vane Operation
A = LOW ENGINE SPEED; B = INTERMEDIATE ENGINE SPEED; C = MAXIMUM ENGINE SPEED.
| ITEM | DESCRIPTION |
|---|---|
| 1 | Engine Control Module (ECM) |
| 2 | Variable Geometry Turbine (VGT) vane actuator |
| 3 | Turbine housing |
| 4 | Variable vanes |
| 5 | Compressor wheel |
Low Engine Speed: At low engine speeds the volume of exhaust gas is low so the vanes are moved towards the closed position to reduce the turbine 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 increases and the volume of exhaust gas increases the vanes are moved towards the open position to increase the turbine inlet area and maintain the gas velocity.
Maximum Engine Speed: At maximum engine speed the vanes are almost fully open maintaining the gas velocity into the turbine wheel.
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 ECM protects the turbocharger by opening the vanes further to reduce the turbine wheel speed. This is known as the altitude margin of the turbocharger, which occurs at altitudes above 2000 meters. Below 2000 meters altitude compensation is not necessary. The ECM uses an internal barometric pressure sensor to monitor altitude.
The maximum position of the turbocharger vanes (fully open) 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.