Tech Tips for the Turbo VGT

Tech Tip Turbo VGT

A small turbo responds quickly and is effective at low rpm. However, it cannot deliver the high flow at high rpm. On the other hand, a large turbo can deliver the high speed flow, but will be slower to respond. This leads to unacceptable response times ("turbo lag"). The VGT has a wider effective range. Varying the turbo vane position allows the turbo to be in small or large capacity configurations. These vanes are managed by an electronic unit.

Strategy

The limitation of the boost setting depends on two maps (engine speed- atmospheric pressure and engine speed - air temperature). A strategy allows detection and warning of any fault which may destroy the turbo. The DCU compares the air flow and the engine speed to check consistency. If a fault is detected, the DCU activates a recovery mode (reduced flow) to protect the system. The "overboost" function is activated according to engine speed variation and requested load criteria.

Two stage VGT

The two stage turbo is managed by 3 pneumatic actuators (diaphragm).

The following sensors are present:

• An intermediate turbocharging pressure sensor: Its role is to provide the DCU with the inlet pressure value between the 2 turbo stages.
• A turbocharging pressure sensor: its role is to provide the DCU with the inlet pressure value at the turbo outlet after the exchanger.
• An exhaust pressure sensor : its role is to provide the DCU with the exhaust pressure before the turbine to compensate the boost pressure request when the exhaust back pressure is too high (example: "DPF" full).