Turbo Boost Control

Turbo boost control









In principle, the turbo control is the same as the unit used on engine B5254T.
The wastegate valve, its pressure regulator being activated by the pressure at the turbo's outlet, controls the turbo's boost pressure. Even if the normal boost pressure is relatively low, the control module can increase the pressure above the basic setting by varying the control pressure via the turbo control valve (TCV). This means that the wastegate valve permits a higher boost pressure.

Normal boost pressure (approx. 130 kPa (18.9 psi) absolute pressure)
When the engine works with normal boost pressure, the wastegate's pressure regulator works with full outlet pressure from the turbo. When the boost pressure increases to max. permitted value, the wastegate's valve opens and part of the exhausts can be routed past the turbo to limit the boost pressure.

Modified boost pressure (max. 150 kPa (21.8 psi) absolute pressure)
When Engine control module (ECM) establishes that a higher boost pressure is needed (and is possible), the turbo control valve (TCV) opens and part of the pressure that acts on the wastegate's pressure regulator is routed to the turbo's inlet. The wastegate closes and the boost pressure increases. The connection cycle of the signal for the turbo control valve (TCV) is used to change permitted pressure level at the wastegate's valve.

Atmospheric pressure compensation
The atmospheric pressure sensor establishes at which elevation above sea level that the car is driven. To prevent the turbocharger from overspeeding in thin air the boost pressure is limited at elevations above 1000 m (0.62 mi). This is hardly noticeable.
When starting at high altitude the fuel mixture is reduced to compensate for the thinner air. The pressure sensor is used as an control of the turbo's boost pressure.

Emergency function (Limp-home)
To create possibilities in emergencies, the mass airflow sensor (MAF) can measure both air mass per hour and air mass per engine rev.
The boost pressure is not permitted to increase above the pressure's basic setting if:
- knocks occur. When knocking starts, ignition is delayed and fuel mixture is made richer. If this is not enough to eliminate knocking, the boost pressure is reduced.
- engine temperature is too high (approx. 125 °C (257 °F)), engine starts to knock. If this is confirmed, the turbo's boost pressure is reduced. (No direct detection of the engine's coolant temperature).
- malfunction occurs on the turbo's control valve (TCV) or one of the hoses. This results in an emergency (limp-home) pressure of 30 kPa (4.4 psi).
Fuel injection is stopped if:
- boost pressure is too high, fuel shut-off is activated. This is calculated with a certain airflow measured by the mass airflow sensor.
- turbo control valve (TCV) has jammed in open position.