Overboost Relay: Description and Operation

Boost Control System:

BOOST PRESSURE CONTROL
Two bypass devices are used for automatically limiting the turbocharger boost, the compressor bypass valve and the wastegate. The compressor bypass valve responds to high intake vacuum during deceleration, and opens to relieve the system of high intake vacuum during deceleration, and opens to relieve the system of high pressure inlet air and the accompanying noise. The other limiting device, the wastegate and its controls, limits the boost to 8.7 psi by regulating the exhaust gas bypass flow around the turbine. This regulation initiates with the ECA and its sensors, which monitor engine speed, inlet air flow, and engine knock, The ECA controls the wastegate actuator and the wastegate valve. Since the amount of boost varies directly with engine speed and load, within the control limits of the system, more boost is generated as speed and/or load increase.

BOOST CONTROL SOLENOID
Under low to medium speed and load, the ECA allows full boost if available, and opens the wastegate solenoid valve fully, thus venting the boost pressure to the compressor inlet and leaving insufficient pressure to move the actuator. As a result the wastegate remains closed and full boost is available.
As load and speed increase, boost pressure reaches its limit and the ECA will gradually close off the venting of boost pressure to the compressor inlet. This increases the pressure to overcome the actuator spring, so the actuator begins to push the wastegate open. This action continues until the solenoid valve is closed and the wastegate is fully open.

OVERBOOST PROTECTION
As engine speed and load increase sufficiently until in an overboost condition, a warning chime will activated by the vane airflow meter through the ECA. Simultaneously, unless the operator reduces his speed and/or engine load, the ECA will execute the fuel cut-off operation and stop fuel injection in order to prevent engine damage.