Boost Pressure Control
Version with turbo only
Boost pressure control consists of a one-stage supercharging system in the form of a turbocharger with fixed turbine geometry.
The Engine Control Module (ECM) receives information about the actual throttle angle, the engine load and knock, among other things. From this information the control module calculates the required boost pressure. The boost pressure is measured using the boost pressure sensor (after the turbo).
The boost pressure in the intake pipe is regulated by the turbo control valve, wastegate. The control module controls the turbo control valve via vacuum regulation so that a correct boost pressure is achieved.
When the pressure rises, the wastegate valve's pressure box is affected. When the boost pressure has increased to the maximum permissible value, the wastegate valve opens, and some of the exhaust gases pass the turbocharger's turbine rotor, limiting the boost pressure.
Control is governed continually by the Engine Control Module (ECM).
Because the control module calculates the boost pressure using the signal from the boost pressure sensor and the temperature sensor, there is automatic compensation of boost pressure control when driving at altitude and at different temperatures. As a result engine power will not be noticeably affected by the air density or temperature.
In the event of overpressure in the intake manifold (boost pressure), such as when the accelerator pedal is released quickly, increasing pressure builds up that can result in noise (pulsations). The relief valve, also called the turbo control valve bypass, then has the task of routing this pressure back to the turbocharger inlet side to limit noise and protect the turbocharger from wear.
The relief valve is governed electrically by the Engine Control Module (ECM).
The pressure that is regulated away is released into the intake manifold after the mass air flow sensor, before the turbo.
The Engine Control Module (ECM) can diagnose the turbocharger control function.
Version with both compressor and turbo
Boost pressure control consists of a two-stage supercharging system in the form of a compressor and a turbocharger with fixed turbine geometry.
The compressor is used at low revs whilst the turbocharger is used at high revs.
In order to disengage the compressor from the crankshaft that drives it, a magnetic coupling is used. The Engine Control Module (ECM) controls the clutch with controlled short slippage.
The compressor is usually already engaged from idle to ensure a good response, In general, engagement/disengagement is fully controlled after the torque request.
Engagement occurs if the torque request to the ECM is above 130 Nm and the engine speed simultaneously drops below 2400 RPM. Disengagement occurs if the torque request is reduced to below 90 Nm.
At speeds above 2400 RPM the compressor only engages if the demand exceeds what the turbo can deliver alone. At high torque requests at speeds above 2600 RPM the compressor does not activate. The compressor can however be switched on if activation occurred earlier at an acceleration from low speed. At engine speeds above 3500 RPM, the compressor is always disengaged mechanically via the electrical connection.
In parallel with the compressor drive being disconnected, the control module governs the supercharger control valve bypass to the open position. This means that fresh air is guided past the turbo.
The turbocharger has on this occasion managed to rev enough to ensure a smooth transition to supercharging via the turbocharger.
Boost pressure control with the turbocharger works in the same way as in the version with only turbocharger, see "Version with turbo only".