chrisd

On the dutch bulletin board we have a discussion about the solenoid valve.When you accelerate hard,boost peaks for a moment and then falls back to 0'9 bar.How is this caused? We have got two ideas; one is that wastegate is not opening fast enough allowing you to run high boost till it opens fully and pressure drops,the other idea is that ECU opens solenoid valve so that air pressure in tube to wastegate stays low.In my opinion solenoid valve is only to protect engine fromblowing up when wastegate refuses .Any experts out there?

alanp

The wastegate solenoid does regulates maximum boost pressure. The wastegate actuator only requires about 0.5 bar to fully open. To achieve a higher maximum boost than 0.5 bar some of the boost supply to the actuator is bled away. By pulsing the solenoid valve open and closed the turbo has to generate a higher boost before the wastegate can start to open and hence we can run at about 0.9 bar. The MAP (Manifold Absolute Pressure) sensor, the ECU, solenoid and turbo system form a closed-loop control system wherby the ECU can set the maximum boost and even turn it down a bit at higher revs. This is probably what happens when the boost reduces from a peak value. It is true that actuator can be slow to fully open or close. This system relies on modulating the wastegate at a partially open position to control the boost such that response time is less of a problem.
If the wastegate fails in a closed position the only protection the engine has is the over boost sensed by the MAP sensor which will cause the ECU to cut the fuel supply......rather abruptly.