The turbo boost is limited by the wastegate mechanism. The wastegate diverts the exhaust gasses from the turbos exhaust rotor causing it to slow and thus reduce the boost.
The waste gate is a "flap valve" and is operated by the turbo's own air pressure routed from the inlet manifold (where the boosted air ends up!) . The air reaches the wastegate via a thin pipe from the inlet manifold to an air valve solenoid which is switched rapidly on and off by the cars ecu. By dint of the ECU switching the air valve solenoid on and off quickly it can vary how much air pressure the wastegate "sees". The wastegate is a sprung mechanism and opens according to the air behind it.
The car ECU uses a Manifold pressure sensor in the engines intake to monitor the level of boost the turbo is producing and can therefore determine at what point the wastegate should open to control the boost level. It does this by juggling the rate at which the air valve solenoid opens and shuts.
For a laugh, I disconnected the air pipe described above that controls the wastegate on my old rover turbo such that the wastegate therfore never saw any boost pressure at all behind it and remained shut!
To say the car went like the clappers is an understatement - this is because effectively the boost is unlimited - the wastegate can never be opened by the ecu as a safety/control mechanism and the boost just goes up and up.
In the case of the rover turbo the ECU has one final saftey measure in that scenario - it uses the manifold pressure sensor to cut the fuel when it see's the boost pressure in the manifold going to excess levels (which is about 14 psi for the rover turbo engines)
The engine just stutters like it hits the rev limiter untill the engine revs and therefore boost drops. Quite fun but not to be advised if you value your engine for a long time!
Any hoo, the ECU programming for speed and load will determine how much pressure is allowed to build up from the turbo and it will be constantly bled off up to a point where the ecu decides just a little more boost is in order and joggles the air valve solenoid to the wastegate a litte less and therefore the wastegate is shut a little more and therfore boost is allowed to build a little more in a controlled loop.
|
TDCi "transient overboost" -- what triggers it? ![2016 Ford Mondeo 1.0 EcoBoost Zetec 5dr [Nav]](http://images.honestjohn.co.uk/imagecache/url/crop/90x60/https/cdn.uk.prod.group-mobility-trader.com/images/cas/13626678e9bb8f885b9a504846aaf25e/02339fa978c2d9b65a7520d12fc823b4.jpg)
.jpg)
