I've been having a search around to see what combination of circumstances triggers / engages the transient overboost on the TDCi motors.
Ford make a big play of this feature in all its tech specs, but so far I've only found fairly vague descriptions such as " it happens when full acceleration is required" or "under hard acceleration". Nothing technical, or more detailed about when it happens, and the circumstances.
I'd be very interested to know:
--> is it only triggered when the pedal is floored (similar to kickdown in an automatic)?
--> or is the ECU clever enough to give overboost if the pedal is 'stabbed' quickly, over a certain defined range of pedal travel?
--> is it only available in certain conditions (i.e. coolant / oil temp above a certain range, exhaust gas temp in a certain range etc)
I would have thought the driver has to actively initiate it with a determined right foot -- especially after the "unintended acceleration" lawsuit that hit Audi in the US in the 80s.
Any thoughts / input / links gratefully received.
|
It's easy to programme-in a temporary overboost for a few seconds - that you wouldn't want a caravan tower using for hours and cooking the engine.
It also means that you can quote the higher power and torque figures in you adverts....
|
It's a while since I looked at a Ford ad or specs, but at one time, they used to quote two torque outputs on their vehicles equipped with this feature - one with and one without the transient overboost (one was bracketed)
It's not a new idea. Lancia were doing it on the Delta HF Turbo back in the 80's, accompanied with a very irritating green warning light on the dash when it was activated.
Cheers
DP
Edited by DP on 07/07/2008 at 15:31
|
The figs for the current 2.0 TDCi are 236 lb-ft from 1750 - 2250 rpm (251 lb-ft with overboost)
I'm just curious about how the ECU knows when to apply it ... is it at full throttle only, or is it a bit smarter than that?
|
It's got to be fairly simple; it's likely the higher boost will be the default setting, but after a set period at that level, the ECU then winds the electronic turbo actuator back to a lower value for the rest of the high-boost period.
|
thanks Screwloose, that makes sense.
Incidentally, from what I can tell, the VNT mechanism on the 2.0 TDCi motor in the new Mondeos is still vac solenoid activated, there are two solenoids near the vac pump. One has tubes going to the EGR valve, the other solenoid's tubes go round to the turbo which is buried behind the block and tucked under the bulkhead.
|
|
Transient overboost is triggered under heavy acceleration. Typically transient overboost will last for a few seconds and provides an extra burst of torque, raising the peak torque output by up to 20 Nm. The lower quoted torque figure is the default setting and the maximum amount of Nm output by the engine when the transient overboost hasn't been triggered (i.e. as in non-heavy acceleration).
|
|
|
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.
|
The turbo boost is limited by the wastegate mechanism.
Does anyone (screwloose?) know if the Focus 2.0 tdci with the variable turbo uses a wastegate as well?
|
The Variable Vane turbos don't have a wastegate, the angle of the vanes, controlled by the vacuum capsule or electronic box, determines the boost pressure. I can imagine the electronic version adjusting the vanes to increase boost but on the vacuum capsule version there must be some way the ECU adjusts the vacuum pressure.
|
|
Yes, the ECU causes the vac solenoid to switch rapidly between applying vac to the VNT and the bypass line. This "pulses" the VNT mechanism (several times per second I believe) so it stays partially, but not fully open.
|
|
Thanks for your rapid answers folks!
|
|
|
