Boost pressure increases during braking

Okay, I've observed the following: When you repeatedly apply the brakes, causing the brake pedal to become 'hard,' the boost pressure increases by approximately 60 mbar and reaches the target boost pressure. (This is for a PD engine... meaning with VVT, etc.)

Okay, so the question is: Is this normal? (I have my own explanation for why things are the way they are, but I'd like to hear your thoughts on it.)

Hmm.

That's a bit less information than I would like.

Please explain the circumstances in more detail.
- which engine?
- What is the engine doing when you observe this?
- Idle
- Engine braking
- at a standstill

Have you heard about the intended braking effect of the VTG?

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

engine? the good old ARL.
observed? when troubleshooting using VAG-COM.
When? At idle/when stationary.

Hi.

I could imagine a standard correction mechanism.

You're causing the vacuum to collapse (that's why you need to press the pedal hard).
As a result, VTG is also falling behind.
The EDC notices this and adjusts the N75 valve more aggressively to restore the boost pressure.
Due to various inaccuracies (electrical control of the N75 valve, conversion to vacuum, and conversion to the position of the VTG turbine blades), the blades are then positioned slightly differently.

What is the resolution of the boost pressure reading in VAGCOM? In other words, what is the smallest possible change that can be detected? It seems to be significantly larger than 1 mbar (I know it's around 21 rpm). Perhaps the aforementioned change randomly triggers the display of the next "interval."

Overall, it needs to function similarly to what's described above, because the brake booster and vacuum-assisted gear (VTG) rely on vacuum, which you are essentially eliminating.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

Okay, as I mentioned, I've also given it some thought, but the correction mechanism is being dropped. The EDC doesn't know what the vacuum pressure is (which would actually be great if it did – it could automatically detect many errors).

Here's the translation:

'What's a fact is that the vacuum pressure drops – and only then can the boost pressure no longer be increased. But I also noticed that the AGR (exhaust gas recirculation) reacts much faster to fluctuations in vacuum pressure than the relatively slow-responding VTG (variable turbine geometry). That's why my thought is: the AGR closes, but the vacuum pressure at the VTG doesn't drop enough for it to move.'
-> More air mass -> The turbocharger can maintain the correct boost pressure.

...regarding the resolution, I'm not entirely sure, but it's somewhere in the range of a maximum of 1-2 mbar. However, the pressure change is at least 60 mbar (more likely 65).

greetings - what do you think
Is that supposed to react like that?

Devilseye503 wrote:

So, I've observed the following: When you repeatedly apply the brakes, causing the brake pedal to become "hard," the boost pressure increases by approximately 60 mbar and reaches the target boost pressure. Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. observed? when troubleshooting with VAG-COM. when? at idle/stopped

It doesn't make sense: You can only meaningfully talk about target NOx emissions when the vehicle is in operation – if the exhaust gases provide enough power that there's something to regulate on the NOx reduction system.

What kind of errors are you looking for?

I think you're talking about a fixed boost pressure, like the 2500 mbar absolute that is only available at full throttle and with the corresponding exhaust flow.

However, the MSG always outputs a target boost pressure in every operating state, which also changes when the engine is idling or running at speeds below 2000 rpm, depending on the load (e.g., simply by turning on consumers like windshield heating or air conditioning). And this target boost pressure of, for example, 1090 mbar absolute (with an atmospheric pressure of, for example, 980 mbar – which is noticeable in the available torque and improved combustion) is only achieved in the specific case described in the first post.
The problem is that even at lower engine speeds, the desired boost pressure isn't being reached. In my opinion, this exacerbates the already significant control issues with boost pressure, because suddenly the pressure increases sharply – leading to large overshoots and, during driving, even more unpleasant undershoots. What I mean is, if the target boost pressure were reached beforehand, the pressure increase wouldn't be so sudden, and consequently, the overshoots and undershoots would be much less pronounced.
And what's my thought about this? It seems like the AGR is working as it should. Is it really okay that the VTG reacts so much slower to a control signal than the AGR does? In other words, how quickly *should* a VTG react? (It is doing something – we can't say it's completely inactive – but it's so slow?)

I hope someone understands my rambling.

Devilseye503 wrote:

ichaber das msg gibt doch in jedem betriebszustand einen soll-ladedruck aus, der sich auch im stand und betriebsdrehzahlen unter 2000 u/min nach belastung ändert (z.b. schon durch anschalten von verbrauchern wie scheibenheizung, klima o.ä.).

Um: Does this change the target value for the LD (which would be new to me), or the actual value?

Quote:

and this desired boost pressure, for example, of 1090 mbar absolute (e.g., with an atmospheric pressure of 980 mbar - which is noticeable in the available torque and improved combustion)

Hm, at lower load levels, the EGR system is already fully active. Consequently, any additional fresh air that might enter due to the higher LD (load demand) would immediately be replaced by more exhaust gases -> whether this improves combustion .

Quote:

das problem ist so, dass schon in den unteren drehzahlen das ausgegebene soll an ladedruck nicht erreicht wird.

It's likely designed that way when using a lot of fuel, to ensure the VTG remains fully closed and therefore allows the turbocharger to spool up as quickly as possible with increasing RPM.

Quote:

was meiner ansicht nach verstärk das dann die enormen regelprobleme beim ladedruck, da dann auf einmal schlagartig der druck ansteigt - was zu mächtigen überschwinger sowie im fahrbetrieb sich noch unschöner äußernden unterschwingern führt. will heißen - bei vorherigem erreichen des soll-ladedrucks, wäre der druckanstieg nicht so schlagartig und dementsprechend die über und unterschwinger wesentlich geringer.

These could be the less desirable consequences, but they were probably considered by the developers as the lesser of two evils.

To address this issue, one can try carefully extending the length of the VTG rod.
A more targeted and nuanced approach would be a software modification (chip tuning).
It's possible that this approach only addresses wear and tear effects (e.g., on LD solenoid valves or VTG units).

Quote:

ist es wirklich ok dass die vtg so viel langsamer auf eine ansteuerung reagiert wie die agr? - sprich wie schnell muss eine vtg reagieren?

A properly functioning VTG (variable transmission gate) opens and closes completely in fractions of a second.
That alone doesn't change the load significantly yet, because the slow-moving component first needs to reach the desired speed.
And that's precisely why "VTG" is so slow to respond to the AGR, which only shifts "weightless" gas flows.

1. - Yes, that is the case.

Regarding point 2, I probably agree with you - decommission the agricultural land .

Here's the translation:

'However, if the target load (LD-target) is already reached in the lower RPM range, why does the actual load (LD) reach its maximum value much faster when the RPM increases?'

'Regarding point 4: Should I make it longer? Absolutely not. If I did, the low-end response would be even more sluggish and abrupt. I actually tried making it shorter because it doesn't fully close at idle.'

Here's the translation:

'Regarding point 5, this occurs when the vacuum is switched from 'full' to 'atmosphere'. However, for example, when adjusting the LD magnet valve from 20% to 50%, the change happens much more slowly – should it be that slow? Should it happen faster?'


Thanks, Ulf, for thinking about it.

Please remember that the boost pressure is not regulated in the lower RPM range. The variable turbine geometry (VTG), or rather, the duty cycle of the N75 valve, is only adjusted according to the defined characteristic maps.

Devilseye503 wrote:

Regarding point 1 - yes, that is the case.

Okay: a perfectly precise answer to the question "Does the target value for LD change in this process, or does the actual value change?"

Quote:

zu 3. warum ist es dann aber so, wenn vorher das LD-soll im unteren drehzahlbereich schon erreicht ist, der LD bei steigenden rpm viel schneller erreicht ist?

It takes the longest to get the engine "out of its low RPM range," and the longer it takes, the deeper it's stuck in that range. If the turbocharger has already started spooling up before you apply full throttle, it will happen faster.

Quote:

zu 4. länger drehen? auf keinen fall - dann wär der ld-aufbau noch träger und schlagartiger - ich hätts mal mit kürzer drehen probiert, da sie im standgas nicht komplett geschlossen ist.

Based on my experience, a shorter rod tends to produce higher oscillations. I can't imagine how the opposite would work.

Quote:

zu 5. das ist der fall wenn der unterdruck von voll da, auf atm geschalten wird. aber z.b. bei einer ansteuerung des ld-magnetventils von einmal 20% auf 50% - das geht wesentlich langsamer, bis sich da was tut - sollte das auch so schnell gehen?

No, it's normal for the vertical tracking generator (VTG) to react more slowly with small TV adjustments because the "adjustment impulse" is weaker.

Regarding point 1 - yes, that is the case.
Okay: a perfectly precise answer to the question 'Does the target value for LD change in this process, or the actual value?'

Sorry, I misread that somehow - yes, both are increasing.

and thirdly - that's exactly what I would like - the pressure should be higher at the bottom (to be fulfilled), and more pressure further up; the VTG valve can then be activated earlier and react more gently (which you confirmed to me in reverse ).

Okay, here's the translation:

'Fourthly, that would certainly be correct if the air-fuel ratio didn't adjust based on the actual boost pressure, but only on a predetermined map. So, I guess I have no choice but to try it out like that? (If only I didn't always run into such problems).'

Devilseye503 wrote:

zu 4tens - hm das wäre sicherlich richtig, wenn sich das tastverhältnis nicht am ist-ladedruck orientieren würde - sondern nur an einem vorgegeben kennfeld.

The current TV (Target Value) is determined based on the pre-tax key figure and the planned vs. actual deviation.

If you turn the VTG rod shorter, you might possibly get a faster pressure build-up, but based on my experience, it will also likely result in certainly higher overshoots.