Differential pressure DPF vs. exhaust backpressure (and SSP 336)

"Since the ash content in the diesel particulate filter of my Passat B6/3C (2006, BMP engine) is close to the (theoretical) limit of 60g, I regularly monitor – especially on longer trips – the differential pressure in the particulate filter (which is normal), the temperatures before the exhaust turbocharger and inside the particulate filter (which are also normal), and the regeneration cycles (which occur approximately every 350km, almost as if I could set the odometer to that value)."

Occasionally, I also check the value for the exhaust back pressure and then I always wonder:

- Where exactly is the exhaust back pressure actually measured?

"I understand that the differential pressure is measured by taking readings before and after the particulate filter, but does this measurement(s) also provide the value for the absolute pressure (which I can read from another measurement block)? Or is the absolute pressure measured elsewhere? And if so, where?"

I have studied the self-study program 336 ("The catalytically coated diesel particulate filter") to learn about the location of the pressure and temperature sensors, but I haven't really become any wiser (because, apart from the pressure measurements before and after the DPF, a dedicated measurement sensor for the absolute exhaust back pressure is not listed anywhere: it always only refers to differential measurement using sensor G450 (see also image S336_030 on page 12).

And in these technical articles, there is, of course, a lot of discussion about exhaust backpressure in general. However, I haven't yet found a clear answer to the specific question of where this is actually measured.

It's obviously more of a theoretical question and not super important, but I'm still curious and would appreciate some clarification.

Take a look under your hood. This is all easily solved.

The differential pressure sensor would theoretically simply send the pressure before the DPF and the pressure after the DPF to the differential pressure meter (which is essentially a membrane that is exposed to pressure on both sides, with a pressure sensor attached). Since it is simply assumed that the tailpipe is always open, only the pressure before the DPF is measured by the differential pressure sensor, and the connection behind the DPF remains open.

If you want to put it that way, it measures the pressure before the DPF, and that pressure corresponds to the differential pressure (plus or minus a little).

Hello Phaidros,
Please see the attached document, page 50. It's getting a bit more figurative.
The engine control unit obtains the "absolute" pressure reading from the altitude sensor.

I'm sorry, but the SSP mentioned above had to be removed due to copyright issues. Please pay attention to the following in the future!
Herbert.

Quote:

And in these technical articles, there is, of course, a lot of talk about exhaust backpressure in general - but the specific question of where this is actually measured, I have not yet found an answer to.

It can be calculated through modeling based on factors such as ambient pressure, speed, load, etc.

Newer models don't even have a second one. Connecting the differential pressure sensor to the "area behind the DPF" works on the same principle.

Best regards, Rainer.

Link removed.

better?

No, that doesn't change anything about copyright. Please do not use any files for which you do not own the intellectual, content, copyright, or other property rights. Even if you have officially obtained an SSP of any kind, you are not allowed to distribute it.

If I'm not completely mistaken, my Passat 3C, also with a BMP engine, but manufactured in 2008, has what's called a "differential pressure sensor" with two hoses. One person leads the way, and another follows behind the DPF.

In reality, it's not actually a "differential pressure sensor," but rather a dual pressure sensor. Therefore, both absolute pressures are measured, and the difference value is calculated. That's how I remember it, anyway, from about two years ago, when I exchanged it...

I believe there was even a picture of the sensor in the open state posted in this forum before. I think it's by Dieselmartin or Rainer.

Greetings.
Guste.

EDIT: Found: /viewtopic.php?t=27732

Thank you to everyone for the answers so far!

According to the repair manual (K0059020800, purchased through Erwin (siehe Fehlerdatenbank)) and the images I have seen of the particulate filter installed in my engine (BMP), it has two connections for the pressure sensor G450, which measures the differential pressure. (And yes, the differential pressure sensor looks like the one in the image linked in Guste's response: it has two connections and two separate piezo pressure sensors inside.)

If I understand the answers correctly, the exhaust back pressure measured in block 033 is not measured anywhere separately (which would be the answer to my original question), but rather is calculated by the vehicle's electronics based on the values obtained from the differential pressure sensor.

During the journey, I observed the differential pressure (below is an excerpt directly from the end of the regeneration) and tried to relate it to the exhaust back pressure (from block 033) in some way.

In layman's terms, I thought: "If the exhaust back pressure is actually derived from the values measured by the differential pressure sensor, then it should somehow change in sync with an increase (or decrease) in the differential pressure."

However, according to the measured values, this is definitely not the case (see examples below).

This actually led me back to my original question: could the (absolute) exhaust back pressure (from block 033) perhaps be measured somehow by another sensor, and if so, where?

If that's not the case (or at least, that's how I've understood the answers so far), then I'll put this question aside for now – as I said, it's rather theoretical.

Apparently, the "exhaust back pressure" is mysteriously calculated from the two values measured by the differential pressure sensor (and some other values), without being directly related to the differential pressure (meaning it doesn't increase and decrease along with it, as I initially assumed).

The fact that the back pressure in the exhaust system does not directly increase or decrease with the differential pressure is, for me, initially a surprising fact. Perhaps it becomes understandable when I consider that the "calculated" measurement point is assumed to be located elsewhere in the exhaust system. (Where that is remains a mystery to me, although I am trying to live with it in peace .)

Below are the values mentioned (while maintaining a constant speed of approximately 120 km/h on the highway, as mentioned, and just at the end of a regeneration phase, which explains the high temperatures, which then decrease significantly).

Sure, no problem!

"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Es ist ein wunderschöner Tag, um spazieren zu gehen oder ein Picknick im Park zu machen. Viele Menschen genießen das gute Wetter und verbringen Zeit im Freien."

Address 01: Engine Control Module (03G 906 021 NK)

22:40:17 Block 033: Lambda control [End of regeneration]
1995 /min Motor speed (G28))
648.0°C Exhaust gas temperature.
1050.6 mbar Exhaust back pressure
42.67 g/s exhaust mass flow rate.

22:40:17 Block 067: Diesel particulate filter (exhaust values I).
Temperature before the exhaust turbocharger: 798.0°C.
636.0°C - Temperature inside the particulate filter.
66.30 Particle filter pressure difference
-10.20 Offset differential pressure.

22:40:17 Block 073: Diesel particulate filter (vehicle data).
21.1 liters - Consumption since regeneration.
360 km distance since regeneration.
336.0 Time since regeneration.

"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Es ist ein wunderschöner Tag, um spazieren zu gehen oder ein Picknick im Park zu machen. Viele Menschen genießen das gute Wetter und verbringen Zeit im Freien."

Address 01: Engine Control Module (03G 906 021 NK)

22:40:20 Block 033: Lambda regulation [3 seconds later]
2016 /min Motor speed (G28))
Exhaust gas temperature: 600.0°C
1009.8 mbar Exhaust back pressure
25.60 g/s exhaust mass flow rate.

22:40:20 Block 067: Diesel particulate filter (exhaust values I).
624.0°C - Temperature before the exhaust turbocharger.
630.0°C - Temperature inside the particulate filter.
43.35 Differential pressure, particulate filter
-10.20 Offset differential pressure.

22:40:20 Block 073: Particle filter (vehicle data)
0.0 l Fuel consumption since regeneration.
0 km Distance since regeneration.
0.0 Time since regeneration.

"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Es ist ein wunderschöner Tag, um spazieren zu gehen oder ein Picknick im Park zu machen. Viele Menschen genießen das gute Wetter und verbringen Zeit im Freien."

Address 01: Engine Control Module (03G 906 021 NK)

22:40:22 Block 033: Lambda regulation [2 seconds later]
1995 /min Motor speed (G28))
Exhaust gas temperature: 576.0°C
999.6 mbar Exhaust back pressure
24.18 g/s exhaust mass flow rate.

22:40:22 Block 067: Diesel particulate filter (exhaust values I).
516.0°C - Temperature before the exhaust turbocharger.
600.0°C temperature inside the particulate filter.
33.15 Differential pressure of the particulate filter
-10.20 Offset differential pressure.

22:40:22 Block 073: Diesel Particulate Filter (Vehicle Data)
0.0 l Fuel consumption since regeneration.
0 km Distance since regeneration.
0.0 Time since regeneration.

"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Ein leichter Wind weht durch die Bäume, und die Blätter rascheln leise. Es ist ein wunderschöner Tag, um draußen zu sein und die Natur zu genießen."
--------------------------------------------------------------------------------

"The sun is shining brightly, and the birds are chirping cheerfully. A gentle breeze is blowing through the trees, and the leaves are rustling softly. It's a beautiful day to be outside and enjoy nature."

Did you also log the ambient pressure / atmospheric pressure?

If I understand the previous posts correctly, the following was suggested: Exhaust back pressure = differential pressure + atmospheric pressure.

Unfortunately, without logging the latter, your measurements are taken out of context.

Greetings.
Guste.

Quote:

If I have understood the previous posts correctly, then the following was hypothesized: Exhaust back pressure = differential pressure + atmospheric pressure. Without logging the latter, your measurements are unfortunately taken out of context.

No, I didn't record the ambient pressure/atmospheric pressure (because at that time, I hadn't even considered the relationship between exhaust back pressure and differential pressure).

However, the ambient pressure (atmospheric pressure) surely didn't really change in those three seconds between the two measurements, did it?

It is therefore still surprising to me that, within those three seconds (with unchanged ambient pressure), a decrease in the differential pressure of 23 (from 66.30 to 43.35) resulted in a drop of the exhaust back pressure by almost 40 mbar – nearly double – from 1050.6 to 1009.8 mbar.

I personally don't believe the connection is that simple, but rather, as Rainer wrote, that the connection and the calculation are more complex.

Most likely, this is because the back pressure in the exhaust system is calculated based on complex parameters for a different (so-called 'virtual') measurement point within the exhaust system.

For now, I'm satisfied that the value for the exhaust back pressure isn't measured somewhere else, but rather is calculated from the values of the pressure sensor G440 (for differential pressure) and other parameters.

I was just surprised that the relationship with the pressure difference isn't as linear as I initially assumed. However, if the back pressure in the exhaust system is calculated for a different location – which would be interesting to know where that is – then I can understand why it doesn't change linearly with the pressure difference (assuming the ambient pressure remains constant, as in my example).

Phaidros wrote:

However, the ambient pressure (atmospheric pressure) probably didn't actually change significantly in those three seconds between the two measurements, did it?

Okay, point to you. To be honest, I hadn't paid any attention to the time at all.

Phaidros wrote:

(...) of the differential pressure by a value of 23 (...) of the exhaust back pressure by almost 40 mbar - so almost double - is "acceptable"

.
I have no idea what the resolution of the ambient pressure is. For engine control, a deviation in the exhaust back pressure and 17 mbar (with an absolute value of around 1,000 mbar) should not have any effect.

Phaidros wrote:

Probably because the back pressure in the exhaust system (calculated using complex parameters) is determined for a different (so-called 'virtual') measurement point within the exhaust system.

Quite possible.
Theoretically, it is also possible for the differential pressure sensor to output both differential pressure and absolute pressure. However, I have no idea whether it is connected digitally or simply via an analog line (which would immediately contradict that assumption).