Boost behavior during throttle lift-off above 3000 RPM

Hi everyone,

I've had an LDA (Lean Defuel Assist) installed in my G4 ALH for quite some time now. It's connected to a thick rubber hose located just before the intake manifold using a hypodermic needle.
So far, everything is working great. The pressure peaks reach up to 1.4 bar (due to the slow response of the VTG control), and the constant pressure is approximately 0.85 to 0.9 bar.

However, it is noticeable that the turbocharger pressure does not drop to zero when, for example, you are driving on the highway at 3800 RPM and then take your foot off the accelerator.
There remains a residual boost pressure, which is approximately 0.3 bar when running on biodiesel and approximately 0.5 bar when running on diesel. Only when the engine returns to an RPM range around 3000 does the boost pressure decrease back to zero as the engine speed decreases.

Does anyone have an explanation for this? The fuel injection amount is zero. The engine seems to be acting only as a pre-heater for the intake air. Does it make enough of a difference that the turbocharger can still reach up to 0.5 bar? And why not use biodiesel? That stuff is supposed to burn even hotter than diesel (which contributes to higher NOx emissions). Also, since the engine is water-cooled, the temperature difference probably isn't that significant.It's kind of weird...

Regards,

Eike.

Hello Eike,

You've almost explained the reason for your turbocharger behavior yourself. At engine speeds above 3000 RPM, the engine already provides such a large airflow in overrun (i.e., as an air pump) that the energy supplied to the turbocharger is sufficient to properly accelerate it to its operating speed.

Ja sicherlich wird auf der Ansaugseite auch wieder eine entsprechend große Luftmenge angesaugt, allerdings lässt sich die Leitbeschaufelung der VTG-Turbine nur in gewissen Grenzen verstellen. Bei großem Massendurchsatz ändern sich die Anströmverhältnisse auf das Laufzeug derart, dass selbst bei der Stellung "wenig Ladedruckaufbau" noch genügend Energie zur Verfügung steht. Soweit My theory!

However, I can't understand the difference between biodiesel and diesel. I think the external conditions were simply too different (temperature, air pressure, engine temperature, etc.), because the boost pressure should actually be completely independent of the fuel type in gliding mode.

Regards,
Alex.

However, I can't understand the difference between biodiesel and diesel. I think the external conditions were simply too different (temperature, air pressure, engine temperature, etc.), because the boost pressure should actually be completely independent of the fuel type in a cruising situation.

Regards,
Alex

Hello,
das seltsame ist, daß es reproduzierbar ist. Ich fahre normalerweise fast nur Biodiesel und habe unabhängig von den Umgebungsbedingungen einen Restfruck von 0,25-0,3 bar. Wenn ich dann mal wieder Diesel tanke, weil ich keinen Biodiesel bekomme, habe ich den erhöhten Druck von ca. 0,45-0,5bar. Ebenfalls unabhängig von den Umgebundbedingungen. Ich komme mit einer Tankfüllung etwa 1 Woche hin, manchmal auch etwas länger. Es ist also keine zufällige Erscheinung, sondern solange die Spritsorte im Tank ist. Ich werde mal Diesel tanken, wenn mein Tank noch halb voll Biodiesel ist. MaLet's see what happens next...

Regards,

Eike.

I suspect your EGR valve is also defective?

'With a faulty AGR valve, the turbocharger starts at 1300-1500 RPM in boost mode and builds up some pressure. It sounds particularly great when decelerating from 2000 RPM to idle.'

As you SEE here, the VTG adjustment will also be reset to 'ready to fire' immediately after the boost pressure is removed. Without the EGR system, the turbocharger will then be pulled along by the engine. (The log was found with a functioning EGR system).

The only thing I find strange is the whole biodiesel thing (SUXX!). In idling mode, nothing really enters the combustion chamber anyway...
Maybe your pump is broken .


CU Gremlin.

...the only thing I find strange is the whole biodiesel thing (SUXX!). In idling mode, nothing really enters the combustion chamber anyway...
CU Gremlin

Hmmm, vielleicht ist aber genau das aber nicht der Fall....

Option 1: One or more nozzles are not completely sealed...

Option 2: The pump is actually defective and, despite being set to a quantity of zero, it's still doing something... (quantity setting mechanism?).

Option 3: He's using a tuning box, and it's malfunctioning, causing it to consistently report an 'insufficient amount' [of something].

Option 4: ??????

Regards,
Michl.

...the only thing I find strange is the whole biodiesel thing (SUXX!). In idling mode, nothing really enters the combustion chamber anyway...
CU Gremlin

Hmmm, vielleicht ist aber genau das aber nicht der Fall....

Option 1: One or more nozzles are not completely sealed...

Option 2: The pump is actually defective and, despite being set to a quantity of zero, it's still doing something... (quantity setting mechanism?).

Option 3: He's using a tuning box, and it's malfunctioning, causing it to consistently report an 'insufficient amount' .

Hi!
Okay, everything is original except for the LDA (Linear Differential Amplifier). The engine itself hasn't been modified in any way. If the metering unit of the pump isn't working correctly, what other symptoms might indicate this? Otherwise, the car's performance is okay, although the VTG (Variable Turbine Geometry) adjustment seems quite sluggish.
Regards,

Eike.

.......Wenn das Mengenstellwerk der Pumpe nicht OK ist, woran kann man das noch nachvollziehen? Die Fahrleistungen des Wagens sind sonst OK ...

Hi Eike!

I suspect that if the metering system malfunctioned, the difference between biodiesel and regular fuel wouldn't be significant because the (incorrect) amount dispensed would be roughly the same, and biodiesel doesn't burn *that* much worse.

Also, nachdem du deinen >Post< verfasst hast, fährst du fast ausschließlich mit Biodiesel und zwar mit einer hohen Kilometerleistung...
As you have probably already noticed, it appears to drip even after being turned off, so it will likely happen during operation as well.

Da Biodiesel etwas 'dickflüssiger' als normaler Diesel ist , dichten die Düsen damit etwas besser und es troft weniger nach... darum glaube ich sind die Düsen am wahrscheinlichsten...

Ich würde mal die Düsen checken lassen (Dichteprüfung,Spritzbild, Öffnungsdruck), denn nicht das mal eine Düse ganz hängen bleibt und Du dann ein Loch im Kolben hast...

Regards,

Michl.

Hi,

Someone really looked closely .

This post is from the time when I was driving a G3. Now it's a G4 with a 66kW ALH engine, which is already 1.5 years old and has almost 72,000 kilometers on the clock.

It's starting to emit a bit of blue smoke in the mornings now. I'm sure it ran on 95% biodiesel for the last 72,000 kilometers. So far, there have been no problems.

But could even a leaky injector still leak enough into the cylinders to provide sufficient boost pressure for at least 5-10 seconds?

How are things with LDA for others? Have they made similar observations as I have?

Regards,

Eike.

PS: If the injectors or the pump are faulty, I will run it dry and then refuel with diesel, run it dry again, and then refuel with diesel again. I doubt they'll be able to detect the biodiesel in the workshop afterwards. The car is still under warranty (or rather, product liability) until February...

Believe me, they'll realize it as soon as they take the cover off the pump...
My dealer also sees a relatively high number of vehicles damaged by biodiesel. The only vehicle that hasn't yet experienced problems due to biodiesel belongs to a company that manufactures and tests biodiesel. This certainly draws some interesting conclusions about the quality of fuel at gas stations...

I will upload some PDFs later this evening and then post the links here.

CU Gremlin.