overshoot in ALH

Hello everyone.

Today I installed my new VDO LDA.

In tuning mode, I have the fourth gear engaged between 2000 and 3000 RPM.
With full throttle at the moment, there's a brief overboost (1.25 bar) that returns to 1.1 bar after approximately 1-1.5 seconds.

When do overspeeding conditions become dangerous? Is it at 1.3 or 1.4 [units]?

In the 90 hp mode, the overboost reaches a maximum of 1.1 and then returns to 0.9.


Are these values okay?

Hi.

I still consider 1.25 bar for a short period to be largely harmless.

If you want to reduce it as a precaution, simply make the VTG rod slightly longer; this will decrease the overswinging.

When do overspeeding vehicles become truly dangerous?

From 1.4 bar? More? Less?

90psler Freund wrote:

From what point on are overspeeding vehicles really dangerous? From 1.4 bar? More? Less?

"I've also experienced pressures of 1.4 bar or more with my small VTG turbocharger – it was a design flaw. However, this only happened briefly and at engine speeds around 2000 RPM, where the turbocharger doesn't have to work as hard to build up pressure."
My charger definitely held up.

Short periods of overload don't seem to be that critical, whereas longer and more frequent overloads can definitely damage the charger, in my opinion.

I don't know when it will be possible to safely and quickly disassemble the charger.
Since, to the best of my knowledge, there are no known cases like this here, but given the large number of incidents involving emergency shutdowns due to overpressure, they should have occurred if it were a real problem. Therefore, I would take the whole thing with a grain of salt.

Hi there,

'I constantly experience pressure spikes up to 1.4 bar. These spikes last for about 1-1.5 seconds. Then, the boost pressure drops back down to 0.5-0.6 bar and builds up again within 3-4 seconds, reaching approximately 0.9 bar.'
The workshop finds this to be normal. However, I am certain that it is not okay.
Let's consider something different: If the turbocharger needs to produce 0.9 bar of boost pressure at 2000 RPM, does it require double the turbocharger speed to achieve the same boost pressure at 4000 RPM?

Regards,

Eike.

Hello Eike,

Your boost pressure control is definitely too slow. I had the same error description on my AHF before. Replacing the charger eliminated the problem.
However, I maintain that simply enabling VTG (Variable Torque Geometry) and replacing the vacuum sensor would have been sufficient.
Regarding your question: take a look over there.
/viewtopic.php?t=1337&highlight=compressor

Best regards, Rainer.

Hello Rainer,

I'm quoting you:

'On the left side are the pressure ratios before and after the compressor, while on the right side are the volumetric flow rate or mass airflow rates.'
For low flow rates, your assumption that the impeller speed is approximately proportional to the boost pressure is correct.
However, at high flow rates (and thus engine speeds), the turbocharger speed increases much more significantly than the pressure ratio.
That means a general statement cannot be made because the relationship between impeller speed and boost pressure varies depending on the operating point.
Unfortunately, I haven't been able to find a characteristic curve for the turbocharger used in the AHF/AFN engines. However, 1.5 bar seems a bit high at first glance. Therefore, it might be beneficial to adjust the adjustment rod slightly longer.

From this, we can deduce that the turbocharger operates at a significantly higher speed when the engine is running at 4000 RPM with full throttle and a boost pressure of 0.9 bar, compared to, for example, 2000 RPM with a boost pressure overshoot of 1.5 bar.

Then these overspeeding situations shouldn't really affect the loader, since it already reaches much higher speeds in normal operation, for example at 3800 RPM.
However, I can imagine that it wouldn't be good for the engine if it was constantly being turbocharged to a pressure of 1.5 bar.
What is the boost pressure in the most powerful production TDI engine?

Next week I'm going on vacation, and I hope I finally have time to build the ODB adapter. Then I'll record a boost pressure curve for my engine and post it here, as well as present it to my workshop.

Regards,

Eike.

Hi,

Quote:

Dann müßten diese Überschwinger dem Lader doch garnichts ausmachen, da er im Normalbetrieb bei z.B 3800 UpM sowieso viel höhere Drehzahlen erreicht.

und wie verhält sichs wenn Du bei 4000min-1 Gas wegnimmst und Vollgas gibst ?

Quote:

I can imagine that it might not be good for the engine if it were constantly being boosted to 1.5 bar. Wie hoch ist denn der Ladedruck im stärksten Serien-TDI?

Well, I think so too. The 150 hp 4-cylinder PD-TDI engine has a constant boost pressure of 1.5 bar, but it has a lower compression ratio, reinforced internal components, and a different turbocharger.
In the long run, this current condition will not be good for your engine.

Best regards, Rainer.

Can we assume that at 4000 rpm, the turbocharger speed is double what it is at 2000 rpm? It's probably clear that the compressor needs to spin faster at 4000 rpm than at 2000 rpm in order to provide 0.9 bar of boost. However, I don't think it necessarily spins twice as fast, because the variable turbine geometry (VTG) reduces the proportion of volume converted into turbocharger speed as the engine speed increases.
Or is the VTG (Variable Transmission Gear) only activated to counteract overshoots?

Best regards,

Can we assume that at 4000 rpm, the turbocharger speed is double what it is at 2000 rpm??? It's probably clear that the compressor needs to rotate faster at 4000 rpm than at 2000 rpm in order to provide 0.9 bar of boost. But I don't think it necessarily rotates twice as fast, because the variable turbine geometry (VTG) reduces the proportion of volume converted into turbocharger speed as the rotational speed increases.
Or is the VTG (Variable Transmission Gear) only activated to counteract overshoots?

Regards,

?!?!?!?!? Please scroll up; this has already been clarified!

At 4000 RPM, my boost pressure only briefly reaches approximately 1.2-1.3 bar. It's likely that the wastegate (VTG) setting is either too low or already at its limit, preventing the turbocharger from achieving higher pressures.

Ultimately, it's a company car, so I don't have to worry too much about the engine.
I'm eagerly awaiting a 'intake manifold control differential alarm' so I can finally get the workshop back up and running.

Regards,

Eike.

Hi there!
I don't have any of those 'Überschwinger' (overshoot devices), but I currently have a different issue, although it's not quite a problem yet!
'For about a week now, I've been seeing a maximum boost pressure of around 2 bar. I have no idea how that's happening; normally, I would expect something closer to 1.5 bar. My gauge only goes up to 1.5 bar, but the needle is almost going all the way around to the bottom. The turbo seems to be working perfectly fine and boosting well. It's a turbo from a 131 horsepower engine, though.'
I'm not sure if the boost control valve is stuck or what.

Data from 1998.

Best regards, corradoTDI.

Hi.

... then also log the boost pressure reading.
You can see the target pressure and the actual pressure, and how the control system is operating at its limit to correct what is likely an excessively high pressure.

There are essentially three main causes of excessive pressure:
Stuck VTG (assuming you have a VTG loader).
Turbocharger wastegate solenoid valve failure.
Problems in the area of tax regulations.