Boost Pressure Surge: Fix with N75 Adjustment

Hi,

Here's how I reduced the boost overshoot on my 2.0 TDI PD engine in my Skoda Superb (engine code BSS) by adding a resistor to the supply voltage of the N75 valve.

First of all: The previous owner had the car "chipped" (ECU remapped) by a well-known company from the Allgäu region. Unfortunately, this resulted in overboost situations that exceeded the measurement range of the boost pressure sensor. Reverting the chip tuning would cost me 350 euros from that company... But I think I can try to fix it myself.

The traditional method for reducing LD peaks is to extend the VTG rod. However, I was unable to loosen the lock nut.

The next option is to reduce the supply voltage of the N75 valve. The N75 is controlled by a pulse-width modulation (PWM) signal. Reducing the supply voltage is equivalent to constantly reducing the clock frequency, which reduces the effective supply voltage while maintaining the same internal resistance of the N75.

Here's a rough estimate for the resistance, calculated as follows:
The PWM current is approximately I = 2 A, and the supply voltage is 12 V (in my opinion, it could also be 5 V). To reduce the voltage by 50%, which is dU = 6 V, a required resistance of approximately 10 ohms can be calculated using the formula R = dU/I. However, this resistor should be properly sized to dissipate the generated heat, so it should have a power rating of approximately 10 W.

I only received a 5W, 10 Ohm potentiometer, but the power rating is sufficient for practical applications.
I connected this to the supply line of the N75 and tried several settings:

*) At 0 ohms, which is the original state, I observe extreme overshoots followed by undershoots. The undershoot is noticeable in the acceleration behavior.
*) At 10 ohms, meaning the potentiometer is turned to its maximum resistance, the system is extremely sluggish, with boost pressure only building up beyond 2500 rpm. While it doesn't over- or undershoot, it's essentially undrivable.
*) I've achieved a good setting at 4 ohms. While the car feels noticeably less responsive at low RPMs and doesn't reach full boost until around 1900 RPM, there's also less of a significant over- and undershoot, and the acceleration is very smooth.

I am attaching 3 diagrams that show the boost pressure behavior as a function of the selected resistance.


Best regards, Georg.

Hello,

It's not really ideal, so please check if the Russian limit (measurement block 8, last value) is already being triggered during the underswing.

Ideally, the dataset would have better (more accurate) control of the VTG. It's unfortunate that this doesn't seem to be a consideration during product development.

Best regards, Rainer.

Hello Rainer


I understand that this isn't as good as a proper software fix. However, I can't bring myself to give more money to the original chip tuning company. It just doesn't sit right with my Swabian sensibilities.

The smoke limit is just barely not being exceeded, or, the lower portion of the waveform is not visible within the smoke limit. See the attached diagram.

What's alarming is that the overshoot is still noticeable even at the 10 Ohm setting, which is a setting where there shouldn't be any overshoot at all.
Not as strong, but definitely present.
That means, to me, that the programmer of this software has very little understanding of control engineering. It's a shameful display for this very well-known tuning company.


Best regards,
Georg

Hello,

Quote:

For me, this means that the programmer of this software has only a limited understanding of control engineering.

Either the latter, or your N75 has a problem, or your VTG is stuck.

Best regards, Rainer.

dieselschrauber wrote:

Either the latter, or your N75 has a problem, or your VTG is stuck.

I've also considered the possibility of a sticking VTG valve or a partially defective N75 valve.

The VTG adjustment system works perfectly during the injection test. Also, the boost pressure regulation behavior has not changed at all during the last 10,000 km. In my experience, VTGs tend to become clogged quickly once they start to become difficult to operate.

Furthermore, this extreme over- or undershoot only occurs in the range around 2000 rpm. When I quickly accelerate at 3500 rpm, there is hardly any overshoot. Therefore, I assume that the maps around 2000 rpm are simply incorrect.

Best regards, Georg.

That doesn't really explain anything. A very powerful engine can overwhelm a pneumatic system. For the standard configuration, the sluggish pneumatic components of older VTG systems are sufficient. It's no coincidence that the current VTG systems of a newer design use servo magnets and a slave function. Monitoring via EDC (Electronic Data Carrier) is hardly possible in a better way. Best regards.