Charging Strategy for Electric Vehicle Chargers

Hello,

Perhaps my random experiences with boost pressure regulation when using an adjustable VTG (Variable Turbine Geometry) rod might be of interest to some people here.

The starting point was overboost conditions, reaching up to approximately 1.5 bar (target = 1.3 bar), particularly during the transition from partial to full load, especially in 6th gear on the highway.

To reduce that, I adjusted the VTG rod (while on vacation in August) to a slightly longer setting.
During the drive back on the highway, I occasionally observed fluctuations in the turbocharger's boost pressure, at a frequency of approximately 1 Hz.

I then adjusted the rod again to make it longer, and since then I haven't noticed any more pressure oscillations. However, I haven't driven as many long stretches of highway since then either.
The overhangs are now significantly smaller than they were originally.

A quite welcome side effect is the overall slightly reduced boost pressure, even though the regulation is operating within its active range (duty cycle from 2000 rpm onwards is between 50 and 65%)!
According to the Vollgas logs, in 3rd gear, the actual pressure is, on average, 20 to 30 mbar below the target pressure.
(It's not a huge amount, but it's a certain offset for the reduction in LLT [low-temperature heating] costs by about 20,000 [currency unspecified] due to the redesign of the cooling and ventilation system for the LLK [low-temperature cooling].)
Even with longer deviations, the pressure is at best briefly increased to the target value and then immediately reduced again.

With the factory settings, the actual value was practically exactly equal to the target value on average.

In my opinion, this means that the boost pressure control system doesn't react abruptly to deviations between the target and actual values, but seems to be linked to "target values" where the desired boost pressure is normally achieved.

Conversely, slight increases in pressure may also be achievable by shortening the rod, without having to push the control system to its limit.

Hello Ulf!

That sounds like a regulation with a permanent deviation from the standard, meaning it doesn't include any "I" component.
Most likely, the boost pressure control system is a P-controller with map-based setpoint adjustment, where the actual value is still filtered before being fed to the controller (or the loop gain is very low).
I don't believe that a "D" gear is typically included in a standard driving strategy, because otherwise, it would be difficult to achieve such impressive acceleration and "kick" when rapidly shifting into lower gears.

In simpler terms, this means:
The EDC reads the actual boost pressure value with a slight delay and, based on other engine operating parameters, determines the corresponding duty cycle from a lookup table.
The remaining deviation from the setpoint (the difference between the desired and actual value) is corrected using a simple P controller (the control signal is adjusted proportionally to the deviation). Therefore, there is a superposition of the feedforward control (based on a characteristic curve) and the feedback control.

This is just my personal assessment of the rule strategy, based on observations and analysis of data logs. If anyone knows better, please feel free to correct me.

Regards,
Alex.

donalexo wrote:

The EDC reads the actual boost pressure value with a slight delay and, based on the other engine operating parameters, reads the corresponding duty cycle from the characteristic map. The remaining deviation from the setpoint (the difference between the desired and actual value) is corrected by a simple P controller (the control signal is adjusted proportionally to the deviation). Therefore, there is a superposition of the feedforward control (based on a characteristic curve) and the feedback control. This is just my personal assessment of the rule strategy, based on observations and evaluations of data logging. If anyone knows better, please feel free to correct me.

Hi Alex,

If I had a good grasp of regulatory Chinese, I would probably describe it in a similar way .

Do you happen to have any regulation-specific hypotheses about the cause of the temporary boost pressure fluctuations I mentioned earlier?

Hello Ulf!
Meanwhile, I know a little more about how the turbocharger pressure regulation works, as I have been provided with in-depth information through certain channels within the company (if... reads this, he will know that he is being referred to. Thank you).

Now, let's get to the point:
Basically, my assessment was correct that a look-up table-based pre-control is being used. There are a variety of look-up tables that take into account various influencing factors (e.g., engine speed, charge air temperature, coolant temperature, atmospheric pressure, etc.). Based on these look-up tables, a target value is first applied to the controller.

This controller is "relieved" to the extent that the pre-control allows it to only compensate for the small deviations between the desired setpoint and the actual value.
However, the control structure is significantly more complex than I initially assumed. It is a PIDT1 controller.

Here's my attempt to explain how it works:

1. The P-component outputs a control signal that is proportional to the deviation from the setpoint.

2. The I component (I = integrator) ensures complete compensation for the control error, thus achieving long-term alignment between the setpoint and the actual value. However, a problem with the I component is the phase shift between the setpoint and the actual value, as well as the potential for destabilizing the control loop. More on that later.

3. The D-component (D = Derivative) ensures a rapid response to sudden changes in the setpoint value. The derivative of the control error is amplified proportionally and added to the control signal by the D-component.
In this case, a DT1 element was implemented, which reacts to a setpoint jump with a "decaying step response." The purpose of this component is, for example, to immediately open the VTG (variable geometry turbine) when there is a sudden acceleration, i.e., to set it to full boost pressure before the damped setpoint can cause a larger control deviation in the sense of the P-component. This is, so to speak, the "oracle function" of the controller, which informs it in advance of the control deviation that it will soon have to handle .

Following the regulator output, there is a monitoring routine that checks certain parameters for plausibility and immediately intervenes if a specified boost pressure is exceeded.

I hope I was able to explain the functionality in a reasonably clear way.

Now, regarding your problem with the 1Hz oscillation. By changing the VTG setting, the pre-control provided by the characteristic curve is no longer as accurate, meaning the controller has more work to do. Furthermore, the gain factors of the individual controller components are no longer precisely aligned, as the mechanical adjustment has slightly altered the mechanical gear ratio.
You are therefore driving around with a poorly tuned controller. Now, especially due to the I-component of the controller, it is very easy for an instability to occur in the sense of control engineering, i.e., the controller does not react cleanly by adjusting the setpoint (e.g., to a disturbance), but instead performs a continuous oscillation (marginally stable ) or even reacts with a self-amplifying oscillation (complete instability).

Could you please turn the VTG rod back to its original position? I bet that will stop the 1Hz oscillation from occurring!

Regards,
Alex.

donalexo wrote:

Could you please turn the VTG rod back to its original position? I bet that will stop the 1Hz oscillation from occurring!!

Hi Alex,

Thank you for your explanations (which are largely understandable to me) .

I actually don't like reversing the VTG (Variable Turbine Geometry), because I appreciate the reduced pressure and smaller pressure oscillations, and the vibration only occurs very rarely.

But I do believe you that the oscillation could be caused by the out-of-tune control loop.