VTG & AGR: Optimize Coordination for Success

Hello.

If my memory of the AGR system from when it was functioning correctly serves me right, the AGR also operates in a load range where a significant amount of boost pressure is already being generated.
So, it means that, on the one hand, energy is being used (ultimately at the expense of fuel consumption) to create boost pressure, but on the other hand, the potential cylinder filling that this boost pressure could provide is not actually realized, and instead, part of the fresh air is replaced by exhaust gases.
That seems a bit inconsistent to me.

Therefore, I considered blocking the control of the VTG turbocharger and, consequently, the build-up of boost pressure, as long as the EGR system is active. Only when the EGR system is completely closed would the electrical circuit to the boost pressure control valve be opened, and the turbocharger would be controlled as usual.

This could be achieved using relatively simple analog electronics that, depending on the duty cycle of the AGR solenoid valve, controls a switching transistor in series with the boost pressure solenoid valve.
"I believe that errors related to the resistance of the charge pressure solenoid valve can be avoided by using a small, dummy resistor."

Before I start wiring everything up to make it work, I'd like to understand why it wasn't programmed that way from the beginning.
Is it just about keeping the turbocharger "pre-spooled" to minimize turbo lag when accelerating out of the EGR range, or are there more important reasons behind it?

(With a wastegate turbocharger, this is not possible because it is always ready to build up at least approximately 0.6 bar of pressure, and with less boost pressure, the wastegate cannot be artificially opened ).

Hi,
I think your plan is a bit risky as long as you don't know the pressures in the exhaust system.
It's not necessarily certain whether exhaust gases are still recirculated when there is boost pressure, or whether fresh air enters the exhaust system.
Certainly, the return rate could be significantly different. This could mean that you would need to control the EGR valve differently than what the EDC (Engine Control Unit) intends. This, in turn, would likely require a considerable amount of effort.

christians wrote:

Hi, I think your plan is a bit risky as long as you don't know the pressures in the exhaust system. It's not necessarily certain whether exhaust gases are still recirculated when there is boost pressure, or whether fresh air enters the exhaust system. Certainly, the feedback rate could be significantly altered.

This could mean that you would also need to control the EGR valve differently than what the EDC wants. } This, in turn, will likely not be possible without considerable effort. }
Hi Christian,

Since the AGR (Automatic Gearbox) operates as a closed-loop system, any external influences can generally be compensated for as long as the system doesn't reach its operational limits.
The feedback value is the LMM signal, which, under idling and partial load conditions, should be reduced towards the instantaneous target value via the EGR rate.
Typically, this is achieved by opening the EGR valve to a corresponding extent.

However, if you reduce the turbocharger's boost pressure, less fresh air will flow through the mass airflow sensor (MAF), and the "benefit" of the EGR control intervention should become apparent with less (or sometimes no) activation of the EGR valve.

I obviously can't estimate the overall behavior of the AGR (Exhaust Gas Recirculation) system in relation to boost pressure (regulation oscillations?) at this point.

Hello Ulf,

Regarding your explanations, I'd like to add that by controlling the VTG (Variable Turbine Geometry) and the associated increase in exhaust back pressure, significantly different amounts of exhaust gases can essentially be forced into the cylinder. I'm not confident enough to speculate on how the pressure ratios between boost pressure and exhaust pressure would change if you were to disable the VTG.
I suspect that with VTG control, it might be possible to implement exhaust gas recirculation for a longer period.
Perhaps the performance is also better, faster, etc.?

Best regards,
Albrecht.

Albrecht wrote:

Hello Ulf, Regarding your explanations, I'd like to add that by controlling the VTG and the associated increase in exhaust back pressure, completely different amounts of exhaust gases can, so to speak, be forced into the cylinder. I'm not brave enough to guess how the pressure ratios between boost pressure and exhaust pressure would shift if you were to disable the VTG.

Hi Albrecht,

"That's an interesting aspect. I'll try to repair the AGR (Exhaust Gas Recirculation) system soon, and then I'll check the AGR duty cycle at idle, both with the VTG (Variable Turbine Geometry) hose connected and disconnected."
This should provide a general indication, as the VTG (Variable Turbine Geometry) is in an idle state and ready for full pressure.

However, at least some of this effect should be offset during operation: when the turbocharger is working, the exhaust back pressure and boost pressure increase, and thus initially "work against each other" to a lesser extent.
Ultimately, what the final outcome will be is something that we can probably only guess at.