Supercharger clutch operation

[Dippy]

I know that there are no S4s with the CREC engine in NA, but I have seen posts from members who know about superchargers so I thought I'd post so see if I can get some insight.

Purely out of interest I have been observing the "compressor speed" of my supercharger in varying driving conditions. I have a 189mm crank pulley so a drive ratio of 2.99, and when using boost the supercharger revs are indeed about 3 times engine revs. I then observed that at idle the supercharger revs fall to around 300rpm. I have assumed that since the supercharger clutch is magnetic, (and the measured current always seems no less than 0.1A) there is enough flux to turn the rotors. I recently observed that when cruising at around 30mph with very light throttle (engine revs a bit less than 1500rpm), the supercharger revs are about 400-500rpm. But even the slightest increase of pressure on the accelerator will raise the revs. I observed that they might reach around 1200rpm with no significant increase in engine revs.

I can only think of two reasons for this. The first, which I doubt, is that even that slight opening of the throttle increases airflow enough to spin the rotors faster. The second which I tend to favour, is that the supercharger clutch is not "open/closed" as I had presumed, but in fact has a progressive action, with variable magnetic flux and therefore variable drive ratio.

Any thoughts?

[SteveYem]

I am not sure, but I recommend that you also post this in the A6 section as I believe the more recent 3.0t engines in those cars also use the magnetic clutch on the supercharger, so they may have a better idea about this.

[fs454]

Wait, they made 3.0TFSIs with a supercharger clutch? Is there any more info on this and what the differences in performance are between what we get here in NA?

[Dippy]

I am not sure, but I recommend that you also post this in the A6 section as I believe the more recent 3.0t engines in those cars also use the magnetic clutch on the supercharger, so they may have a better idea about this.

Thanks Steve, I might do that but the 3.0 TFSI, and particularly the CREC version is a minority engine for the A6 forum. Besides it appears that the more technical members, and those who know superchargers, are here on this forum.

[Dippy]

Wait, they made 3.0TFSIs with a supercharger clutch? Is there any more info on this and what the differences in performance are between what we get here in NA?

Yes, but it was never used in the S4 or S5 in NA, only in the A6/A7/Q5. Same performance. It was produced to meet EU6 emission regulations, but it did have a few useful revisions (such as to timing chains, fuel injectors and intercoolers).