this post was submitted on 22 Jan 2024
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Not running the extra 20kg or whatever of rotating blower mass would increase efficiency for cruising. A supercharger doesn't have a good way of doing active bypass when you don't need boost like a turbo wastegate so just turning it off can save some mpgs.
If the size of the turbo on my VW is anything to go by, I think the rotating mas of an automotive supercharger would be more likely on the order of 2 kg, not 20 kg. In my mind, that has two implications: (a) the gain from bothering to disable it is perhaps not actually all that significant, and (b) the additional mass that would come with attaching a clutch to it might be large compared to the total mass you're trying to control, so maybe it wouldn't be worth it. Then again, the Previa supercharger the other reply gave (which certainly wouldn't be a very large supercharger) might be a counterexample...
Turbos spin far faster than (Roots-type) superchargers, and can therefore be much smaller.
Besides that, I don't think rotating mass is really the issue. Yes, more inertia is like having a bigger flywheel so the engine will be slower to spin up/down, but that doesn't consume much energy, especially in steady-state cruising.
Superchargers compress air - that takes energy. You then restrict it through the throttle body, because you're not cruising with a wide-open throttle. That throws away all the compression.
You also have pumping losses and bearing/gear/belt losses.
With a Roots style supercharger like the 8-71 on Mad Max's, if the supercharger isn't spinning then there's no path for air to enter the engine. You'd have to implement another full-size throttle body as a bypass to allow enough airflow into the engine when the supercharger isn't rotating. SCs are very parasitic, hence their use mostly being limited to larger displacement engines that have sufficient low-end torque offset the draw. You could definitely resolve this with a clutched pulley and a bypass throttle-body, the complexity, space requirements, and engineering needed to make it work isn't worth it. Multi-sized sequential triple turbos are clearly the superior solution to boost at any RPM.