[RedDevil]

Originally Posted by planejob

I don't think that would work in theory either. You are restricting the air intake, making the air do "work" and "work" is never free. The engine would be doing the same or more work to make the same electrical power as a belted alternator.

Same theory as putting a windmill on top of your car. It's not free to push the blades through the air.

Putting half of a turbo on the exhaust side and running it through a planetary gear reduction seems pretty plausible.

Take a look at the centrifugal superchargers. They use a belt off of the accessory pulley to spool a compressor wheel... just use the same gearing and use an exhaust turbine to spool your alternator.

A broken centrifugal supercharger on eBay could yield some fantastic parts... I really like the idea.

Well, the windmill setup would work (impractically, still) if the wind is strongly behind. Which is analogous to the situation here.
We have a lossey vacuum because of the need to restrict power on partial load. The name for that (pumping loss) is telling. The vacuum is created by the engine drawing air in against the vacuum.

The "work" therefore is already happening. It is controlled by air friction over the edges of the throttle plate.
It makes no difference whether you do that restriction by a throttle plate or a small turbine, as long as the restriction matches the need.
But the turbine could drive an alternator - for free, basically.

The vacuum traditionally gets used for other purposes as well - to power brakes and there have been cars where it was even used to power the windshield wipers. When you wiped the windows on those while idling the engine would rev up slightly for the extra air flowing in.

You can hold your hand ahead of the carburetor but you cannot sense the vacuum behind the throttle plate. That can be as low as 20% of atmospheric pressure at idle. Which you can easily monitor if you have an OBDII device... I saw it go down to 20% on my Insight, I guess that is a normal value at idle.

Then again, even if you manage to control the turbine well enough to double as a throttle valve it will still be hard to maintain the power needed for the 12V system. It would need all the work done with a regular alternator delete, and compared to that it would be hard to make a ROI on the extra parts for the turbine.
Technologically possible, but economically feasible just for special cases.