[Crazyrabbit]

The turbo with an alternator is feasible and has been done. I don't know if they sell them now, but Garrett designed and built some turbos with a three phase motor/alternator between the turbine and compressor. Their objective was lag reduction, but with the appropriate controls, one of those would do what you want. That said, the design and development of such equipment is well beyond the capability of the vast majority of even professional engineers, it requires a team of engineering specialists. Like you might find at Garrett. As far as possible alternator speeds go, I have worked on the design, development, and test of alternators that operated WAY over 100,000 rpm. Windage, bearing, and magnetic hysteresis losses do add up, but I think you could go 1,000,000 rpm, if you could get the rest of the system to support that speed. That may have also have been done, on some space based cryo-coolers. There is a lot of stuff that never makes it to consumer media.

The comments about nothing being free are largely true, the only free energy I associate with a turbo, comes from the reduction of normal shock losses in the exhaust port due to the back pressure of the turbo. That shows up as about a 0.5% decrease in BSFC if you compare turbo, and non-turbo versions of the same engine from Yanmar, Kubota, etc.