Here’s some general information about turbo compound engines.
Here’s some good reading and the author has included some very interesting data that demonstrates an 18% increase in efficiency at high loading. Article is not free at this time, sorry.
I think the compound turbo used to run an alternator at turbine shaft speed rpm might be a great application for the hybrid auto, since all the other systems are already part of the design, the blow down turbo alternator might be a cost effective add on, and it’s output might feed directly into the motor to improve hill climbing performance, or routed back to the battery as the regenerative breaking system does.
Seems there’s a bunch of liberal dope smoking pot heads out there ready to harness energy from over unity schemes, but modern materials and the hybrid concept may give humanity reason to revisit the proven turbo compound engine, and the application of exhaust driven turbines to recover energy. As mentioned in both articles, the system was complex, but that was using 1940s technology. Using a microprocessor and rare earth materials for a high speed alternator running at turbine shaft speeds may provide cost effective efficiency gains in modern piston engines, especially at high loading. Worth exploring in the COGEN and HYBRID application I think.
MY thanks to retired Engineer Paul Lamar for writing the great article found in August 2009 KITPLANES magazine. If you don’t like aircraft, you’re not really a Motor Head.