I saw this system on a mercedes site and I have been thinking about it ever since. turbocompound The application of recovered exhaust energy directly to the crankshaft is very intriguing. I think this could be adapted for automotive use. the recovered energy could amount to (pie in the sky guess) 30-40hp and a good bump in FE.
A 16:1 gear reduction and a small torque converter would do the trick something akin to a giant air ratchet system of small pinion spiders and a sun outside then into a small torque converter (possibly running a viscous liquid of some sort) from the converter a further 4:1 reduction (small driving big and then once more to complete the 16:1 reduction. The trick it seems is the converter that takes the shock load off the turbine shaft and acts as a torque multiplier.
The old radial aircraft engines in the 1930-1940's did this. It was a supercharger/exhaust recovery system. Nowadays supercharger means gear/belt driven intake pressure system but back then it was directly linked to the crankshaft. They would make literally thousands of horsepower by doing this. Those engines only operated in a narrow power range so it wasn't difficult to develop a system that would work. But for a car that uses a very wide operating range I see the need for a torque converter.