[Big Dave]

Compounding is an idea that has been tried a number of times.

It actually works quite well for aircraft engines. The reason that B-29s could hit nearly all of Japan was the super-efficient R-3320 engine. It was also very temperamental and probably more B-29s were lost to engine failures than Zeros.

Also the Napier Sabre was considered very efficient.

Airplane engines often cruise at a high percentage of rated power. Even running "Lindbergh lean" those R-3320s were running at 65% of rated power just to keep the heavily-laden planes at altitude.

But on cars, not so much. Especially cars driven in the low-power, low speed(efficient) part of the envelope. Most cars run around 10% of rated power. Lower fuel burn at lower perecentage of rated power leaves you with not much available energy.

My truck barely makes any boost (turbocharger = turbocompounded air compressor) at all at 70 MPH. At lower speeds the needle is super-glued to the peg. Very little compounded power being recovered.

For ground vehicles, you rapidly hit the point that all the weight & complexity of the compoinding exceeds the recouped energy.