The turbo compound was developed in the 1940's as a way to increase the power output of large piston engines.

The only production engine to eventually use turbo compounding was the Wright R-3350-32W. These engines were installed on Lockheed L-1049 Super Constellations and Douglas DC-7C's. They were finicky and hard to maintain properly but when well maintained they were good engines.

The "turbo" part of turbo compounding works just like a turbocharger in that there is a turbine in the flow of the engine's exhaust, Wright called these "PRT's" or "Power Recovery Turbines." The pressure from the expansion of the exhaust gas turned the turbine which was connected by a torque converter and transmission back to the engine crankshaft. Each R-3350 had three PRT's. R-3350 turbo compound engines were able to convert 20 to 30 percent of otherwise wasted heat energy into horsepower. Where a normal turbosupercharged R-3350 could produce 2,800 horsepower, the R-3350-32W turbo compound engine produced 3,500 horsepower.

Today some European truck manufacturers are claiming this as a revolutionary advance... but like most piston engine technologies it's been around the block a few times.