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Where-will-we-get-more-power-placement-626x382Turbochargers successfully recycle some of that energy, but there are other recovery technologies carmakers haven't tapped. One that's especially promising is turbo compounding.

Like a turbocharger, the turbo-compound approach recovers waste-exhaust energy, but, instead of powering a compressor, the turbine wheel is connected to the crankshaft through a train of gears. Turbo compounding works with or without a conventional turbocharger upstream.

Just after World War II, the Wright R-3350 18-cylinder aircraft engine used three turbines to recover 20 percent of the exhaust energy in Douglas DC-7 and ­Lockheed Super Constellation airliners. Freightliner and Western Star semis powered by Detroit Diesel DD15 TC engines employ this technology today.

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To avoid the complexity and cost of turbine-to-crankshaft gearing, the turbo-compound approach alternatively can be used to drive a generator. Such a device is used in Formula 1 where it's called a "motor/generator unit-heat." Here the exhaust turbine, compressor wheel, and generator are all mounted on a common shaft. The recovered electrical energy powers the wheels out of a turn (through a second device called "motor/generator unit-kinetic"). The generator can also operate as a motor to preemptively spin the compressor to eliminate boost lag.

Production cars could use the electrical energy recovered by turbo compounding to eliminate engine-driven alternators. In hybrid applications, turbo compounding is an efficient way to charge batteries and to power electric-drive motors.

With electric assist expected to become commonplace in gas-powered cars, turbo compounding might be the technology that further solidifies automakers' current affinity for exhaust-driven turbines.