TECH TALK: Turbo-Compounding

Is it “the next Big Thing” in energy recovery?

Car and Driver magazine recently proposed that "turbo-compounding is the next big thing in energy recovery."

We pull down any one of the standard internal combustion engine texts and see that the chemical energy of the fuel goes into three big channels: power to the crankshaft, heat rejection to coolant, and exhaust energy.

Turbochargers recover some of the exhaust’s blowdown energy—the outrush of hot exhaust gas at high velocity—and use it to drive a supercharger. But as we cruise up the freeway, we seldom need turbo boost. So why not do as Wright Aeronautical Corp. did with their mid-to-late 1950s TC-18 radial aircraft piston engines: let the exhaust spin one or more turbines, and then gear their power back to the crankshaft? This is called turbo-compounding, and the ‘TC’ in TC-18 stood for guess what.

Turbochargers were terribly exciting back when we could allow ourselves to enjoy acceleration, but in today’s world we, and automakers alike, stare in fascination as 2025’s coming 54.5 mpg Corporate Average Fuel Economy requirement rushes at us. Even Formula 1 is furiously recovering every scrap of energy, so as a Helmeted Hero downforces his/her way through a medium-speed corner on part-throttle, no wastgate opens to noisily dump energy not needed for the turbocharger. No siree, that excess exhaust turbine power spins a generator that industriously stores up power in ultra-capacitors, to be sent to the electric traction motor for full acceleration at the next opportunity.

This will work just fine in future hybrid autos, for they already have the battery and electric traction motor aboard to put that recovered turbine power to work. The same idea has been in use for years on giant marine diesels and some long-haul trucks; make every scrap of recoverable energy work for you (and for the stockholders)!

With current vehicle regulations, motorcycles are not now under mandate to improve fuel economy. If they were, a very effective first round of improvement would come from doing what cars have been doing for some years—providing more gearbox speeds (as many as nine in some autos) so that engine revs (and engine friction) are reduced in highway cruise, while keeping lower gears as they are.

Motorcycles are presently so tightly package that it’s not easy to figure out where future designers might put energy- recovery systems. Once upon a time, simplicity was thought a virtue.

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