Technologue

“THE INTERNAL-COMBUSTION-POWERED AUTOMOBILE IS RIFE WITH OPPORTUNITIES FOR FREE (OR REALLY CHEAP) ENERGY HARVESTING UPON WHICH SCIENTIFIC ADVANCES ARE POISED TO BEGIN CAPITALIZING. ”

power4free

Generating hot-watts and jolt-volts

■ illustration doug fraser

THE SIREN SONG of the free

lunch, the gratis timeshare weekend, the complimentary hotel happy hour can be incredibly alluring, but each often comes with unsavory strings attached—aged buffet fare, a halitosis-scented sales pitch, watereddown well-brand booze. But the internalcombustion-powered automobile is rife with opportunities for free (or really cheap) energy harvesting upon which scientific advances are poised to begin capitalizing. You may know that - percent of the chemical energy in the fuel you burn wafts right out the tailpipe and radiator as waste heat. BMW has proposed using this heat to power a two-stage steam turbine capable of boosting fuel economy by  percent (Turbosteamer, “Technologue,” June ), but the mass and cost of the plumbing and hardware have yet to shrink to production-viable levels. Now some savvier new ways of harvesting heat—and even vibration—are coming to light.

The first employs shape-memory alloys of titanium and nickel (NiTinol) to drive a heat engine that spins a generator. Remember NiTinol from my June  column? It can be bent or stretched when cold and then returns to its original shape when heated. The simplified concept of the SMA heat engine dates back three decades and works like a bike-chain drive, with the chain made of SMA wire. Imagine the chain bathed in cool water on the bottom and heated by hot air on top. As the top of the chain constricts and the bottom stretches, the rear sprocket is pulled along. Make these sprockets slightly different diameters, force them to turn at the same speed, and the expanding and contracting NiTinol does the pedaling.

GM envisions SMA generators replacing vehicle alternators for a fuel economy boost of  percent. BMW is developing a thermoelectric material that generates electricity across a temperature differential via the Seebeck effect (the reverse of the Peltier effect used in some electric seat coolers). Employing this material on an engine’s exhaust-gas-recirculation cooler could generate  watts of energy (about half the accessory load in a  Series), for a two-percent fuel savings. Larger ones on the exhaust might one day power the entire electrical system. Using this material to electrically heat the catalyst, transmission fluid, oil, etc. could speed warmup and reduce friction for fuel savings of five percent or more. Both heat-to-electricity ideas are far more mass efficient than the Turbosteamer.

Meanwhile, Duke University engineers are at work on piezoelectric devices mounted to cantilevers that can generate electricity from vibration. You’ve heard of piezo fuel-injector pucks expanding to open a fuel nozzle when electricity is applied? These piezoelectric laminates produce electricity when they’re bent back and forth at a certain resonant frequency. In the past, that resonant frequency, or sweetspot in the vibration spectrum, has been too narrow to be of much use. But the Duke folks discovered that, by putting a magnet on the end of the cantilever oscillating between two other magnets that can be moved in or out along the cantilever, they can greatly expand the range of resonant frequencies at which the piezo materials produce electricity.

Duke’s Brian Mann says engine and suspension vibrations are ripe for harvesting, and while the milliwatts or s of milliwatts of piezo-power these things produce won’t extend hybrid range meaningfully, it could allow wireless powertrain/chassis sensors to be installed in hard-to-access locations (like inside tires) or elsewhere without the need for complex wiring harnesses. And, hey, in a .mpg world, every milliwatt counts. ■