Algorithms Will Let You Coast Your Way to a Lower Fuel Bill

Illustration: iStockphoto

Every small child on a bicycle knows that the payoff for pedaling hard up a hill is coasting down the other side. It's a trick also practiced by the hypermilers, a fanatical brotherhood of fuel-economy nuts who coast their cars, streamline every protuberance and jettison every bit of extraneous weight in order to save a drop of fuel.

All cars could benefit from coasting if only their cruise control systems possessed the right algorithms and knew the road's profile—its highs and lows. The savings could rise as high as 10 percent, says Hermann Koch-Groeber, a professor of automotive systems at the University of Heilbronn, in southern Germany. He and his graduate student, Jue Wang, have worked out algorithms [pdf] and modeled their effectiveness.

At first Koch-Groeber tried out his model by hand, by operating the clutch and throttle of the manual transmission in a 2012 Ford Focus sedan. "From my knowledge of the stretch of highway, I knew when it was proper to inaugurate the coasting phase, when I put it neutral," he says. "It's not that important to get the optimal point; it is a flat optimum, so if you initiate the coasting a bit earlier or later, it's not dramatically different."

Then he began automating the algorithm in a German-government-supported research project done jointly with Getrag, the world's largest supplier of passenger-car transmissions. Getrag provided the key ingredient: an automated gearbox. Unlike the more familiar automatic transmission, an automated gearbox uses direct friction rather than hydraulics to convey torque from the engine.

"That’s interesting because to automate coasting I need actuators that shift into neutral and open the clutch, and this has all the actuators already on," he says. "So I can use bits and bytes to initiate coasting—a huge benefit." 

He expects to have a prototype by March of 2015, in time to validate the idea if not to profit off it.  "I can tell you from my industry network and my former employer, Bosch, that there is quite a bit of work in that direction," he says. "This will be in series production in 2015."

You can coast strategically on flat terrain only if you're prepared to cycle between periods of acceleration and coasting, repeatedly. It's a dizzying ride that only a hypermiler could abide. That means you'll really only see savings when driving up and down hills in places that have a lot of them, like southern Germany. Elsewhere the improvement would be small.  "People may say, hah, I don’t care about a 5 percent savings," Koch-Groeber notes. "But the European Union and the United States have set limits to fuel consumption, there is a lot of effort to meet those goals, and so it's well worth doing, even if it only saves 5 percent."





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