Robocars Love the Draft

Illustration: Randi Klett

Drafting—trailing behind a car to take aerodynamic advantage of its motion—is something novice drivers are taught never to do: if you get too close, it’s curtains.

That’s why it figures so prominently in discussions of autonomous cars, which never lose their concentration. And when such tirelessly attentive cars and trucks run together in platoons, the resulting fuel savings can be as big a selling point as the leisure you get from not having to pay attention to the road. Up ahead of you are other cars, similarly in thrall to the lead car. That lead car can be controlled by a professional driver, as in the case of a European experiment that concluded in 2012 (described in IEEE Spectrum by Erik Coelingh and Stefan Solyom, engineers at Volvo, which participated in the project). Of course, when fully autonomous driving finally arrives, that professional will lose his job, too.

Robots can draft safely because only they can keep their beady digital eyes on the bumper of the car in front and converse with that car’s electronic overlord, without for a second allowing the car-to-car gap to shrink by more than a few centimeters. The only humans who even try to do the same are tailgaters (Jerkus automobilus vulgaris) and professional athletes.

Was I the only one shocked less by the drug-taking of Lance Armstrong and his entourage of cyclists than by the fact that he had an entourage? And that all top-notch cyclists must, to have any chance of winning? In these platoons, or pelotons, the designated main cyclist travels with a bodyguard of windbreakers, who shift in and out of the lead position to shield him from the wind, thus saving his energy for the final stretch.  The race is not to the swift, indeed.

In that European platooning project, drivers could join the convoy or peel off from it at will, but so long as they were entrained they were free to disengage from all driving duties. That's why the system required not only longitudinal control, to mind the gap, but lateral control, to keep everyone in their proper lanes while following curves and maneuvering through traffic. It involved the sophisticated pooling of sensor data, managed by vehicle-to-vehicle communications.

But if you are willing to leave much of the driving in the hands of the guy behind the steering wheel, and all you want is to manage tight spacing for efficient drafting, then the solution to the problem is much easier and cheaper. And those two words are music to the ears of the truck fleet operators, who have been known to refuse to install air conditioning.

One such stripped-down platooning system is being studied in a project funded by the U.S. Federal Highway Administration, led by researchers at Auburn University, with the participation of Peloton Technology (a specialist in platooning technology), Peterbilt Trucks, Meritor-WABCO, and the American Transportation Research Institute. In a highway test not far from Salt Lake City, Utah, two Peterbilt 386 trucks—a leader and a follower—traveled at 103 kilometers/hour (64 mph) while maintaining an 11-meter gap. 

Results: the following truck saved 10 percent on fuel, the same as in the more elaborate platooning experiment conducted earlier in Europe. Greater savings are expected in future tests involving a narrower gap.

In the more distant future, when many, most or all cars and trucks are fully automatic, at least on the highway, the gains may be much greater. Not only will gaps be smaller; speeds will also be higher. And the importance of drafting rises exponentially dramatically with speed.

A tightly packed bunch of cars going at breakneck autobahn speeds could therefore save a great deal of fuel. And, of course, the ultimate savings will come not on the land but in the air.

Did you ever wonder why birds fly in v-shaped formations? And did you know that they alternate the lead position? It turns out that drafting is for the birds, as well. (Though, to be technical about it, the birds aren't just drafting; they're also synchronizing their flapping.)

Drafting's also for the planes. Fighter jets, it's true, need to stay white-knuckle close to wring much of an energy advantage, but big, fat transport planes are another story. In a recent test, the U.S. Air Force flew a pair of C-17 transports from California to Hawaii and back again and estimated that such practices, carried out routinely, could save the service US $10 million a year. 

(Yes, I know, to the Pentagon, that's small change. But remember, little things add up. Like removing a single olive from the salad served to passengers on American Airlines.)

Anyway, that C-17 that flew in the second position was able to reap the full fuel advantage while hanging back a kilometer or two. Such a distance would put very little strain on a pilot—and none at all on an autopilot.

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Cars That Think

IEEE Spectrum’s blog about the sensors, software, and systems that are making cars smarter, more entertaining, and ultimately, autonomous.
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