Software Looks at the Road Ahead to Boost Hybrid-Car Efficiency

Algorithm uses trip plan to maximize the use of battery power

PHOTO: David Lewis/iStockphoto

3 February 2009—Drivers use all manner of data these days to travel efficiently, and vehicles should follow their lead, according to University of Wisconsin–Milwaukee mechatronics expert Yaoyu Li. He predicts that vehicles privy to data in the latest GPS-enabled electronic navigators—which download real-time traffic data to update route suggestions on the fly—will provide substantial fuel savings in the decades to come.

Li has developed control algorithms that use route and traffic data to allow hybrid vehicles to plan how and when to use stored battery power so as to burn as little gasoline as possible. He hopes to enhance the plug-in hybrid vehicles already in development at major automakers, such as a grid-chargeable Prius that Toyota plans to lease starting later this year, or the Chevy Volt that General Motors promises for late 2010.

The idea has merit, according to Tom Robinson, senior manager, control and electronics, at automotive-systems supplier Ricardo, in Shoreham-by-Sea, England. ”If you know what’s forthcoming, you can inform vehicle systems to operate more effectively,” says Robinson, whose company has worked on similar algorithms for conventional hybrids.

An uninformed plug-in is almost certain to discharge its battery power either too quickly or too slowly. If it simply uses the battery until it is discharged, it will lack an electric option for later stop-and-go situations where running the internal combustion engine is inefficient. Alternatively, if the plug-in acts like a conventional hybrid and lives in the moment, blending its electric and gasoline energy based on the driving conditions that second, it is likely to arrive at its destination with leftover battery charge. Either way, the plug-in will have consumed more gasoline than necessary.

Li’s algorithms use data from electronic navigators to optimize the mix of combustion and electric propulsion to suit the trip. First, one algorithm cuts the driver’s chosen route into segments and, based on traffic data and the plug-in’s current state of charge, predicts how the vehicle should balance its use of electricity and gasoline in each segment. Li likens this macroscale algorithm to his family’s monthly spending plans for the year ahead. A microscale algorithm then takes over to make en route adjustments, much as Li might adjust his spending if friends pay a surprise visit to Milwaukee and blow his budget. ”As the vehicle approaches the next route segment, I use my current state of charge as a start point to solve a new optimization problem. I’m trying to force my actual expenditure toward my preplanned budget,” explains Li.

By last year, Li’s dynamic programming was generating results good enough to win support from Honda Motor Co., which enabled the team to rewrite his code for practical onboard use. Optimization of a 27-kilometer trip (his old commute from the suburbs to his office in Milwaukee) took 4.5 hours on his dual-core Pentium 4 desktop in 2007. The revamped code crunches the same route in under four minutes. And that’s in a Windows-based mathematics package. If you run the program in a vehicle controller, Li predicts the same algorithms will plan out most routes by the time you leave your driveway.

A more nimble program also means that the system can be more dynamic, incorporating incoming traffic data or impromptu route changes. ”Let’s say I decide to exit the freeway early and take a local shortcut. Our algorithm is fast enough to foresee the possible route I would take and adjust the power-splitting plan accordingly,” says Li.

Li’s simulations suggest that giving a plug-in SUV a view of the road ahead could deliver 20 percent better fuel efficiency. ”Actual fuel economy will depend on many factors, but you can see the potential there,” says Li.

Ricardo’s Robinson projects up to 10 percent fuel savings from trip-smart algorithms it developed for a conventional hybrid—the Ford Escape Hybrid SUV—and agrees that the plug-in could benefit even more. However, he cautions that only by accounting for the carbon footprint of the electricity used to charge the plug-in can one determine if its greater fuel savings will do the environment a favor.

Li’s next step is testing his dynamic programming on a real plug-in. With car sales down, getting research funds from automakers such as Honda is hardly assured. Then again, says Li, a cash-strapped economy favors plug-in vehicles with smaller, cheaper batteries, such as Toyota’s plug-in Prius prototypes, whose 13 ampere-hours of charge capacity provide barely 10 km of all-electric travel. And in those cars, optimizing algorithms matter more.

This entrepreneurial engineer has another pitch for automakers: Move his software to the sales floor. His speedy algorithms could help sell customized plug-ins based on a prospective buyer’s daily commute. ”The dealer can calculate payback curves for different battery sizes based on the commute and the current gas price,” says Li. ”We have already illustrated that to Honda.”

About the Author

Paris-based journalist Peter Fairley has tracked the energy story from the coalfields of Inner Mongolia to the powerless villages of Bolivia’s Cordillera Real. He’s the energy editor for IEEE Spectrum Online’s blog EnergyWiseand covers electric power and sustainable transportation for IEEE Spectrum. In the January 2009 issue, he explained how and why Europe is starting to replace its old wind farms.

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