In less than ten years, the average consumer vehicle in the US will need to be about 50 percent more fuel efficient, and that will require big advances in vehicle design.
In gas-powered cars, the powertrain is the whole system that connects the engine to the wheels. The Volt’s hybrid electric powertrain includes the engine, battery and two motor-generators. In different driving circumstances, combinations of these components provide power to turn the wheels. It is also possible for the engine to send power to the battery via a motor-generator. Recharging on the fly is important because the engine and battery need to work together to provide enough power for acceleration.
The Volt—like Toyota’s Hybrid Highlander, Hybrid Camry and Prius—uses two sets of so-called planetary gears to connect up their power systems and wheels. This gear design offers good efficiency, strong torque and plenty of design flexibility.
Even limiting the system to three clutches—or points at which the powertrain elements can be connected or disconnected during operation—Xiaowu Zhang, a recent doctoral graduate in mechanical engineering, and his colleagues were left with million different ways to link up the Volt’s powertrain. They needed a quick way to assess which designs are worth a closer look.
They mathematically described each design and then ran an initial test to weed out any that couldn’t make if from zero to 60 mph in under 7.1 seconds. The most accurate way to assess powertrain designs takes about half an hour for each, but the team developed a simpler method so that it took a fraction of a second to estimate acceleration time. This cut the number of competing powertrains to less than 2,000.
The acceleration algorithm was quick and dirty, but accurate enough for the task. To assess fuel economy, they improved on another simplified algorithm so that it could take into account the different ways that powertrain components operate together. This brought the number of designs down to 49. This part of the assessment was 10,000 times faster than the most accurate method.
Fuel economy didn’t change by much, but zero-to-60 acceleration could fall by almost 2 seconds. Chevy could make the Volt almost as fast as a basic Tesla, or it could turn that gain into fuel economy by shrinking the motors. In 20 cases, the Volt would see no loss in performance by shrinking the motors, one by 10 percent and one by 20 percent.
Zhang emphasizes the importance of the method itself as automakers sift through billions of designs looking for the improvements that will take them closer to the lofty fuel economy goals. It’s impossible to run a full analysis on a million cars, but 20 is a different story.
Huei Peng, the Roger L. McCarthy Professor of mechanical engineering and director of the Mobility Transformation Center; Jing Sun, a professor of naval architecture and marine engineering; and Shengbo Eben Li, an associate professor of automotive engineering at Tsinghua University in Beijing, China, also contributed to this work.
The study, titled “Design of Multi-Mode Power Split Hybrid Vehicles—A Case Study on the Voltec Powertrain System,” is to be published in an upcoming issue of IEEE Transactions on Vehicular Technology.