Your car is eerily quiet at a stop light. You think it stalled. But, no, it is equipped with fuel-saving start-stop technology. Models as varied as the Kia Soul, Jaguar F-Type, and Ram 1500 V6 pickup are employing the system.

How It Works

“Start-stop technology shuts down the engine when it doesn’t need to be running, like at a stop light,” said Craig Rigby, vice president of production management and strategy at Johnson Controls, supplier of start-stop batteries and other auto technologies. “After traffic clears, and the driver lifts off the brake, the engine automatically re-starts.”

To work this magic, cars need high-performance batteries and more robust starters. Batteries must be capable of powering climate control systems, radios, and other accessories while the engine is not running. Starters also endure repeated stress with start-stop. More advanced systems employ regenerative braking to re-charge batteries during deceleration.

How much does it improve efficiency?

“Estimates range from 5 to 12 percent,” said Mike Calkins, manager of technical services at AAA. “But real-world savings vary significantly with driving conditions. Drivers who find the systems annoying and turn them off realize no savings. It will take a period of adjustment to get used to the engine shutting down at a stop.”

Urban drivers who stop often will realize more benefit than highway drivers.

What Comes Next

While the concept of shutting off the engine to save gas is simple, getting the EPA to agree is tougher.

“Stop-start systems can save fuel in real-world urban driving, but the federal test protocol used to generate EPA fuel economy ratings does not include sufficient driving conditions of that type for the systems to provide a significant difference in estimated economy,” Calkins said.

If automakers cannot get mpg credit, they will quickly drop start-stop, which AAA estimates adds $300 per vehicle. Favorable testing standards in Europe resulted in about 50 percent of vehicles and virtually every automaker having the system. Start-stop is expected to save 1.2 billion gallons of fuel globally during the next five years, Rigby said.

“Start-stop doesn’t require a driver to significantly change how they drive,” Rigby said. “Ultimate value is triggered by consumers saving gas.”