Car Tips | Drive Smart

Drive Smart Start-Stop Systems

Start-Stop Systems

By Peter Bohr | Illustration by Steven Salerno

Q: I’m about to buy a new car that comes with a standard stop-start feature—whether or not I want it. Do I want it?

When I started driving, I worried about the old clunkers I owned stalling at stoplights. That was back in the days before fancy things such as microprocessors and fuel injection tamed temperamental engines. So it seems ironic that automakers now are designing car engines to stall on purpose—the so-called stop-start feature.

Here’s how it works: When a car is motionless for several seconds in jammed-up traffic or at a traffic light, the stop-start feature automatically shuts off the idling engine and then immediately restarts it when the driver lifts his or her foot off the brake pedal.

It’s all about saving fuel. Full hybrid gasoline/electric cars (the Toyota Prius, for example) and mild hybrids (Chevrolet Malibu), as well as many conventional nonhybrid cars sold in Europe, have used the technology for years. And because automakers are under a U.S. government mandate to bring the average fuel economy of the nation’s passenger-vehicle fleet up to 54.5 mpg by 2025, the technology is also beginning to appear on conventional cars and light trucks sold in America.

How much fuel does it save? Last year, the Auto Club’s Automotive Research Center (ARC) performed tests using two conventional cars equipped with stop-start: a Ford Fusion and a Mercedes-Benz CLS550. ARC compared results with start-stop both enabled and disabled. Overall, there was a 5 to 7 percent improvement in fuel economy with stop-start, says Steve Mazor, ARC’s chief automotive engineer. At $2.50 a gallon, 20 mpg, and an annual mileage of 15,000, a 7 percent savings adds up to about $650 over five years.

Moreover, stop-start systems are planet-friendly because they reduce carbon dioxide (greenhouse gas) emissions. Carbon dioxide production is directly correlated to fuel consumption. ARC found a 5 to 8 percent reduction in carbon dioxide emissions with the stop-start systems enabled on the two test cars in their study.

Stop-start systems aren’t without challenges, however. First, all that stopping and starting taxes an engine’s starter motor and battery, requiring them to be more robust and, consequently, more expensive. (The extra cost is built into the price of the car.) You could also face higher costs if you need to replace the battery or starter down the road.

Second, when the engine shuts off, so does the car’s air conditioner. Hybrid vehicles avoid the problem by using an electric A/C compressor that operates off the hybrid’s large battery pack. But crank up the A/C on a hot day and the stop-start system on a conventional car may automatically disable itself to allow the engine to remain idling and the A/C to continue working. Or the driver can push a button and disable the feature. Either way, it defeats the purpose of stop-start.

Finally, there’s the annoyance factor. On some cars, the engine restarts with a noticeable jerk (on others, the restart is so smooth you barely notice). “Some automakers implement stop-start better than others,” Mazor says. So if you’re considering a car with a stop-start feature, take the car on a long test drive before you buy to be sure you can live with it.

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