Do you really save gas when you stop your car at stoplights? We looked into it.

Imagine sitting at a stoplight, the rumbling of your car’s engine beneath you.

Suddenly it stops, and your car shuts off. When you take your foot off the brake, the engine cranks back up again. In the car industry it’s called a start-stop system.

“Originally it was on hybrids, but now has moved into more of the mainstream of vehicles as a way to address fuel economy and exhaust issues,” said AAA spokeswoman Georjeane L. Blumling.

But does it really save gas? And can you get the same benefits by just turning off your traditional engine at stoplights?

A curious reader wrote in to The Pilot’s Glad You Asked initiative to find out.

Jim Bennett, who owns Carmasters Automotive in Norfolk, said a friend of his had a Malibu that helped him save 20% in fuel.

It depends on how someone is driving the vehicle, he said.

When asked if the start-stop systems are effective at saving fuel, he said it depends on who’s driving the cars and how. Someone who’s sitting in traffic for a long period of time will probably save more than someone who’s stopped for a few minutes or seconds.

“They get to the light and they stop, and as soon as they stop, it turns green,” he said. “If it’s not stopped very long, they’re going to notice probably a very little difference.”

But there are some downsides to the systems, AAA said, like the high cost of the batteries needed to run them. It has gone down over the years, however. A 2011 hybrid replacement battery would have been $7,000 or more, but they cost half that for replacement now.

Bennett knows a lot about the four types of hybrid cars, which all use some combination of gasoline engines and electricity.

“For a while, we owned a Toyota Prius that we let our customers drive when we were working on their cars until a customer insisted on buying it,” Bennett said.

He also said drivers will see the biggest fuel savings with a full hybrid; the most common type is the Prius.

As for the folks who manually stop and start their cars at red lights, they may save some fuel, but Bennett doesn’t recommend it. You risk damaging parts that your car uses to start.

Start-stop systems use an alternator to start the car with a belt. Traditional cars use dry gear-to-gear contact, Bennett said. A circular plate called a flywheel has an outer ring with gear teeth that rotates and meshes against the starter gear to crank up the engine.

It’s a very dry process, and if done repeatedly, it can damage the car parts, he said.

“A starter or flywheel that might last hundreds of thousands of miles, you could wear out in a fraction of the time,” he said. “If the car isn’t designed for it and you’re doing that on your own, you might have short-term gains but long-term losses."

Cars designed to shut on and off automatically are made with electric circulating cooling pumps and electric-driven A/C compressors, he said.

"When you come to a stop, you still have some heating and cooling to keep the passengers comfortable and you keep your windows clear and defrosted,” Bennett said.

“If you’re completely shutting the engine off and it’s not designed as a hybrid, you’re going to lose those features.”

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