Ford will offer stop-start as a $295 option on the 2013 Fusion.

You come to a stoplight and press the brake pedal and the car suddenly seems surprisingly quiet.  As the light turns green and you move your foot back to the throttle you hear a brief rumble from under the hood and pull forward as if nothing had happened.  That’s because the vehicle’s stop-start system worked as it was supposed to, briefly powering down your engine instead of idling in order to save gas.

The technology first appeared on the original Honda Insight and Toyota Prius hybrids, a dozen years ago, and is now a standard feature on the growing number of gas-electric vehicles offered in the U.S. market. But a number of manufacturers are starting to add the fuel-saving technology to their more conventional car, truck and crossover lines.

In fact, a new study by the Lux Research estimates stop-start will be offered on as many as 8 million vehicles sold in the U.S. by 2017 – and some forecasters believe it will be even more common in Europe and Japan, where fuel costs justify the technology’s modest price premium.

The technology isn’t entirely new.  In fact, some heavy truck manufacturers were offering it nearly 30 years ago under the name “idle stop,” and it has become essential for truckers who face increasing restrictions on idling their engines in urban centers. But the technology has only recently migrated to passenger vehicle applications.

“Engine stop-start isn’t a brand new technology, but the latest systems benefit from significant advances made in the last few years,” said John Nielsen, AAA’s Director of Automotive Engineering and Repair. “This technology is only going to gain momentum as vehicle manufactures work to meet the more stringent Corporate Average Fuel Economy (CAFE) standards set for 2016.”

Stop-start is a generic name for a variety of technologies that all do essentially the same thing.  When a motorist comes to a stoplight, for example, or is waiting in a fast food or bank line the engine automatically shuts off.  When the driver’s foot lifts off the brake the engine instantly restarts.  Some of the systems are so subtle motorists might not even be aware anything has happened.

Motorists have long been advised to shut their engines off, rather than idle.  Several studies have indicated that shutting the engine off for as little as 30 seconds will save a measurable amount of gas.  But drivers typically avoid that advice, especially if they’re stopped only briefly.  Stop-start does it for them.  And AAA estimates the technology can improve a vehicle’s fuel economy by as much as 12%.

That figure is a matter of debate.  “Today’s vehicle’s idle at such a low speed they don’t burn a lot of fuel,” cautions John Krafcik, a veteran automotive engineer who now serves as CEO of Hyundai Motor America.  Nonetheless, most experts agree that the figure is at least a 5% savings and can run significantly higher for those who spend a lot of time in stop-and-go urban driving.

In fact, stop-start technology is one of the ways hybrid vehicles like the Prius and the latest version of the Honda Insight deliver such good fuel economy.  The technology is standard issue with all hybrids now on the road but it is starting to show up on a wide range of conventional vehicles, as well.

Among the models that now offer the feature are economy models like the Kia Soul, as well as BMW’s sporty 3-Series and the luxurious Porsche Panamera.  Even more products are set to debut the feature in 2013, including the all-new versions of the Ford Fusion and the V-6 version of Chrysler’s Ram 1500 pickup.

Some products – typically those at the high end, like the Panamera, are making stop-start standard issue.  The Ford Fusion’s system will be offered as a $295 option.

Ford estimates the technology will save anywhere from 3.5% to 10% of an owner’s annual fuel bill.  For its part, AAA estimates the average American will see a $167-a-year reduction in fuel costs with a vehicle offering stop-start.  That’s based on a vehicle getting 20 mpg and being driven 12,000 miles a year using gas at $3.75 a gallon.  Clock 15,000 miles – as a significant number of Americans do – or pay $4.00 and up and the savings will grow.

There are a few potential downsides to the technology.  The biggest challenge for a manufacturer is ensuring that a stop-start system operates quickly, quietly and imperceptibly.  The Prius system has been praised for its seamless operation but BMW has taken some hits for its somewhat abrupt operation.

Part of the problem is that not all stop-start systems are the same.  Some simply rely on a beefed-up version of a conventional engine starter.  Others adopt a more robust – but notably more expensive component that combines the function of a starter motor and a generator.

Meanwhile, in extreme weather conditions that might require plenty of heat or cooling, the feature could degrade the performance of a vehicle’s HVAC system – though most stop-start systems are programmed to keep running until the cabin has reached a comfortable temperature.

The technology is likely to be more expensive to replace, though most makers claim they have designed their stop-start technology to last the life of the vehicle – longer than the typical, conventional starter.

Stop-start systems have become increasingly common in Japan and, especially, in Europe where fuel costs as much as $8 a gallon.  The lower price of fuel – which means it takes longer to recoup the savings from a stop-start system – delayed the U.S. introduction, as did the initial reluctance of the EPA which provides the mileage ratings for individual vehicles sold in this country.

But the federal agency recently began giving manufacturers credit for offering stop-start and with fuel economy now the single biggest concern for American car buyers you can expect to see it show up in more and more new models over the next few years.