Low-price Kia Soul, Rio get idle-stop to boost mpg

Kia plans to add so-called idle-stop-go as an option on its Soul and Rio small cars this year, making them the least-expensive models on which the fuel-saving feature is available.

Mazda could be in the running for low-price idle-stop vehicles, but probably not as soon. It sells Mazda3s equipped with idle-stop -- which it calls iStop -- in Japan and Europe. The feature is under consideration for the North American market, but Mazda says it's made no decision and thus can't say when it might happen.

Toyota has used idle-stop on the European Yaris for several years, but not in the U.S.

Gasoline-electric hybrids have idle-stop, but it's rare on ordinary gas-engine machines. As you'd guess, it turns off the engine when the car's stopped awhile, saving fuel.

The engine restarts immediately when the driver's foot comes off the brake. The engineering trick has been to eliminate the shudders that accompany the gas engine's awakening. They're much like a hybrid's shimmies when it switches between electric and gasoline power.

Engineers say the seemingly minor vibration is maddeningly difficult to eliminate, both in a hybrid and a conventional model with idle-stop.

Still, the shiver is easier to mask in a hybrid because the electric motor can move the car away from a stop. And once the car's rolling, the driver is less likely to notice a slight vibration from the drivetrain. In non-hybrids, such as Soul and Rio, the gas engine has to restart and get the buggy rolling without the electric-motor buffer.

In Kia's case, its so-called ISG boosts mileage just 1 or 2 mpg, and will be priced $500 or $1,000, depending on the model.

Seems like a lot of money for a little mileage boost, but there might be no choice.

The Society of Automotive Engineers expects idle-stop to spread because it can improve mileage 3% to 8%. That's roughly the amount it helps the Kias. Though modest, the boost is considered significant at a time when car companies are turning over every possible rock to improve mileage even slightly as fuel economy regulations continue to tighten.