The term "hybrid" means many things, but to most Americans, it's a car like a Toyota Prius driven by a combination of engine power and electricity. Usually it can run a short distance, up to a mile or two, on electricity alone.

But there's a more basic type of "micro-hybrid" that's actually the simplest way to use stored electricity. It does just one thing: quickly, automatically restarts an engine that's switched itelf off at a stoplight to save fuel. Hence, it's more commonly called "idle-stop".

These systems are spreading quickly throughout cars sold in Europe and Asia. Many carmakers, including BMW, Mazda, Mercedes-Benz, Mini, Citroen, and Peugeot, now fit idle-stop systems or plan to do so. Soon, idle-stop may appear on several million cars a year as it becomes standard across entire lines, especially in regions with explicit limits on vehicle emissions of CO2. Nost notably, that means Europe.

Stop-start systems can substantially cut fuel consumption and air pollution from idling vehicles, especially in crowded city centers. Parts maker Valeo, which supplies stop-start systems to PSA Peugeot Citroen, estimates that in cities, cars spend up to one-third of their time idling at a standstill. China, with its enormous traffic jams and horrendous air pollution, might benefit most from inexpensive and universal stop-start systems.

2010 Mazda3

2010 Mazda3

But we won't see them in North America. Our traffic patterns are different; believe it or not, we don't spend enough time in stop-and-go urban traffic to offer a payback. Take the idle-stop system that Mazda offers on Asian and European models of the new 2010 Mazda3, for instance. The company says flat out that it only makes sense in "extremely congested" areas. Like Tokyo, perhaps. Or Beijing.

These systems are pretty limited: The electricity stored to restart the engine doesn’t power the vehicle itself. And battery recharging is provided solely from engine power, not from regenerative brakes that blend with the regular brakes. It's composed only of an energy storage device—usually an enhanced, high-output 12-Volt battery—and a beefed-up starter motor that also acts as a generator.

The car’s electronic engine-control unit shuts down the engine on deceleration, or when the car has come to a stop. As soon as the driver puts in the clutch, lifts off the brake pedal, or moves the shift lever, the battery turns the starter motor, which starts the engine.

The closest thing to an idle-stop system is the more capable mild hybrids offered by Honda and General Motors. They are the 2009 Honda Civic Hybrid, 2010 Honda Insight, 2009 Saturn Vue Hybrid, 2009 Saturn Aura Hybrid, and 2009 Chevrolet Malibu Hybrid.

They too shut off their engines when the car stops. But each also has a separate battery pack and regenerative brakes--much pricier--and the electric motor blends power with the engine to give the car more kick on the road.

[SOURCE: Detroit Free Press; photo of Beijing smog by Flickr user michaelhenley]