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What's a mild hybrid system?

Mild Hybrid Systems: Engine Assist
Electric motors in mild hybrids can't run the vehicle on their own, but they do help the gasoline-engine start and stop at the right times to save fuel.
Electric motors in mild hybrids can't run the vehicle on their own, but they do help the gasoline-engine start and stop at the right times to save fuel.
AP Photo/Kirsty Wigglesworth

In reality, mild hybrids don't have much in common with commercial hybrid cars at all, at least not the kind we generally think of. In fact, mild hybrids are much closer to conventional cars with gasoline engines rather than the ubiquitous gas-electric hybrid cars we hear so much about.

To define a mild hybrid, it helps to know the different degrees of hybridization. The most common hybrid vehicle is the full hybrid, or strong hybrid. These vehicles usually use a 30- to 70-kilowatt electric motor alongside a gasoline engine. The electric motor in a full hybrid will most likely be in use entire time the car is running and will use a large battery for power. Common examples of full hybrids are the Toyota Prius and the Ford Escape.

Mild hybrids, on the other hand, are on the other end of the spectrum. They're still classified as gasoline-electric vehicles, but the extent of how much -- or rather, how little -- the car uses the electric motor is what defines them as mild. The big difference between a mild hybrid and a full hybrid is that the electric motor in a mild hybrid cannot (and does not) actually propel the vehicle on its own. The gasoline engine in a mild hybrid is the piece of machinery doing all of the grunt work and the electric motor serves only to assist.

So, the electric motor in a mild hybrid is acting as a power booster. The real benefit of the mild hybrid system is that it saves fuel by shutting off the gasoline engine when the vehicle is stopped, braking or cruising. Also, the electric motor helps the gas engine restart with improved efficiency -- that is, much more efficient than say, a driver using the ignition to switch the engine on and off. Depending on the system, some mild hybrids can also capture mechanical energy during braking.

Although they don't have the same fuel efficiency as full hybrids, mild hybrids still offer increased savings at the pump when compared to a conventional gas-powered car. In fact, because they're not burning gas at certain points during your drive, a mild hybrid can improve fuel efficiency between 10 and 15 percent. Another positive side to mild hybrids is their low price tag: because they're not quite as sophisticated as a full hybrid, mild hybrids cost less to produce, and are therefore less expensive at the dealership. And although they're not as prevalent as full hybrids, several companies have produced mild hybrid cars, including BMW, Chevy, Honda and general Motors. So, mild hybrids are out there -- you just have to know where to look.

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