DETROIT—The old-fashioned fan belt is losing its grip.

The familiar toothed rubber belt, a staple of car engines for nearly a century, is driving fewer mechanical components as electricity takes over more functions under the hood.

The belt, originally dubbed the fan belt because it connected the crankshaft to the engine's cooling fan, now typically is called the accessory drive belt.

Not only has the number of accessory drive belts decreased from as many as four on some 1970s and "80s engines to just one today, but so has the length.

The Chevrolet Volt is down to one small belt that drives only the water pump.

Engineers have been replacing belt-driven accessories, such as the power steering and water pumps, with more energy efficient, electrically driven components.

Belt-driven systems typically draw power constantly. Electric systems use energy only when needed.

Other parts that are candidates for removal from accessory belts are the air conditioner and alternator, and in diesel engines a vacuum pump.

Three 2014 gasoline-electric hybrids—the Toyota Prius, Ford C-Max and Ford Fusion Hybrid—have no belts and no components bolted to the front of the engine.

More beltless engines are coming as auto makers shift belt-driven accessories to electricity.

The main reason for the disappearing belts: higher fuel economy.

Auto makers are rushing to meet 54.5 mpg fuel economy standards by the 2025 model year.

"We are being asked to do whatever we can to improve fuel economy," said Scott Willis, Ford Motor Co.'s North American technical specialist for front engine accessory drives.

Because today's serpentine fan belt runs under high tension on a series of rollers and pulleys on the front of the engine, it generates friction, which lowers fuel economy.

A typical Mercedes-Benz V-8 engine, for example, has one long serpentine belt that turns the water pump, alternator, air conditioner and power steering pump. It snakes around 10 pulleys, rollers and tensioners.

"Everywhere the belt touches something and bends and stretches, you have (friction) losses.

"More pulleys and more tension mean more losses," Willis said.

Engine designers can remove the alternator from the belt when the starter and alternator are merged into one unit—called an integrated starter generator—and moved to the back of the engine.

Some hybrids, such as Honda's Civic Hybrid, Insight and CR-Z, use an integrated starter generator, which is sandwiched between the engine and transmission.

Systems made for hybrids, such as the integrated starter generator, could be used in regular autos to spread development costs over higher volumes.

Japanese supplier Denso Corp. has in production an electric air conditioning compressor that is used on Ford and Toyota hybrids.

General Motors, Ford and other auto makers are migrating to electric power steering, which takes the belt-driven hydraulic pump off the engine.

Auto makers are using electrically driven water pumps not just to save energy but also to warm the engine quicker, which reduces emissions.

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