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SANTA MONICA, Calif. In a crowded field of conventional sports cars, the BMW i8 is a high-tech wonder.

It offers all the performance you'd expect in a $135,700 sports car, but does it with tiny three-cylinder engine and fuel economy that is expected to shame a Toyota Prius hybrid.

The new BMW i8 plug-in hybrid, due in showrooms in September, is intended as a sports car for a new age, breaking through engineering and technological barriers.

While it seems exotic now, the lessons learned in building the i8 point the way to energy-efficient future for mainstream cars that combine power plants in new ways and use airplane-like lightweight construction with extensive use of aluminum and carbon-fiber.

'The combination of incredible styling, performance, and efficiency wrapped in carbon-fiber reinforced plastic is significant on many levels,' says Ron Cogan, publisher of the Green Car Journal, which follows advanced-car developments. 'It's a breakthrough.'

The i8 shows production-ready answers for the high-wire act that the auto industry must perform as it faces tightening government-fuel economy and carbon emission mandates. On one hand that is creating intense pressure to cut engine sizes and performance and sell smaller, lighter vehicles. On the other, it has to try to sell those cars to consumers who not only crave performance, but also space in a vehicle and higher safety standards.

The i8 shows how one automaker has leaned on advances in technology in meeting competing demands:

Power. The i8's three-cylinder engine displaces only 1.5 liters, about the size of the 1.4-liter, four-cylinder turbocharged engine in the popular Chevrolet Cruze compact sedan. But, while the Cruze engine generates 138 horsepower, the i8's engine has twin turbochargers and develops 231 horsepower.

That gas engine in the back of the car then is coupled to an electric motor in the front, good for another 131 horsepower, and a lithium ion battery in between for low center of gravity and weight balance. The result is an all-wheel drive car with with a combined maximum power and torque of 362 hp. and 420 lb. ft.

As a result, i8 can rocket from zero to more than 60 miles per hour in about 4.5 seconds, nearly the same as a Porsche Boxster.

Gas savings. As a plug-in hybrid, actual fuel economy will vary, based on how far owners drive on electricity only when they have a full charge in the battery pack, which can be recharged in 1.5 hours. While the car doesn't have a government rating yet, BMW hope for an overall mileage rating in the 60- to 70 mile-per-gallon range. A Prius averages 50 mpg.

The i8 can drive up to 23 miles on electricity alone at speeds up to 75 mph and drivers can choose from five settings that balance performance vs. fuel conservation. In hybrid mode, the i8 can go 310 miles between fill-ups, BMW says,

Aerodynamics. Designers turned the i8 into an aerodynamic wonder in a quest for extra battery range and also high-speed, sports-car performance. But it looks like a beauty, not a science project. The car was given a teardrop shape that tapers wind passing from the bow to the stern, but a port in the hood that passes it through the car to the gas engine in the rear.

Materials. The i8's body and 'passenger cell' is made of lightweight carbon fiber. One of the most impressive breakthroughs for the i8 and the sibling, all-electric i3 sedan -- was to be able to produce the material, long used in race cars, at a cost and in a volume for production cars. It produces the material in a venture with SGL Group in Moses Lake, Wash. at a plant that runs entirely on renewable energy, such as hydro and shapes it in Germany. The black fiber weave is on display to passengers every time they open the upward-tilting doors.

The i8's use of carbon fiber underscores how it is slowly becoming a mainstream material as makers find more efficient composites and ways to produce them. 'Carbon fiber is really poised to take off,' says Steve Russell, vice president for plastics for the American Chemistry Council.

BMW so proud of its use of the material that it commissioned Louis Vuitton to make a set of matching luggage to go with the i8. Sounds like a throwback to the grand touring cars of the 1930s, but the bags aren't leather, they're carbon fiber.

BMW may be among the first, but certainly isn't the only maker involved with plug-in sports cars. Porsche has its 918 supercar. Audi has talked about a 'e-tron' version of the R8 supercar.

But as BMW learned, creating one isn't a simple matter of sticking a hybrid powertrain in a conventional sports car.

In the case of the i8, BMW had to solve how to get its 3-cylinder gas engine to work in conjunction with the electric motor. The solution, says Carsten Breitfeld, director of the i8 program, was a big generator that not only could recharge the car's batteries, but give extra oomph to the gas engine. The gas engine has a six-speed automatic transmission, while the electric motor has an integrated two-stage automatic.

BMW also had to make compromises. The i8 carries 440 pounds of battery and electric motor and tops out at 3,285 pounds, about 200 more pounds than the Cruze. The interior is limited, partly because of the space taken up by its powertrain components. The space under the rear hatch, just behind the gas engine, won't hold much more than a couple of laptop bags. BMW spokesman Manfred Poschenrieder says i8 should be looked upon a 2-plus-2, with the back seats more for cargo than people.

But BMW doesn't sound particularly worried about finding buyers.

'Selling the car is going to a walk in the park,' says Henrik Wenders, head of product management for BMW's 'i' division it's unit for alternative cars. He says that none of i8's rivals offer 'the driver so many different facets, different characteristics' -- the ability to save gas around town, to go on a long trip without range worries or to put it through its sports-car paces on a track.

The car will be sold in 40 markets around the world as a true 'global player,' he says.

BMW i8 plug-in hybrid sports car

Seats: four
Engine: 1.5 liter twin-turbocharged three-cylinder, 231 horsepower
Electric motor: 96 kilowatt, paired with a lithium-ion battery, 131 horsepower
Performance: 0 to 60 miles per hour: 4.5 seconds
All-electric driving range: about 22 miles (then goes into hybrid mode)
Construction: Aluminum chassis and carbon-fiber body
Length: 15.4 feet
Width: 6.4 feet
Weight: 3,285 pounds
Price: $135,700 plus delivery fees

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