Volkswagen is redefining the automobile with the L1, a bullet-shaped diesel hybrid that weighs less than 900 pounds, gets an amazing 170 mpg and might see production within four years.

The L1 concept car unveiled at the Frankfurt auto show pushes the boundaries of vehicle design and draws more inspiration from gliders than conventional automobiles. The only question the company's engineers asked when designing the L1 was, "How would a car have to look and be built to consume as little energy as possible." Their answer was small, light and extremely aerodynamic. Those guidelines led to a car that requires just 1.38 liters of diesel fuel to go 100 kilometers.

Although the L1 is just a concept, VW says the tandem two-seater points the way toward a future model.

"Volkswagen is making a clear statement," the company said in a statement. "The L1 represents a step forward into the future with completely new technology and new design; revolutionary yet close to production readiness."

The L1 updates an idea Volkswagen has been kicking around since 2002, when Dr. Ferdinand Piëtch, then the chairman of the board, drove the 1-Liter microcar from Wolfsburg to Hamburg. The car drew its name from the fact it needed one liter of fuel to go 100 kilometers. The 1-Liter was a technological marvel built of carbon fiber and magnesium, but it was so ridiculously expensive VW knew it wasn't practical. Piëtch shelved the project, figuring it wouldn't be financially viable for another decade.

VW now thinks the car's time has come.

The L-1 is more refined than the 1-Liter we saw last year when Britain's Car magazine said VW had greenlighted the project. The drivetrain features a two-cylinder turbodiesel displacing just 800 cubic centimeters. It runs in two modes: "Eco," offering 20 kilowatts (26 horsepower); and "Sport," with 29 kilowatts (39 horsepower). Maximum torque is 74 pound-feet. The engine is mated to a 10 kilowatt electric motor, which VW integrated into a seven-speed DSG automatic gearbox.

Electric motors provide great torque, and VW says the motor in the L1 kicks in under hard acceleration to provide up to 40 percent more grunt. It also can propel the car on its own, though VW doesn't say how far the L1 will go on electricity alone. VW also didn't offer any specs on the lithium-ion battery, which gets a boost from regenerative braking.

Start-stop technology helps improve fuel economy. The fuel tank holds just 1.7 gallons, and VW says says the car gets 1.38 l/100 km. That works out to 170.4 mpg by our math. VW claims the L1 emits just 36 grams per kilometer of carbon dioxide. For the sake of comparison, the 2010 Toyota Prius emits 89 g/km.

The L1 looks unlike anything else on the road, which is exactly what VW had in mind.

"The design of the L1 redefines classic and aesthetic vehicle traits," Walter de'Silva, head of design for the VW Group, said in a statement. "Especially significant, of course, is how the nearly rocket-shaped lines catch one's attention. All of its moving parts are integrated so accurately that the body resembles a rocket or jet. It is a body that cuts through the wind with minimal aerodynamic resistance."

Indeed. The L1 has a claimed drag coefficient of just 0.195. That matches the General Motors EV1, which is the most aerodynamic production car ever built, but is slightly less aerodynamic than the first-gen 1-Liter.

A carbon-fiber monocoque and carbon fiber exterior skin, both developed with technology derived from Formula 1 and aerospace manufacturing, account for the L1's super flyweight 380 kilograms (837 pounds). The body alone weighs just 124 kilograms (273 pounds). The car is 12.5 feet long and 3.75 feet tall, making it about as low as a Lamborghini Murcielago. There are no doors, only a jet-like canopy that swings up from the left side. There's room for two, one behind the other, and everyone sits in carbon fiber seats. Instruments are laid out in a 180-degree radius in front of the driver, and there are no mirrors, only cameras that display images on organic light-emitting diode displays.

VW says the L1 is perfectly safe, with the driver and passenger safely ensconced in a carbon fiber safety cell and protected by head and side-curtain airbags. The front of the car features an aluminum crash structure.

The word out of Frankfurt, according to The New York Times, is VW will build a limited number of these next year. Officially, VW says we'll have to wait a little longer.

"2013 is the target year for this future dream car to become a reality," the company said in a statement.

Photo and renderings: Volkswagen