Seven years ago, Volkswagen revealed the tiny 1L Concept, touted as the most fuel-efficient car in the world thanks to its 0.3l one-cylinder diesel engine and astronomical 238-mpg capability. Since then, VW has continued to kick around the idea and has now revealed an updated model targeted for production in 2013.
Dubbed the L1 Concept, the tandem two-seater is all grown up in every sense of the phrase. The L1 is larger, heavier, faster, and more powerful than the 1L, and its efficiency has suffered for it, though few will likely complain about achieving 158 mpg.
The first thing to grow up was the drivetrain. The original 1L featured a 0.3l one-cylinder diesel engine making 8.4 horsepower and mated to a six-speed automated manual transmission. The L1, though, sports a larger 0.8l two-cylinder common-rail turbodiesel derived from Volkswagen's new 1.6-liter four-cylinder turbodiesel. Mounted behind the rear seat, the 800cc engine produces 27 horses in Eco Mode and a sprightly 29 horses in Sport Mode, both at 4000 rpm. In either mode, it churns out 74 pound-feet of torque at 1900 rpm and, with Sport Mode engaged, will hit a top speed of 99 mph -- considerably faster than the 1L's 75-mph top speed.  
Unlike the 1L, though, the L1 isn't only diesel-powered. Sandwiched between the diesel engine and a seven-speed DSG dual-clutch gearbox, which is optimized for fuel economy, is the E-motor, a 14-horse electric motor. Powered by a lithium-ion battery mounted in the front of the vehicle and regulated by a 130-volt control module, the E-motor can boost performance by adding up to 40-percent more torque at any speed and can move the L1 under electrical power only for short distances. The E-motor also acts as the starter for the diesel engine and as a generator to recapture braking energy.
Altogether, the diesel engine, E-motor and DSG help the L1 achieve an impressive 158 mpg, giving it a range of 416 miles on its 2.6-gallon tank. It might take a bit longer to get there, though, as 62 mph is reached in a leisurely 14.3 seconds.
As with the drivetrain, the rest of the L1 has also been upsized compared with the 1L. Dimensionally, the L1 has grown in length and height, though it has slimmed down a bit in the middle. At 150.1 inches long and 45 inches tall, it's 13.5 inches longer and 5.5 inches taller than the original. Across the middle, however, the L1 has trimmed two inches and slips through the air at only 47.2 inches wide. The extra size has, predictably, had an ill effect on the 1L's incredible 0.159 coefficent of drag. The L1 now pushes through the air with a Cd of 0.195, which is still far better than the Prius' 0.25, which seems portly by comparison.
The increased girth also takes its toll on curb weight, which is up 200 pounds over the original concept. While it may not seem like much, for a car that now weighs just 838 pounds, it's an enormous difference. Like the original, the L1 uses Carbon-Fiber Reinforced Plastic (CFRP) for nearly all the vehicle's structure and body panels. The monocoque chassis also uses aluminum reinforcements, particularly in the sides, for crash safety. VW says that, in the intervening years between the 1L and the L1, they've developed a process for creating CFRP that significantly reduces the cost and makes it far easier to mass produce.
With efficiency as its ultimate goal, the L1's funky bodywork is all functional. Gone is the round, bubbly shape of the old 1L, and in its place is a vehicle that looks like a cross between the cockpit section of a fighter jet and an original Honda Insight. To keep the L1 as aerodynamic as possible, the radiators have been moved to the rear of the car, negating the need for a grille in front. Cooling the engine is achieved by opening special ducts in the bodywork that direct air to the radiators, but the ducts are opened and the water pumps activated only when the diesel engine and/or electric motor -- they're on separate cooling circuits -- need to be cooled down to reduce aerodynamic drag and drag on the engine for maximum efficiency.
Meanwhile, tandem seating keeps the L1 as narrow as possible and a Kamm-back rear end maximizes aerodynamic efficiency while minimizing overall length and weight. Covered rear wheels streamline airflow along the sides of the car while a smooth underbody does the same for airflow under the car. LED head- and taillights reduce weight and electrical draw and thus engine load while skinny Michelin Energy Saver tires, sized 95/60 R16 in front and 115/70 R16 in rear, minimize rolling resistance.
VW carries on its minimalist strategy inside the cockpit as well. Rather than doors, a single canopy cover is hinged on one side of the car and opens at the touch of a button to allow occupants to climb into the vehicle. Inside, the front seat is made of CFRP with aluminum supports while the rear seat is molded right into the CFRP monocoque chassis, as is the instrument panel. Traditional mirrors are replaced by cameras that display on an energy-sipping OLED screen behind the steering wheel, along with other instruments. Volkswagen's Park Distance Control aids in parking the vehicle.
Driving the L1 should be fairly easy. Push the starter button next to the steering wheel, then twist it over to Drive, which automatically releases the electronic parking brake. Climate controls are accessed via a touchscreen while the onboard computer, navigation, and entertainment systems are accessed through buttons on the steering wheel. Travelers are advised to pack light, though, as the L1's trunk offers only 1.8 cubic feet of storage space. In the event of an accident, occupants are protected by the rigid CFRP shell with its aluminum reinforcements as well as a steering wheel-mounted airbag and side-curtain airbags on both sides of the canopy.
If and when the Volkswagen L1 reaches production, it will likely be found in cities where its hybrid system can best be utilized. Offering more than three times the mileage of today's best Prius, the L1 will be attractive to those willing to give up a great deal of practicality for unheard of fuel economy. At this point, Volkswagen isn't even talking pricing, production volume, or markets, so whether or not the "1-Liter Car" ever makes the leap from concept to reality remains to be seen.

Volkswagen L1 Concept