Hyper-efficient VW XL1 drives to production

  • 04-Mar-2013 03:38 EST
VW XL1 2-13 aero front.jpg

A Cd of 0.189 makes the VW XL1 the most aerodynamically efficient production car in the world. The vehicle is just 1153 mm (45.4 in) tall. Overall vehicle length is 3888 mm (153.1 in). In pure electric mode, the car needs less than 0.1 kW·h to cover 1 km (0.6 mi). 

Settling into the tight cockpit confines of the production-ready Volkswagen XL1 is to experience a step-change in automotive engineering and design. After a gestation period as long as that of a stealth aircraft, the “series” production XL1 is potentially as significant as anything the automotive industry has ever produced or the public has seen—as the 2013 Geneva Motor Show beginning this week will prove.

But a sense of proportion is required. Production in XL1 terms is two a week to an initial maximum of 50 units. To get that into focus, worldwide VW is building on average two Golfs in little more than a couple of minutes of every working day and 18,224 a week.

However, for the halo model, technology demonstrator, and attention getter that the XL1 may really be, the figures that emerge from the integration of its advanced materials, mechanical and electronic technologies, and manufacturing processes make for an extraordinary achievement.

A two-seat plug-in parallel hybrid powered by a two-cylinder, 0.8-L 35-kW (47-hp) diesel engine and a 20-kW electric motor, the CFRP (carbon fiber reinforced polymer) bodied XL1 has a curb (unladen) mass of a mere 795 kg (1753 lb)—the monocoque is only 89 kg (196 lb), the result of a rigorous mass-reduction campaign. It is the most aerodynamic production car ever, with a Cd of 0.189.

The integration of its technologies delivers a combined fuel consumption of 0.9 L/100 km, CO2 emissions of 21 g/km, an electronically limited maximum speed (and cruising speed) of 160 km/h, 0-100 km/h (0-62 mph) time of 12.7 s, and a minimum 50-km (31-mi) e-drive range with a total range of more than 500 km (310 mi). In driving mode and to maintain the battery level without a plug-in boost, consumption is 2.1 L/100 km and emissions 70 g/km.

Safety elements include an expected EuroNCAP five-star rating. The car’s styling separates it from everything else on the road and taken all together it has no peer.

Compressed technology

At a very small but high-level media event at the former Karmann facility at Osnabrück, Germany, AEI was given access not only to the car and its advanced "compressed" technology but also to its production facility. Hand-built, the introductory limit of 50 examples could burgeon to give it a more realistic claim to series production status but VW is loath to give a specific figure.

The gestation period of the production XL1 has been, to say the least, protracted. Conceived as a millennium project by Dr. Ferdinand Piëch when Chairman of the VW’s Board of Management (now Chairman of the Supervisory Board of VW AG), it was first seen in 2002 as the “one liter” (in fuel consumption terms) car—a pure concept with tandem seats, rear wheels so close together it looked like a three-wheeler, and powered by a one-cylinder 0.3-L 9.3-kW (12.5-hp) diesel engine. Piëch drove it from Wolfsburg to Hamburg to demonstrate that it worked.

But after this, the project had languished until 2007 and the arrival of Dr. Ulrich Hackenberg from Audi to become VW’s Board Member for Development, which equated to engineering boss. Under the aegis of VW Chairman, Dr. Martin Winterkorn, he put it back on the priority list, deciding that it formed the basis of a viable solution. A small team was formed involving about 50 specialists—and the result is today’s production LX1.

Advanced RTM

The pause in the project had allowed development of CFRP—and RTM (resin transfer molding) processes, Hackenberg told AEI: “I took the idea and started to make a migration of the original concept into a new one. A gateway was to bring CFRP technology for the monocoque into a process which is able to make larger volume at reasonable costs using what we term Advanced RTM. [A one-piece injection molded part, the whole body, dressed, has a total mass of 230 kg (507 lb).] Our car’s monocoque is one shell, not pre-pregs. We use a single injection, introducing the resin to all areas. It is much faster than a lay-up, but we have had to invest in large, complicated tools, which are expensive. But the most difficult thing about the project was the integration of all the technologies: to make the whole car.”

By 2009, a conceptual study of a hybrid version was completed and shown publicly, fitted with a two-cylinder diesel and small electric motor. And by 2011 this had been further developed. “Now we can have small-volume series production,” said Hackenberg.

The spaceframe car, with an overall torsional rigidity at more than 30,000 N·m (22,100 lb·ft) per degree, has upward swinging doors and side-by-side seats, the passenger’s slightly staggered toward the rear to provide sufficient shoulder room within the very slim 1665-mm (65.6-in) body cross section. The two-cylinder diesel with balancer shaft engine—half a production VW 1.6-L TDI—is placed just ahead of the rear wheels and drives through a seven-speed DSG gearbox with magnesium housing; in this application, the DSG has a triple clutch to integrate with the electric motor. It has the ability to cope with a motor output of up to 100 kW (134 hp).

A 120-L (4.2-ft³) capacity trunk is positioned behind the engine. The car’s 5.5-kW·h battery is positioned near the front axle to provide satisfactory fore/aft weight distribution. Power steering is deemed unnecessary.

Challenge of CFRP accuracy

Developing production processes for the XL1 has proved a huge challenge, said Friedrich Wegert, Head of Development at Osnabrück. “When we started we had particular challenges in achieving necessary accuracy. It took a year to achieve this as we worked to manage tolerances and build reproducible cars.”

Virtual design of tooling was insufficient; it was necessary to develop and try to optimize tools physically to create the required BIW (body-in-white) dimensional accuracy. “We have over 100 years’ experience of sheet metal for automotive production; for CFRP there is no satisfactory simulation," he said. "Our development was about how to build the tool to build the part. To find out what you can do you have to do it.”

Wegert explained that the manufacture of the XL1 involves a nine-stage process. The first sees the CFRP monocoque delivered from its Austrian supplier (that VW prefers not to name) and mounted to an assembly support plate, with parts positioned by fixtures. Components are joined by gluing, and the roof section is sited above the CFRP and “hovered” before being lowered to be bonded. The trunk pan (extreme rear of the car) is glued and screw-fastened. Other structural and exterior components (including rear crossmembers and front and rear side panels) are put in place via a sled fixture and also screw-fastened.

The car’s two forward-swing doors are produced with aluminum side impact reinforcements in a special tool and fitted to associated body elements. The accent is on absolutely minimal production tolerances; steel or aluminum can be reshaped and trimmed for detail fits, but CFRP cannot, said Wegert. CFRP thickness varies from 2 mm (0.08 in) to 6 mm (0.24 in) at the A-pillar. Epoxy and four types of polyurethane glue are used, including high elasticity glue in specific areas.

Opening doors with Lamborghini

The third manufacturing stage involves the monocoque assembled with all parts, including doors, hood, trunk lid, and front bumper. VW worked with sister VW Group company Lamborghini on door fit; later in the assembly sequence, they get explosive bolts to be triggered in the event of an accident to facilitate door opening.

Painting involves the surface treatment of 32 exterior skin parts. During the RTM process, a resin film (fleece layer) is used as a topcoat. To save weight, paint is only 100 microns (100 µm) thick but gives a Class-A finish. A matt pearl gray paint is used for the interior with a matt clearcoat on exposed CFRP areas including the door sills.

The next step in the manufacturing process is final assembly, where the front body section joins a prefabricated floorpan that carries double wishbone front suspension with a die-cast aluminum swivel bearing, CFRP anti-roll bar of 700 g (25 oz) (there is a similar rear component), and front ceramic brake discs (similar at rear) that are more than three times lighter than conventional brakes, aluminum ZF motorsport-type dampers, magnesium wheels of 3 kg (6.6 lb) each, and low-rolling-resistance Michelin tires (front 115/80R15, rear 145/55 R16). Further mountings at the front are the air-conditioner (it is also used to cool electronic components) and a regular 12-V battery, plus the hybrid system’s high-voltage battery.

Next added is the rear section, which is constituted mainly of the powertrain. The rear axle is die-cast aluminum.

The dashboard is of molded wood-fiber just 1.4 mm (0.06 in) thick and produced by what VW terms a “special pressing process.” It is dressed on line. The dashboard has a magnesium cross-beam and with all instruments has a mass less than 20 kg (44 lb).

As the car is completed, the swing doors are fitted. VW describes them as the most complex add-on components of the car’s body.

Polycarbonate moves forward

The car’s laminated lightweight windscreen comprises two glass layers separated by a plastic layer; total thickness is 3.2 mm (0.13 in). Side windows are lightweight polycarbonate (of Lexan with Exatec plasma coating by Sabic) in two sections, the lower opening via a manual winder.

It is said to be the first time polycarbonate has been used on a production car in Europe ahead of the B-pillar. Dedicated assembly fixtures are used to glue the windows in place.

The XL1 uses Visteon lighting. Rearview cameras (the car has neither exterior mirrors nor a rear window) are fitted at this stage, with only final systems checks to follow.

The scalable drivetrain will form the basis of systems for future conventional VW Group small models including the Up and some Audis, using two-, three-, or four-cylinder internal-combustion engines.

Audi will have its new A3 e-tron with 1.4-L TSFI four-cylinder gasoline/electric hybrid at Geneva.

From the XL1, through high-performance models, and on to purely practical compacts and superminis, plug-in technology is the way forward in the VW Group.

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