Components of the XL1. Click to enlarge.

Conceptually, the XL1 represents the third evolutionary stage of Volkswagen’s 1-liter car strategy. Prof. Dr. Ferdinand Piëch, who is today Chairman of the Supervisory Board of Volkswagen AG, formulated the goal of bringing to market a production car that was practical in everyday use with fuel consumption of one liter per 100 km.

In the two-seat XL1, the developers successfully came up with a body concept which delivers more everyday utility than in the two previous prototypes. While the driver and passenger sat in a tandem arrangement for optimal aerodynamics in the L1, the 1-liter car presented in 2002 and in 2009 (earlier post), in the XL1 two occupants sit slightly offset, side by side, nearly as in a conventional vehicle.

XL1’s engine. Click to enlarge.

Powertrain. The powertrain consists of a 0.8-liter, two-cylinder diesel TDI engine (35 kW / 47 hp), E-motor (20 kW / 27 hp), 7-speed dual clutch gearbox (DSG) and lithium-ion battery. Offering 140 N·m of torque, the electric motor works as a booster to support the TDI engine (120 N·m of torque). Together, the TDI and E-motor deliver a maximum torque of 140 N·m and 51 kW in boosting mode. With a top speed of 160 km/h (99 mph), the XL1 can accelerate to 100 km/h in 12.7 seconds.

The entire hybrid unit is housed above the vehicle’s driven rear axle. The actual hybrid module with electric motor and clutch is positioned between the TDI and the 7-speed DSG; this module was integrated in the DSG transmission case in place of the usual flywheel.

The 5.5 kWh lithium-ion battery pack is integrated in the front section. Operating at 220 volts, the power electronics manage the flow of high voltage energy from and to the battery or E-motor and converts direct current to alternating current. The body electrical system of the XL1 is supplied with the necessary 12 Volts via a DC/DC converter and a small auxiliary battery.

In all electric mode, the TDI is decoupled from the drivetrain by disengaging a clutch, and is shut down. Meanwhile, the clutch on the gearbox side remains closed, so the DSG is fully engaged with the electric motor.

Restarting the TDI while driving uses “pulse starting”: the electric motor’s rotor is sped up and is very quickly coupled to the engine clutch. This accelerates the TDI to the required speed and starts it. The entire process takes place without any jolts, so the driver hardly notices the TDI engine restarting, Volkswagen says.

When the XL1 is braked, the E-motor operates as a generator that utilizes the braking energy to charge the battery (battery regeneration). In certain operating conditions, the load of the TDI engine can be shifted so that it operates at its most favorable efficiency level. The gears of the automatically shifting 7-speed DSG are also always selected with the aim of minimizing energy usage. The engine controller regulates all energy flow and drive management tasks, taking into account the power demanded at any given moment by the driver.

Some of the parameters used to realize the optimum propulsion mode for the given conditions are: accelerator pedal position and demanded engine load, as well as the energy supply and mix of kinetic and electrical energy at any given time.

The 0.8L two-cylinder TDI was derived from a four-cylinder TDI (1.6 liter displacement), and so the 0.8 TDI exhibits a cylinder spacing of 88 mm, its cylinder bore is 81.0 mm, and its stroke is 80.5 mm. The 0.8 TDI of the XL1 also shares key internal modifications for reducing emissions with the 1.6 TDI. They include specially formed piston recesses for multiple injection and individual orientation of the individual injection jets. In addition, a balancer shaft that is driven by the crankshaft turning at the same speed optimizes smooth engine running.

The TDI’s aluminium crankcase was constructed to achieve high dimensional precision, which in turn leads to very low friction losses. With the goal of reducing emissions, exhaust gas recirculation and an oxidation catalytic converter as well as a diesel particulate filter are used. Equipped in this way, the 0.8 TDI already fulfills the limits of the Euro-6 emissions standard.

Engine management only cools the TDI by activating the regulated mechanical water pump when engine operating conditions require it. This cooling system includes an automatically controlled air intake system at the front of the vehicle to reduce cooling system drag. This thermal management strategy also contributes towards reduced fuel consumption. A second electric water pump, which is also used only as needed, circulates a separate lower temperature coolant loop to cool the starter generator and power electronics.

CFRP. Volkswagen is producing large parts of the XL1 in carbon fibre reinforced polymer (CFRP). The monocoque with its slightly offset seats for driver and front passenger, all exterior body parts as well as functional elements such as the anti-roll bars are all made of CFRP.

The preferred process for producing CFRP components at Volkswagen is the RTM process (Resin Transfer Moulding). The density of this material or its specific gravity is only around 20% that of a comparable steel exterior skin. While the CFRP parts exhibit a level of stiffness and strength that is not inferior to that of comparable steel or aluminium parts, the exterior skin of the XL1 is just 1.2 mm thick.

Volkswagen says that compared to other methods such as manufacturing CFRP in a prepreg process, CFRP production via RTM is more economical—with lower costs at higher part volumes—because it can be automated. The RTM parts are produced in multi-shell, heated and vacuum-sealed tools. This involves injecting liquid resin at high pressure into the tool containing the semi-finished carbon material whose interior has the shape of the part to be produced. The part cures afterwards in the mould.

Of this 795 kg overall weight of the XL1, 227 kg represents the entire drive unit including the battery, 153 kg the running gear, 80 kg the equipment and 105 kg the electrical system. That leaves 230 kg for the weight of the body. A total of 21.3% of the new XL1, or 169 kg, consists of CFRP. In addition, Volkswagen uses lightweight metals for 22.5% of all parts (179 kg). Only 23.2% (184 kg) of the new XL1 is constructed from steel and iron. The rest of its weight is distributed among various other polymers (e.g. polycarbonate side windows), metals, natural fibres, process materials and electronics.

In a crash, the high-strength CFRP monocoque provides the necessary survival space for the driver and passenger. This is achieved by design of load paths, including the use of sandwich structures in the monocoque. In addition, the front and rear aluminium car structures absorb a large share of the crash energy. These principles were also implemented in the design of the CFRP doors, where an aluminium impact beam is used to absorb crash energy; a stiff CFRP door frame also minimizes intrusions into the CFRP safety cell. If the XL1 is resting upside down on its roof after a rollover accident, pyrotechnic separating screws simplify opening of the doors (swing doors).

Manufacturing. The XL1 will be manufactured by Volkswagen Osnabrück GmbH. For the small production series for the XL1, Volkswagen will use automotive handcrafting. The production process for the new XL1 entails:

• Production stage I – bodyshell frame. Production of the XL1 begins with delivery of the CFRP monocoque, which is produced by a supplier in Austria using the RTM process. In Osnabrück, the monocoque is mounted to an assembly support plate; this is where the body is built, but without doors or lids. This first body production stage is referred to as the “bodyshell frame”. At this station, all parts are moved to their prescribed design positions by special fixtures. This approach is necessary to maintain the tight manufacturing tolerances.

  The various interior and exterior surfaces of the monocoque itself are pretreated in advance. This pretreatment is necessary to attain tight gaps and smooth surfaces. The individual CFRP components are joined to one another in the bodyshell frame by gluing—a highly complex and unique process in manufacturing technology.

  Over the further course of production stage I, the boot pan is brought into position with the water channel, and it is glued and screw fastened. In addition, all structural and exterior skin parts (rear crossmembers, rear termination part, side panels front and rear) are positioned and screwed via a sled fixture.

• Production stage II – door assembly. In parallel to production stage I, the two wing doors are produced in a separate production stage, including their crash reinforcements. Volkswagen developed its own tool for this, which is used to fit the doors to adjoining body parts with millimeter precision to satisfy the extremely tight production tolerances.

• Production stage III – body assembly. At the third production station, the bodyshell frame is placed on a new fixture. Here, all body parts are assembled to achieve the specified gap dimensions and flush mounting precisions. These parts include the wing doors, bonnet, hood lid, front bumper and various small parts. Adjustment of the wing doors is a special challenge, because a precise fit must be assured to the roof and side body surfaces.

• Production stage IV – painting. A total of 32 exterior skin parts are painted on the XL1; six of them are visible carbon parts. The CFRP parts are specially prepared for painting in the framework of XL1 production. To fulfill the quality standard of a Class A paint job despite a minimally thin and therefore lightweight paint layer, in the RTM process a special fleece layer or resin film is added to the parts as a cover coat. Compared to conventional CFRP paints used in the industry, this yields a weight reduction of more than 50%. This innovative CFRP painting process owes its implementation to intensive fundamental work by the Volkswagen Technical Development Centre in Wolfsburg and an associated series of tests by paint experts at the Osnabrück plant.

  The paint itself consists of three layers. The primer with a filler material is followed by the base paint—the colored layer. Then the final layer or clearcoat is applied, which provides a high level of scratch resistance and UV resistance. In the interior, on the other hand, a decorative “matt pearl grey” paint is applied, or—on visible carbon parts such as the sills—a matt clearcoat. The same applies to the roof structure on which Volkswagen omitted trim parts in the interest of attaining optimal weight and maximizing open space for the occupants.

• Production stage V – front car section. Following painting, all components are transported to final assembly. The first step here is to join the front body section to the prefabricated floor pan. The module of this floor plan consists of components such as the double wishbone front suspension with swivel bearing (produced from die-cast aluminium), anti-roll bar (produced from CFRP), a small 12-Volt battery for the vehicle electrical system and the front ceramic brake discs.

  Also integrated in front is the high-voltage battery for the plug-in hybrid drive. Another special aspect is the mounting position of the air conditioner: the unit is typically mounted in the vehicle interior. For packaging reasons, however, this is not possible in the XL1. Therefore, the air conditioner is installed in a special insulated capsule in the car’s front section. Automatic testing of the vehicle’s electrical system and preliminary startup of all electronic components are also performed at the ITC (Startup and Test Centre) in this production stage.

• Production stage VI – rear section and interior. The classic merging of the drive unit with the body occurs after assembly of the front end. The entire drive unit (two-cylinder TDI engine, E-motor and 7-speed DSG) is installed in the rear section of the XL1. The rear axle produced from die-cast aluminium together with final drive shafts and ceramic brake discs, as well as the CFRP anti-roll bar, complete the components integrated at the rear.

  In parallel, the cockpit is installed at this station via its magnesium supports. Unlike in mass production, no provisions were made for preassembling the XL1 cockpit due to the small production volume. Instead, all individual cockpit parts are mounted inside the vehicle superstructure. The dashboard itself consists of a moulded wood fibre material, which is just 1.4 mm thick and is produced in a special pressing process.

• Production stage VII – windshield, doors and wheels. After assembly of the drive unit, the laminated 3.2 mm thick glass windshield is installed. The wing doors (including outside door mechanisms) are reinstalled; their exact positions and alignments were already set in production stage II. The hood is also mounted on the monocoque with centering pins. The XL1 also gets its magnesium wheels. They are fitted with low rolling resistance tires sized 115/80 R15 (front) and 145/55 R16 (rear).

• Production stage VIII – final assembly of the doors. The wing doors are the most complex add-on components of the XL1 body. After installing the painted door and integrating the window mechanisms, special assembly fixtures are used to glue the polymer side windows into place. The larger part of the windows is permanently joined to the exterior door skin for packaging reasons, while a segment of the lower area of the side windows can be opened. Finally, the reversing cameras are placed in their housings, and the e-mirrors that serve as digital door mirrors are mounted to the exterior CFRP of the door.

• Production stage IX – startup. All electronic control modules and their individual software and wire harness are checked. At the same time, the control modules are interconnected with the vehicle’s specific wire harness. Now, final startup of the XL1 is performed. First, the entire high-voltage system is checked. For this purpose, “simulated” isolation faults are introduced to test the system’s emergency shutoff functionality. The next step is to start up the internal combustion engine; all actuators and sensors of the TDI engine are checked, and parameter values at its first startup are compared to target values. In parallel, employees adjust the camera-based door mirror (e-Mirror); the correct visual field of the e-Mirror is optimally tuned using a special computer program.

  After all systems have been started up, a check is made of all electrical equipment; this too is done according to a precisely observed checklist. A test drive checks dynamic vehicle functions.