The XL1 - The Car

Future mobility is one of the most stimulating topics of our time. The key question here: By how much could the energy consumption of cars be reduced if all the stops were pulled out for efficiency? There is now an answer to this question from Volkswagen. It is the new XL1, with combined fuel consumption of 0.9 l/100 km. No other production car with a diesel plug-in hybrid is more fuel-efficient.

Lightweight construction (monocoque and add-on parts made of carbon fibre), very low aerodynamic drag (Cd 0.189) and a plug-in hybrid system - consisting of a two-cylinder TDI engine (35 kW / 48 PS), E-motor (20 kW / 27 PS), 7-speed dual-clutch transmission (DSG) and lithium-ion battery - enable a fuel consumption of 0.9 l/100 km, the new Volkswagen XL1 only emits 21 g/km CO₂. Since it is designed as a plug-in hybrid, the XL1 can also be driven for up to 50 kilometres in pure electric mode, i.e. with zero emissions at point of use. The battery can be charged from a conventional household electric outlet. Naturally, battery regeneration is also employed to recover energy while slowing down and store as much of it as possible in the battery for re-use. In this case, the electric motor acts as an electric generator.

Despite the very high levels of efficiency, developers were able to design a body layout that offers greater everyday practicality than in the two previous prototypes: the XL1 now incorporates the comfort of slightly offset side-by-side seating, nearly as in conventional vehicles, rather than the tandem arrangement seen in both the first 1-litre car presented in 2002 and the L1 presented in 2009 for optimal aerodynamics. In the new XL1, wing doors make it easier to enter and exit the car.

Most efficient car in the world

The new XL1 shows the way forward for extreme economy vehicles and clean technologies. It also demonstrates that such cars can also be fun. The feeling when driving the XL1 is truly dynamic - not based on pure power, rather on its pure efficiency, as illustrated by two examples. First, to travel at a constant speed of 100 km/h, the XL1 only needs 6.2 kW / 8.4 PS – a fraction of the performance necessary from today's cars. Second, in electric mode, the XL1 needs less than 0.1 kWh to complete a one kilometre driving course. These are sensational values that have not been attained by any other production vehicle in the world.

When the full power of the hybrid system is engaged, the Volkswagen accelerates from 0 to 100 km/h in just 12,7 seconds, and its top speed is 160 km/h (electronically limited). Yet these numbers alone do not tell the whole story. Since the XL1 weighs just 795 kg, the drive system has an easy job of propelling the car. When full power is needed, the electric motor, which can deliver 140 Newton metres of torque from a standstill, works as a booster to support the TDI engine (120 Newton metres of torque). Together, the TDI and E-motor deliver a maximum torque of 140 Newton metres and 51 kW in boosting mode.

Plug-in hybrid concept

With the XL1, Volkswagen is implementing a plug-in hybrid concept, which utilises the fuel-efficient technology of the common rail turbodiesel (TDI) and the dual clutch transmission (DSG). The TDI generates its stated maximum power of 35 kW / 48 PS from just 0.8 litre displacement. 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 lithium-ion battery (capacity: 5.5 kWh) which is integrated in the front section supplies the E-motor with energy. Operating at 220 Volts, the power electronics manages 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.

The E-motor supports the TDI in acceleration (boosting), but as described it can also power the XL1 on its own for a distance of up to 50 km. In this mode, the TDI is decoupled from the drivetrain by disengaging a clutch, and it is shut down. Meanwhile, the clutch on the gearbox side remains closed, so the DSG is fully engaged with the electric motor. Important: The driver can choose to drive the XL1 in pure electric mode (provided that the battery is sufficiently charged). As soon as the electric mode button on the instrument panel is pressed, the car is propelled exclusively by electrical power. Restarting of the TDI is a very smooth and comfortable process: In what is known as "pulse starting" of the TDI engine while driving, 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.

When the XL1 is braked, the E-motor operates as a generator that utilises 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 favourable efficiency level. The gears of the automatically shifting 7-speed DSG are also always selected with the aim of minimising 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 realise 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 two-cylinder TDI (0.8 litre displacement) was derived from a four-cylinder TDI (1.6 litre 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. The excellent, smooth running properties of the common rail engines were transferred to the two-cylinder engine. In addition, a balancer shaft that is driven by the crankshaft turning at the same speed optimises smooth engine running.

Meanwhile, 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 fulfils the limits of the Euro-6 emissions standard.

Also designed for efficiency is the vehicle's cooling system. 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.

Design for a new era

The XL1 is 3,888 mm long, 1,665 mm wide and just 1,153 mm tall. These are extreme dimensions. The Polo has a similar length (3,970 mm) and width (1,682 mm), but it is significantly taller (1,462 mm). The wing doors of the XL1 are reminiscent of those of a high-end sports car. They are hinged at two points: low on the A-pillars and just above the windscreen in the roof frame, so they do not just swivel upwards, but slightly forwards as well. The doors also extend far into the roof. When they are opened, they create an exceptionally large amount of entry and exit space.

Visually, the XL1 also adopts the styling lines of the L1 presented in 2009. However, the XL1 has a more dynamic appearance thanks to its greater width. The body design was uncompromisingly subjected to the laws of aerodynamics. In front, the XL1 exhibits the greatest width; the car then narrows towards the rear. Viewed from above, the form of the XL1 resembles that of a dolphin; especially at the rear, where the lines optimally conform to the air flow over the car body to reduce the Volkswagen's aerodynamic drag.

In side profile, the roofline traces an arc from the A-pillar back to the rear. The rear wheels are fully covered to prevent air turbulence; the air flows here are also optimised by small spoilers in front of and behind the wheels. Observers will look for door mirrors in vain; replacing them are small cameras integrated in the wing doors known as e-Mirrors (digital outside mirrors) that send images of the surroundings behind the car to two displays inside the vehicle.

The front of the XL1 no longer exhibits the typical radiator grille; however, it still reflects the styling of the current Volkswagen "design DNA" with a predominance of horizontal lines. Specifically, there is a black cross-stripe that combines with the energy-efficient dual LED headlights to form a continuous band. The actual air intake for cooling the TDI engine, battery and interior is located in the lower front end section and has electrically controlled louvres. The narrow turn indicators are also designed in LED technology; these form an "L" shape which vertically follows the wheel housing and horizontally a line beneath the headlights. This creates a front end, which – although it is completely redesigned and extreme in its dimensions – can immediately be recognised as a Volkswagen design by its clean lines.

At the rear, however, the design takes an entirely new path, but the brand values of precision and quality are clearly evident. The XL1 represents a new dimension of Volkswagen design. Four characteristics are discernible: First, there is the characteristic dolphin body form, once again, that narrows towards the rear with very precise trailing edges for perfect aerodynamics. Second, there is the coupé-shaped roofline without rear windscreen. Merging into the roofline is the large rear boot lid that covers the drive unit and 120-litre luggage space. Third, there is a strip of red LEDs that frames the rear section at the top and on the sides. Integrated in this LED strip are the reversing lights, rear lights, rear fog lights and brake lights. Fourth, is a black diffuser, which exhibits nearly seamless transitions to the completely covered underbody.

Running gear with high-tech materials

The running gear is characterised by lightweight construction with maximum safety. In front, a double wishbone suspension is used, while a semi-trailing link system is employed at the rear. The front and rear suspension are both very compact in construction and offer a high level of driving comfort. The running gear components mount directly to the CFRP monocoque in key areas.

Running gear weight is reduced by the use of aluminium parts (including suspension components, brake callipers, dampers, steering gear housing), CFRP (anti-roll bars), ceramics (brake discs) magnesium (wheels) and plastics (steering wheel body). Friction-optimised wheel bearings and drive shafts, as well as optimised low rolling resistance tyres from Michelin (front: 115/80 R 15; rear: 145/55 R 16), contribute to the low energy consumption of the new XL1. Safety gains are realised by an anti-lock braking system (ABS) and an electronic stabilisation programme. That is because sustainability without maximum safety would not really be a step forward. The new XL1 shows how these two parameters can be brought into harmony.

Technical data of the XL1

Notes:
TDI, TSI, DSG and Twincharger are registered trademarks of Volkswagen AG or other companies of the Volkswagen Group in Germany and other countries.