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Volkswagen's Eco-Stunner

Less than 1 litre/100km fuel economy from this stunning concept vehicle

Courtesy Volkswagen

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The new Volkswagen XL1 attains a CO2 emissions value of 24 g/km, thanks to a combination of lightweight construction (monocoque and add-on parts made of carbon fibre), very low aerodynamic drag (Cd 0.186) 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.

The results: with fuel consumption of 0.9 l/100 km, the new Volkswagen XL1 emits only 24 g/km CO2.

Since it is designed as a plug-in hybrid, the XL1 prototype can also be driven for up to 35 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. 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, incorporating side by side seating rather than the tandem arrangement seen in both the first 1-litre car presented in 2002.

In the Volkswagen XL1, wing doors make it easier to enter and exit the car. The body parts are made from carbon fibre reinforced polymer parts (CFRP). Together with suppliers, Volkswagen has developed and patented a new system for CFRP production in what is known as the aRTM process (advanced Resin Transfer Moulding).

Efficiency

The Volkswagen XL1 is highly efficient. Two examples:

1) To travel at a constant speed of 100 km/h, the prototype only needs 6.2 kW / 8.4 PS – a fraction of the performance of today’s cars (Golf 1.6 TDI with 77 kW and 7-speed DSG: 13.2 kW / 17.9 PS).

2) In electric mode, the XL1 needs less than 0.1 kWh (82 Wh/km) to complete a one kilometre driving course.

These are record values for any car.

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When the full power of the hybrid system is engaged, the Volkswagen prototype accelerates from 0 to 100 km/h in 11.9 seconds; its top speed is 160 km/h (electronically limited). 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 100 Newton metres of torque from a standstill, works as a booster to support the TDI engine (120 Newton metres torque). Together, the TDI and E-motor deliver a maximum torque of 140 Newton metres in boosting mode.

Plug-in hybrid concept

With the Volkswagen 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 integrated lithium-ion battery supplies the E-motor with energy. The high voltage energy flow from and to the battery or E-motor is managed by the power electronics, which operates at 220 Volts. The XL1’s body electrical system is supplied with the necessary 12 Volts through a DC/DC converter.

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The E-motor supports the TDI in acceleration (boosting), but as described it can also power the XL1 prototype on its own for a distance of up to 35 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.

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.

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 shared between the TDI engine and the electric motor can be shifted so that the turbodiesel is operating 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 engine load, as well as the energy supply and mix of kinetic and electrical energy at any given time.

Two Cylinder TDI

The 0.8 litre TDI engine (35 kW / 48 PS) was derived from the 1.6 litre TDI, which drives such cars as the Golf and Passat. The 0.8 TDI exhibits the same data as the 1.6-litre TDI common rail engine in terms of cylinder spacing (88 mm), cylinder bore (79.5 mm) and stroke (80.5 mm). In addition, the XL1’s two-cylinder and the mass produced four cylinder share key internal engine features for reducing emissions. They include special piston recesses for multiple injection and individual orientation of the individual injection jets. A balancer shaft, driven by the crankshaft and turning at the same speed, optimises smooth engine running.

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The TDI’s aluminium crankcase was constructed to achieve high rigidity and 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 cools the TDI by activating an externally driven electric water pump only 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 body

The development team made extraordinary strides in designing the CFRP body - in terms of its lightweight construction as well as its aerodynamics. A comparison to the Golf illustrates just how innovative the body concept of the Volkswagen XL1 is:

The drag coefficient of the Golf is very good for the compact class: Cd (0.312) x A (frontal area 2.22 m2) equals a total drag figure of 0.693 m2 (Cd.A). Meanwhile, the XL1 exceeds this performance with a Cd value of 0.186 and a frontal area of 1.50 m2. The product of these two parameters yields a total drag, or Cd.A value of 0.277 m2 which is 2.5 times lower than that of the Golf.

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The Volkswagen XL1 is 3,888 mm long, 1,665 mm wide and just 1,156 mm tall. These are extreme dimensions. The Polo has a similar length (3,970 mm) and width (1,682 m), but it is significantly taller (1,462 mm). The height of the Volkswagen XL1 is about the same as that of a Lamborghini Gallardo Spyder (1,184 mm).

The wing doors of the Volkswagen XL1 are 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 free up an exceptionally large amount of entry and exit space.

In front, the Volkswagen 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 reflects styling lines that trace 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 on the wing doors are small cameras which take on the role of digital outside mirrors that send images of the surroundings behind the car to two displays inside the vehicle.

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The front end of the Volkswagen XL1 no longer exhibits the typical radiator grille. 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.

Large sections of the Volkswagen XL1’s body consist of carbon fibre reinforced polymer (CFRP) - which is as lightweight as it is strong. Specifically, the monocoque with its slightly offset seats for driver and passenger and all exterior body parts are made of CFRP. The layers of carbon fibre, which are aligned with the directions of forces, are formed into parts with an epoxy resin system in the aRTM process. This material mix produces an extremely durable and lightweight composite. For a long time, it was considered impossible to manufacture a body of CFRP, like that of the Volkswagen XL1, to industrial standards. Nonetheless, Volkswagen successfully found a cost-effective way to mass produce CFRP parts in sufficient volumes as early as 2009 – in the framework of the XL1 development project.

CFRP is the ideal material for the body of the Volkswagen XL1 because of its light weight. The XL1 prototype weighs only 795 kg. Of this figure, 227 kg represents the entire drive unit, 153 kg the running gear, 80 kg the equipment (including the two bucket seats) and 105 kg the electrical system. That leaves 230 kg, which is precisely the weight of the body – produced largely of CFRP - including wing doors, front windscreen in thin-glass technology as in motorsport and the highly safe monocoque. A total of 21.3 percent of the Volkswagen XL1, or 169 kg, consists of CFRP. In addition, Volkswagen uses lightweight metals for 22.5 percent of all parts (179 kg). Only 23.2 percent (184 kg) of the Volkswagen XL1 is constructed from steel and iron materials. The rest of its weight is distributed among various other polymers (e.g. polycarbonate side windows), metals, natural fibres, process materials and electronics.

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The Volkswagen XL1 is not only lightweight, but very safe as well. As mentioned, this is due in part to the use of CFRP as a material. In the style of Formula 1 race cars, the Volkswagen has a high-strength monocoque. In contrast to Formula 1, however, this safety capsule is enclosed on top – for safety. Depending on the type of collision, the load path may be directed through the A- and B-pillars, cant rails and sills, all of which absorb the impact energy. Additional side members and crossmembers in the front and rear perfect the car’s passive safety.

Running gear with ESP

The running gear is equipped with anti-roll bars at the front and rear and 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.

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Running gear weight has been reduced by the use of aluminium parts (including suspension components, brake calipers, 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 an entirely new generation of 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 Volkswagen XL1. Safety gains are realised by an anti-lock braking system (ABS) and electronic stabilisation programme (ESP). That is because sustainability without maximum safety would not really be a step forward. The Volkswagen XL1 shows how these two parameters can be brought into harmony.

Technical data

Body

Construction method: CFR monocoque and add-on parts

Length / width / height: 3,888 mm / 1,665 mm / 1,156 mm

Wheelbase: 2,224 mm

Drive system

Type: Plug-in hybrid, rear wheel drive

Internal combustion engine: TDI, two cylinder, 800 cm3, 35 kW / 48 PS, 120 Nm

Electric motor: 20 kW / 27 PS, 100 Nm

Gearbox: 7-speed DSG

Battery: Lithium-ion

Emissions class: Euro 6

Weight data

Kerb weight: 795 kg

Performance / fuel economy

V/max: 160 km/h (electronically limited)

0-100 km/h: 11.9 s

Fuel consump. (Ø NEDC): 0.9 l/100 km

CO2 emissions ((Ø NEDC): 24 g/km

Range: E-drive: 35 km

Range: TDI + E-drive: approx. 550 km (10 litre fuel tank)

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