Volkswagen Staging World Debut of its XL1 Diesel Plug-in Hybrid Electric Vehicle (PHEV) Prototype at Qatar Motor Show

The XL1 prototype PHEV. Click to enlarge.

Volkswagen is staging the world debut of its XL1 diesel plug-in hybrid electric vehicle (PHEV) prototype at the Qatar Motor Show this week. The prototype, representing the third evolutionary stage of Volkswagen’s 1-liter car strategy, features combined fuel consumption of 0.9 L/100 km (261 mpg US), according to VW.

The new Volkswagen XL1 attains a CO₂ emissions value of 24 g/km, due 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 0.8L two-cylinder TDI engine (35 kW / 47 hp), E-motor (20 kW / 27 hp), 7-speed dual-clutch transmission (DSG) and lithium-ion battery. The XL1 prototype has an all-electric range of up to 35 kilometers (22 miles); total range is approx. 550 km (342 miles) with a 10 liter fuel tank.

Hybrid system. 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 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.

The E-motor supports the engine in acceleration (boosting), and can also power the XL1 prototype on its own for a distance of up to 35 km (22 miles). 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 uses “pulse starting”: 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 utilizes 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 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 engine load, as well as the energy supply and mix of kinetic and electrical energy at any given time.

The 0.8-liter TDI was derived from the 1.6 liter TDI, which drives such cars as the Golf and Passat. The 0.8 TDI exhibits the same data as the 1.6-liter 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. These include special piston recesses for multiple injection and individual orientation of the individual injection jets.

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 only cools the TDI by activating an externally driven electric 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. 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-liter car presented in 2002 and the L1 presented in 2009. In the new XL1, wing doors make it easier to enter and exit the car.

The new 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 new XL1 is about the same as that of a Lamborghini Gallardo Spyder (1,184 mm). In other words, the XL1 is as long and wide as a Polo, but with a low profile like a Lamborghini.

Material. Large sections of the new XL1’s body consist of carbon fibre reinforced polymer (CFRP); the prototype weights 795 kg. Of that, 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.

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 materials. The rest of its weight is distributed among various other polymers (e.g. polycarbonate side windows), metals, natural fibres, process materials and electronics.

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 (advanced Resin Transfer Moulding) process. This material mix produces an extremely durable and lightweight composite.

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.

Source: Green Car Congress