The earlier, first-generation 1-Litre concept. Click to enlarge.

The L1 combines a two-cylinder 0.8L TDI (the smallest diesel engine intended for production applications ever built by Volkswagen); a 10 kW motor; and 7-speed DSG installed at the rear. As a unit, they represent the most fuel efficient hybrid drive in the world, according to Volkswagen. The aerodynamic (Cd = 0.195) L1 offers combined cycle fuel consumption of 1.38 L/100km (170 mpg US), with CO₂ emissions of 36 g/km. Top speed is 160 km/h (99 mph).

0.8L TDI. In the standard ECO mode, the 800 cm³ TDI develops a power of 20 kW / 27 hp (at 4,000 rpm); in Sport mode—used to reach the car’s top speed, for example—the car’s power increases to 29 kW / 39 hp (at 4,000 rpm). The maximum torque of the TDI is 100 N·m / 74 lb-ft (at 1,900 rpm). The L1 also has a Stop-Start system, which automatically shuts off the drive unit when the vehicle is at a stop and restarts it when the gas or E-pedal is pressed.

The two-cylinder 0.8-liter TDI unit has been derived from the 1.6 TDI just introduced a few months ago. The 1.6 TDI is making its debut at the IAA in cars such as the new version of the Golf BlueMotion (3.8 l/100 km) and the Passat BlueMotion (4.4 l/100 km). (Earlier post.)

Based on their common origins, the 0.8 TDI and 1.6 TDI have identical cylinder spacing (88 millimeters), bore (79.5 millimeters) and stroke (80.5 millimeters). These TDI engines also share key internal engine features for reducing emissions. They include special piston crowns, multi-injection and individual orientations of the specific injection jets. On both drivetrains there is exhaust gas recirculation, an oxidation catalytic converter and a diesel particulate filter. Equipped this way, the TDIs in each Volkswagen fulfil the limits of the Euro-5 emissions standard.

The 1.6 TDI, thanks to its common rail injection, is also an exceptionally quiet and low-vibration diesel engine. These positive properties have been successfully transferred to the two-cylinder unit. The TDI’s aluminium crankcase was also constructed with high precision to achieve very low friction losses. The oil pump, designed to operate at a maximum oil pressure of 4.0 bar, also contributes to engine efficiency.

Another example of how the entire drive system is configured for high efficiency is the L1’s cooling system. Its external water pump is controlled by engine management so that cooling is only activated while engine operating conditions require it. This thermal management also contributes to reduced fuel consumption. A second electric water pump, also activated only when needed, provides cooling required for the starter generator and the power electronics in a separate water circulation loop operating at a lower temperature level.

The E-motor. The hybrid module has been integrated into the housing of the 7-speed DSG (Direct Shift Gearbox). It is located between the TDI engine and the DSG gearbox and consists of a 10 kW / 14 hp electric motor and a clutch. The E-motor is supplied with energy from a lithium-ion battery located at the front of the car. An electronic power control module, operating at around 130 volts manages the flow of high voltage energy the battery and to the E-motor. In parallel, the vehicle’s low voltage electrical system is supplied with the necessary 12 Volts through a DC/DC converter.

In normal operation the electric motor can support the TDI engine in conditions such as by electronic load point shifting and in acceleration. If necessary—generally during acceleration—the E-motor can supply 40% additional torque over the entire speed engine speed range. The E-motor can also propel the L1 over short distances by itself. In this case, an auxiliary clutch decouples the TDI from the drivetrain.

Restarting the TDI is a very easy process. In so-called “pulse starting” of the TDI, the electric motor is sped up and is then coupled to the TDI unit to provide almost instant starting. The entire process takes place automatically and without jolts, so the driver hardly notices the restarting of the TDI engine.

In braking phases, the E-motor operates as a generator to charge the lithium-ion battery by recovering braking energy. The gears of the automatically shifting DSG are always selected with the aim of achieving the best possible fuel economy. The engine controller regulates all energy flow and drive management tasks taking into account the moment by moment demands for power made by the driver. Some of the parameters used to calculate the optimum propulsion mode for the given conditions are: accelerator pedal position, engine load, momentary fuel demand, energy supply and the mix of kinetic and electrical energy at any given time.

Transmission. Gear shifting work aboard the L1 is handled by the 7-speed DSG. Compared to the version equipping the new Polo, the design of the L1 Direct Shift Gearbox has been developed to include clutch control for the hybrid module. Furthermore, individual gear ratios have been optimized to attain responsive driving performance despite the car’s extremely low fuel consumption. The hybrid module is located where the flywheel is usually to be found.

Body. The dimensions of the L1 are distinctive. While the length of the L1 at 3,813 millimeters is still similar to that of a Volkswagen Fox, and its height of 1,143 millimeters nearly matches that of a Lamborghini Murciélago, the car’s aerodynamically optimized narrow width (1,200 millimeters) has no comparisons in the world of today’s production cars.

The two-seat monocoque, including the tubular frame driver’s seat and passenger seat as well as the exterior body skin, all consist of carbon fibre reinforced plastic (CFRP). There are no doors. Instead, the driver and passenger climb into the L1 from the top. An electrically actuated entry canopy above the seats is opened and closed for this purpose.

Headlights and taillights all utilize LED technology. The rear wheels are completely covered; their wheel covers can be removed to change the Michelin low resistance tires. The underbody is also completely enclosed. The 0.8 TDI is cooled via adaptive air channels integrated in the sides of the car body. These automatically open and close based on the hybrid unit’s operating state and vehicle speed. The tailgate is opened in the usual, manual way. It too consists of CFRP. Inside is a stowage space of 50 liters.

Of the 380 kilograms curb weight, 122 kg are taken by the drivetrain, 79 kg by the chassis, 35 kg by interior furnishings and 20 kg by the electrical system. The remaining 124 kg is the weight of the body.

These 124 kilograms can be further broken down: 64 kg are accounted for by the CFRP monocoque including integrated passenger seat; 28 kg is the weight of the entire CFRP exterior skin; 19 kg for the CFRP entry canopy; 9 kg for the CFRP driver’s seat; and 4 kg for the LED lights.