The “Fleet study in electric mobility” is being conducted by 6 project partners from research and commerce under the leadership of Volkswagen AG. Representing the energy industry is energy provider E.ON. From the research area, the Fraunhofer Gesellschaft ISIT (representation of the battery systems and development of new battery chemistry); Heidelberger Institute for Energy and Environmental Research (IFEU; creating eco-balance); the German Aerospace Centre (DLR; analysis, forecasting traffic scenarios); and the Westphalian Wilhelm University in Münster (development of methodologies, laboratory testing of battery cells) will contribute their expertise and know-how to the fleet study.

The German federal government is targeting one million electric vehicles in Germany by 2020. These vehicles must be operated sustainably—i.e., from renewable energy sources—to attain significant progress in environmental protection. Before the transition in German energy policy away from nuclear, Volkswagen had already appealed for the intensified use of renewable energy sources.

More than 16% of Germany’s electrical needs are already covered by renewable energy sources, and plans are to extend this share to 30% by 2020. Volkswagen, for example, has installed one of the highest performance wind power systems in the world at its German plant in Emden; it already supplies one-third of the factory’s energy requirements. In parallel, more and more electricity is being generated from solar and water power. These forms of energy must also be used for mobility, Volkswagen says.

However, the amount of renewably generated energy is subject to fluctuations due to natural factors (e.g. sunshine duration, wind strength). This means that it is necessary to intelligently control electrical demand to avoid load peaks. In this context, the “Fleet study in electric mobility” is analysing the usage behavior of drivers of cars with electrical charging, electric load control and intelligent strategies in the charging process. In addition, a scenario is being tested in which some of the daily peak electrical demand might be buffered by the cars’ lithium-ion batteries in the future.

One of the goals of Volkswagen AG is to launch numerous plug-in hybrid cars on the market in the years 2013/2014. In the area of electric mobility, they are intended to supplement the Group’s hybrid models that are already being produced today (these models currently include cars by Audi, Porsche and Volkswagen), as well as the pure electric vehicles that will also debut from 2013.

The plug-in hybrid, according to Prof. Dr. Martin Winterkorn, enables what many customers expect: an unlimited driving range with internal combustion engine mobility and an attractive electrical driving range in everyday driving.

Under the hood. Click to enlarge.

Golf Variant twïnDRIVE. The electric traction motor of the plug-in hybrid powertrain outputs 85 kW and supplies 600 N·m (443 lb-ft) of torque from standstill. When combined with the TSI engine that is currently used in the latest generation of the Golf Variant twïnDRIVE (charged gasoline direction-injection engine, which also outputs 85 kW / 115 PS), a peak power of up to 120 kW / 163 PS is produced with powerful acceleration.

The entire drive unit of the Golf Variant twïnDRIVE is housed in the engine compartment; it consists of the fuel-efficient 1.4 TSI, generator (30 kW / 250 N·m), E-motor, electrically actuated separation clutch between the TSI and the E-motor as well as a 1-speed transmission. The flow of electrical energy is regulated by a high-voltage power distribution unit, which, like the battery, is located at the rear of the Golf Variant. In addition, a DC/DC converter is used to supply the 12V onboard electrical system with its required voltage.

The plug-in hybrid drive of the Golf Variant twïnDRIVE is equipped with either an 11.2 kWh or a 13.2 kWh battery pack—Volkswagen is testing packs from two vendors (more on this below). The gasoline engine is used to support the electric heating system when outdoor temperatures are low.

Over mid- to long-range drives, an intelligent hybrid management system automatically optimizes load distribution between the electric and internal combustion engine drives to minimize fuel consumption. In this process, the vehicle’s control system always selects the optimal operating mode—individually tailored to the current route and situation—which represents the most energy-efficient combination of electric motor and combustion engine. If necessary, the driver can manually activate the pure electric driving mode (E-MODE), provided that the battery has sufficient charge.

In phases of battery regeneration (during braking) or coasting (with anticipatory deceleration), only the E-motor is active; drag torque losses are reduced by disengaging the petrol engine via a separation clutch. The results: less energy is consumed during E-mode driving; when the accelerator pedal is released, the Golf Variant coasts significantly longer and further.

Based on guidelines for determining the fuel consumption of plug-in hybrids, a fuel consumption value of 2.1 L/100 km (112 mpg US) is attained (equivalent to 49 g/km CO2). When the battery is fully charged, the strategy is to maximize the share of pure electrical energy used for driving. Only when longer distances are driven does the share of supplemental gasoline fuel increase.

Top speed of the car is 170 km/h (106 mph); the Volkswagen accelerates to 100 km/h in under 12 seconds. When operated in pure electric mode, the Golf Variant twïnDRIVE can reach a top speed of 120 km/h (75 mph).

The Golf Variant twïnDRIVE has 3 primary operating modes, each of which is automatically set according to the driving situation and the battery’s charge state.

• Electric driving (E-drive): The car is powered exclusively by the E-motor that is supplied by the battery. The TSI is off, and the separation clutch is open. Up to 65 kW of power is available, which leaves hardly any wishes unfulfilled in normal everyday driving. The internal combustion engine can be started whenever it is needed to provide additional drive power via the generator or to charge the battery.

• Driving with coupled internal combustion engine (E-motor plus TSI drive): When the TSI engine is used for driving at speeds above 50 km/h, the separation clutch closes, and the TSI is directly coupled to the drivetrain. This also enables efficient driving with pure internal combustion energy transmission at high speeds in long-distance operation. When powerful acceleration is required, the battery-powered E-motor and the TSI work together in the Golf Variant (boosting). Whenever necessary, the battery can also be charged via the generator.

• Battery regeneration (E-motor active): When the car is braked, the E-motor – in this case acting as a generator – converts the car’s kinetic energy into electricity that is used to charge the battery. The TSI is off, and the separation clutch is open; this also applies to anticipatory coasting.

Li-ion packs. Volkswagen is studying two different lithium-ion battery systems for the Golf Variant (Golf Generation VI). A total of 10 vehicles are equipped with batteries from the American-German manufacturer GAIA (cathode type NCA). 10 more vehicles are powered by lithium-ion batteries (cathode type NMC) from the Korean-German joint venture SB LiMotive (Samsung and Bosch); these 10 vehicles have been in use since early this year. Both battery systems offer high power and energy density. They each weigh about 150 kg.

The GAIA battery (302V, nominal capacity 37 Ah per cell) consists of 86 cells and offers an energy capacity of 11.2 kWh. The battery from SB LiMotive (315 V, 42 Ah) with its 84 cells supplies 13.2 kWh of energy. As a comparison, the Golf blue-e-motion concept car with full electric drive has a battery capacity of 26.5 kWh, while the Touareg Hybrid battery stores 1.7 kWh.

Cooling of the twïnDRIVE batteries is performed by the Battery Management and Monitoring System (BMS), which was developed by Volkswagen for vehicles with the SB LiMotive battery.

Smart charging. An innovative charge management strategy can be implemented via intelligent charging stations that can be operated bidirectionally. The vehicle optimizes the time point of battery charging based on driver inputs (such as “Start time of next drive” and “Desired electric driving range”). These parameters are input via the radio-navigation system. By exchanging data with the electrical energy provider, the Charging Manager can optimize the charging process based on predictive tables, so that battery is charged cost-effectively and with electricity generated from renewable sources.

It is even possible to feed electricity back into the electrical grid; in this type of discharging, acting as a small component of a large network, the car is essentially used as a module that helps to temporarily offset fluctuations in the electrical grid. In this case, the system operates with AC power (AC, 230 V, 3 kW).

Stations with a pure charging functionality also operate with AC power, e.g. a household electrical outlet in the garage. Charging with AC power takes a maximum of 5 hours. The third option is to charge the battery very quickly using an off-board charger with DC power (DC, 230 to 400 V, 30 kW). The charging time in this case: 20 minutes. All 3 charging modes are being studied and analysed within the framework of the fleet study.

Data logging and saving. A wide range of data is saved for analysis. During the drive, data is continually recorded by a data logger (Car-PC) integrated in the Golf Variant. The data is transmitted to an online server via a UMTS mobile radio connection together with geo-coding of GPS position. Volkswagen Research then preconditions the acquired data to make it accessible to project partners.