Two drives — one goal

Calls for more economic vehicles and alternative drives have become even louder since the global economic crisis began. In addition to conventional small cars, the focus of public interest is on electric vehicles in particular. Above all, hybrid drives play a major role. However, the technical challenges cannot be dealt with quickly enough to allow electric vehicles to take to the roads in large numbers. In the area of battery technology in particular, fundamental problems still need to be solved in the storage of energy.

Volkswagen has been researching electric drives with numerous partners for several years as part of its fuel and powertrain strategy. At the start of the “Electro Mobility Fleet Trial” in the summer of 2008, Professor Dr. Martin Winterkorn, Chairman of the Board of Management of Volkswagen AG, and German Environment Minister Sigmar Gabriel unveiled the Golf twinDRIVE® at the Automobil Forum in Berlin. twinDRIVE® is a new kind of plug-in hybrid concept, which combines an electric motor with a combustion engine as usual, but this prototype can cover distances of up to 50 kilometres exclusively on electrical power. Its batteries can be charged simply using conventional mains power.

 
“At present, cars would be unconceivable without highly efficient petrol and diesel engines. One thing is sure: the future, however, will belong to electric motors — ‘filled up’ at a mains socket,” commented Prof. Dr. Winterkorn at the presentation of the twinDRIVE®. The first prototype presented in Berlin was equipped with a diesel engine (1.5 l TDI) as the combustion unit. Beginning next year, a test fleet comprising a further 20 test vehicles with 1.4l TSI petrol engines will take to the roads of Berlin and Wolfsburg to try out the electrical drive in everyday use. Special long-life batteries will also be developed in the project, which involves seven other companies and research institutions and will run until 2012.

Government backing
The German federal government is the initiator of the research project and is supporting the tests as part of its climate protection initiative. Environment Minister Gabriel says: “The fleet trial launched with partners from research and business shows which way future mobility is heading: The efficient use of renewable energy in transport. Using the latest drive concepts like the twinDRIVE®, drivers can in future cover everyday journeys with electricity produced from sun, wind and water with the same freedom as with conventional vehicles.”

In general, Volkswagen is expanding the research and development programme for long-term electrification of drives with this commitment. The three essential goals of the fleet trial are:

• The electric motors used in the test fleet should be powered with regenerative electricity.
• The test vehicles store this energy using the latest battery technology.
• These batteries need to be particularly durable and low cost to allow the electric drive to make the breakthrough to mass production.

The next test vehicles will conclude the first step towards cars that are powered exclusively by electricity. In electric mode, twinDRIVE® offers new perspectives for local traffic since it has a high range and produces no emissions. Furthermore, the system also allows low fuel consumption figures on long journeys thanks to the hybrid function. The fuel consumption of the prototype with diesel engine is around 2.5 litres of diesel and eight kilowatt hours every 100 kilometres in combined operation. The TSI version used for the fleet test achieved 3 litres and also 8 kilowatt hours per 100 kilometres.

Two-digit million sum investment
A total of eight German cooperation partners from research and business are involved in the fleet trial. In addition to Volkswagen, the company leading the project, energy supplier E.ON, GAIA and Evonik/Li-Tec (battery technology) are participating. From the field of research, the Fraunhofer Gesellschaft (international research organisation), the Heidelberg Institute for Energy and Environmental Research (Ifeu), the German Aerospace Center (DLR) and the University of Münster are providing their expertise for the project.

Volkswagen is investing a two-digit million sum in the research project for the emissions-free mobility of tomorrow. Furthermore, the German Federal Ministry for the Environment has granted a million-figure subsidy for all project partners. “This fleet trial can contribute to combined mobility and climate protection in a completely new way. We will be able to use electricity from regenerative sources in individual transport with a high level of efficiency only through the close cooperation of partners from the automobile industry, the battery industry and the energy industry,” adds Environment Minister Gabriel with conviction. “For the first time, energy producers and energy consumers are joining forces to become an even more powerful unit. A successful fleet trial will send a positive signal across the world for climate protection,” says Prof. Dr. Winterkorn underlining the significance of the project.

Combustion engine supports electric drive

In contrast to other hybrid vehicles, the twinDRIVE® allows medium ranges in the city in exclusively electrical mode. While in hybrid models, the electric motor supplements the combustion engine, exactly the opposite is the case with twinDRIVE®. Not just reducing fuel consumption is at the foreground in this case, but electrical and thus emissions-free driving as far as possible with regenerative energy. The efficient TDI diesel power plant ensures that you can also cover long ranges. The intelligent vehicle operating system always selects the best operating mode. The system is individually tailored to the respective route and driving situation so that the most efficient combination of electric motor and combustion engine is always available.
 
In addition to the pure electric mode with a range between 30 and 80 kilometres, the plug-in hybrid has further operating modes: In the so-called series mode, the combustion engine and alternator work together as an electrical generator to extend the range. The diesel engine is started to recharge the battery while the vehicle is driven exclusively by the electric motor. If the driver demands more power by accelerating, the diesel engine will then contribute drive power and support the electric motor. Together, they both provide the desired propulsion. The power is always transferred to the drive wheels via a single-speed gearbox. Electrical energy is also produced by means of regeneration. The energy that is normally lost as heat is stored in the battery.

Good driving dynamics in electrical mode
While the first twinDRIVE® prototype still had four-wheel drive with two additional wheel-hub motors on the rear wheels, the planned test vehicles have only one electric motor and front-wheel drive. The electric drive power totalling 90kW is shared by three electric motors each delivering 30 kilowatt (kW) in the first test vehicle. The required energy comes from a lithium-ion battery, which has a rated voltage of 300 volt, an energy content of approximately 12kW per hour and weighs around 150 kilograms. Good driving dynamics and a top speed of 120km/h are achieved with a usable output of over 60kW in exclusive electrical operation without any support from the combustion engine.

The first Golf twinDRIVE® uses a 1.5-litre TDI diesel engine delivering 75kW for long-distance journeys. The car can reach a top speed of over 170km/h. Both the combustion engine and the electrical components of the drive have the same rotational axis and can be fitted as a so-called inline solution in today’s series models on the Golf and Passat platforms. This is possible not just with the four-wheel drive version using additional wheel-hub motors, but also with the exclusive front-wheel drive concept. “On the road to the future, our TDI and TSI in particular are being forged together with electric motors and highly efficient battery systems to make a new drive system,” says Prof. Dr. Winterkorn. “The twinDRIVE® is therefore a major step on the way to cars that are exclusively powered with electricity.”

The future belongs to energy generated in environmentally friendly ways
Of course, it is clear that this road can be taken only in an environmentally friendly manner using regenerative electricity. Even now, more than 6 percent of the electrical energy required in Germany is produced with wind power. This share is to be increased to 20-25 percent by 2020. At the same time, more electricity will be generated from solar and water power. It is very worthwhile to use this energy for mobility too. However, the amount of regenerative energy obtained in this way is subject to fluctuations due to natural influences (periods of sunshine, wind strength). Therefore it is necessary to always keep a permanent reserve of energy. The “Electro Mobility Fleet Test” also includes an analysis of whether the lithium-ion batteries in the cars can supply this reserve energy in future.

Project partner and energy supplier E.ON is providing the infrastructure and the necessary knowledge to “fill up” the test vehicles directly at the mains socket and incorporate the car batteries via a centralized remote load control. The local infrastructure required for this is simple — you just need a normal household mains socket. However, public and private car parks will need to be gradually equipped with “electricity filling stations” to create a dense network of charging stations.

Lithium-ion battery is key technology
To allow electric vehicles to be produced in large numbers in the future, we need lithium-ion batteries with a high energy density and long life, which are suitable for fast charging. They would allow traffic to be “electrified” first in large cities, starting a gradual move from combustion engines to electro mobility. The “Electro Mobility Fleet Test” serves not least to test the usage of the pioneering lithium-ion technology under representative operating conditions. Volkswagen is working together with companies from different areas of battery technology towards the goal of presenting the first vehicles with highly efficient lithium-ion batteries as early as 2010.