Scientists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have developed an entirely new kind of drive concept to extend the range of electrically powered vehicles. The free-piston linear generator (Freikolbenlineargenerator; FKLG) is a combustion engine that generates electricity. This electrical power drives the electric car when its battery is flat. In contrast to conventional range extenders, different fuels can be used in the free-piston linear generator.
Researchers at the DLR Institute of Vehicle Concepts in Stuttgart have demonstrated the feasibility of this technology on a test bench specifically developed for this purpose. This makes them the first in the world to succeed in commissioning this kind of energy converter, comprising an internal combustion component, a linear generator and a gas spring. “Innovative solutions like the free-piston linear generator will help to make electrically powered mobility an everyday reality and demonstrate the scientific strengths of Baden-Württemberg as a location,” said Rolf Schumacher, the Ministerial Director at the Ministry of Finance and Economics in Baden-Württemberg, at the official inauguration ceremony for this project.
New structural approach with powerful feedback control
Engineers have been aware of the principle of this drive unit for some time. Through the installation of a gas spring, DLR researchers have now succeeded, for the first time, in operating this system in a stable manner. The challenge here was to develop a particularly powerful mechanism with a highly dynamic control unit that regulates the complex interactions between the individual components,” said Ulrich Wagner, DLR Director of Energy and Transport, as he described this innovation.
The free-piston linear generator works in a similar manner to a conventional combustion engine. But instead of converting the linear movement of the piston into the rotational movement of the crankshaft, it generates electricity directly. A fuel-air mix is ignited in the combustion chamber. This expands and pushes the piston towards the gas springs. These springs decelerate the piston movement and push it back. The linear generator converts the kinetic energy of the piston into electricity and this in turn powers the electric motor. The control system devised by the DLR engineers is able, for example, to control piston movement accurately to within one tenth of a millimetre. At the same time, it recognises fluctuations in the combustion process and compensates for them…
“With our functional demonstrator, we have shown for the first time that our free-piston linear generator principle can be implemented. In the next step, we need to work with industry to develop this technology and build a prototype,” explained Friedrich. To accomplish this, DLR has concluded a technology transfer contract with Universal Motor Corporation GmbH and will provide scientific support during further work. One of the tasks ahead is to optimise the weight and size of the free-piston linear generator in such a way that one or more of the assemblies can be located in the underbody area of a vehicle. In this way, initial estimates suggest that an additional range of about 600 kilometres could be achieved without increasing the weight of the car.