DLR team develops demonstrator of free-piston linear generator as range extender for EVs; technology transfer to Universal Motor Corp.

http://www.greencarcongress.com/2013/02/dlr-20120220.html

Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Vehicle Concepts in Stuttgart have developed a demonstrator multi-fuel free-piston linear generator (FPLG, or Freikolbenlineargenerator, FKLG in German) as a range extender for electric vehicles. The FPLG comprises an internal combustion component, a linear generator and a gas spring; the researchers have demonstrated the feasibility of the technology on a test bench specifically developed for this purpose.

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. 

Research on this type of drive unit has been fairly widespread. As one example, GM Global Technology Operations LLC and the Regents of the University of Michigan recently were recently awarded a US patent (Nº 8,261,860) for a plug-in series hybrid or range-extended electric vehicle powertrain using multiple free piston linear alternator (FPLA) engines. (Earlier post.) As another, researcher at the Nanjing University of Science & Technology has proposed a novel opposed-piston free-piston linear generator for use in series hybrid electric vehicles. (Earlier post.)
The DLR researchers says that through the installation of a gas spring in their system, they have now succeeded, for the first time, in operating such a system in a stable manner. The control system devised by the DLR engineers is able, for example, to control piston movement accurately to within one tenth of a millimeter. At the same time, it recognizes fluctuations in the combustion process and compensates for them.

The challenge here was to develop a particularly powerful mechanism with a highly dynamic control unit that regulates the complex interactions between the individual component.

—Ulrich Wagner, DLR Director of Energy and Transport

The core module operates at high efficiency even at partial load, with an indicated power of up to 35 kW per module; operating frequency is 40-50 Hz.

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