DLR team develops demonstrator of free-piston linear generator as range extender for EVs; technology transfer to Universal Motor Corp.
20 February 2013
|35 kW free-piston linear generator module. Click to enlarge.|
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.
|Sample layout of an extended range EV powertrain with FPLG modules. Click to enlarge.|
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.
|Intake, exhaust and fuel injection. Click to enlarge.|
In contrast to conventional drive technologies, the free-piston linear generator enables the compression ratio, piston speed and cubic capacity to be adjusted flexibly. For this reason, different fuels can be used—from gasoline, diesel and natural gas through to ethanol or hydrogen. For example, with gasoline, the unit could use a compression ratio of 10.0:1; for natural gas, 13.5:1; and for ethanol, 14.0:1.
By virtue of its versatile properties and depending on vehicle speed and driving characteristics, the settings of the DLR range extender can be adapted to deliver the optimum operating strategy.
We can therefore set the operating point of the engine when driving to ensure that we can drive as efficiently as possible and in the most environment-friendly manner.—Horst E. Friedrich, Director of the DLR Institute of Vehicle Concepts
The free-piston linear generator also functions with fewer components. For example, certain crankshaft and camshaft components normally essential in a conventional combustion engine can be dispensed with altogether.
DLR’s next step is to work with industry to develop this technology and build a prototype, said 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 optimize 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 kilometers (373 miles) could be achieved without increasing the weight of the car.
Cornelius Ferrari, Horst E. Friedrich (2012) Development of a Free-Piston Linear Generator for use in an Extended-Range Electric Vehicle (EVS 26)
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