DLR’s free-piston linear generator redesigns the range extender
By Darren Quick
May 31, 2013
Technically, the combustion engine in any hybrid vehicle is a range extender, but the term commonly refers to gasoline-fueled generators that are used to charge an electric vehicle’s battery pack but aren’t used to directly power the wheels. This is the set up used in “series” or “inline’ hybrids like the Chevy Volt, which differs from parallel hybrids like the Toyota Prius, where the wheels can be driven by the electric motor or the internal combustion engine (ICE). Researchers at the German Aerospace Center (DLR) have developed a new type of range extender that can be powered by a range of different fuels.
The free-piston linear generator comprises an internal combustion chamber, a linear generator, and a gas spring. It works in a similar way to a conventional ICE with the ignition of a fuel-air mix in the combustion chamber pushing the pistons. However, rather than converting the linear movement of the piston into the rotational movement of the crankshaft like a conventional ICE or the Range Extender engine from Lotus, the device converts the piston’s kinetic energy directly into electricity.
The explosion of the fuel-air mix pushes the pistons on either side of the central combustion chamber towards the gas springs, which decelerate the pistons and push them back. The device operates at a frequency of 40 to 50 Hz and produces up to 35 kW of power.
“Engineers have been aware of the principle of this drive unit for some time,” says Ulrich Wagner, DLR Director of Energy and Transport. “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."
The control system created by the DLR engineers is able to accurately control the piston movement to within one tenth of a millimeter, while recognizing fluctuations in the combustion process and compensating for them. It also allows flexible adjustment of the compression ratio, piston speed and cubic capacity, which enables different fuels to be used, including gasoline, diesel, natural gas, bio-fuels, ethanol and hydrogen.
The free-piston linear generator’s versatility also allows the device to adapt to operate at optimal efficiency based on the vehicle’s speed and driving characteristics to reduce emissions. And without the crankshaft and camshaft components found in conventional combustion engines, the device is also constructed with fewer components. Its low-profile design also allows one or more units to be easily installed in the underbody area of a vehicle to provide an additional range of around 600 km (373 miles) without increasing the weight of the car.
Researchers at the DLR Institute of Vehicle Concepts in Stuttgart have demonstrated the feasibility of the range extender on a specially developed test bed and has partnered with Universal Motor Corporation GmbH to develop the technology and build a prototype. DLR believes the device will act as a bridging technology to make electric vehicles a more attractive option for car buyers who are still concerned about the limited range of electric vehicles.
DLR’s video outlining the free-piston linear generator can be viewed here.