Automotive

DLR’s free-piston linear generator redesigns the range extender

DLR's free-piston linear generator could extend the range of electric vehicles by around 600 km (Photo: DLR (CC-BY 3.0))
DLR's free-piston linear generator could extend the range of electric vehicles by around 600 km (Photo: DLR (CC-BY 3.0))
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The test bed used to demonstrate the feasibility of DLR's free-piston linear generator
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The test bed used to demonstrate the feasibility of DLR's free-piston linear generator
DLR's free-piston linear generator can be located in a car's underbody area
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DLR's free-piston linear generator can be located in a car's underbody area
DLR's free-piston linear generator recharges an electric vehicle's battery to increase range
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DLR's free-piston linear generator recharges an electric vehicle's battery to increase range
The fuel-air mix ignites in the free-piston linear generator's central combustion chamber
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The fuel-air mix ignites in the free-piston linear generator's central combustion chamber
DLR's free-piston linear generator
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DLR's free-piston linear generator
Ignition of the fuel-air mix pushes the pistons towards the gas springs
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Ignition of the fuel-air mix pushes the pistons towards the gas springs
DLR's free-piston linear generator could extend the range of electric vehicles by around 600 km (Photo: DLR (CC-BY 3.0))
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DLR's free-piston linear generator could extend the range of electric vehicles by around 600 km (Photo: DLR (CC-BY 3.0))

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.

DLR's free-piston linear generator
DLR's free-piston linear generator

“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.

Source: DLR

20 comments
John Routledge
Would it really kill these people to tallk about engine efficiency when they put out press releases like this? How many kilojoules of electricity gets generated per gramm of petrol, and how does that compare to a regular four-stroke/generator? Great it's compact. But if it uses twice as much petrol as a regular engine, that's not so good, especially in an alleged 'green' car.
Conny Söre
I like it. I have actually been waiting for solutions like this. But gas springs? I suppose the moving parts are packed with neodym magnets. Why don't they use them and some windings to create a linear motor to push the pistons back. Would that be less flexible and precise than a 'mecanical', spring?
MirceaDrac
"from Florian Kock, FPLG project manager, DLR Thank you for your comments. I am going to provide some more information concerning your questions: - Upscaling: Upscaling to a greater power output is possible in two different ways. Firstly, the swept volume per cylinder can be increased. Secondly, it is possible to use several units. This is the equivalent to today’s multi-cylinder engines – but in contrast to those, some of the cylinders / units of the FPLG can be deactivated completely in case they are not needed currently. - Cooling: Water cooling for combustion section, linear generator and gas spring with different temperature levels for the 3 subsystems. - Gas exchanged: Indeed, it is necessary to turbo- or supercharge the engine making sure the scavenging process works satisfactorily. - Power output: Our current function demonstrator generates up to 12 kW from a single piston module operated at a frequency of 20 Hz. For a production version the frequency will be increased and a layout with two pistons will be used. We expect an electrical power output between 20 and 35 kW per module to be most beneficial."
BigGoofyGuy
I like the part where it can use a range of different fuels. IMO; this would make it more green than a conventional ICE that uses gas. I think it could help eletric vehicles by extending the range in a green sort of way; IMO. I just think its cool. :)
jdk
I think it's a great concept - multi-fuel, reduced mechanical parts, and a size that provides a fairly high power-density (e.g., volume or weight). What I think is a problem is a strong focus on using it in an automotive application. Sure, it does indeed offer great potential there (though like an earlier comment, I'd love to know more about it's true efficiency). You see, there are MANY other applications for a generator with just these characteristics - backup generator for solar/wind off-grid system, backup generator for regular grid power (part of an UPS), primary power for emergency services or construction sites, etc. So while the automotive environment might be the most challenging to to design to, please don't neglect consideration for other (more mundane) applications!
Adam Lund
Is it a two stroke? Definitely not green. If it is a four stroke, what in the hell pushes the cylinder through the bang-blow cycle?
DonGateley
What's the (projected) gas consumption per kWh?
Slowburn
This looks like it will not produce a lot of vibration and with Homogeneous charge compression ignition (HCCI) and burning high octane fuel you can get some really high efficiency do to all the heat being generated at the very beginning of the power stroke do to all the fuel igniting at the same time rather than starting at the spark plug and burning across the chamber or from the outside of the fuel droplets in like in a diesel. I would rather have the vibration of having cylinders at both ends and a linear generator in the middle and have it run without springs. re; Adam Lund It is not a law of nature that 2-stroke engines have to be dirty. Simply giving one direct injection will massively improve its efficiency and clean up its exhaust.
ClauS
Every time I see a similar design I am thinking about mechanical advantage, actually the lack of it. The required magnetic circuit is more heavy and you need more permanent magnets comparing with the "usual" design with crankshaft, gearbox and generator. Until I see some figures I'm not convinced.
Slowburn
re; clau_sav Even If the generator is heavier than a conventional generator the weight increase in the generator is significantly less than the weight decrease from not having a crankshaft, and rotator bearings. There is less friction in this design. Only a fool has an ICE powered generator with gears adding friction between the engine and generator.
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