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


May 31, 2013

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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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.

Source: DLR

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag. All articles by Darren Quick

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.

John Routledge

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?

Conny Söre

"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." MirceaDrac

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. :)


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!


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?

Adam Lund

What's the (projected) gas consumption per kWh?


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.


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.


iirc the last or some part of the work stroke pushes fresh air from behind the piston into the chamber for the next cycle, something like that ...


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.


This is not new, and I wrote about it last year. It's a good idea and there are a lot of other designs vying for attention out there. The spinning compression disks are also promising.

Drayson Racing Technologies has developed great ideas such as recouping lost kynetic energy form shock absorbers into electricity, which powers its flaps and spoilers.

Also: "Technically, the combustion engine in any hybrid vehicle is a range extender", this is stretching the definition a lot. A mild hybrid requires the gasoline engine to spin the wheels with the electric motor helping once in a while. With other more advanced systems, such as the Volt, the electric motor does most of the work with the engine acting 95% as a generator and spinning the wheels under heavy loads, such as steep hill incline. And finally, you have systems like the Fisker Karma where the electric motor only spins the wheels, relegating the gasoline engine to a generator. It's understandable how the term "extend range" is confusing most people with the colorful PR marketing pitches carmakers throw at us.


Free piston engines are not new. But they are almost always tied 'to ground' mechanically speaking, in practical applications, and not tied to something where humans are going to be in the nearby vicinity to feel the effects of the piston firing. Managing the shock that will happen from both the firing and the return of the piston is going to present some very difficult challenges both from a durability standpoint as well as from a human factors standpoint. I'll be very surprised to see a free piston engine in near-term use as a generator on a vehicle (10 years or less). While the reduction in friction is tempting, the lack of vibration management will keep this off the market for likely all time. The car companies have very smart people working on all manner of alternative propulsion - even though the general public have been led to believe that they are in some kind of collusion with the oil industry, that simply isn't true.


This concept might also lend itself to a steam engine approach with pressure in the middle and on each end driving the piston shafts.

Cecil Hutchins

I"m curious , if so much attention was given to our boots , like using the bump of the first endagement with the pawment to srind us back , and usung all sorts of springs that are known , may be the cars would be perfectly useless , at least in the cities .

Angel Vardjiisky

Piston Free, you sure? Then what are those two things on each side of the fuel called that are opening and closing in the photos? Looks like a piston function to me. I tough they were going to use a rotary engine from Mazda which is compact. Wait, that sounds like a great idea. I want royalties on my idea..


Well I can see that this has a shorter stroke, But unburnt fuel seems like it would build up on the pistons and valve assembly. making this unreliable source of power. unless they made it at a slight angle. I would have liked to see how the valves,cooling and oil system worked and how they stopped the piston at the end of stroke from making the ((( bang ))) noise ? and a spring, do what ? don't magnets repel if oriented right, fighting a spring ? why man why. this is one of them ideas that look good on paper. I would like to have a crack at building one of these.

Jay Finke

I agree with JDK and would like to see more information on broader application of this technology. Specifically I'd like to know how well does it scale. If a generator build for car can produces 35 kW per cell, a 1/100 scale generator would conceivably fit inside a AA battery and generate 350 Watt. Even if a mini generator only produced 1 Watt per cell but would run on common consumer fuels like Ethanol, butane, or methane, it would lead to a new technological revolution. Many devices would be possible that are currently limited by our current battery technology.

Hasan Long

All Rockstar Industrial nonsense engineers always head over to the hydraulic cylinder shelves - and thinks they just re-invented the wheel using 130 yr old 2 stroke smoking technology for everything -

These same minds thought they were going to pull off the 600 HP air-conditioning compressor 2 Stroke engines- like 30 yrs ago. Don't see any of those do ya.

Not once in 50 years has any of it ever gotten past the screen of a CAD Program and placed into a true application.

They come back with - Oh it needs a supercharger to flush the crud in & out - and we'll need more than one - and it will add 500 lbs to the shopping cart.

How about Keep it Simple first - Use the technology to create the highest output Generators in regenerative Braking. Oh yea - This is already the power plant & Brushless.

Maybe Plastic this time is where it needs to head - For battery & eliminate 60 other fittings of suspension & body mounts.

If it's not Nitrogen fueled - all further 2 Stroke Ideas should be made illegal by the government.

I along with less than two others built the highest performance turbo 6 cyl dragbikes in Honda's History - Mr Honda was impressed enough to show up at the shop to learn how - and the following yr - Honda builds the first factory turbo bikes.

Long before Apple got out of the garage.

I like Gizmag where some guys really do make a huge out of the box idea technically fit for true function.

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