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

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.




It would be interresting to buy if ever that can achieve high mpg like 60 mpg or more. But they are not near a good proven working prototype and they didn't say anything about mpg. probably we will never heard again fron them before 20 years, LOL.


Interesting for compact PHEVs? However, by the time it is fully developed, affordable extended range BEVs may be common place.


Ultra quick future chargers will recharge EVs in 10, 12 or 16 minutes (at 60%, 80% and 100% respectively) negating the requirement for on board ICE range extenders.


For years, commenters have ask for simple, cheap, BEV range extenders.

~30-50 hp to recharge a ~60+ mph rate, should plug-in battery be discharged. A cheap, seldom used, constant speed 2-cycle, 4-cycle, rotary, ..backup - at least till better batteries.

Finally, 2 moving parts and ".. initial estimates suggest that an additional range of about 600 kilometers (373 miles) could be achieved without increasing the weight of the car."


The reciprocation could be two more pistons on the other side of the linear alternators for more power and less vibration.


What I'd really like to hear is the thermal efficiency of such a genset. There are reasons to think it could be substantially more efficient than most ICE technologies, but it would be nice to hear it confirmed.


It might be too good to be true, but "..have developed a demonstrator multi-fuel free-piston linear generator" plus diagrams, including vehicle layout looks like something that could be on a test track pretty quick.

Not sure why more than one module(47 hp) would be needed to occasionally recharge and "get home."


You don't need rotary motion to just generate electricity, so you can go with a contraption like this.

If you can generate electricity with good efficiency and low emissions, you have a real winner here for a PHEV or range extender design.

+ as Kelly says, 47 hp is quite a bit of power and could propel you along a Motorway at quite a speed, well above the 60 mph, "get you home" level.


Forget it. It is BS. If the combustion engine makes 35kW in electricity expect waste heat of at least 3*35kW = 105W. There is no cooling system in the shown drawings so this is BS. And what about notice and vibration. I expect it to be far worse than in a normal combustion engine. Also I do not get what moves the cylinder back after the central combustion. Is it a spring?


In a typical free piston engine, a spring force is used to move the pistons back to center, most commonly compressed gas as in the engine above. However, with linear generators, you could just use some of the stored electric energy to push the pistons back for compression by using the generator (or alternator) as a motor. Free piston engines are not new but there have been problems in the past in controlling the engines. The engine in question is an opposed piston 2 stroke so it should run as least as smooth as a 4 cylinder opposed cylinder 4 stroke.

An interesting free piston engine is the Innas CHIRON engine. http://www.innas.com/home.html I looked at the design of the engine a few years ago as a possible means of actuating a mobile hydraulic robot.


Maybe the displacement will be released.

This FPLG may be liquid cooled, like the CHIRON, but it isn't clear on either diagram nor is a radiator shown.

Perhaps finned/forced air cooling could be sufficient.

On a private test track away from the media, hang the FPLG under a Leaf, let it recharge the battery, and see what happens for how long :)


I can't help but believe we are very close to some company announcing the more energy dense mass produced battery we all are waiting for. Nissan has been building their 600 lb battery pack for three model years now and it still uses the same NEC modules at the same energy density level, 141 Wh/Kg. Most currently manufactured battery modules are around this level of density. Surely Nissan can't continued offering this battery much longer because it cripples their ability to offer decent range performance.

When this new high density battery is made available, hybrid cars and fancy onboard gensets will be rendered moot.


Hope plus an interesting, exciting concept makes a great proposal or prospectus but if it in not superior, sound engineering will slap it down just as firmly as if it were an ugly crude contraption.

And the longer a technology breakthrough is overdue, the more likely it will remain so, and be passed by.


It looks very compact and light, and it can burn anything!

Henry Gibson

Pescara would be pleased as would Junkers and others. Pescara invented opposed piston engines of this type that did not directly generate electricity but used turbines to generate electricity from the expanding exhaust gases and also used gas springs. The electric assisted turbocharger of OPOC could be added to this design. Van Blarigan, of SANDIA has been working on direct generation for decades. There was the steam free piston generator of the defunct LION.
Patent US 8,091,519 was issued in 2012 for an electric generator of similar design and computer control for flexible operation and possible tiny size for operating on jet fuel to supply electrical energy for the personal electronics of combatants it also will allow self cooling cycles much as a gas turbine is self cooling. ABB has recently introduced the synchronous reluctance motor which is more efficient than the induction motor and is related to the switched reluctance motors, and both are highly efficient and light weight without any expensive and rare permanent magnets. They also allow more compact coils.
It is good that the INNAS NOAX CHIRON was mentioned. An opposed piston hydraulic pump has been tested, and a CHIRON opposed piston version could also be used for an automobile that would double the fuel efficiency of present cars by just using the ARTEMIS digital displacement pump/motors to drive the wheels and no expensive batteries need to be used. With well designed pistons and cylinders the efficiency can be almost as high as fuel cells. The huge ship engines have %50 efficiency. Such units could eliminate half of the automotive fuel use and any need for biofuels and greatly reduce carbon in the process. Rather than having these electric generators in automobiles they should be used in homes and other buildings as combined heat, cooling and power units also known as cogeneration systems. They could compete with the Honda Ecowill units. ..HG..

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