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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..

Henry Gibson

Pescara and Junkers engines were produced and used for many years. ..HG..


The difference is that now we have better sensors and much faster and more sophisticated control computers and algorithms.
Thus, we have a better change of getting it to work now - especially if it is only generating electricity.

Henry Gibson

Quite small generators can be used as range extenders for town use only. Two to Six kilowatts will keep a battery car flowing with the town traffic. Throw a HONDA inverter generator into a TESLA and you will get 40 MPG and 40O miles on ten gallons of gasoline at town speeds.


Maybe I've misunderstood you but I and the rest of the world want to abort pollution. I consider any type of fossil driven extender solely as a means of transition. I'm confident that it's just a matter of time until batteries with appropriate energy and power density, at reasonable prices, appear on the market rendering extenders obsolete. Beyond that, I would only accept FCs (with reasonable prices) as range extenders.

It's not only a matter of dwindling resources but also the increasing pollution problematic. I'm not keen on keeping on going as we have been in the past until we have "Beijing conditions" worldwide.


Now we have an interesting point - do you supply enough power to "get you home" or enough power to cruise on the motorway at 75mph ?
IMO, you may as well go to the 75mph (or higher) limit as once you have installed an ICE (or whatever kind) in a car, you may as well install a reasonable one.
As HG says, 6 KW will do for traffic, 15KW will do for 60mph, 47hp should be enough for most things, especially if you have some battery storage as well.

Anyway, let them build and test one at any power, and then start optmising the capacity.


Read up on BEVx here:

"battery-electric vehicle with a small “limp-home” range extending engine or APU (auxiliary power unit)—i.e., not a series-hybrid type vehicle such as the Chevrolet Volt equipped with a full-capability engine"

I would say make these units about 25kW, weight, cost and size (not economy) being the critical items..for larger vehicles just use two. What's the highest thermal efficiency attained from such a small engine?


Fascinating discussion. Imagine how this free-piston linear generator would have screwed the NYTimes/Broder story.

An EV should run on battery >90%, but if the FPLG is cheap and "~adds no weight(?)".. A spare tire adds no performance, but they have been in trunks a century.

Lad said, ".. very close to some company announcing the more energy dense mass produced battery" and I've had that feeling too.

Battery makers still battle phase one - works/reliable/no media 'fires'. Perhaps only the GM Volt altered it's original 1.5 year old battery and went from 35 to 38 mile(+8.5%).

It also seems there have been many more positive battery tech articles since the election & a advanced Google search might confirm this.


You would limp home pretty smartly on 25kW.

A question: would it be better to run the engine at maximum efficiency for some of the time, or all the time at lower efficiency ?

I am thinking of the 47Hp engine, in particular here.


Germans developed free piston engines for gas compressors all the way back in WWII. This is not a new concept althought the uses of electronic controllers and their optimization in order to burn a variety of fuels is a new development.

Check out the following web site to see a smooth running working engine being tested and evaluated in a lab environment. It's not clear if the engine uses water cooling.


The engine is shown on the right with the electronic controlled variable gas spring on the left in the video.

The director of the project claims a 30% reduction in CO2 compared to a similar size displacement engine using similar fuels. Unfortunately he is speaking in German which may be a language problem for some posters of this site.


You would limp home pretty smartly on 25kW.

You got that right! I think the average car only needs 7hp to overcome drag at 55mph. That's just a bit over 5kw.

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