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Novel opposed-piston free-piston linear generator for hybrids seeks to overcome drawbacks of earlier designs

Design of opposed-cylinder free-piston linear generator from earlier works with single, centered linear generator. Click to enlarge.

A researcher at the Nanjing University of Science & Technology is proposing a novel opposed-piston free-piston linear generator for use in series hybrid electric vehicles. According to his simulations, Ling Huang says, the linear generator could deliver 15 kW average output with a generating efficiency of 42.5%.

A number of research groups have explored the development of a free-piston linear generator, including work at West Virginia University (WVU), Sandia National Laboratory (SNL), Chalmers University of Technology (earlier post), and Shanghai Jiaotong University (earlier post), among others. Most of this work focused on a similar design: a single linear generator placed between two opposed cylinders, as in the diagram at right from Shanghai Jiaotong.

Although most of the [earlier] work was focused on a similar design [as above], few designs have been successfully developed up to now. This is mainly because the motion of the free-piston in this kind of design is difficult to control under the interaction of the combustion in the two opposed cylinders.

To overcome the drawbacks in the reported opposed-cylinder free-piston liner generator...a novel opposed-piston free-piston linear generator is proposed...The main parts of the new designed system are a central combustion chamber and two linear generators. The central combustion chamber has two opposed pistons.

Firstly, inherent mechanical balance of the system can be realized when the two linear generators are synchronized. Secondly, the central combustion chamber is benefit for highly efficient scavenging via intake and exhaust ports without using valves. Besides, higher compression ratio can be obtained for the same stroke length of linear generators.

—Ling Huang 2012

Sketch of novel opposed-piston free-piston linear generator with two linear generators. Huang 2012. Click to enlarge.

In an SAE paper, Huang describes a modeling and simulation study of the new free-piston linear generator. Each of the two linear generators in the system is equipped with a mechanical spring, which is used for storing mechanical energy in the power stroke and releasing mechanical energy in the compression stroke.

One working cycle of the proposed free-piston linear generator includes two strokes (compression and combustion). In the compression stroke, driven by the fore of the mechanical spring, the piston moves to the middle of the cylinder; combustion then drives the piston backward.

One of the important control variables is ignition timing. As there is no crankshaft or rotational motion to provide a crank angle for timing, Huang proposes a piston position for use for ignition timing. Ignition advance (piston at 4.4 mm) provided a higher peak pressure in the cylinder (67 bar) and better generating efficiency (42.5%).

For further study, an accurate model of the linear generator and combustion will be developed. The control method of linear generator and the movement of free-piston will be investigated, and special attention will be [paid to] the synchronization of th left and the right pistons.

The effects of [HCCI] operation will be investigated by future accurate combustion model. Furthermore, natural gas will be investigated as the fuel of the system.

—Huang et al.


  • Ling Huang (2012) An Opposed-Piston Free-Piston Linear Generator Development for HEV (SAE 2012-01-1021) doi: 10.4271/2012-01-1021



This is an old hat with a new lining. The Germans used this type of engine on their subs during WWII as compressors.
Frank Stelzer, a German inventor, hade made a new design of this engine type approx. 20 - 25 years ago. Around 2005 I paid Stelzer a visit at his premises in Oberursel by Frankfurt, Germany. I was impressed by his efforts but not from the results.
The engine was difficult to start and once it ran, it was difficult to keep it running.
My personal impression was, from the complete frequency band of the engines capability, it was able to operate reliably only within a very small frequency spectrum / window.
I sincerely doubt, that due to the very physical attributes of the basic design, any respectable improvement can be expected of this engine type.


Thanks for the info, over the years I've heard this idea proposed many times and I've always wondered why I would hear nothing more.


The linear motors could be handy for controlling the position of the pistons.. it would be better if they could get rid of the permanent magnets.


Stelzer didn't have 32 bit microcontrollers and the associated electronics and sensors.
It may be that it is not possible.


Piezoelectric generators in the springs are an obvious advance. The short stroke is a problem for PM generators (high linear forces are required) but piezo elements already operate with minuscule strain and can be engineered for mechanical advantage.

The operating frequency is a function of the spring constants and mass of the pistons (ω=√k/m, where k includes both the mechanical and air spring constants and is thus not linear).  Ideally, the pistons would be very heavy right at ignition so combustion occurred under constant-volume conditions and maximum expansion work was recovered.  Maybe the right controls on an engine like this can get closer to the ideal.


Ideally, the pistons would be very heavy right at ignition

Just a thought here but, could it be done with hydraulics? I'm thinking that at the start of each stroke the expansion would be working against the inertia of the oil added to that of the piston.


the linear motors can fine tune all those characteristics


But at what cost?


I suspect yoatmon's evaluation is optimistic.

Henry Gibson

Pescara had a company that made machines that operated with air-springs and air compressors, but the exhaust was fed to a turbine to generate rotational energy for vehicles or electric generators.(Raúl Pateras Pescara de Castelluccio) Improvements in jet engine technology eliminated some of the advantages of the Pescara engines. For years, comments have promoted the OPOC small engines for range extenders which have dual sets of pistons in two cylinders with mechanical linkages.

Patent US 8,091,519 was issued this year 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.

Fairbanks and Pescara and "Commer Knocker" and Junkers and Deltac and OPOC should become known to all who might like this idea of opposed piston engines. A giant low speed one is illustrated in an old Encyclopedia Britanica.

Junkers build a free piston air compressor that was used in German submarines and Japanese contractor air compressors.

INNAS NOAX built and demonstrated a hydraulic hybrid Fork-Lift that used its Chiron hydraulic free piston engine to supply fluid energy. This engine should be modified to become the basis for most automobiles as hydraulic hybrids. Artemis technology can supply the rotational function at high efficiency. Electrical devices are more expensive. A tiny TURGO wheel and generator can supply electricity as needed from the high pressure fluid. ..HG..

D Hagen

Do you really know what a free-piston engine is? It seems so simple but the absence of a crankshaft makes it a bit complex. Its worth it to look at it because striking advantages are really there. Just look at this site where you find any information about free-piston linear alternators:

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