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Modeling a Free-Piston Engine Genset for Hybrid Applications

Cross-section of the FPLA. Click to enlarge.

Researchers from Shanghai Jiaotong University in China have developed a new and more accurate computer model of the alternator component of the free-piston linear alternator (FPLA)—a two-stroke, free-piston engine combined with a linear alternator to generate electricity, with potential application in a hybrid electric vehicle. A paper on their work is scheduled for the 17 Sept. issue of the journal Energy & Fuels.

The free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.

The concept has thus stimulated a good deal of research, including that by Dr. Clark at West Virginia University (WVU), Dr. Peter Van Blarigan at Sandia National Laboratory (SNL) and Dr. Fredriksson at Chalmers University of Technology (earlier post). Much of this work has been focused on the combustion process, however, and not on the alternator component, according to Qingfeng Li and his colleagues.

The alternator, which is another main part of the FPLA is ignored. The commonly used modeling method is using different thrust force profiles to simulate the applied alternator loads. Considering that the free-piston motion is only dependent upon the acting forces, the used profiles could not represent the real working process of the alternator. This paper uses the finite element method (FEM) to establish the alternator model.

By this method, the actual thrust force is integrated to the final model of FPLA. Besides, the final model consists of another two main parts, the piston dynamic model and the chemical kinetic model. The piston dynamic model is solved by Matlab/Simulink. The cylinder kinetic process is simulated in Senkin, which is a subroutine of Chemkin. By integrating these three models, the more accurate model for FPLA is successfully established. It can help us to obtain a further understanding of this engine.

—Qingfeng Li et al. (2008)

There are three types of free-piston engines, according to the researchers:

  • Single piston;

  • Double piston (dual piston and opposed piston); and

  • Four pistons (dual piston, opposed piston, and complex piston configuration).

They chose to work with the double piston configuration because of the benefits in the compact structure and higher power. The free-piston linear engine in the study, which uses HCCI combustion, contains dual-opposed pistons, connected by a connecting rod and two relevant cylinders. Three intake ports and one exhaust port are placed around the circumference at the bottom end of each cylinder. The fuel is supplied to the cylinder though the injector arranged in the intake port to form the homogeneous mixture.

The linear alternator is used as the start device at the very beginning. After the engine exceeds a certain reciprocation frequency, auto-ignition occurs alternately in each cylinder, making the connecting rod move back and forth. Then, the magnetic flux linked with circular wound coil changes periodically to produce the current.

Among the conclusions drawn by the modeling work are:

  • During most of the piston strokes, the velocity of FPLA keeps constant. It reverses to the maximum values in the opposite direction just before the TDC. This makes the acceleration at least 3 times larger than the traditional ICE. In addition, it avoids the longer residence time at TDC, which seems a better way to reduce the peak temperature.

  • The TDC of FPLA can change with the external loads as well as other parameters. It can make sure that the ignition occurs right around TDC. This allows more energy used in the expansion process. Using lean mixture to acquire a higher compression ratio can improve the indicated thermal efficiency effectively.

  • The FPLA has the shorter combustion duration under a high compression ratio. The peak temperature and pressure are lower than the traditional ICE. This could reduce the temperature- dependent emissions. It is an environmentally friendly power source for the future.


  • Qingfeng Li, Jin Xiao, and Zhen Huang (2008) Simulation of a Two-Stroke Free-Piston Engine for Electrical Power Generation. ASAP Energy Fuels doi: 10.1021/ef800217k



Can't wait to see the prototypes, I'm a fan of this layout.


Sounds like what you want for future serial PHEVs and BEV range extenders.

Either way, there is no need for a small generator to have the same characteristics as a traction motor, so it should be possible to make them very efficient, cheap and small. It is a matter of enough research getting done and tested.
Let a thousand gensets bloom.


As it is shown on picture, perm magnet will obviously be exposed to a very high temp, and it is known to negatively affect its field and therefore genset performance.

Some other solution that ensures perm magnet won't get overheated must be devised.


Ummm Sweet. An unbalanced two stroke.
This sounds like a good fit for the new German Retro-Trabant hybrid.


A PHEV, with two e-motors, (front and rear drives) one of which (front drive?) could be automatically reconfigured as part of a light weight rotary motor genset. This arrangement could be just as small and more flexible. Both e-motors could be used for better accelleration, passing and hills.


@ Harvey:
Very interesting and original idea, to use the genset e-motor as a drive motor, on demand.
Some mechanical device would be required to do the switch, some sort of transmission.
It would allow either the use of a smaller main drive e-motor for 2WD, or one e-motor less for part-time 4WD setup. The price to pay would be an extra transmission.

As for the use of a light weight rotary engine in genset - there are advantages and disadvantages.
If you look at the rotary engine used in Mazda RX-8 (I think it is the only rotary currently sold outside Japan), it is about 20% less fuel efficient then comparable (in power terms) in-line ICE designs. Plus reports say it also burns oil too much, even when new.
It's true it offers higher power output per kg, then conventional ICE's. But in terms of total car weight it might not make big difference.

Some transmission expert could comment on complexity (and cost) of a mechanical switch required for your proposed solution.


A free piston Stirling could combine cycle with an SOFC and gas turbine to produce better efficiency. Run natural gas from ANG tanks and it would be clean and quiet with V2G and co-generated heat for the home.

... but would be complex and expensive to the extreme.

The free-piston arrangement lends itself to HCCI without the control problems of having to manage the moment of ignition, but free-piston engines have historically been extremely difficult to control and can't operate over a wide speed range. The latter is not a problem for some specific applications (e.g. a constant-load generator).

The HCCI self-ignition and extremely fast combustion process should lead to lower thermal losses than a standard engine. The direct actuation of the alternator eliminates a lot of sources of friction. Theoretically, this potentially has a better fuel-to-electricity efficiency than a standard diesel engine, and the HCCI operation should give low emissions if correctly implemented. Also, this arrangement keeps the number of moving parts to the absolute minimum.

Vibration is a big problem with the layout shown.

Piston ring lubrication is an issue with all piston-ported two-stroke engines (including diesels). The lack of side thrust on the pistons should help a little, but it can't be ignored. There are some emission-related implications of this because a little of the lube oil unavoidably gets out the exhaust ports.


"Vibration is a big problem with the layout shown."

The classic way around this is a two opposing two piston design. The two pistons provide the reciprocation and then two of those lined up in opposing phases cancels a lot of the vibration.

Fred H

Also remember that since the compression ratio can be freely varied almost instantaeously, the FPLA engine can run on almost any liquid or gaseous fuel, and almost any mixture of fuels. Also, it is less than half the size of a comparable standard four stroke engine.
The range extender of a PHEV needs to be only about 10 to 20 kW, so the vibration of such a tiny engine is probably manageable with special engine mounts.
For these reasons, the FPLA engine is also my current favorite for this purpose.
However, if any of the technical problems already mentioned above prove to be unsolveable then it will be out of the running.
I second the notion of getting some prototypes running as soon as possible to see if the practical problems can be solved, because the basic concept is excellent and ideal for PHEVs.

I’m sure some type of synchronous liner generator could be developed and applied to this to get away from the heat effects on the permanent magnets. I would like to see a diesel version made with electronic fuel injection and turbo charged.


I’m sure some type of synchronous liner generator could be developed and applied to this to get away from the heat effects on the permanent magnets. I would like to see a diesel version made with electronic fuel injection and turbo charged.


I’m sure that a synchronous linear alternator could be developed to eliminate the permanent magnet over heating problem. I would like to see a diesel version with electronic fuel injection and turbo charged.


I am never sure of anything, but an inductive linear alternator might be constructed. You may need to excite the motion to get the whole thing started in the first place, so the alternator could become an actuator as well.

Brian P

These motors indeed use the alternator as a motor to get the engine started; that part of it is not difficult to do.

Regarding "diesel" ... The whole purpose here is to NOT use high-pressure injection at the moment of ignition. The idea is to squirt the fuel in early during the compression stroke and let it mix and ignite when it wants to ignite. This is how HCCI operates. It gives quite a bit of flexibility in the choice of fuel, and it allows the combustion to be done faster than even with a diesel, which makes better use of the compression ratio. The short time of exposure to high temperature minimizes heat losses. This type of arrangement has the potential to be *more* efficient than a diesel but without the complex and expensive high-pressure injection system. The two-stroke scavenging process inherently gives some EGR for further dilution. The bad thing from the emissions point of view, is the piston ring lubrication ...

Turbocharging and intercooling is certainly possible, if necessary in some applications. For an automotive-scale range-extender engine (20 - 50 kW), it probably isn't necessary. For an over-the-road truck or similar application, it can certainly be done.

Henry Gibson

If ever a free piston generator were comercialized, Peter Van would lose some lunch money. The Pescara free piston machines and especially the Junkers were made in large numbers. The Pescara had a turbine output and the Junkers was an aircompressor. The NOAX machine can be converted into a four-cycle machine if necessary, and it should be made as a balanced dual piston device. It is the logical succesor to the Pescara and computer control makes it a winner. It should be used in series hydraulic hybrids, but if you want electricity, use a turgo wheel and standard alternator. Efficient linear alternators are hard to build in large sizes. ..HG..

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Yes mahonj you are right. Why not then use compressed air to drive the piston? Easy, cheap, no heat :)


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It is good to see free piston engines starting to be looked at hard again. They have been shown to have much better fuel conversion efficiency due largely to combustion at higher pressures, and this was proven about 50 years ago. The major issue was how to extract power out of these engines. The linear alternator is a good option to investigate, but as indicated in this blog, there are vibration issues with this linear design. There are other designs that eliminate this vibration, and operate in the 4-stroke mode. These charateristics are new to FPE develop, and some of the benefits haven't surface yet.


Qingfeng Li,

Very interesting work, as an engineer with a interest in ICE designs I found it very well explained in terms of emission reductions.

Now, for my two cents,

Remove the loading at or near TDC, this may allow the piston to spend less time near TDC, further reducing emissions via reduced peak tempuratures

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