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Toyota R&D continues work on free piston linear generators for EVs; novel resonant pendulum control method

In 2014, a team at Toyota Central R&D Labs Inc. published two SAE papers on their work in developing a prototype 10 kW Free Piston Engine Linear Generator (FPEG) for B/C segment electric vehicles. (Earlier post.) The FPEG consists of a two-stroke combustion system, a linear generator, and a gas spring chamber; the unit offers potential for compact build, high efficiency and high fuel flexibility.

Now, the Toyota researchers report on a new FPEG control method to realize stable and flexible piston motion control for efficient electric power generation. They presented their work in a paper at the recent 2016 SAE World Congress.

Structure of the Toyota FPEG. The FPEG comprises a cylinder block combined with the stator of a linear generator, inside of which is a piston combined with a slider. The piston and cylinder head form a combustion chamber, which includes an exhaust valve, ignition plug, and fuel injector.

The slider has permanent magnets and is able to move freely along the internal circumference surface of the stator without a crank mechanism. The other side of the slider, opposite the combustion chamber forms an air spring chamber, which is sealed with seal rings in the ring grooves of the slider, against the molded inner cylindrical surface of the stator. The air spring chamber has a check valve to ensure the required pressure is maintained, and the base pressure of the air spring chamber is controlled with a regulator valve. Moriya et al. Click to enlarge.

There are some technical challenges in ensuring an FPEG can achieve continuous operation over a long period, including lubrication, cooling, and piston motion control. Among these technical challenges, the piston motion control is the most significant factor in improving the robustness and efficiency of the FPEG because the combustion characteristics depend strongly on the piston motion, which is controlled by the linear generator. This paper describes a novel linear generator control method which realizes the simple harmonic oscillation governed by the piston mass and the air spring pressure.

—Moriya et al.

Control of the linear generator is a key to stable operation of the FPEG over long period of time because the combustion depends on the piston motion—controlled by the force of the linear generator. Linear generator control is also important to the generating efficiency.

The Toyota researchers earlier had developed a target positon feedback control method. Although this method was successful for control of the linear generator at low-power operation points, it was not able to maintain the piston swing motion when the output power of the FPEG was increased.

To address this issue, the team developed the new method, called “resonant pendulum” control.

In their description of the study, the team noted that one of the major problems in the practical application of FPEGs is the vibrations arising as a result of the piston motion. To solve this problem, they said, two sets of FPEGs can be installed in opposing positions on board and controlled such that their pistons are synchronized. Before working on that, however, they needed first to establish a method for precise piston control; as a result, they used a single FPEG in the current study.

Position sensors—gap sensors on the cylinder block and scale lines on the piston—detect the position of the piston. The resolution of the position sensor is 0.55mm/count, and the count value from the sensor is interpolated by software.

The load of the FPEG can be manipulated with the amount of the fuel which is injected by the fuel injector. The air flow rate is adjusted responding to the fuel amount to keep the targeted A/F.

Resonant Pendulum control. The resonant pendulum control method is based on speed control; its purpose is to adjust the positions of top dead center (TDC) or bottom dead center (BDC) to set positions by manipulating speed control commands.

The piston moves freely following the principle of simple harmonic oscillation governed by the piston mass and the air spring pressure during the remainder of the piston stroke. The current of the linear generator is set to zero during this period.

In general, the linear generator has to drive the piston just before the TDC and BDC to adjust the positions of the TDC and BDC to the commands. When both the positions of both the TDC and BDC exceed the commands, the amplitude of the piston swing is decreased. Similarly, when both positions are insufficient to meet the commands, the amplitude is increased.

Conversely, when the position of the TDC exceeds the command and that of the BDC is insufficient to meet the command, the piston swing is shifted in the negative direction, thus decreasing the offset. Similarly, when the position of the TDC is insufficient to meet the command and that of the BDC exceeds the command, the piston swing is shifted in the positive direction, thus increasing the offset.

The new values of the amplitude and offset for the next cycle can be calculated from the previous values of ETDC and EBDC because the piston is considered to periodically repeat the same swing motion.

During motoring operation, the piston is accelerated by the linear generator to approach the speed command values. In this case, the linear generator works as a linear motor and uses electricity from the battery. During firing operation, when the piston speed exceeds the speed command, the linear generator decelerates the piston and generates electricity, which charges the battery. There is no need to switch between the motoring and firing operation modes, as these operations change automatically based on the piston speed.

The team also devised and tested a method of improving the generating efficiency by manipulating the pressure of the air spring chamber; basically, the air spring chamber can function as an energy buffer to suppress the peak of the generated electric power and distribute it to the compression stroke.

The Toyota team investigated the new control method both via simulation and experimentally.

Among the findings reported in the study:

  • Resonant pendulum control delivers reliable and stable operations in all the modes of start-up, motoring and firing.

  • The resonant pendulum method controlled the positions of TDC and BDC within 1mm difference in both motoring and firing modes.

  • Requiring that the generating action occurs only in the range of high-speed piston motion enables high-efficiency generation.

  • Using simple harmonic oscillation realized generation in both the expansion and compression strokes. The ratio of the generated powers in both strokes changes depending on the air spring chamber pressure. When the generated power during the expansion stroke is larger than that during the compression stroke, the maximum DC power is generated.


  • Moriya, K., Goto, S., Akita, T., Kosaka, H. et al. (2016) “Development of Free Piston Engine Linear Generator System Part 3—Novel Control Method of Linear Generator for to Improve Efficiency and Stability,” SAE Technical Paper 2016-01-0685 doi: 10.4271/2016-01-0685



What I said in 2014:
It looks like a great idea to me - a 20Kw range extender.
if you had one, you could reduce the battery range of BEVs to your daily requirements and use the extra weight and space for the range extender.
I suppose it all comes down to noise and vibrations and neat packaging - you don't want to lose your trunk to fix one of these.
Also, it had better be quieter than the BMW I3 range extender, which, by all accounts is very noisy.

A good range extender really would be a great boon.
A fuel tank that could hold 200-300 miles worth of gasoline and off you go, 300 miles travel, 5 minutes refueling, anywhere.

Henry Gibson

An INNAS NOAX engine with Artemis Digital displacement technology would be cheaper and more efficient and save half the fuel over present automobiles of same weight.

To this we must consider the Capstone turbines and the Wrightspeed turbines and the Bladon Jets turbines.

To this article we must remember the Pescara dual piston air spring engines that used turbines for power output. One of which powered an experimental US automobile after locomotive and power stations and ships were built that used the cheapest heavy petroleum fuels.

Highly related is the free piston Stirling commercial co-generation unit recently installed in a UK Hotel by Qnergy which also has wood pellet prototype. ..HG..


Why not increase efficiency by putting another combustion chamber on the left hand side, firing in anti-phase to the one on the right? Compressing the mixture in either chamber would still act as the required 'air spring' to decelerate the piston just fired from the other side, but without the need for compressors and complicated pneumatic controls, or battery input via the linear motor, to return the piston on each stroke.

I suspect the answer is that it is already very difficult to effectively cool this thing, so doubling up on the energy input would only exacerbate the problem that the permanent magnets will be having to survive an environment where maybe 20% of combustion energy is being transferred into the piston as heat rather close by … the best Neodymium PMs start to irreversibly lose magnetisation at ~65°C.


@ barbar
What you're suggesting is already reality and market-ready. It's only the thumbs-down attitude of Mercedes' management that is preventing market launch.


@ yoatmon,

hmmn … DLR's approach looks good, neatly solving the vibration/balance issue and being cooler running, but is still one combustion chamber, now having two free pistons, each decelerated and returned by its own 'air spring'. Problem is, this term is deceptive, as rebound alone will not provide the power for a compression stroke, thus, as with Toyota's system, it still requires an external compressor, pneumatic reservoir and sophisticated control valves with feedback from finely resolving piston position sensors to individually meter the air into each 'spring' on every compression stroke. This all runs to complexity, weight, cost and breakdown susceptibility, so Mercedes may have a few valid reasons for not commercialising it until some remaining kinks are ironed out.

An alternative FPLG range extender might be one integrated into the vehicle's existing suspension system: mount the PMs on the shock absorber gas piston head and let them move through a stator coil in the housing tube during the normal damping cycles. Each wheel would then act as a separate generator, recovering electrical energy from the road vibrations, without the heat, noise or fuelling problems.


are you sure about the insufficiency of the air spring for the exhaust/compression stroke?Also, I see no reference to a need for a compressor. It would seem possible that they have not gotten that far, but perhaps the linear piston could be the compressor and just a reservoir would be sufficient.

Not to be raining on your parade, but a suspension/shock absorption based system of range extension would clearly depend on how bumpy the road is. Probably great in the boonies!


are you sure about the insufficiency of the air spring for the exhaust/compression stroke?Also, I see no reference to a need for a compressor. It would seem possible that they have not gotten that far, but perhaps the linear piston could be the compressor and just a reservoir would be sufficient.

Not to be raining on your parade, but a suspension/shock absorption based system of range extension would clearly depend on how bumpy the road is. Probably great in the boonies!

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