Toyota Developing a Free Piston Engine Linear Generator - No more engine AND generator, the engine IS the generator

Toyota free piston engine linear generator - FPEG, generates electricity from linear motion moving a magnet past a coil

Toyota free piston engine linear generator - FPEG, generates electricity from linear motion moving a magnet past a coil
Image credit: Toyota

Quite a few years ago, we wrote about a free piston engine design being tested in research labs. It's a pretty interesting concept where a piston is used to generate electricity by moving a magnet past a coil. The piston isn't attached to a crankshaft, it just connects to a magnet and the linear motion does all the work. Now it looks like Toyota is working to make this engine a reality.

It looks a little different than the design we showed you earlier, but it's the same principle, it doesn't do any mechanical work to drive the wheels, it charges the batteries or could be used as a range extender when the batteries are discharged. It's fairly compact, too.

This could have a lot of uses if they get the kinks out, but with six years already gone since we last wrote about this, there's obviously some tricky engineering involved.

Link: Toyota R&D via Road and Track


  1. Yeti2bikes says

    I must be missing something. What force acts on the piston to create the compression stroke?

    • GenWaylaid says

      There’s a gas spring behind the magnet, so the expansion of that gas should drive the piston back. It looks like it has a larger diameter than the piston bore, so the gas spring should be able to compress the gas in the cylinder to higher pressure than it itself experiences.

      In addition to that, the coil could be charged as a solenoid during part of the compression stroke to boost the piston’s motion.

      What I’m wondering is whether the magnet is moving in a bath of the same oil that lubricates the cylinder. There doesn’t seem to be any separation between the coil chamber and the piston rings. I might add a second set of rings just over one stroke length behind the first, and inject oil between them. The magnet experiences electromagnetic centering forces, so it doesn’t have any sliding contact that would need lubrication.

        • says

          I would assume the generator is used as a motor when starting the engine. I’d also expect that the linear motor/generator would be used to control the piston motion during operation.

  2. David Duarte says

    Wouldn’t it stop producing electricity at the top and bottom of each stroke? It seems like a rotary engine design following a similar principal would be more efficient at producing power in this fashion.

  3. SKoo says

    Interesting: it’s a two-stroke, marine diesel style. How will they control emissions?

    As it needs forced air induction, I’m guessing Toyota will use an E-motor assisted turbo.

    Other interesting details are the two outlet valves with electronic actuation.

    If they get this to work properly there no reason why they wouldn’t use this engine architecture for a mechanical drive train. Weight, size and efficiency reduction could be impressive.

  4. Paul says

    Toyota could use it to share more components between an electric and combustion model:
    1. Combustion model: fuel->electricity generator + electricity->mechanical engine
    2. Electric model: battery + electricity->mechanical engine
    So, engine, chassis, some fairings, wheels (possibly even in-wheel if the unsprung mass issues can be resolved)… Essentially almost everything expensive can be consolidated in a way that should allow them to react quickly to market demands…
    Also, since an electric engine is being used instead of a drive-train, there should be about the same or fewer moving parts and the efficiency should be about the same or possible even better.

    Short of emissions (which we simply don’t have any information on), this looks VERY promising.

  5. M. Mitchell says

    They’re fighting a steep uphill battle here with sundry tricky hardware issues and even trickier software control architectures competing against extremely mature and reasonably efficient conventional internal combustion/generator technology. A purpose built direct-injected small 3 or 4 cylinder IC engine/generator package is going to be very tough to beat in the relevant metrics, and a great deal easier to design and manufacture with very few, if any, technological show-stoppers to overcome.