• May 1, 2014
We often hear how an electric vehicle powertrain architecture allows vehicle designers much more freedom than a traditional ICE powertrain does. With differently shaped battery modules and small electric motors, there are lots of way to put the pieces together. With today's plug-in hybrid technology, engineers still need to put a decent-sized ICE somewhere, but new technology from Toyota could free up the gas-electric vehicle designers of the future.

Presented at the recent SAE World Congress in Detroit, the idea from Toyota Central R&D Labs Inc. and involves what is called a Free Piston Engine Linear Generator (FPEG). Think of it as a sort of one-cylinder, two-stroke mini-engine that can work either as a generator (thank to magnets and a linear coil) or to directly drive a vehicle. The current prototype is a 10-kW unit that Toyota say would provide enough power to get a B- or C-segment electric vehicle up to highway speeds (75 miles per hour) when paired up to offer 20 kW. Pairing the FPEGs is also important to minimize vibrations. One system tested by Toyota had a 42 percent thermal efficiency, but the engineers are working to improve the overall efficiency even further.

You can watch an animated video of the piston in action here (click on "Outline") and see the SAE papers here and here. More technical details are available at Green Car Congress.


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  • 57 Comments
      • 7 Months Ago
      Snowdog "With only 26 HP, it wouldn't matter what reasonable size your battery is. On a road trip it will run out unless you drive SLOWLY. Once your battery runs out, then you will have trouble with inclines or a even a strong headwind. 26 HP is limp-home range extender, not a road trip range-extender (like the Volt)." No. It is better than the Volt (Toyota or Ford plug-ins) which are constantly lugging around a bunch of weight you should rarely need. For these small sized, battery dependent range extenders you must start range-extending long before running down the battery. If you are going to be traveling beyond the battery range, the extender must start as soon as needed to maintain battery charge throughout the trip which could be as soon as you pull out of the driveway. If your battery fully depletes at any point and you are relying solely on this extender to provide motive power, you've effectively "run out of gas" and are darned lucky you can limp anywhere.
        Snowdog
        • 7 Months Ago
        " If your battery fully depletes at any point and you are relying solely on this extender to provide motive power, you've effectively "run out of gas" and are darned lucky you can limp anywhere." Which is why I call it a "limp-home" range extender. So I guess you agree. The volt provides about 50KW from it's range extender, making the car MUCH more drive-able on the range extender, and much more suitable for general purpose, including road trips. It does much more than limp after the battery is run down. For myself I consider the Limp-Home solution one I would skip entirely, either go PURE EV and skip ICE maintenance/weight altogether and learn to drive within your range. Or buy one like the Volt that is more than just a limp home range extender, so you can drive across country like a normal car if that is your wish. BMW i3 gives you the choice between pure EV and Limp home range extender. The limp-home model is thousands more expensive, ~250+ lbs heavier, slower, and gets worse EV range, all to have a limp home range extender you hope you never actually have to use.
          • 7 Months Ago
          @Snowdog
          Snowdog "Which is why I call it a "limp-home" range extender. So I guess you agree." I don't agree with your characterization. The vehicle would only be limping anywhere because you've effectively "run out gas". Limping at that point is still better than leaving you stranded but it isn't remotely a basic performance characteristic inherent due to small size of the range extender. There zero reason that any EV should ever need a range extender to provide enough energy for motive power. A 26kw range extender can provide equivalent range to adding 26kwh of additional batteries over the course of an hour. Over the course of two hours, a 26kw range extender can provide additional range equivalent to adding 52kwh of batteries. If 26kwh of battery will provide an EV an hour of driving, a 26kw range extender can help the same EV provide 3 hours of driving at exactly the same performance level.
      cointel
      • 7 Months Ago
      Nice design, minor changes and you can boost the amount of electricity generated.
      Snowdog
      • 7 Months Ago
      20 KW paired up? That is still less than 30 HP. That is more of a "Limp-Home" range extender, than a drive on long road trip, range extender.
        rubley00
        • 7 Months Ago
        @Snowdog
        30 HP is plenty to keep a car moving at highway speed. That's all it takes.
          Joeviocoe
          • 7 Months Ago
          @rubley00
          The Volt normally only allows SOC to drop to 30% (16*.7=11.2kwh keeping 4.8kwh in reserve)... in mountain mode, it will prevent the SOC from going below 45% (16*.7=8.8kwh keeping 7.2kwh in reserve) which allows the battery to assist the 1.4L 84HP engine up steep hills. The smaller the range extender, the more battery SOC kwh will be required. This 20KW of this engine should NOT be put into an EV with 38 miles of battery range (you would have to put 8 inside instead of 2). Most likely, since these are very small and light... 2 would be placed inside a 100 mile EV... with a mountain mode, to ensure nobody is trying to take a drive up a steep hill after depleting down to 5 miles remaining. GPS trip evaluation with topography maps should also be used to set the mountain mode in advance.
          Joeviocoe
          • 7 Months Ago
          @rubley00
          A proper battery does not simply "run out". The generator is supposed to kick in way before it gets so low, as to be too weak to handle a few hills. If the i3 cannot do this.. that is the fault of BMW engineers. They need to put in a "mountain mode" like GM did. The computer should consider the drivers request and be able to anticipate needing higher peak power, and turn on the RE earlier. The i3 is an EV first, and the RE seems like an after thought, the battery is sized for EV only operation. The Volt had considerations for much more, and thus, the Volt only uses 11 kwh out of the 16 kwh... so the power output would always be there.
          Joeviocoe
          • 7 Months Ago
          @rubley00
          --" If you have to climb a hill," That is PEAK power. The battery handles that. The output of a range extender doesn't have to meet peak power, but only average power over several miles. If the BMW is suffering up hill, they did not size the battery properly. Luckily, with the extremely small size and low weight of this linear engine, the batteries can be beefy enough to support any hill.
          Joeviocoe
          • 7 Months Ago
          @rubley00
          At its root, Mountain Mode is meant to force the Volt to switch from 100% battery-powered mode to charge-sustaining mode (where the engine is burning gas to generate electricity, and sometimes even powering the wheels directly—basically acting as a hybrid) long before the battery is fully drained. GM engineers have said this is to preserve battery power to help get the car up steep slopes with an assist from the electric motors—hence the name "Mountain Mode." In contrast, in both Normal and Sport modes, the car will use up every last drop of stored battery power it can before switching to charge-sustaining mode. http://www.plugincars.com/chevy-volts-mountain-mode-vastly-underrated-yields-new-driving-strategies-107176.html
          Snowdog
          • 7 Months Ago
          @rubley00
          It's not plenty. It is just barely enough. If you have to climb a hill, you will slow more than most laden 18 wheeler. Check out the BMW i3 review with a similarly weak range extender. They slowed to 44MPH climbing what they described as a slight incline. Actually I read the i3 range extender was 34 HP, this is 26 HP, so it would be even worse. You can't escape physics when you downsize the range extender to the point that there is no headroom to climb a hill.
          Snowdog
          • 7 Months Ago
          @rubley00
          With only 26 HP, it wouldn't matter what reasonable size your battery is. On a road trip it will run out unless you drive SLOWLY. Once your battery runs out, then you will have trouble with inclines or a even a strong headwind. 26 HP is limp-home range extender, not a road trip range-extender (like the Volt).
      BipDBo
      • 7 Months Ago
      That's a really cool idea. I have doubts it will ever make it though, if for no other reason, then just for the need for electromagnetiv valves. EMVs that are reliable, durable and affordable for use in cars has been the holy grail for engine performance and efficiency for decades, but has never been achieved. Seems like they could substitute in some mechanical linkage and still make the concept work.
      CoolWaters
      • 7 Months Ago
      - So, they'll actually deploy two into a car. - Looks like they could be moving in the Volt design direction, as this would be lighter weight then a conventional engine, with no camshaft to turn. But, those two engines will only produce electricity, because it's not turning a camshaft. But, that's ok, if the battery increases in capacity.
      Joeviocoe
      • 7 Months Ago
      Two stroke designs have certainly worse emissions. But with the increased efficiency potential from not having so many other things... combined with the infrequent use of this engine as range extender... the greater emissions might still be well worth it.
      JB
      • 7 Months Ago
      Well, someone has SolidWorks. I like it.
      Dave
      • 7 Months Ago
      Just wanted to point out that this is not, by any stretch of the imagination, a new idea. "The first free piston generator was patented in 1959,[9] and since then, a number of variations have been proposed..." http://en.wikipedia.org/wiki/Free-piston_engine#Generators
      Dave
      • 7 Months Ago
      Funky. I wonder what they will do to maintain proper pressure in the "gas spring chamber."
      diffrunt
      • 7 Months Ago
      Looks like my EV will be from the owhatafeeling bunch
      Aaron
      • 7 Months Ago
      The animation shows energy being generated only on the downstroke of the piston. What's true is you will also have energy generated on the upstroke of the piston too! I wonder why they didn't show it in the animation.
      danfred311
      • 7 Months Ago
      From the geniuses at Toyota whom we know have the finest of grasp on winning technologies :) I have considered linear generator like that but I hunch the math isn't favorable. You have a heavy mass that has to come to a complete stop and return many times per second. The variable speed means variable generator voltage although that can be overcome and then the shaking from that thing will be amazing. Continuous rotation is just better I'm afraid. What they can do is a rocker piston. A curved piston in a curved cylinder on an arm such that the piston never scrapes the sides. Should help efficiency and wear. But ultimately there is no combustion engine at all.
        Levine Levine
        • 7 Months Ago
        @danfred311
        your regular reciprocating ICE has pistons that come to a stop and then reverse direction, too. "A curved piston in a curved cylinder ......." means a type of Wankel engine. We know Wankel has problems.
          danfred311
          • 7 Months Ago
          @Levine Levine
          not a wankel. like a normal cylinder, just bent sideways. And hinged off to the side. So it isn't sliding on the walls but held in place by the arm to the side. a wankel is a rotary engine. mine is a reciprocating rocker still working a crank shaft like a normal piston. as for start and stop, a normal engine makes a sine motion dictates by an arm connected to a continuous rotation. sure there are heavy forces but a piston and rod can be much lighter than a freeflying permanent magnet clump. I'd take a bet that this design never sees the light of day. It's just flatulence from the flatliners at toyota. toyota's R&D seems oddly headless. well, even more than the other car makers which is saying a lot.
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