Circle Cycle Internal Combustion Engine

Circle Cycle orbital, non-reciprocating internal combustion engine

Circle Cycle orbital, non-reciprocating internal combustion engine

We haven't looked at any new internal combustion engines for a while and I just spotted this one, the Circle Cycle engine. It's described as an orbital, non-reciprocating internal combustion engine, having pistons and cylinders and able to run on a variety of combustible liquids or gasses. It has no engine block, no crankshaft or connecting rods, no separate flywheel, no intake or exhaust valves, no water pump and no radiator or any of the hardware usually associated with these items.

Circle Cycle Engine - operational prototype

Circle Cycle Engine - operational prototype

The engine uses direct injection of fuel into the center of the combustion chamber and it can be configured to run as either spark ignition or diesel. Potential applications, according to the website, are pretty much what you would expect, which is anything you would use any other internal combustion engine for.

The company has been developing the engine for five years and believes it is at the stage where it can be taken to the next step, which is design and production for specific applications, so they are looking to sell or license the patent or enter into a joint venture with someone who can carry it forward.

The only part of the design I question is the gearing necessary to keep the cylinders and pistons in alignment while the large geared wheels are turning. Any hiccup in operation or broken tooth on one of the gears could result in catastrophic failure, but they may have addressed that somewhere in the design and it may be more apparent if we were to see the actual engine up close.

There are a lot of photos, drawings and animations on the site plus a great deal more explanation of the concept. Is it better, more efficient or in some way an improvement on what we already have? I can't say, but it's pretty ingenious and cool to watch in operation, just the sort of thing any gearhead should enjoy.

Link: Circle Cycle

Animation and videos below:

Animation to show the process.

Prototype hand cranked in slow motion.

Prototype running on propane.


  1. B50 Jim says

    Aha! Paul scooped Popular Science on this one!

    Looks good and it addresses some aspects of a standard engine that are least efficient. But Paul is right — One broken gear tooth and you’e got a boxfull of scrap. And what happens when the gears get slightly worn? But I like that it uses nearly all of the stroke for power production. That alone could make a big improvement in efficiency. Issues to address, however, are emissions, longevity, production gremlins, etc., etc., etc. Usually, no matter how well engineered, rotational engines incorporate some weaknesses or flaws that sink the project. This is because the inventors put their entire fortunes into development and a prototype, and can’t locate any more R&D money to overcome design flaws, production costs, etc. As promising as Circle Cycle is, it has a long way to go before we see them in our cars, trucks and bikes. Still, I wish them all the best of luck on the project. A diesel version would be a formidable powerplant.

    • AlwaysOnTwo says

      Not sure about the longevity issue or wear characteristics of the materials being able hold tolerances, but I’m damn sure a broken tooth on a camshaft, a broken link in a timing chain, or a stripped drive gear on an oil pump will result in a ” catastrophic failure” in virtually any internal combustion engine.

      Yeah, yeah, I know, this engine demonstrates potential higher stress points in the design, yada yada yada.

      • lee says

        How about gearing made out of 300M? That should solve the stress issue on gears as well as fluxation

  2. Miles says

    As interesting as this is, you have to admit that in the end they are converting linear motion to reciprocating motion.

    The problem arises of the mass and number of the moving parts. In my opinion this isn’t as efficient as a piston engine, much less a Wankel.

    Interesting to see though. If you see anyone with a hybrid electric/turbine for traction motor use I would love to see that. Nothing is better than an Electric for traction, or a Turbine for energy production.

    • Les says

      Read all the information about effeciency. this thing is 10 times more effecient than a standard internal combustion engine. That is if the facts as reported are true

  3. Miles says

    On a look at the engine based on its own merits I have a few questions:

    Where does the fresh Oxygen enter,(and fuel for that matter)

    Related; where does the exhaust go?

    I see that you can trade the valves and valve-train for an “open” cylinder. But is the complexity of the motor offset by the loss of the valves?

    Can this motor change the “valve timing” to allow for greater efficiency throughout the rev range?

    It is sometimes overlooked how much R&D has gone into the old piston and valve type of internal-combustion engine, and how steep the path really is when you try a new one.

    My hat is off to these guys for their hard work.

    • todd says

      This doesn’t need valves. It looks like the intake and exhaust cycles are around 270 degrees duration. The intake and exhaust “ports” within the chamber are the same diameter as the piston itself (which itself acts as the “valves”). This design relies on the supercharger on top to pressurize the entire piston chamber. The fuel is directly injected in the cylinder after the complete intake cycle. The exhaust escapes into the same chamber but is forced out by the supercharger.

      Brilliant, I’d say.


  4. says

    I thought it was pretty informative and you have to admire the guys behind this bit of equipment. Amazing to think that people are still striving to make things better!!

  5. Norm says

    Interesting but I don’t think it is something we will see in a practical day to day application. As others have noted, the gearing is the spot most questionable. One broken tooth and disaster. Another questionable spot is the “ring” packs. With a little wear and tear they could easily cock in the bore and reduce power or fail altogether.

  6. Paulinator says

    These computernauts already did the impossible by creating porno of an actual moving prototype. It took NASA billions of dollars and a decade to master re-entry…and even then there are calculable risks. This engine defies probability with every passing second on the hobbs meter.

  7. Aichbe says

    ANY part, like a nut, comes loose and gets between the piston and cylinder, and it just got expensive. Wear, or even dirt on the gears would throw the timing, and cause misalignment. The cylinder to piston alignment is another critical area, as if even one out of 8 got cockeyed a few degrees, it would all tear apart. All materials would have to be the best available; not too many cast parts would be used, mostly billet. Great in a Rolls Royce. Might be too heavy for aviation, though. How much torque and horsepower is it capable, and how do you hop it up?
    The concept kind of reminds me of how old bowling alleys worked, but with 500% less slop. Worst. Job. Ever.

  8. Aichbe says

    Oh, yeah; regarding combustion timing; wouldn’t it need to have a very sensitive ignition advance/retard setup to compensate for burn time and engine speed variences. Does it start easier on “retard ” setting, or make more power advanced?
    With a diesel, it wouldn’t matter, but with a spark plug, it might. Changing plugs looks daunting, too. It would need a supercharger to work well, as long as it was a timed intake, and not continuous. $$$$$$

  9. Fred M. says

    And this answers what problem? That traditional internal combustion engines are too compact, too lightweight, too reliable, too mechanically simple, and too inexpensive?

    Look at the amount of mass that the engine is moving! The flywheels are huge! Not only do you have the mass of the piston, now you also have the mass of each entire cylinder spinning around.

    A traditional four cylinder internal combustion engine has one crankshaft, four rods, four pistons, and four piston pins. No gears necessary. This has 24 gears and it appears that even a slight bit of gear wear or backlash could cause cylinders and pistons to droop, slamming the crown of the piston into the edge of the cylinder.

    To make matters worse, this dumps its exhaust into the same chamber that houses this incredibly complex, tight-tolerance gear assembly, putting combustion byproducts, including carbon soot, directly into oil that lubricates all of this.

    I love innovative engineering, but I’m not a big fan of Rube Goldberg “solutions” to non-problems.

    • todd says

      Since there is no reciprocating motion the (assumed) extra mass has little consequence. Yes, it would be a bit slower climbing up and down through the RPMs. It is best used with an electric transmission where it can run at a constant RPM.


      • Paulinator says

        Todd, reciprocating masses require enormous amounts of energy to accelerate – then decelerate (up to 600 times per second when screaming at F1 tempo). That energy, however, comes from the inertial effect of the rotating masses that store and release it – aka: the flywheel. Its net-zero.

        • todd says

          just like I said; in this design there is no reciprocating mass to accelerate or decelerate. In engines where there are reciprocating masses, it’s not the energy that’s required but the effect of the forces imparted on the component being accelerated. How many times can a connecting rod or piston dome be stretched and compressed until it fails?


  10. Cameron says

    This is an absolutely fascinating engine. The shear fact that they could get a rotating piston to insert into a rotating cylinder and fire is mind boggling. I know packing machinery does this all the time but it doesn’t then try to fire the piston out with an explosion! It’s hard enough to get three pistons into the cylinder block of a twostroke motorcycle engine. This may be feasible with a large slow moving engine but I wouldn’t want to be within firing range when the piston missed the cylinder. I love the ingenuity and you never know when something as obscure as this will lead to the big break through.

  11. Cameron says

    Check out Capstone Turbine Corporation. They are putting 65kw turbine generators into big rigs with traction motors as range extenders. They also make a 30kw unit that I think would fit nicely under the hood of a 3/4 ton pick-up with a large traction motor where the trans sits. A few batteries under the box and you have a Chevy Volt Silverado. There are also micro turbines designed for large scale model airplanes that would fit on a motorcycle. Jay Leno eat your heart out.

    • todd says

      That info is a bit old. Capstone have not been able to bring pricing low enough to be a viable APU…


  12. jScottj says

    Gear-tooth concerns seem trivial.
    Imagine jet-engine compressor blades, automobile cam drives & transmissions, bicycles …
    Except for my cheap mini-blinds, I can’t think of when gear-tooth failure has loomed large.

  13. F0ul says

    Love it! 😉
    I think that the issues of excessive wear are perfectly valid criticism, however, Used as a generator, and built using plastic rather than metal, it could be a lot cheaper than the equivalent 4 pot. It could even be developed into a disposable generator due to its compact nature making it easier to store. In that guise the wear issue would be less of a problem! 😉

  14. Mark in Sydney says

    Impressive. I do wonder about the maintenance, though. Something lets go in there and the results would become very interesting indeed.

  15. Gerry says

    1. How do you lubricate the cylinder/piston walls? Oil mist? what about emissions from a total loss oil system?
    2. Thermal expansion/uneven heat warp-age?
    3. Max RPM? Large diameter rotating parts usually mean low RPM and low power.
    4. Single offset support bearing on the piston/cylinder limits RPM and power.
    Fun exercise but too complicated and limited by design

  16. says

    Im not so sure this design would stand up the the filth of a automotive or transportation power. It may make a great design for a industial air compressor or pump.
    The engineering on it is genius for sure.

  17. B50 Jim says

    There’s an ad for it in this month’s Popular Science — the kiss of death for alternative engine technology. PS even acknowledges this with a whimsical MAD fold-in on a back page. Too bad — looking at this engine again, I think it has potential, flaws and all. With sufficient engineering and durable parts, it could make a strong-running powerplant for any number of applications, although I think it would work best as a diesel. Emissions, however, would be a problem, unless some means of treating them downstream were made available. And the inside would get pretty gunky; I don’t know how well those gears and bearings would tolerate such a corrosive and destructive environment. I might make a good compressor, either for air or refrigeration.

  18. mustridemore says

    The fact that the pistons come completely out of the cylinders is a recipe for disaster. No matter how good the timing is, at high RPMs parts are going to deflect, when they do, boom goes the dynamite.

  19. brandon says

    nice too look at but rather useless in the real world. aside from wear on the tolerances causing a scrap heap, the exhaust solids will build up on every surface in the engine and grind it to death. plus, combustion’s primary output is heat, where is that going to go? I assume we run the unit for 20 seconds like they do in the video, let it cool for a hour, then tear it down and scrub off all of the oil and soot.

  20. cas says

    What about piston rings? The expansion stroke of the engine will be very inefficient and compression very poor.

  21. Kevin says

    I think this is very cool how it operates, but cool doesn’t mean it is good. I usually like radical ideas, but this looks to have many flaws.

    1) Lubrication: How? if it is an oil bath it will just suck that in and burn in. You could run pre-mix like a 2-stroke, but then their are no uses for it with such bad emissions.

    2) Ring seal. With the piston coming completely out of the cylinder you can’t use a ring. So the compression leakdown will be terrible. You will also need loose tolerances to account for thermal expansion, don’t want the piston slamming into the cylinder, so even more leakdown. In fact loosen up those tolerances some more because any vibration will cause it to slam the cylinder.

    3) Intake/exhaust. Again, if it is lubricated then it will be burning oil. And because the exhaust is dumped right where the intake is it will be breathing exhaust. That exhaust will cause heat and wear problems in the engine.

    4) Rotating mass. That is a LOT of rotating mass. Not only will it rev slow, it will cause a vehicle to have bad handling. It will also have very slow response as that is a lot of mass to spin.

  22. George says

    As I understand it, the “crankcase” is pressurized by a supercharger sitting on top of the engine. Exhaust flows out the bottom. But in order to have a positive pressure in the crankcase, the exhause flow has to be restricted- backpressure has to be equal to crankcase pressure. Hardly a recipe for efficiency- but it is fun to watch.

  23. lee says

    Seems the gear issue would not be too hard to solve with Ti-Moly gears.
    The science of this is sound. The materials can be amended as they have been amended/improved for years in our I.C. Engines. The fact that no big brother has stepped in to buy this amazes me. SO many viable possibilities.
    It would seem as always that the engineers need a salesman.

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