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Concept: Oscillating Piston Engine (Another Toroidal Engine)

A cutaway view of the toroidal engine showing the pistons. Click to enlarge.

Rotoblock, a startup company, is developing a new toroidal internal combustion engine: the Oscillating Piston Engine (OPE). The company has also signed a letter of intent with Apollo Energy Systems, a manufacturer of lead cobalt batteries and akaline fuel cell systems (earlier post), to develop a hybrid electric drivetrain with integrated OPE engine.

The OPE is one of a number of toroidal internal combustion engines currently under development, including the MYT engine (earlier post), the VGT RoundEngine, the Rotary Opposed Piston Engine (ROPE), and the Trochilic engine. Multiple designs for toroidal engines extend further back in time to the Tschudi engine (1968) and earlier.

A toroidal engine is one in which the power pistons rotate in a perfectly circular chamber with the drive shaft at the geometric center. (A torus is the doughnut-shaped surface of revolution generated by revolving a circle about an axis coplanar with the circle.)

One of the theoretical benefits of the design is a dramatic increase in power-to-weight ratio compared to a conventional reciprocating piston engine. The Rotoblock engine fires 16 times on one revolution of the crankshaft, compared to a V-8’s four times per crankshaft revolution, for example.

The original patent for the OPE was issued in 1993, with four follow-ons for specific applications, including the use of the OPE as the genset for a series hybrid (awarded 1994).

Rotoblock acquired the original OPE prototype engine and rights from the inventor (Dr. Monti Farrell). The company in now on its second-generation implementation of the technology.

A schematic of the Rotoblock design. Click to enlarge.

The current Oscillating Piston Engine design incorporates four pairs of pistons, each alternately attached, via two opposed oscillating adjacent thrust disks, to two coaxial driveshafts extending from one face of the cylinder block.

The thrust disks use coaxial shafts and a dual scotch yoke mechanism to couple the motion of the oscillating pistons to a single crankshaft.

A sketch of the engine.

The round cylinder block containing the pistons, connecting discs and coaxial output shafts continuously rotates in a counter-clockwise direction, with a 90° rotation for every complete revolution of the rotating crankshaft. This action is accomplished by a four-to-one ratio gear reduction mechanism that couples these two components together.

Rotation of the cylinder block causes a pair of inlet and exhaust ports, as well as two diametrically positioned spark plugs, to regularly appear in the intervening spaces formed between the faces of the oscillating pistons as they move back and forth through 22.5 degrees of axial rotation within the confines of the toroidal cylinder housing. The movement of the pistons form combustion chambers of variable volume in the toroidal cylinder.

Rotoblock had intended to show its 1st-generation OPE during the New York International Auto Show las month, but the display case that contains the prototype OPE was damaged in shipping.

The company is putting its 2nd-generation design through testing in its Santa Rosa, California, facility.

(A hat-tip to Robert Cummings!)




So the prototype was damaged in shipping? Hmmmm.....


Technically, no. They say that the "display case" was damaged in shipping, the engine itself was not. And what was to be on display, according to the company, was the first-generation engine.


I'd like to see an animation of the combustion cycle these use.

Also, the concepts seem very simular, what is sufficiently different to warrant seperate patents?

John Allison



This is interesting and ingenious, perhaps, but I don't see how it's particularly 'green'.


the MYT's claim to greenness is that it spends a much greater period of time burning the fuel, leading to more complete burning, less CO.

I imagine this engine would make a similar claim.

also, less reciprocating parts, higher operating temperature. these may lead to claims of higher efficiency.

but, as was observed on the post for the MYT engine, these claims seem to come from nowhere and that is where they usually go.



Roger Pham

Spend much greater time burning the fuel, and melt the piston as well as the cylinder altogether, as the temperature goes as high as 4000 degrees F. If you don't melt the engine, you will burn your lubricating oil into carbon with the heat of combustion. In a four-stroke-cycle engine, a rapid combustion is followed by an intake stroke which significantly cools the piston and the cylinder prior to the next power stroke. Any piston-ported engine will give unacceptable exhaust emission due to the fact that lubricating oil will get carried from the cylinder port out into the exhaust. The increasing exhaust emission standard won't help, either. That's the reason that no two-stroke engines ever can pass stringent exhaust standard and be acceptable in the market place. This toroidal engine is in the same situation.


I can't see why this engine would be more efficient than a normal engine 'cause as far as I know, combustion between the two pistons will push them tangentially outward i.e. in a straight line. But as the engine body is curved, component of force pointing towards the center of curve will be the only force that drives the piston. And frictional force will be much greater between pistons and walls due to the tangential component of force. Whereas in normal engine, combustion force pushes the piston down, exactly where it is supposed to go.
Tell me if I'm wrong somewhere!


I get really excited about these things and then they just die. I used to believe in government conspiracies keeping these technologies down, but now I am gravitating towards just snorting "bullcrap!" when this sort of thing shows up with all these great claims. For instance, see the McMaster Motor ( The info hasn't changed for 6 years. Hmmm. And what about the "skycar?" Check out ...again, the info doesn't really change much over time. But look! You can buy life-extending almond-butter on the main page. These engines and concept cars and things that bait us just seem to never come to light...

Lee McLane

for Roger Pham: Regarding your comment about straight line tangential forces incresing friction between the piston and the cylinder; in fluid mechanics, pressure forces operate normal (perpendicular) to the surface of a containment vessel(i.e. the face of the piston in this case), so I don't see a problem. However, I will hasten to qualify my observation by saying that am not an engine design engineer but rather a physicist in a completely different area of study who happened to accidentally stumble across this discussion while googling to see if VW was making any noise yet about a fuel-electric hybrid of some sort in the near future.

Lee McLane

Oops! It seems I misread the thread; I should have addressed my previous post to Mridul - not Roger Pham. My apologies.

Barry R. Guthrie

How is lubrication going to be applied to the pistons to minimize friction when the chamber is always expoised to high temperature combustion gases? How is the lubricant going to be prevented from generating emission problems?


Sort of reminds me of the rand cam engine thing.


Yes, how exactly do you lubricate the inside of the 'doughnut' when it is ALL combustion chamber? It can only be done with a total loss oil system and don't tell me that's green....

And as with the MYTE (which I do not believe ever ran under its own power) how do you get rid of so much heat in such a short amount of time? The limiting factor with high speed piston engines tends to be piston cooling, above all else. I applaud efforts to come up with something better then the old crank and piston jobs, but there always seems to be a flawed principle involved with most new designs.



Our current ICE design is flawed too. It is just that we have applied a century or more of engineering to refine the designs.

Daren Chidester

hello ,your engine design seems to work,I am also working on a design but my engine design is a bit different then yours but similar to a car engine.

Daren Chidester

I almost forgot my engine design is electric no fuels of any kind

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