Years ago, I saw a documentary about engine efficiency where people were testing a pivoted, curved-path piston engine. The claim is that the standard piston engine, with straight up-and-down path, causes lots of friction with the walls while the piston travels through the chamber. This friction forces use of expensive rings, and ultimately reduce life span of engine block itself. It also limits linear speed of piston in order to prevent melting and engine failure.

The alternative was to attach the piston to an axle in the top and make it follow an angular path through the chamber. They even displayed a prototype. But I can't find that on the web. If you know about it and can send references, I would greatly appreciate.

If it helps, the engine was being tested for a motorcycle.

EDIT: Added the sketch below to help. While sketching, I found that it requires a sliding window around the rod connecting to the axle, to prevent leaking of gas/liquid. That's probably the biggest weakness. Still I wish to find the team and prototype to confirm that.

enter image description here

  • This is an interesting concept. Will have to look into it when I get home. – Pᴀᴜʟsᴛᴇʀ2 May 29 '16 at 15:23
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    Does this image look anything like what you are talking about? If not, what is different, or what can you further describe about what you saw? – Pᴀᴜʟsᴛᴇʀ2 May 29 '16 at 18:54
  • @Pᴀᴜʟsᴛᴇʀ2 Thanks but no. I added a sketch. As you can see, the piston head is attached to an axle which forces a circular/angular movement, which is supposed to reduce stress on the walls. While sketching, I realized that we need a sort of sliding window in the connection to the axle, otherwise gas/liquids would leak. That's probably a big weakness in this design. Still I'm interested to find the project/prototype so I can grab more information. – fernacolo May 29 '16 at 19:31
  • Question here? None was asked. – geoO Jun 1 '18 at 15:14

We built the engine, we called the company Pivotal Engines. worked well but fell victim to management issues.


enter image description here

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    This sounds like it could be the start of just the answer the OP is looking for but it needs more information. – motosubatsu Jun 1 '18 at 9:49
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    @Phil: Can you elaborate? You seem to be able to give a very good answer to the OPs question. – user5626466 Jun 1 '18 at 10:32
  • Wow, was not expecting an answer after so much time. Thanks @Phil, that's precisely what I recall from the documentary. Here's the explanation about less "piston rock and slap": pivotalengine.com/pistonrocking.html. – fernacolo Jun 6 '18 at 7:25

The only curved bore designs I am aware of are toroidal

I have been unable to find a match to the drawing, not even close. I took a hard look at steam engines as well.

Here is an example of a toroidal design

enter image description here

Here is a modern mock up of a similar design.

enter image description here

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