Stirlingengineforum.com

Hi

As u know friction is one of the biggest enemy of stirling engine. I was thinking to reduce friction can we use a ribbed piston? We cut groves along the lenght of piston same as piston ring grooves. This will reduce the surface area of piston which will further reduce friction. Also the displacer rod seal can be made this way but with inside grooves. Sealing wont be a problem as far as piston is a perfect fit in cylinder. Or we can use labyrinth piston and cylinder. What do u guys think? U have more experience than me. Please give a feedback as i dont want to waste time and money on idea that will not work.
Sorry for bad english and spelling mistakes.
Thanks in advance.
Rustybarrel

This technique is used in many larger engines. By having two rings near the top side, and one near the back side, you achieve a good seal, and prevent some side loading. However, rings almost always require lubrication, and as such would best be designed to use a non petroleum based lubricant to prevent gumming up of the interior.

Thanks for the info. I am not thinking about using rings. i just want to cut the ring grooves so that piston surface area will reduce. Its the piston that will seal the gases.

Whether it's o-rings or ribs, it's the same geometry in the end. So long as the central structure is strong enough (which pretty much demands a single piece), you would only need a couple of high points at the ends, with most of the middle length removed as much as possible without compromising the structure. It will decrease both friction and weight, both pluses. Your main disadvantage is reducing it's strength.

Your concept seems sound enough for experimentation. Many syringe designs incorporate this idea, especially plastic ones. It's rather hard to push an inch long piston through a plastic syringe, but a piston made of three "ribs" (a common design) is easy enough to push with a single fingertip. Turning the ribs into a graphite piston, and comparing against one without ribs on a spring loaded measuring device would be an easy enough experiment that wouldn't be expensive. Of course, you'll need to test the two in the same cylinder before turning the ribs to ensure they both start with the same friction (use as light a spring as possible).

Thanks for the feedback. I will make a ribbed piston and compare its performance with existing piston. I will try to post the results as soon as possible.
And can u guide me about labyrinth piston. Is the concept possible for stirling engine? I have worked on big compressors where this design is used to compress gases upto 20 bars pressure.

The big question with a labyrinth piston design is, will the increased air friction counter the decrease in surface friction? I imagine one could be designed that would have a net reduction in friction, but it would take a lot experimentation (computer modeling can only go so far with something like this), and would be dependant upon speed and pressure among other things.

rustbarrel, laberrinth seals in pistons need only to be .010"/ .020" wide, and no deeper than that, how about more of a cotton reel shape, ie., full diameter at each end, with the bit in between reduced by a few thou", you only need a short bit for compression, the length is required for stability (look at the piston in a steam engine with a ridgid piston rod and guide). The laberinth seal is used in bearings, but they are usually of alarger bore than those used on most of the engines that would be built on this site, the largest displacer gland that I have, on a Ringbom motor is 1/2", and it has carbon impregnated teflon for a bush, as do many of my motors. Ian S C