Why not piston rings every time?

[tk]

Before I start work on another engine I have a general question. Some engine plans require a piston with close tolerances in a cylinder, maybe even lapping. Other plans describe a less close piston with piston rings. Why couldn't I use piston rings for every engine (except low power Stirling)?

 

[kvom]

Cast iron rings are challenging to make. So a small engine operating on air can work without them.

 

[lohring]

Small engines are more easily overwhelmed by friction losses than larger engines. Model engine builders developed a system of chrome plated brass cylinders with high silicon aluminum pistons that can have very low friction when fitted correctly. Other very small engines (Cox) used precision ground, hardened steel pistons and cylinders like hydraulic valves. Lapped cast iron piston and cylinders (Fox) also have been very successful.

It's easier to get a good seal with piston rings, though, so there's a point where they will work better. Gasoline as a fuel results in higher temperatures than alcohol based fuels so differences in expansion are tougher to deal with in gasoline engines. A recent example is the CMB 35 cc gasoline engine. CMB tried to use their ringless piston design that's successful in their 16 cc engines that run alcohol based fuels. It was a failure and they ended up with a ringed piston. The aluminum pistons usually seized in the chromed liners at full power, even after careful break in.

Lohring Miller

 

[Tin Falcon]

IMHO most of the little air steam engines do not need rings.
I have several I built with no rings.
the PM research engines call for teflon rings(included in the kits) these seem to work well. I have an engine with a teflon piston in a lucite cylinder I can run it on breath power.
Tin

 

[tk]

Thank-you all for the information - it would have probably taken me a lot of failed experiments and I still might not have understood why. Thanks again!

 

[Chriske]

Stirling engines and vacuum engines(small ones) will not work well if piston rings are used. And I even think they will not work at all imo. If applied friction will stop these little engines.

Chris

 

[vederstein]

So what defines "little?" 1/2" Bore, 3/4" Bore, 1-1/2" etc? Where's the break even point for which type of engine.

I know it wouldn't be exact, but are there guidelines?

...ved.

 

[lohring]

I'm not aware of any successful gasoline engines without rings, but most are bigger than 15 cc with over 25 mm bores. While many successful 15 cc engines running glow fuel have been built with ringless pistons, high production engines of this size with around 25 to 28 mm bores tend to run rings. Below the 25 mm bore size, most glow engines use ringless piston designs.

Lohring Miller