pistons without rings?

[maxaccel]

hello

its my understanding that piston rings create the biggest drag
inside an engine,
so to reduce this friction,I was wondering if it
would be possible to make pistons without oil rings?

what type of material would be required for such pistons and block to make it tight enough to prevent oil blow by?

has anyone attempted this yet?

thank you

 

[mattsooty]

Well....

The piston would need to have a coefficent of thermal expansion which was in keeping of that of the bore liners. If not massive friction forces or blow-by/lack of compression would ensue as the engine warmed up.

However, if the above could be controlled the piston material would need also need to have a very low coeeficient of friction to render the use of rings obsolete.

On the whole it is, at present, an unfeasible idea.

MS

 

[patprimmer]

Pistons are made without rings.

The rings are a seperate item fitted to the piston before assembly.

For an engine to run without rings, the piston would need to be virtually size for size with the bore and remain that way during operation.

If the fit was good enough to have the appropriate level of seal for oil and combustion gasses, the friction would be at least as high as the seperate devices (rings) designed to specifically to optimise this seal.

Even with an identical co-efficient of expansion, the piston temperature will vary at a different rate to the block, and different parts of the piston and block will have different relative temperatures, so they will expand differently and lose the seal or seize.

The internal combustion engine as we know it today is highly developed and manufactures are under constant pressure to reduce costs while improving fuel efficiency, emission performance, power, drive ability and durability.

If the cost of fitting rings could be left out without decreasing some other aspects, it would be done.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

...and it has been.

Small high speed engines don't need rings as the effective blow by rate is slow enough that compression can be maintained.

Starting is an issue.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[alehman]

Small 2-cycle model engines are frequently constructed without rings. They are either lapped steel on steel or aluminum piston with chrome or nickel plated cylinder.

 

[reidh]

A simple patent search of ringless piston engines on Google provides a good bit of material on the subject.

-Reidh

 

[waross]

I have heard of several instances where an engine that had a very easy life for years was called upon to make a sustained high speed run. Over the easy years carbon had built up behind the rings. Under high output conditions, the piston expanded more than usual. As the piston expands the rings recede into the grooves. When the piston gets hotter than normal and there is carbon behind the rings the rings can not recede as normal. Friction increases greatly and the engine is destroyed.
Remember that the bore is cooled by water and the piston is cooled by a spray of oil and the average temperature of the gas. Pistons tend to run hotter than the bore and get even hotter as the load is increased. Resilient piston rings allow enough clearance between the piston and bore to compensate for uneven heating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kenre]

Model engines as used in RC cars, boats, planes, are nearly all ringless.


Most common is a high silicon piston, varying from 17% to 30% Si, In a hard chrome plated brass sleeve that is tapered approx .002 thou smaller at the top.

When cold the piston is a very tight fit at TDC, almost a press fit sometimes. With the engine at running temp the top portion of the liner, being hotter expands reducing the 'Nip' at TDC. To get this to work well with low drag and minimal blowby while avoiding piston scoring, means holding tolerances far greater than a standard ringed setup.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[Lou Scannon]

Here is a page describing one of my favorite examples of this type of engine:

http://www.gregorie.org/freeflight/f1j/cox051/cox051.html

 

[Lou Scannon]

This article has some interesting facts & construction details.

http://www.mh-aerotools.de/airfoils/documents/cox_test_teedee_010_aeromodeller_october_1979.pdf

 

[BrianPetersen]

Halfway-house solution, used in many racing engines, is to use only a single compression ring instead of the customary two, and make the rings thin with low spring pressure, and organize gas channels to apply cylinder pressure to the inside of the ring to seal it against the cylinder wall only when there is pressure to seal. An additional benefit is that this lets the piston crown be shortened a little, to cut down the reciprocating mass a little.

The lack of redundant sealing means leak-down will generally be higher, and engine life will trend shorter because if that one piston ring starts leaking, there's no back-up function at all.

 

[patdaly]

Much of the information that rings cause the largest friction component in an engine date from the 1970's.

Most modern engines have very little ring friction.

 

[drwebb]

This is news to me. SAE 2002-01-3355 estimates that rings account for about 24% of the total engine friction power loss for a typical European 2.0L engine operating under motorway conditions. Is this way out of line?

 

[patprimmer]

The engine I just built would spin free when trial assembled to short block level without rings, but I needed a 16" long bar to turn it once the rings were in, and not much more once the valves were also operating.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JSteve2]

drwebb,
I don't think that figure is out of line. Piston rings at 24% are a huge factor of the engine friction, and may well be the biggest single factor. I think that's what was meant by the OP. It's hard to think of another single factor that would account for 1/4 of friction losses.

 

[GregLocock]

Valvetrain must be up around that

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TDIMeister]

Do ringless pistons in an automotive-sized engine (not RC engines) and let us know the result.

 

[GregLocock]

Honda used them in high speed engines.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TDIMeister]

And how did it go? I'm not trying to be a smart-aleck here, just an honest question because I don't know. Honda builds engines for MotoGP, F1 and IRL, which would to me qualify as high-speed engine applications. Are they using ringless pistons in them (my guess is no)?. And if not, why not? I mean, a potentially drastic reduction in friction is quite a panacea for an engine, even at the cost of some extra blow-by, isn't it?

I guess what I'm getting at is that people think cylinders are- and stay perfectly round, and ditto the pistons. Thermally- and mechanically-induced bore- and piston distortion do not exist (or is negligible) so it's easy to distill things down to a simple problem with a simple solution.

And I should go back to the original poster's question. If I understand the context correctly, he asked if it's possible to make (and run) pistons without oil rings in a bid to reduce friction, not eliminating ALL piston rings altogether.

 

[ivymike]

you could definitely do that. you'd probably find that the oil ring is not the biggest contributor to ring friction (and that some amount of the oil ring friction loss "moves up" to the second compression ring if the oil ring is gone). you might have trouble measuring the difference in engine power, although you could probably measure a difference in friction if you use a floating-liner friction test rig.