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pistons without rings?

pistons without rings?

pistons without rings?

(OP)
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

RE: pistons without rings?

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

RE: pistons without rings?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: pistons without rings?

...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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: pistons without rings?

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.

RE: pistons without rings?

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

-Reidh

RE: pistons without rings?

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

RE: pistons without rings?

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.


www.retallickeng.com.au

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

RE: pistons without rings?

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.

RE: pistons without rings?

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.

RE: pistons without rings?

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?

RE: pistons without rings?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: pistons without rings?

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.

RE: pistons without rings?

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

RE: pistons without rings?

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.

RE: pistons without rings?

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.
  

RE: pistons without rings?

Valve train friction contribution is the most sensitive to engine speed.  The paper calculates ca. 65% of total at idle condition, 33% at urban, and 7% at motorway ('direct acting bucket tappet valve train system').  Of course the total engine friction power loss increases from 392 to 1845 to 7462 W under those conditions.

The paper seems to indicate that the major factor in piston ring friction is the oil film thickness of the top ring, and this is controlled by the lubricant viscosity and the design of the oil control ring.  The oil ring being always fully flooded (and I'm assuming lower tension) will have lowest friction, so presumably has minimal effect on total ring pack friction.

2-stroke engine pistons require no oil control ring, so that's one way to eliminate them.

RE: pistons without rings?

Another important function of the ring is to transfer heat away from the top of the piston.
I have a real world experience with that, in an old SC 500 Yamaha 2 stroke that would hole a piston on a regular basis.
After seeing the new models (400cc) came with 2 rings I machined a second ring grove and ended the problem.

Cheers

I don't know anything but the people that do.

RE: pistons without rings?

Valve train friction will change with rpm, but not with load.

Ring friction will change enormously with load and a bit with rpm. It will also change quite a lot with the age of the engine, wear and carbon build up in the groves.

Bearings will change somewhat with rpm and a little with load.

Oil drag will change a lot with rpm and not at all with load.

How these add up as a percentage of total friction varies enormously with speed and load and design so trying to allocate percentages is really pointless unless a specific set of conditions is specified. It is also not much real world value unless the conditions relate to real world use and lead to a real world benefit, like to reduce fuel use at the most typical load and speed.

Having said that, I am sure ring friction is on average a high portion of friction and I am sure there is a lot of work done toward improving seal, improving durability and reducing friction.

Reducing the ring tension, reducing the ring width and reducing the number of rings can all reduce friction, but may reduce seal quality and or durability. It is all a trade off.

Precision machining, accurate prediction of size change and distortion of pistons and bore, type of materials and bore finish can all help reduce friction, but at the end of the day, all these things are compromises and the designers set the best balance of properties they can with consideration to performance, market acceptance and cost.

Regards

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

RE: pistons without rings?


One of the main advantages of rings is their ability to compensate for wear.  

Ringless pistons are common in compressors, particularly refrigeration compressors.  But refrigeration compressors operate under different conditions.  They are not subject to the rigors of combustion and wear products of combustion.  

A problem with slug pistons is cold starts.  They require significantly more torque to start.  It is doubtful that a ringless piston, which would have to be fitted tighter would exhibit less drag than a ringed piston with standard clearance.

RE: pistons without rings?

Dragging a sheet of plywood across sticky mud at 60 mph is hard work.
With that in mind I wonder how much "friction" the piston skirts, which slide just as fast as the rings, have relatively large contact area if lower pressure, and are subjected to unscraped, possibly oil flooded cylinder walls on each down stroke, contribute.
The shockingly skimpy skirts on race engines suggests to me that the rings are not the lone culprit in creating "friction" or drag.

Looking at engine power charts that include BSFC, it appears to me that reducing power losses to friction, whether aerodynamic or mechanical, really need a change to taller gearing to get full benefit.
http://www.tc.gc.ca/programs/environment/climatechange/subgroups1/vehicle_technology/study2/Final_report/image/Final_27.gif
There are some rates of friction/power reduction at low throttle settings that just move vertically (no rpm or speed change)along the map from higher power with better BSFC to lower power with worse BSFC, for little or no net fuel economy benefit, even if right at the torque peak rpm where the best BSFC lies. On the other hand, simply going to taller gearing looks to rarely fail to improve theoretical mileage since lowering the rpm and opening the throttle is so likely move things up and to the left, heading toward a better BSFC number even if the road HP requirement remains the same.

Actually I got the idea from an 1980-something SAE paper by VW about their process for optimizing fuel economy via aero details.

RE: pistons without rings?

I can't find the reference to the Honda ringless engine (I'm beginning to think I may have made it up), but here's a NASA paper pushing a barrow http://www.techbriefs.com/content/view/2258/32/

Heywood has a longish section on friction sources, but he does point out that measuring the effect of individual rings is tricky.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: pistons without rings?

It was a Honda F1 engine with ceramic pistons... and a big oil tank. Apparently some drag racers used steel ringless pistons.

Sorry, no good references for either.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: pistons without rings?

All of the small RC engines I have worked with (albeit many moons ago) called for (and burned) copious amounts of oil (in mix), which I always assumed was needed to cope with the fact there were no rings, and the engine needed the heavy oil film to insure a decent seal.

Another point that has not been discussed- bore and ring wear as an analogue for ring friction.  If you pulled apart any engine with 100K miles on the clock back in the 60's, you often required a ridge reamer to get the damn pistons out of the bores.  How many mechanics these days even know what a ridge reamer is?  The point is, most engines today with 100K miles have little discernable ridge, due to better air filters, better cylinder boring/finishing methods, better cylinder/piston materials, better piston machining methods, yada yada.  Shouldn't this imply that ring friction is possibly lower in operation that many people think?  Also, many papers have been written that show that cylinder wear is much greater upon start-up and/or at low temperatures, implying to me that any attempts to characterize ring friction from static or cold observations is misleading.

RE: pistons without rings?

The reason RC engines use large percentages of oil in the fuel is mainly due to the dry nature of the Methanol/Nitromethane mix, and the bushed conrods.   20% oil is common being a mix anwhere from 100% castor, to 100% synthetic or a mix in between.   Engines have been run on as little as 6 to 8% pure synthetics, with needle roller conrods.

Im sure that modern engines dont develop a large ridge from wear primarly due to better fuel control, especially durng startup.  Nothing like a manual choke (anyone remember them?)
pulled out to far for too long to wash all the oil off the bore.

Modern engines may use a narrower, lower  tension ring set conpared to earlier designs.


www.retallickeng.com.au

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

RE: pistons without rings?

JWaterstreet, that's an interesting observation.  I always assumed that the reason we don't see so much wear anymore is that, for whatever reasons you choose (weight, emissions/wear limitations, NVH?) they aren't casting engine blocks out of "magnetic cheese" anymore, along with better fuel control leading to reduced cylinder wash.


RE: pistons without rings?

I put it down to better cast iron, better precision, better piston shape, better piston composition, better ring design better fuel control and better oil.

Regards

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

RE: pistons without rings?

I've seen it written that the last two stages of cylinder finishing have been found to be critical to cylinder life, mainly to obtain the proper microprofile that holds oil most effectively.  (I know I've seen it a few places, but could only get to it quickly in my copy of Romance of Engines, written by Takashi Suzuki.  I don't know if any of you guys have this book, but my wife about flipped out when she saw it, as she maintains I have absolutely no knowledge of romance whatsoever)

RE: pistons without rings?

I am a hopeless romantic ABOUT ENGINES. My ex wife probably agrees with your wife.

Regards

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

RE: pistons without rings?

I think the gain from ring less pistons lower friction would be lost in engine efficiency.

As a manufacture of hydraulic cylinders, and patent holder on a high pressure seal design my observations are that seal width has little effect on moving friction, the higher the pressure the greater the friction, a piston ring that has circumferential oil grooves in the surface will have lower friction and better gas sealing capabilities, gas at high pressure will leek through a .0002” clearance which is less than the oil film thickness required for friction reduction that a piston must have to guide it in a bore.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: pistons without rings?

My experience is The relationship between "friction" and wear in various classes of machine with sliding surfaces and even rolling surfaces is so variable as to not exist.
 
- File on soap, or my daughter's Volvo heading off the snowy road, low friction and immense wear
- Full skirted piston sliding fast and bathed in high viscosity oil = lots of "friction" and no wear.  
- Brake pad lining on brake rotor = plenty of friction and acceptable, even surprisingly low wear.  

The more reputable purveyors of piston coatings state (admit?) the piston coating comes into play only during emergencies. At all other times the piston (not ring) friction is from oil shearing a peak sliding velocity approximately comparable to road speed.

RE: pistons without rings?

How would you keep cylinder lining/wall and piston skirt scuffing within acceptable margins without lubrication?

The clearing would have to be much tighter to maintain reasonable compression without rings. Tight enough to scrub most if not all the oil off the cylinder wall until the piston skirt and cylinder wear enough to start burning oil and lose compression.

RE: pistons without rings?

I have a 2001 Ferrari F1 piston, and it has two rings, an oil control ring, and a compression ring.  

When I used to build racing engines, we built a jig to hold a ring on a lathe, so we could machine the inner diameter to reduce ring tension.  Then on the dry sump oil pump, we would add and extra scavenge stage, seal the engine case well, and draw a vaccum inside the crackcase.  This eliminated almost all oil consumption.  The ring drag with all three rings installed was measured at about 5 pounds on a ~3" bore.

HTH

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