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

 

[drwebb]

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.

 

[thundair]

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.

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[CCycle]

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.

 

[Tmoose]

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/enviro...nology/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.

 

[GregLocock]

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

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

 

[GregLocock]

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

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

 

[JWaterstreet]

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.

 

[kenre]

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.

http://www.retallickeng.com.au

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

 

[izzmus]

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.

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[JWaterstreet]

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)

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[EdDanzer]

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

http://www.danzcoinc.com

http://www.dehyds.com

 

[Tmoose]

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.

 

[SonyAD]

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.

 

[vwenginebuilder]

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