piston seals

chhitiz

as far as i understand, piston have cast iron seals because of the properties of cast iron, which make it a sealing material. but i was wondering, if the cylinder and piston are both made of cast iron and closely fitted together, could we avoid using seals all together?
friction doesn't depend on surface area, so there shouldn't be extra friction, right?

SteamKing

What happens when the piston wears out the bore of the cylinder? Instead or replacing a few rings, you have to replace the entire piston/cylinder.

Ranger Mike

are you talking about internal combustion engines and related pistons?

256bits

So the whole piston and cylinder would have to be machined to 0.001, 0.0001 or 0.00001 inch radial tolerance, have no runout, etc, etc. for a close fit. Cost of manufacture increases dramatically.

You would probably have to match parts. Even so, any highs on the piston and highs on the cylinder meeting up would increase friction unrelated to the surface area of the whole piston-cylinder contact mating surface area. If liquid lubricant is used as a friction and wear reducer, as the highs and lows meet and disperse, microscopic pressure fluctuations could lead to microscopic metal fatigue, small flaking of material, and subsequent spalling and total failure.

Application dependant

Q_Goest

Piston rings are pressure energized. Pressure has to get behind them to push them against the wall in order for them to seal. Simply making a tight fitting piston won't provide the same sealing effectiveness.

Ranger Mike

I gotta jump in here...If we are talking about internal combustion engines..
Piston rings function in sets of three rings, starting with the top compression ring, followed by the
2nd groove ring and the oil control ring. Their function is to seal off combustion gases, aid in the
heat transfer to the cylinder wall, and both lubricate and scrape down oil from the cylinder wall. The
top ring serves to seal off the majority of the combustion gases while the bottom ring provides most
of the oil control. The 2nd ring helps with both functions, playing a finishing role in the combustion
sealing as well as the downward oil scraping. Top ring transfers 45% of the combustion heat from the piston to the cylinder wall , middle ring 20% and oil scraper 5%.

Depending upon the engine and its purpose typical end gap of the piston ring can be 0.003” to .005” per inch of bore and will close up to near zero when heated to proper operating temperatures. The thermal expansion will effectively seal the ring although you can drill gas ports to the rings to assist in the sealing. The top ring is effected by combustion gags pressure and uses a unique design to assist in sealing against the cylinder wall..the other two rings do not use this technique. You need to properly hone the cylinder bore sleeve to provide enough “ valleys” in the surface profile to hold lubrication. You can over machine finish this area and ruin an engine. There are numerous surface texture parameters used to control this surface.
Contrary to common belief a piston is NOT round. It is oval shaped to better fit the bore when up to operating temperatures. The piston is elliptically turned so the diameter at the pin boss is less than the diameter across the thrust face. Under operating temperature expansion forces force the piston pin bosses away from each other causing the piston to assume a more nearly round condition. You need to run at least .002” to .004” piston to cylinder clearance with cast production pistons and as high as 0.010” with aftermarket forged pistons depending upon the thermal expansion properties..see the piston manufacturer for this.

Averagesupernova

Not always. I don't know how it can work very well but I have seen pistons come out of Caterpillar diesels with only a compression ring and an oil ring.

pantaz

My vintage RM400 2-stroke motocross bike uses a single piston ring.

Ranger Mike

My post was directed toward OP to find out if he / she was asking about automobile engines. I then went on to discuss typical OTTO cycle internal combustion engine piston / ring things. I should have been more precise in describing this..you guys are good!
keep me on my toes!!!

i would say that in the case of diesel engines where you only have two rings, the top compression ring is one big beefy affair. And diesels are large volume relatively low oil pressure systems. Diesels are not hi revving so the second ring may not be required since these engines require large volume due to more massive bearing area and heavier component weight. Because you do not have a high pressure oil stream slinging oil everywhere in the engine case one oil ring
/ scraper may be sufficient.
A two stroke engine may have only one ring since the oil control problem is not present.

chhitiz

ok i get it.
but what about the gaps in piston seals? do they seal off by expansion?

the seals themselves are manufactured with that clearance. would manufacturing costs really increase that much for making pistons and blocks of desired clearance?

Ranger Mike

piston ring end gap must be controlled. Heat causes the gap to close to almost zero end gap. Usually a cast piston will have .003" per inch diameter so if you have a 3 inc hcylinder bore you need .009" end gap..soo much end gap causes blow by contaminating crank case oil..too little end gap will freeze the ring and break it...

Q_Goest

Consider how you could calculate that. What is the smallest reasonable clearance one could manufacture without excessive cost? For example, consider a 3" or 4" diameter piston. To get much better than +/- .001 or .002 inches, you need to do a lot of extra work. But part of that work is simply getting the surface finish required so let's say you could get down to around +/- 0.0001" consistently. I don't think that would be economical, but that's not the only issue.

The second issue is what clearance can you tolerate for a cylinder/piston arrangement. For automotive use, or even for reciprocating compressor use, if there is a difference between the temperature of the piston and cylinder, there will be differential thermal growth of one compared to the other. A difference in temperature of roughly 30 degrees F will produce a change in length of 0.001" over a distance of 3" for steel. In other words, if the difference in temperature between the cylinder wall of your engine and your piston is 30 degrees, and the diameter is 3", then the piston will have grown 0.001" more than the cylinder wall. So consider what the minimum gap must be between your cylinder and piston in order to prevent the piston from expanding to a size that results in it being as large or larger than the cylinder. As you can see, that's a limiting factor. The piston has to have at least 0.001” clearance or more (perhaps much more) to prevent the piston from binding up in the cylinder. So even if you could get the gap down to something like 0.0002”, you couldn’t run an engine like that. There has to be a gap that will vary depending on engine running conditions. The temperature difference between parts has to be accomodated.

Now that you have considered what the minimum gap will be, calculate the clearance area between the cylinder and piston. In other words, calculate the area that the high pressure gasses can flow through. Equate that area to a single hole of some diameter. You should get a pretty good sized hole.

Finally, calculate how much air will flow through the hole during the compression stroke and the power stroke. You can do that rigerously if you’d like or you can just do a rough order of magnitude calculation by making some assumptions such as assuming a constant pressure difference over a period of time. You should find that the amount of air that will flow through that hole will be very significant. That step by step analysis should help you understand why seals such as piston rings are used to reduce that flow rate. How those seals are designed and what different types of rings are available is another interesting topic.

Ranger Mike

automotive manufacturers have been thru this drill before. In fact, many gage makers made automatic gages that would measure the diameter of pistons and classify them ( there were 10 classes of pistons in the 1960s now here are 6 classes, I think).

The Automatic Piston Gage

Gage Parameters
GAGING SYSTEM: Electronic and Air
OPERATION: Automatic
NUMBER OF CHECKS: 12
READOUT: CAGTM
CLASSIFICATION: Size
FEATURES: Automatic measuring, crack detection, roller burnishing, part washing and drying, marking, and sorting.

The Challenge
An automotive manufacturer's system of manually gaging its range of pistons during final inspection was slow, cumbersome, and did not provide 100% quality control. The company wanted an efficient method to measure each piston after machining, sort the good parts from the bad, and gather data that would provide information to improve their process.

The Solution
Edmunds Gages developed a multi-station automatic gage placed in the manufacturing area. Basically, here's how it works: Once a piston's entire OD profile and pin bore are machined, it is transferred to a staging area where it is picked up and cycled through the gage. The first station is a chip removal device, consisting of a slide with three steel hooked fingers. As the part rotates, chips are cleaned away from the ring grooves.

The next stations incorporate a part rinsing and drying system. At the rinsing station, coolant is removed from the piston in a tank with nozzles located on the ID and OD of the part. A remote pump applies pressure through the nozzles. A retractable cover is on the tank to prevent splashing of the rinse. The drying station is similarly designed, but with compressed air pushing through the nozzles. Further, vacuum devices attract the water droplets out of the air that were blown off the part.

The subsequent operation conducts an eddy current crack detection test while the part rotates 360º. If no cracks .005 inch or larger are detected on the head and at several land diameters, the part moves on to a pin bore orientation area. A laser determines by reflection whether the part is aligned properly or not by the pin bore location. If not, the programmable logic controller interprets that the part is 180º off, and directs the part to be rotated into the correct position for gaging.

The part is then conveyed to a roller burnishing station to impart a smooth surface finish. From there, the part is presented to a gage nest where three ODs are measured along with the concentricity of the skirt diameter to the oil ring groove. The measurements are accomplished via LVDT heads with pneumatic retraction for part loading. The pin bore is then air gaged in four places to check variances in taper and roundness at both ends. Compression height and pin bore offset measurements are also taken.

All of the measurement data is fed into the CAG microprocessor for acceptance or rejection. The part is then sent to a scale for weighing, and passed on to a dot peen marking station. Then it travels to the sorter where the piston is categorized as "good" or "reject" on the basis of part status as determined by the CAG. Good parts exit from the gage; reject parts are transferred to a reject chute.

The Result
With the automatic piston gage, the company is getting 100% of its parts measured and sorted to ensure no reject parts are shipped. Statistical information from the CAG microprocessor is analyzed to make adjustments in the machining process, reducing the total number of reject parts produced.

Operations Performed

Chip removal from ring grooves
Wash
Dry
Roller burnish pin bore I.D.
Measure:
Skirt diameter
Skirt profile @ 2 places
Concentricity or ring groove to skirt
Pin bore diameter @ 4 places
Pin bore taper
Pin bore roundness
Pin bore offset
Compression height
Categorize by size
Detect cracks
Weigh
Mark machine would sort these as well. The main problem as I stated in post above...pistons are NOT round...

chhitiz

yes, but wouldn't there be much less of a difference if both piston cylinder were made of cast iron? isn't that why cast iron is used in the first place? because it retains shape/size even under high temperatures? if a cast iron seal can make up for expansion difference between piston and cylinder, why would a cast iron piston not work in a cast iron cylinder under similar clearance?

Q_Goest

Cast iron and steel have roughly the same coefficient of expansion. I used the word "steel" but you can equally say the same for cast iron.

The problem is that the cylinder is water cooled and the pistons don't have any kind of active cooling. Cylinder walls are always lower in temperature than the piston. In fact, pistons have some temperature distribution throughout that results in them being hotter in some areas and cooler in others such that they don't expand perfectly. Regardless, the piston is always significantly hotter than the cylinder wall. So there is always going to have to be some clearance between the piston and cylinder.

Also, seals work not simply by taking up the space between the two parts, but because there is also a contact pressure between the seal and the surface it is sliding against. That contact pressure is created in a piston ring by the pressure it is sealing, so the higher the pressure it has to seal, the higher the contact pressure between the ring and cylinder wall. It's this contact pressure that is providing the sealing, not the fact it is simply filling in the gap. If a piston ring were made of a solid ring instead of being cut at one point, it couldn't seal properly, even if it were made the same diameter as the cylinder because the pressure it was attempting to seal couldn't push the ring out against the cylinder wall. To your point, it would restrict the flow of gasses through whatever clearance you have, but that isn't really what a piston ring is doing. The ring is sealing by being pressing against the cylinder wall and against the groove in the piston by the gasses it is sealing with enough contact stress to stop the flow of gas.

Ranger Mike

Whoa up fellows...the Chevy Vega tried to use aluminum alloy engine block ( aluminum cylinder bore and iron coated pistons and it was a dismal failure. Why would anyone want a piston made of Iron? Aluminum is a lot lighter and has better cooling properties.
Sorry Q - Pistons do have two engineered cooling systems. The Piston rings as I have noted above in my post and the engine oil lubrication system cools the inside of the piston in a controlled and predictable manner. To say it is

“In fact, pistons have some temperature distribution throughout that results in them being hotter in some areas and cooler in others such that they don't expand perfectly”. Uh...half correct and good try....but let us look at this a little more detailed.. Pistons, as noted in my above post are nor round and do not have the same degree of material. True there are areas of the piston that have different temperatures At top dead center position during combustion, but when rotating thru the Otto cycle, the piston top quickly stabilizes in temperature. So the hottest temperatures the piston top sees is for about 30 degrees of one cycle of 4 cycles and you better not have significant deviations at the piston top or we have premature detonation...kaboom. As far as expanding perfectly, better words are that Aluminum casting with differing amounts of material and these do not expand linearly. The pin boss area is significantly heavier than other areas.
One more thing, the clearances for the piston to cylinder wall are are relatively small ( .002” to .004”) and is almost zero AFTER the engine reached operating temperature other wise the piston would rock in the bore and wear out very quickly.

“Also, seals work not simply by taking up the space between the two parts, but because there is also a contact pressure between the seal and the surface it is sliding against. That contact pressure is created in a piston ring by the pressure it is sealing, so the higher the pressure it has to seal, the higher the contact pressure between the ring and cylinder wall. It's this contact pressure that is providing the sealing, not the fact it is simply filling in the gap. If a piston ring were made of a solid ring instead of being cut at one point, it couldn't seal properly, even if it were made the same diameter as the cylinder because the pressure it was attempting to seal couldn't push the ring out against the cylinder wall.”

If this was true, which it is not, the oil scraper would not work. The only piston ring that uses combustion pressure is the top ring. the other two are isolated from any combustion pressure. To think they are pressurized by combustion pressure is not understanding the design intent.
Note the attached pic..the top ring has a notch engineered into the ring. When combustion gas passes between the top ring and piston land during combustion it forces a gap. This gas then fills the notch and forces the ring toward the cylinder wall to more effectively seal the combination.
This pressure is controlled and is stopped from blowing pass this combination if all other factors are at design specification. Any resultant blow by is stopped by the second ring tension only. During initial start and warm up of an Otto cycle engine, we need this or you would be changing engine oil every 1000 miles.

Attached Thumbnails

 

Q_Goest

That's the point. Cooling of the piston is obviously not nearly as efficient as cooling of the cylinder. The piston runs hotter so the design has to accommodate differential thermal expansion.

Yes, thanks for talking down to me. Chhitiz is asking about why not use the piston to seal as opposed to questions about piston circularity. We should be responding to the OP, and the point I was making is that there must be a gap between the piston and cylinder in order for the engine to operate. I'm sure you agree with that. Whether or not that gap is uniform all the way around doesn't matter. Any gap provides a leak path.

First, the scraper is not a piston ring, it is a scraper similar to other types of scrapers. Scrapers don't seal as you know. The second piston ring aids in sealing just like the first as noted in your attachment. In addition, it helps wipe oil. Regardless, multiple seals on a single piston all act to provide a seal just as multiple teeth of labyrinth seals are all used to provide a seal. They are a restriction to flow and there is a pressure differential across both of them which is a function of compression chamber pressure and the leakage path through each seal (primarily the ring end gap for simple butt cut piston rings). How seals work, whether dynamic or static, is straightforward. They must have a contact stress greater than or equal to the pressure differential across the seal. How much greater is a function of the sealing materials and surface finish. That contact stress is aided by the gasses they seal. The rings are literally pushed out against the cylinder wall and against the piston land by the pressure of the gasses they are sealing.

Ref: http://courses.washington.edu/engr10...on%20Rings.htm

To get back to the OP, the point about seals is that they must remain in contact with the surfaces they are sealing with sufficient contact stress to operate as seals. Without the contact stress, they can't act as seals and excessive leakage results. That's why pistons can't operate as seals simply by taking up the gap.