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Piston seals

by chhitiz
Tags: piston, seals
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chhitiz
#19
Jan8-13, 02:40 PM
P: 221
Quote Quote by Q_Goest View Post
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.
ok.
then again, if piston heats up more and expands more, wouldn't it create enough contact stress?
you might argue that it would need precise clearances, but wouldn't you need that for seals as well?
that is what my doubt keeps coming back to. whatever function the seals are serving, could be performed by the piston. no?
also, someone please tell me at what stage of ic engine development did seals come into the picture. how did engines operate before seals?
Q_Goest
#20
Jan8-13, 03:13 PM
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Quote Quote by chhitiz View Post
ok.
then again, if piston heats up more and expands more, wouldn't it create enough contact stress?
Not really. The pressure from the combustion process doesn't aid in sealing between the piston and wall. Even if you allow only enough clearance between the piston and wall to prevent rubbing and allow some lubricant in, that amount of clearance and the resulting leakage would be huge compared to simply putting in a piston ring. You should try doing some calculations on it, they're not that hard.
you might argue that it would need precise clearances, but wouldn't you need that for seals as well?
Piston rings are cut at one point around their periphery so they can more easily deform to take the shape of the cylinder and also to allow for differential thermal expansion. If they weren't cut, they couldn't spring out against the cylinder wall and any pressure behind the seal wouldn't be able to force the seal outwards. Cutting the ring at one point allows the ring to act similar to a beam in bending. They can spring outwards. Solid rings can't spring outward so they don't seal. They have to be cut at one point to allow them to take the shape of cylinder. The only time a solid ring can be used is when they are soft enough to deform such as rubber or plastic seals used in hydraulic applications. Obviously, those types of seals can't handle the operating temperature of an engine.
that is what my doubt keeps coming back to. whatever function the seals are serving, could be performed by the piston. no?
They really can't. Piston rings are unique types of seals designed for the issues faced by IC engines. Getting rid of them really ins't an option.
also, someone please tell me at what stage of ic engine development did seals come into the picture. how did engines operate before seals?
Don't know enough about the history of them. Perhaps Ranger Mike knows?
Ranger Mike
#21
Jan9-13, 02:27 AM
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Thanks Q, lets see now..if I remember correctly Mr. Otto and I used technology that was currently available ...we borrows the piston ring technology used on steam engines. The pressure of an internal combustion engine is significantly greater and we had to modify the rings to use the combustion pressure to properly seal the combination.. see Stanley steamer 735 engine piston rings attached..( hey..I'm old but not that old..people!)


one more point.. Sprint cars must be push started as the builders eliminate the starter and clutch assembly to get the lightest combination. Consequently the engine spews fuel and oil for the first couple of laps until the engine heats to operating temperature..every race night these cars are out on the track means a super slick asphalt track..I hate it..can you imagine what would happen if we made a piston with no rings and relied on the heat to expand the piston to close the clearances..What a mess..EPA sure would be all over us like Ugly on an Ape!
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Atonsis
#22
Feb25-13, 07:27 PM
P: 7
In aircraft engines, the cylinder walls are either chrome plated, nitrided, or plain steel. Because of this the piston rings are different materials as well. As for the end gap, they're staggered in even portions (i.e. 4 rings at 90 degrees each, 3 at 120, etc.) This is done so that you don't have leakage from the combustion chamber into the crankcase, which creates a loss of power, and mucks up your oil faster. The rings on the piston are a little bit spring-like, so they press against the cylinder wall. The top rings seal the combustion chamber and the ring at the very bottom ring it the oil control/scraper ring which keeps and excessive amount of oil from entering the combustion chamber (keeps just enough on the walls for lubrication) so you don't get fouled plugs.

The majority of this has been said, but I just wanted to throw in from an aircraft engine point of view, even though the basics of the Otto-cycle engine are the same.
chhitiz
#23
Feb26-13, 01:31 AM
P: 221
now that i have a clear idea of piston seals, i have real doubts regarding how seals are maintained in wankel engines. if there is no force pushing the apexes and side seals, how are they sealed properly?


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