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Old 26th October 2006, 18:08   #1
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Default How do piston rings work?

Here is a brief description of " How does a piston ring seals the combustion chamber from crankcase? "

The piston rings have two type of clearances in their grooves. One is radial and other axial. Due to this clearances, the piston ring is free to move in the groove. Also the piston rings have some springness and they will always be pressing against the liner wall. But this springness is not enough to seal the combustion chamber against the gas forces.

When working, the piston is sitting on the bottom of the piston ring groove and they are pressing against the cylinder liner wall. Due to this there is a small clearance at the top and at the back (towards centre) of the piston ring. Due to this clearance, the gases can go to the back of the piston ring from the top clearance and press the ring hard against the liner. This causes it to become almost gas tight.

There is another clearance provided at the ends of the rings. This is to compensate for the change of length of the piston ring due to temperature changes. Due to this, the small amount of gases leak past the rings. Just like a air leaking through small hole. So to redue the pressure to a small level, number of rings are provided. Each one reduces the pressure a fair amount and the last one reduces it to crankcase pressure.

If these gaps get blocked due to carbon deposits and piston rings are not free to move in there grooves, the gas pressure do not act from inward side, but due to taper on the rings will act from outward side towards inwards. This will lead to sudden collapse of the ring and breakage.

Last edited by Rehaan : 5th November 2006 at 07:31.
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Old 27th October 2006, 04:27   #2
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no offense...but why are you copying and pasting data from the net.members can look it up on the net .
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Old 27th October 2006, 12:49   #3
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do you have a question in all of this or is this a lecture for all of us ??
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Old 27th October 2006, 18:23   #4
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What is the intent of this thread if there are no questions asked ? People who have free time can browse or read books (old style) to know more. Working of Piston rings is not rocket science right ?
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Old 27th October 2006, 19:28   #5
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It is in deed informative. I pressume this forum is not only for asking questions.

I agree, it is good if you had provided the linky to the source and some recommended reading.
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Old 27th October 2006, 21:25   #6
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My god! What a reaction!!

Sorry guys if I have offended you. Did not realise that posting such an innocent thing will attract so much attention.

How did you guess that it is a copy and paste thing? Well! almost everybody is copying pasting here on this site. Isn't it? So I decided to try my own copying and pasting;-) I tried to make it as simple as possible for everybody to understand.

There is neither a question nor a lecture. While going over many posts, I realised the there are quite a few here who would like to know or if given a start will find out more on piston rings performance. Reasons are very simple. If you are interested in performance, fuel efficiency or simply longer life to the engine, pistons rings are one of the weak links.

Working of piston ring is not a rocket science. But believe me, for desgin consideration, it is still a nightmare for design engineers. If you go in details of working of piston ring, you will realise it. Just ask yourself following questions:
Which is the ideal place for piston rings - at the top end of piston or bottom end of piston?
How do you control the temperature of piston rings?
How to avoid or reduce the taper (sloping) wear down of piston grooves in which the piston rings moves? How to lubricate the piston rings when they are TDC with such high temperature and pressure and not much speed to piston rings? And how to get the lube oil in good quantity to such a place without overlubricating the bottom part? In diesel engines, how do you avoid sulphuric acid corrosion at low load operation and high humid areas? etc etc

Thanks. And sorry, I could not provide link because I typed it on my own and hence cannot provide any linky to it.

Anyway, thanks to you all for the feedback. Don't take it otherwise. I will ask one question again which I have asked before. Why can't we make our own site a complete database for which everybody in the world will refer to instead we going to other sites? Definitely, it has pros and cons about it. But we can think over it. Right guys!

Last edited by jat : 27th October 2006 at 21:28.
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Old 27th October 2006, 21:32   #7
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Yes, as DRC mentioned it isnt compulsary to follow every content with a query or a question. This forum encourages posting knowledgeable stuff for ignorant people like me to learn Most of the times a post like this encourages people to come forward with queries, suggestions etc...

JAT: Dude kindly mention the link of your extract for further learning...
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Old 28th October 2006, 21:51   #8
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Ok.. a Good Explanation.
But u forgot to mention.... There r basically 3 Types of Piston Rings!!

1. The top Piston Ring - which is made of Hard metal and is coated with hard Metal.

2. The Middle Piston Ring - Which is made of Cast Iron.

3 The Bottom Piston Ring - Oil Scrapper Ring, has a zig zag type scrapper ring and two rail rings on both side.

The Function of 1st ring is in initial period of engine life, wen the wear and tear is maximum. At that time the hard metal coating comes off and mixes with lube oil and smoothens the wear and tear areas!

The function of 2nd Ring is to Dissepate heat quickly from piston to cylinder. that is y made of cast iron which is good heat conductor.

The 3rd ring makes sure that the Lube oil sprayed from crankshaft on to the cylinder walls does not go above the ring in excess!

Besides that the rings are generally fitted at 120 Degrees wid each other from the clearance in the ring.
This is to ensure that oil or gases do not leak easily.
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Old 2nd November 2006, 15:54   #9
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In the past, when the sealing methods were not advanced enough, there was a tendency for design teams to put a number of rings on there!!
The oil ring which is the bottom most ring as Vipul has pointed out, aids in adequate lubrication of the liner and on the downstroke, ensures the oil gets scraped off the liner to ensure it doesnt get burnt off and add to combustion/emission nightmares.
I have seen pistons of past design that have had 5 rings with 2-3 oil control rings.
Nowadays, the mantra is to move towards having as few piston rings as possible. (Its simple, more the rings, more is the friction as the rings press against the liner during operation)

It would be interesting to know why is it that we cannot eliminate the second ring i.e. go with just the top ring and the oil control ring...??
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Old 5th November 2006, 03:32   #10
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Above are the few of the links, which may prove worthwhile reading.

http://www.springerlink.com/ is paid one, you have to buy the documents but seems that it has got the best reading materials.

There are basically two types of piston rings. Compression rings and oil control rings. The material used for piston rings varies but for running in a softer coating and not hard coating is used.

The top ring has got the highest thermal stress and not the second ring.

Why multiple rings are used rather than one ring? 1. The pressure differential across multiple rings are much less than single ring and hence mechanical and thermal load are less and therefore easier to design a multple ring system. 2. The sealing in more effective with multiple rings as pressure differential is less. 3. If one ring breaks, you got a back up :-)
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Old 5th November 2006, 07:33   #11
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Hey Jat (& vipul261 & MC Mayank),

Some great info here, and its a good starting point now that you have posted some links as well.

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