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How piston rings seals?


ethan

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I am really curious to know how does a piston ring seals inside the cylinder and create such an airtight, explosion proof sealing whereas both rings and cylinder is made of metal. There must be something in between that I am missing. Appreciate if someone can kill this curiosity. Lately watched a nice video of how engine works and ended up with this puzzling thought.

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Generally, there are 3 rings. The top one is the primary compression ring which holds the compression in the cylinder when firing. In the middle there is a secondary compression ring which acts as a backup for the primary compression ring. The secondary compression ring will be a bit tighter than the primary ring to help with holding the compression in. The bottom ring is the oil ring. This distributes oil right around the circumference of the base of the piston which lubricates and helps to keep the piston cool. The oil ring also prevents oil from travelling up into the combustion chamber. Think of a piston ring a bit like a round spring. It has to be compressed to fit into the cylinder. When fitted, they spring out and press against the cylinder wall and create the seal. If they were made the same size as the cylinder, they would simply slide up and down and not do anything. Hopefully that makes sense to you. 

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This is damn interesting answer of a very interesting question. May I add further: Is the middle or last ring is made of rubber? to provide such strong combusion sealing.

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All the rings are metal. If they were made of rubber, they would simply melt as soon as the engine was started. There's a lot of heat inside the cylinder. 

Lucky for you I'm currently working on an engine with a piston ring problem so I can show you a pic. The engine was overheated and the rings stuck inside the grooves on the piston so they weren't pushing against the cylinder wall and creating a seal. This resulted in the engine losing compression and not starting unless oil was put into the cylinder. 

You can see from left to right, primary ring, secondary ring and oil ring. You will also notice that the oil ring is grooved to let it carry oil around the base of the piston. 

IMG_3474.JPG

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