Piston rings

[BigWallyB]

The "ceramic engines" thread mentioned piston rings and gave me a question to ask. The rings on a typical 4 stroke IC engine are not keyed and thus do move around. However, the rings on my SeaDoo jetski (2 stroke) are keyed presumably to prevent this. Being an EE I don't follow the reasoning of either, as it seems not to be very important if its done both ways. Comments?

Life is what happens while we're making other plans.

Wally

 

[SMOKEY44211]

The rings in your 2 stroke are keyed to prevent the end gaps from "catching" in the exhaust ports.------Phil

 

[tbuelna]

SMOKEY44211 is correct, any engine with cylinder ports (ie. loop-scavenged or uniflow 2-strokes) "pin" the compression rings. The ring ends tend to be the hottest part of the ring, and thus will deform the most. This can cause "catching" or "snagging" at the upper and lower edges of the cylinder wall port, as the ring passes by. For this reason, the compression rings are oriented so the the ring gap glides over a bridge area between the ports.

With 4-strokes, the rings are allowed to rotate freely. This arrangement allows a random, and thus uniform, wear pattern between the rings and cylinder bore, and maximizes ring life. A ring that is free to rotate in its groove is, by far, the preferable arrangement.

Regards,
Terry

 

[BigWallyB]

That splains' it.

Thanks to both for expanding my knowledge base!

Life is what happens while we're making other plans.

Wally