Hi, what I don't quite get is how do you construct a seal that
allows the piston to move easily, yet doesn't allow any of the gas
to escape under expansion pressure (or let any air inside under
contraction). How is such a seal made?
Thx.
gas-tight seal
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Response to gas-tight seal
David,
The answer to this one could be very different depending of the other operating conditions.
For engines with lower temperature differences the problems is not as difficult because there are more material choices. Also, thermal expansion of the piston and cylinder are less of a problem. Usually very low friction material combinations are used. Graphite pistons in honed glass cylinders are very very good here.
As temperatures and pressures rise the materials used will need to be more carefully chosen. Graphite continues to be a good choice because it won't melt and has a low thermal expansion. It is not very strong mechanically so this would limit its usefulness. Glass would not be good for high pressure due to its brittleness.
When conditions are high temperature and pressures a metal piston and cylinder are usually needed. A separate seal ring is then needed to allow for lower friction and expansion of the materials. PTFE(Teflon) rings are good here. They could be mounted at the cold end of a long piston to keep them from melting. Longer wear properties are available if a reinforced Teflon called Rulon is used. It is also very beneficial to use a drive linkage that has very low side loading. A seal should not be expected to resist the high side forces from automotive type slider dives.
To assure maximum sealing, rings should be continuous. A backing ring can be used as a spring to keep the lip of the seal pressed to the cylinder wall.
Rick
The answer to this one could be very different depending of the other operating conditions.
For engines with lower temperature differences the problems is not as difficult because there are more material choices. Also, thermal expansion of the piston and cylinder are less of a problem. Usually very low friction material combinations are used. Graphite pistons in honed glass cylinders are very very good here.
As temperatures and pressures rise the materials used will need to be more carefully chosen. Graphite continues to be a good choice because it won't melt and has a low thermal expansion. It is not very strong mechanically so this would limit its usefulness. Glass would not be good for high pressure due to its brittleness.
When conditions are high temperature and pressures a metal piston and cylinder are usually needed. A separate seal ring is then needed to allow for lower friction and expansion of the materials. PTFE(Teflon) rings are good here. They could be mounted at the cold end of a long piston to keep them from melting. Longer wear properties are available if a reinforced Teflon called Rulon is used. It is also very beneficial to use a drive linkage that has very low side loading. A seal should not be expected to resist the high side forces from automotive type slider dives.
To assure maximum sealing, rings should be continuous. A backing ring can be used as a spring to keep the lip of the seal pressed to the cylinder wall.
Rick