Piston grooves

[henryspringer]

I have two designs of piston for the Stuart D10 I am building. One with 3 fine grooves and the other woth one large groove to accept graphited packing. any recommendation as to which to use please

[Tel]

I always use the larger groove & graphited yarn packing. The oil groove system will work, but you have to hold things that much closer.

[SteveW]

Henry,

I think there is more to the multi-groove thing than just oil retention. If you look at opening wooden window frames there is a groove around the frame in the same way the groove around a piston, this I'm told acts as a seal.

I recall someone explaining the fluid dynamics of the flow speed/pressure changes that happen as the gases/steam squeeze through the gap and then encounters the opening that is the groove, it goes from high to low velocity and a major pressure reduction. I think the point is to absorb the energy better.

Anyone else hear of this one?

[chrissyvine]

The grooves work a little like a labyrinth seal on a jet engine. Huge pressures are sealed with little leakage with no rubbing surfaces at all. Each time the fluid goes past a restriction it must accelerate, this takes a force or pressure. When it enters the space afterwards it will slow down again but the pressure will not be fully recovered. A small pressure ratio is set up. If this is repeated four or five times the pressure ratios are multiplied together and quite a big pressure ratio results, hence the seal.

This is the same reason why lots of thin piston rings are more effective than one wide one. It took locomotive engineers many years to realise this and sort out the problem of sealing piston valves.

Chris.