Page 13 – The Spool Piston
”Getting a Stirling to
run, is an exercise in friction reduction”,
said
Malcolm on completion of the assembly of his first Stirling engine
- a kit.
Yes, when the power source is limited, it is
essential to minimise the friction.
Andy
Ross
initiated the idea
of the spool piston.
It reduces the friction between the piston and its cylinder.
For
the beta engine, it has long been the practice to drive the
displacer by a con rod that
passes through the centre of the
piston. A good sliding fit that has to be gas tight.
The
consequence is that usually, the fixing for the piston con rod is
off centre.
This generates slight side forces on the
piston. Also, it can be seen that, with every stroke
of the
piston, it does a slight dance. It twists one way, then on the
return, twists back again.
Andy’s piston has its
con rod attached in the centre. Friction is reduced and power
increased.
The piston is made up of two discs joined
with a tube. The top disc is the gas seal.
The piston con rod
can now attach to the lower disc in the centre.
The
displacer con rod passes through the centre of the upper piston
disc as usual.
Between the two discs – the
connecting spool tube is cut away on one side.
A peg
is fixed to the end of the displacer con rod at right angles and
protrudes out of the tube.
This peg can then connect,
via a ball joint, to a second, lower, displacer con rod.
This
lower con rod passes through a slot in the lower spool piston
disc.
It’s hard to describe. Have a look at the
photo.
The result, is that of the two items, piston and
displacer, the piston has the better linkage.
The piston is
the one that actually has the force of the engine on it.
