In this discussion a "hot-air"
engine is either a stirling cycle engine or an atmospheric (flame
eater) engine. Over the years I have made pistons from many
different materials and of the ones I have tried, graphite is far
A piston operating in a cylinder is one object in sliding contact
with another object. Applications that involve substantial material
stress - machines that generate or have inputs of large amounts of
power - such as internal combustion engines, air compressors etc.,
dictate that metal components be used because of it's strength. When
metal slides over metal, a liquid lubrication (called oil) is
required to control wear. Actually, the purpose of the lubricant is
to prevent the metal to metal sliding contact
which causes wear. In low power applications such as model hot-air
engines, an oil film between the piston and the cylinder wall
creates a viscous drag which will consume a high percentage of the
Furthermore, even if a very non viscous
lubricant is used or the piston/cylinder fit is made looser, any oil
inside a stirling engine will eventually work its way into the hot
cap where the high temperature will turn it into varnish or carbon
residue. This will tend to insulate the inside of the hot cap which
is not at all what we want. In an atmospheric engine the by products
of the flame will react with the oil film causing gum residue which
will eventually drag the engine to a stop.
A piston/cylinder material combination which will operate dry while
having desirable thermal expansion properties, self lubricating
properties and stable mechanical properties would be ideal for our
purposes. Many materials are ruled out as unsuitable due to the
elevated temperature conditions that the cylinder/piston may be
subjected to. Most self lubricating plastics such as Teflon have a
high thermal expansion rate which will cause the piston to bind in
the cylinder as it warms up. If the piston is made smaller to
compensate for this, then the fit will be too loose at start up when
the engine is still cool. Plastics such as delrin are considered
self lubricating but can not stand the heat. Another problem is that
few plastics have stable mechanical properties, in other words they
are not rigid. There are some self lubricating, heat tolerant,
thermally and mechanically stable plastic products that could be
used but the cost is extremely prohibitive for our application.
Machinability is another important factor.
Now lets consider graphite. It is extremely self lubricating due to
its molecular structure. The molecules are arranged like a stack of
cards and the bond is so weak that they readily slide over one
another. Even so, graphite is quite hard and mechanically stable but
it is easily machined. It can easily tolerate temperatures up to red
hot, yet has a very low thermal expansion rate. No other material
has all these properties! Are there any drawbacks to graphite? There
is one, it is dirty to handle and makes a mess when machined. The
dust and granules make a mess that even resist being wiped away dry.
I keep my shop vacuum wand as close to the work as possible and try
to prevent any dust from getting away. The dust won't hurt your
machine tools though and if anything it may be a benefit.
I make the cylinder first then turn the piston to fit. It is easy to
make the piston fit within less than .0005" of the cylinder
bore. Stop turning the piston when the cylinder is still a snug fit
over it. Part the piston from the stock piece and by rubbing
ordinary paper around the piston, remove material until the piston
will just fall through the cylinder by its own weight. The paper
rubbing process also produces a nice burnish on the piston.
The cylinder is of course just as important as the piston. Never use
aluminum as the power cylinder unless
it is fitted with a liner of some other metal. Aluminum is notorious
for scratching and galling under any circumstance, and when used as
cylinder material even the self lubricating graphite will scratch
it. Ordinary steel would work just fine but there is always the
possibility that the bore may develop rust - especially in a damp
location. I use only stainless steel and brass for power cylinders.
I bore them to size then with some 400 or 600 wet/dry sandpaper
wound around a wooded dowel, polish to give the bore a near mirror
polish. Care must be taken to not make the bore bell mouth or barrel
shaped. Graphite pistons will operate in these cylinders for
thousands of hours and not cause any scratching at all. Some of my
engines do have well over a thousand running hours and the cylinder
bores remain polished and some of the pistons have a polish on them
but none show any wear. Although graphite is a form of carbon, do
not use ordinary carbon for pistons as it is somewhat abrasive and
not self lubricating.
One last very important point: NEVER allow oil to get on the
cylinder wall or the piston.