Jerry E. Howell Model Project Plans & Kits
Outpost Enterprises, LTD - 695 Godfrey Road - Hollansburg - Ohio 45332 - USA
(World Wide Supplier of Quality Model Project
Plans & Kits Since 1992)
Hot-Air Engine Power Pistons
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 superior.
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 power output.
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.
© Copyright 1999 - 2007 Jerry
E. Howell - All rights reserved