MODEL ENGINEERING TIPS > 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.


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