There are a number of different combinations of materials for the pistons and liners of engines. I’ve listed a number below and comment on their merits, if I’ve missed one then do drop me a line along with a description and if possible supporting images – or post a comment.
Aluminium Piston and Cast Iron Cylinder
This shows the cylinder and piston from my wood and metal beam engine.
As you can see from this obscure view of the piston I did not use piston rings and even so the engine has kept compression.
With steam engines it is quite common to lag the cylinder to insulate it.
Aluminium pistons reduce the reciprocating mass and so reduce vibration (this particular example is rather poor as I offset that advantage by making a brass connecting rod).
Aluminium Piston and Steel Cylinder
This is quite a common combination and what you will find in most commercial model 2-stroke engines, although the cylinder bore will often be plated in the small commercial engines.
This shows the cylinders and liners in the image below from my 5 cylinder rotary engine. The drill that is shown in place of the gudgeon pin is just 2mm in diameter.
The piston and master conrod are shown on the far left.
In this particular case the pistons are plain with no rings, in very small engines such as this with a bore of just 9mm it is difficult to make rings. However, slightly larger engines will normally have piston rings.
You need to take care honing the cylinders and you must wash them to remove any grinding paste before fitting the piston.
After I got a good fit I then ran the engine with lots of oil to bed the pistons in.
Cast Iron Piston and Cast Iron Cylinder
Favoured by ET Westbury. This combination works very well as the expansion rates match. This is the combination I’m using for my single cylinder poppet valve engine.
Cast Iron Piston and Steel Cylinder
This is the easiest setup to machine and get a running engine.
Phosphor Bronze Piston and Cast Iron Cylinder
This is quite a favourite of Stuart Models and many others.
The piston is quite shallow in most double acting steam engines – this is due to the fact that the piston does not need to react any side loads as the conrod is supported in a bearing in the cylinder end cap.
This piston has two cast iron rings in the one groove. This particular cylinder and piston combination belong to a Stuart Beam engine. I used this combination in my oscillating engine, images of the complete engine are in the gallery and in these you can see the supporting bearing in the cylinder end cap.
Steel Piston and Phosphor Bronze Cylinder
This combination took quite a lot of time to hone, but this was partly due to the fact that the cylinder and head are one piece.
After a few hours of running time this now runs as sweet as sweet
I must note that I have only ever run this on compressed air and not on steam and so not sure how well this combination will work at higher temperatures.
More details of this lamp post engine are in my gallery pages.
Engine Design – a number of short articles discussing aspects of engine design and in particular model engine design.