Cast Iron Piston in an Aluminum Cylinder?

[vtsteam]

Has anyone used or heard of a cast iron piston running in an aluminum cylinder? No, not Nikasil or chrome plated.

This would be for a model sized hot air engine (say 1", 25mm  bore), run dry, not oiled. Depending only on the natural aluminum oxide formation to harden the cylinder interior, and the graphite of the cast iron for lubrication. High degree of polish for both to start with. No rings, of course.

Natural inclination would be to guess issues might be galling, and differential expansion due to heat opening up too much clearance.

But maybe not.......?

[BillTodd]

I can't see why it wouldn't  work . There are plenty of aluminium pistons running in cast iron barrels.

The piston thinks the barrel  is moving most of the time .

Your barrel would probably best be made of a suitable piston alloy .

[vtsteam]

Thanks Bill, good suggestion.  I have automotive pistons I can use as the melting stock.

[vtsteam]

There is a difference though.

Aluminum expansion with temperature is several times as much as cast iron's. When an aluminum piston works in a cast iron cylinder the piston clearance reduces with temperature. When a cast iron piston works in an aluminum cylinder, the piston clearance increases with temperature.

I would expect that in any case, the aluminum alloy used for a piston has a relatively low thermal expansion coefficient -- for aluminum, so it would still be a good idea to try to use that material as a cylinder in this experiment.

Also, the use I have in mind probably means the piston is operating at higher temperature than the cylinder, since the cylinder is a cooling heat exchanger for a hot air engine. So that might offset to some extent their different material rates of expansion.