I am wondering if there is some rule of thumb for the happy middle.
IMHO the starting point to look at is is square.
That is piston thickness about the same to slightly less than diameter .
And the stroke and piston diameter equal. this would make the cylinder three times the piston diameter in a double acting engines so some thinning of the piston can be desirable. like 1/2 thickness . Too thin you get blow by too long and you add some friction and piston length.
Application has a lot to do with e ratios as well.
I just looked at two rudy K plans as examples. his marine engine has a 5/8 bore and a 5/8 stroke. (square)
his beam engine however a slow mover has a .500 bore and a 1.125 stoke. or a 1:2.25 ratio. I expect one could push to 1:2.5 with no problems.
The pm research mill engine has a .5 bore and a .75 stroke for a 1:1.5 ratio
probably the happy medium you are looking for.
When in Doubt I look at proven designs for guidance.
Also keep in mind the above examples are for double acting engines.
I have several engines that are single acting oscillators and the piston and con rod is a piece of drill rod. So piston length is more or less mute.
Lucy and several of the oscillators I built have a 1/4 bore and a 1/2 stroke or a 1:2 bore to stoke ratio.
I am sure there is published data on this as well as many opinions.
Hope this helps you.
Tin
