Compressed air engine design basics

[bullpeters]

Is there a rule of thumb concerning
Bore/stroke/piston length. I can imagine the extremes, long stroke, small bore, long piston or big bore, short stroke with very thin piston. I am wondering if there is some rule of thumb for the happy middle.
Is there?

 

[Tin Falcon]

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

 

[Tin Falcon]

If your next question happens to be connecting rod length that was discussedCan a connecting rod be any length ? here.

The short answer is con rod length should be 1.5 to 3 times the stroke.

Tin

 

[bullpeters]

Thank you
I couldn't find any published info on web that pertained to models. I was unsure if life sized info would transfer.
I am a bit unsure about double acting and single acting, I will do some research
I am to green to burn

 

[Tin Falcon]

I was unsure if life sized info would transfer.

That is another discussion.it mostly does but the laws of scaling can play funny trick.

a single acting engine has an open ended cylinder air/steam peshes on the piston the inertia of the fly wheel cases the exhaust.
double action has valving that allows air to push in one direction then the other so the piston is always under power and the cylinder is closed on both ends.
Tin

 

[bullpeters]

This is engine i ended up with. I just started with the bearing i found in a mower and made the rest to fit. Its nothing special but was a lot of fun to make, and by god it worked!! Its got a *******ized C fellows slave arrangement for the exhaust, but with poppet valves instead of bearings. [ame="http://youtu.be/3sKbChNwkjA"]http://youtu.be/3sKbChNwkjA[/ame]

 

[robcas631]

Nice work!