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David Kerzel,     Pompano Beach, Florida

Rotary Valve Compressed air engine  First posted 6/27/2004

Tube engine is built out of standard hobby shop tubing and hardware store items. The design requires no lathe or mill but they may help improve the appearance (mine is a mix of hand and power tools).  It is a fast fun build with a lot of opportunities for changes.  It has 9/32 bore and .300 stroke.


It was inspired by a 3 cylinder radial design by Joseph S. Ott, in the Model Craftsman Magazine May 1933. It uses a rotary valve to pressurize and exhaust the cylinder. The bore and stroke were reduced and parts were modified for hobby metals from the hardware store.

After reading the article I had to build one.  I started with this simple single cylinder.  The fit of the tubes is rather loose (.002 clearance size to size) but it should work.  It also shows how easy it is to use brass to fabricate a small engine with limited tools.

Click this line for free plans and to view the drawings.

All the parts are made from material from the hardware store.  A lot of the brass tube and sheet was used.

The rotary valve is 2 layers of to be soldered to a 6-32 screw with flats cut or filed.  5-40 would have been the ideal size but it was not available so 6-32 was used.  the diameter needs to be reduced to 1/8 to fit in the first tube.

The valve body is made of 2 layers of tube.  The frame is made from sheet brass soldered together.


I need a fly wheel or a prop for this one.  It turns free and it leaks a considerable amount of  air at the valve.  I had to resolder the inlet tube to fill a pin hole and ended up with too much solder on the joint.  Turning it by hand has a definite power stroke.  Construction time was about 5 hours so its a fast fun build.

   (too much solder & clogged tubes)

7/3/2004, It would not start when I tried it for the first time with a fly wheel this morning.  There was some compression but no real power stoke.  The rotary valve leaked some but itís made of layers of KS brass tube so the fits are loose.  I started taking it apart to see what was actually happening.  I found the rotary valve worked well and the piston sealed with a drop of oil. 

Reviewing the design there should be no compression on the up stroke or suction on the down with no air supply.  For the up stoke the exhaust valve is open and on the down stroke the air inlet is connected to the cylinder.  At the top and bottom of the stroke the valves are closed but the piston travel is very small.

Then I noticed the air tube from valve to cylinder top was full of solder. 

I think the bright clean material I am using is puling the solder in by capillary action as I solder.  For the most part the joints are weak but joints with a fillet of solder for sealing and a little mechanical support should work.

7/4/2004  I put on a new tube.  I heated it till it changed color so solder would not stick on the inside.  I polishes the outside and tinned the ends.  The solder flowed and the tube remained open. I cleaned the flux off and oiled it with #30 oil.  this time there was no sign of compression which is correct for this design.  I put 15 PSI of compressed air in the inlet and gave a light spin and it ran.

The engine have very little exhaust noise.  It sounds more like a continual hiss.  The rotary valve leaks some but that is expected with the loose fits.  Speed picked up as air pressure increased and at about 60 PSI the hose popped off.  The engine could run at 8PSI with the propeller and at about 5 PIS with the flywheel.

I chose the propeller because the engine in the article was for a airplane, steam powered.  I cut the ends off so it will be easier to display latter.

I t was fun and fast and  can be built without a lathe or mill. 

Click this line for free plans and to view the drawings.  The drawing set includes a 1 and 3 cylinder version.


Copyright 2004,  Florida Association of Model Engineers and engine builder as noted above, All rights reserved.