Single Cylinder Air Engine, Experimental

An experimental single cylinder air engine.


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A few months ago (June, 2017) I posted a video on my YouTube channel ( of Single Cylinder Air Engine, Experimental. After experiencing more than a few propeller strikes on my fingers, I put in on the shelf and decided not to publish it. Since then, I've had a few requests for the STL files, so here they are. Remember, this engine is experimental and when spinning the 10" propeller is potentially hazardous, so be prepared for precision filing, assembly, and fine adjustments if you decide to print it, and wear full protective gear (gloves, face shield, etc.) if you decide to run it.

This design utilizes a cam, pushrod, ball bearing and spring to pressurize the cylinder at the correct time. The design depends on cylinder blow by for exhaust, and as such it is not as efficient, but again it does make an interesting conversation piece and impressive noise.

As usual, I probably forgot a file or two or who knows what else, so if you have any questions, please do not hesitate to ask as I do make mistakes in plenty.

Materials and methods

1) Purchase, Print and Prepare the Parts.

I have attached a PDF file containing the list of printed parts which includes the part names, part counts, print settings and colors I used. Carefully note that all black parts are printed in nylon. I did not attempt to print these components in any other material, so print and test with extreme caution. Also, there are some fairly small parts in some very tight spaces in this model, so patience is required (wonder how I know that?).

You will need to have a 1/4 thread compressor adapter and compressor, 6.5mm ID star washer, 6.5mm ball bearing, 10.5mm by 6mm 5 turn spring, two 5mm ID 2mm section O-rings and a 10 by 3.8 model aircraft propeller to operate this model.

Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Carefully file all edges that contacted the build plate to make absolutely sure that all build plate "ooze" is removed and that all edges are smooth. I used small jewelers files and plenty of patience to perform this step.

2) Assemble the Air Engine.

Position the smaller end of "Arm Piston.stl" into "Piston.stl", then place "Pin Arm Piston.stl" into position as shown, and finally press it fully into the piston as shown.

Slide the piston assembly into "Block.stl" as shown. It must slide easily with no binding or sticking.

Slide "Journal Male.stl" into the block as shown.

With the larger end of the piston arm centered over the hole in journal male, press "Journal Female.stl" into the male journal as shown. Note this is a very difficult process as you will need to carefully expand the journal mounting arms to perform this step. Once completed, the assembly must rotate freely.

Insert the inner and outer O-rings in the air engine assembly as shown.

Insert "Valve Pushrod.stl" into the air engine assembly as shown.

Position the piston assembly at the top of its stroke, then press "Eccentric Valve.stl" onto the male journal as shown, carefully noticing the orientation of the pin on the eccentric as the engine will not run if the pin is not positioned as shown.

Position "Arm Valve Pushrod.stl" into the air engine assembly as shown, then position "Axle Arm Valve Pushrod.stl" as shown. Press axle arm valve pushrod fully into the air engine assembly as shown.

Position the ball bearing onto the outer O-ring in the air engine assembly as shown.

Position the spring on the ball bearing as shown.

Thread "Adapter Compressor.stl" into the air engine assembly as shown. The adapter secures the spring an ball bearing into position.

Thread "Axle Threaded.stl" into the female journal as shown. This must be a secure fit and you may require cyanoacrylate glue to secure it.

Press the propeller onto axle threaded as shown.

Place the star washer onto axle threaded as shown.

Thread "Spinner.stl" onto axle threaded as shown.

Thread the 1/4 thread compressor adapter into the compressor adapter as shown.

3) Test the Air Engine.

Mount the air engine in a suitable mounting device (e.g. a vise as shown). Turn on the air compressor and set the pressure output to 5PSI. Connect the air compressor quick connect to the air compressor adapter. Using a screw driver or other similar tool, gentle nudge the propeller counter clockwise (as viewed from the front of the air engine) to start it.

I've included a slow motion video of the engine in operation using an air compressor at 5psi of air pressure.

That's how I made my Single Cylinder Air Engine. If you decide to print and operate it, please exercise extreme care during printing and assembly, as well as during operation!

Hope you like it!

4) A Brief "How It Works".

Assume the model is correctly assembled, with the piston at top dead center in the cylinder, and air pressure from the compressor and spring force from the spring firmly press the ball bearing against the outer O-ring effectively cutting off air pressure to the engine cylinder via the air passage way.

To start the engine, a slight nudge counter clockwise on the propeller (as viewed from the front of the propeller) rotates the crankshaft assembly, which in turn lowers the piston in the cylinder and rotates the eccentric. Within a few degrees of rotation, the pin on the eccentric exerts force on arm valve pushrod and with the assistance of mechanical advantage, forces valve pushrod against the ball bearing. As the ball bearing is pushed into adapter compressor, air pressure is allowed to flow around the ball bearing and the smaller diameter of valve pushrod and into the air passage way to the top of the cylinder. Note also at this time the larger diameter of valve pushrod is forced into the inner O-ring which assists in minimizing air loss to the atmosphere.

With air pressure entering the top of the cylinder, the piston is forced further downward which continues rotating the crankshaft assembly counter clockwise. After a few more degrees of rotation, air valve pushrod drops off of the pin on the eccentric, allowing both air pressure and the spring to re-seat the ball bearing on the outer O-ring which in turn effectively removes air pressure from the air passage way.

With air pressure removed, the mass inertia of the propeller continues rotating the crankshaft assembly until the pin on the eccentric again exerts force on arm valve pushrod which in turn allows air to enter the air passage way to the top of the cylinder, and the process repeats.

If you have any questions regarding this model, please feel free to comment and I will do my best to answer them.



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D39082cd4eba714392c842b52246f060?default=blank&size=40dowd13 commented ago

So, Looked around and couldn't find the exact dimensions of the spring, O-rings, and ball bearing. This is what I am looking to order soon from McMaster-Carr unless you can point me to the source you used, if you remember.

* Hard Wear-Resistant E52100 Alloy Steel Balls 6.500 mm Diameter

* 302 Stainless Steel Compression Spring, with Closed Ends, 0.375" Overall Length, 0.3"

* Water-Resistant Neoprene O-Ring, 1/16 Fractional Width

ID: 0.070", OD: 0.210", Hardness: Durometer 70A (Medium)

Let me know what you think.

Mini lora and iGreg Zumwalt commented ago


Beautiful, congratulations!

Again, just be careful with it!


D39082cd4eba714392c842b52246f060?default=blank&size=40dowd13 printed this ago
Carousel thumb zum air motor
Printed on:
Da Vinci 1.0
Excellent, once the parts were cleaned up everything fit nicely together. Although some parts had to be persuaded by a vice.
Deviations from specification of the designer: * No nylon parts, all parts are ABS * 0.2mm pitch used instead of 0.1mm * Unit shown in picture does not have O-rings, springs, or ball bearing. Will purchase those soon.
D39082cd4eba714392c842b52246f060?default=blank&size=40dowd13 commented ago

Words of caution heard clearly, thanks guys, I promise to be careful.

Before hooking up to a CO2 I will use a hand pump apparatus from a science experiment kit I had on hand. It too had a compressed air motor but with such a novel design I didn't care to experiment with it.

Greg, what sizes are the two ends of the threaded rod?


9063e7c57154f4d38afb99bbbc548723?default=blank&size=40greenbriel commented ago

Thanks Greg!

I'm also extremely paranoid around compressed gases. I've made a couple of high powered potato cannons in my time and always over-engineered the safety after reading that PVC fails catastrophically and the shards don't show up on x-rays :0

I used to wrap the pressure chamber in old jeans, layers of duct tape, and then clamped metal gutter pipe around it!



Mini lora and iGreg Zumwalt commented ago


Excellent suggestion!

And again, proceed with extreme caution! During tests and the making of the video, I used a full face shield, leather gloves (after sticking my finger in the propeller path) and chest protector.


9063e7c57154f4d38afb99bbbc548723?default=blank&size=40greenbriel commented ago

@dowd13, yes, good luck, don't blow yourself up!

If it works with a cartridge, try hooking it up to a 20lb CO2 cylinder! That would actually probably be safer than the cartridge because you can gradually raise the pressure with a regulator.

Mini lora and iGreg Zumwalt commented ago


Good luck, and proceed with extreme caution!

Thanks, I'm glad you enjoyed the design!

D39082cd4eba714392c842b52246f060?default=blank&size=40dowd13 commented ago

Printing this now in all ABS, wish me luck, I have no idea of the results.

If it works at all, I have thoughts of connecting this to a CO2 cartridge and see how long it runs. From there to see about optimizing it for longer runs.

I have to say I do like the design, great job on that.

D39082cd4eba714392c842b52246f060?default=blank&size=40dowd13 added this to the Air Engine collection ago
Mini lora and iGreg Zumwalt commented ago

@ greenbriel,

Thank you very much for your kind words, I truly enjoy it when a design I've created brings a smile!

Great minds think alike. An early version channeled air from the fan back into the compressor so efficiently, I couldn't stop it! ;-D.

Thanks again!

9063e7c57154f4d38afb99bbbc548723?default=blank&size=40greenbriel commented ago

Another brilliant design, Greg! I love this one.

Now if you could just figure out how to channel the air from the fan back into the compressor you could have another perpetual motion machine in your portfolio ;-D

Mini lora and iGreg Zumwalt published this design ago