Four Cylinder Air Engine, Experimental




Introduction: Four Cylinder Air Engine, Experimental

About: Formerly the owner of a company that designed software for avionics (EFIS, FMS, etc.) and video games (Tetris, Robocop, Predator, Michael Jordan in Flight, and a number of others), I'm now retired and finall...

A few months ago (February, 2017) I posted a normal and slow motion video of a four cylinder air engine on my YouTube channel ( After experimenting with the design, given its weight and air flow requirements, I put in on the shelf and decided not to publish it. Since then, I've had a number of requests for the STL files, so here they are. Remember, this engine is experimental. so be prepared for precision filing and fine adjustments if you decide to print and experiment with it.

This design utilizes a spinning rotor to direct air into each of the four cylinders. The rotor contains four holes, spaced equally along the length of the rotor, and rotated 90 degrees from each other. "Gear Crankshaft.stl" and "Gear Rotor.stl" are designed to maintain synchronization between the rotor and the crankshaft to make sure air is delivered to the cylinders at the proper time. The design depends on cylinder blow by for exhaust, and as such it is not too efficient, but it does make an interesting conversation piece.

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.

Designed using Autodesk Fusion 360, sliced using Cura 2.7.0, and printed in PLA on an Ultimaker 2+ Extended and a dual extrusion Ultimaker 3 Extended.

Step 1: 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.

You will need to have a 1/4 thread compressor adapter and compressor 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.

Step 2: Assemble the Block.

Assemble a piston by placing the smaller end of one "Connecting Rod.stl" into one "Piston.stl", and secure in place with one "Piston Pin.stl". Repeat this process for the remaining 3 pistons. Test fit the piston assemblies in "Block.stl" to make sure they slide smoothly with no binding or grabbing.

Position on "Journal Main.stl" as shown in "Block.stl". Slide one piston assembly into the cylinder from the top of the block as shown. Center the large hole in the connecting rod over the hole in the journal main, then press one "Journal Rod.stl" into the hole in journal main as shown. Test to make sure the assembly rotates with ease. Repeat this process for the remaining pistons and journals carefully noting the orientation of each journal assembly as shown.

Press "Cap Crankshaft.stl" onto the first main journal as shown.

Press "Gear Crankshaft.stl" onto the last main journal as shown.

Press "Head.stl" onto the block assembly as shown.

Slide "Rotor.stl" into the head as shown.

With the rotor aligned as shown and the first journal rotated such that the piston is at the top of its stroke, press "Gear Rotor.stl" onto the rotor as shown. The position is important for successful operation of the engine.

Thread a compressor fitting into "Adapter Compressor.stl" as shown.

Press the adapter assembly into the head as shown.

Step 3: Test the Engine.

I've included a slow motion video of the engine in operation using an air compressor with about 8psi of air pressure.

That's how I made my Four Cylinder Air Engine.

Hope you like it!



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    21 Discussions

    The third photograph in step 2 shows the alignment of the rotor with piston #1. Primarily, the rotor is aligned such that compressed air is introduced into a cylinder about 20 degrees after top dead center.

    Hope that helps!


    You may try running it on the exhaust port of a shop vac. While I never tried it, I did use a shop vac on the single cylinder version.


    That looks pretty cool! You should try putting an airplane into the gears to see if it works as a airplane launcher.

    Great instructable!


    10 months ago


    Lovely air engine!

    Have you seen these... you can run a plastic engine with steam. I have one and it cranks like the devil with the tuna-can boiler. My guess is the cylinder and piston are UHMW or some other lubricious engineering plastic that can take a little heat. Bill

    1 reply

    Thank you very much!

    I have not seen those. I can only imagine what a PLA version would look like after a few seconds of runtime with steam. ;)

    Thanks again!

    very nice project, it can produce air if rotated ?

    1 reply

    Thank you very much, I'm glad you enjoyed it.

    No, unfortunately it cannot since it uses piston blow by for exhaust.

    Thanks again!

    My 2nd comment is partially reiterated from my first: would a re-adaptation of the rotory Wankel internal combustion engine fueled by compressed air be more effective and simpler to produce than your current re-adaptation of a 4 cylinder internal combustion engine feuled by compressed air. I haven't the time or brains to figure this out but I wonder if you had given it any thought?

    1 reply

    Thanks again for your inquiry, it makes me think...

    I cannot answer your question at this time. While I do understand the mechanics of the rotary-wankel engine, it does depend heavily on lubrication which in this design using blow by exhaust would not work. However, with the addition of an exhaust valve system...

    Thanks again!

    Evidently, your mind was operating on all 4'cylnders when you conceived this idea.

    I understand your engine operates on compressed air (8 psi). At home, I have have a pnuematic rotory motor built at the turn of the last century (1900) that operates on Forced Air between about 3 and 15 water column inches. Forced air is produced by a rotary blower or pumped bellows in an open circuit, as opposed to a compressor, which is nay effective in a closed circuit. The pneumatic rotary motor I have was taken from an old pipe organ and its job was to rotate a 4' long 2" diameter wood shaft to which 4 vanes were attached to disturb the air above the organ pipes and create an undulating effect from the sound produced by the pipes. The design of the motor is much simpler than your compressed air motor and shares some of the features associated with the design of the rotary-wankel internal combustion engine. I suspect the forced air motor I'm referring to is relatively inefficient. The core of the engine is made of machined brass and brass fittings.. it could have been built of the same material your engine is made of but back then, plastic was made of shellac or the material ping pong balls are made of ( very flammable) and in both cases, of very limited usefulness. Keep the good work!!

    1 reply

    I truly enjoy your thought process (mind, 4 cylinders, etc.).

    I understand your thought and design process, and must include the striation layers of 3D printing. Depending on the angle of influence, the striation levels of 3D printing can be both a hinderance and a help. In this design, with the tolerances allowed, the striation influence is minimize by the clearances between the piston and the cylinder walls. Including the design constraints of using blow by exhaust, the piston and cylinders should not be lubricated since doing so would obstruct the blow by exhaust. The rotary-wankel engine depends heavily on thick viscosity lubrication and as such in this design would hinder its performance.

    But, I'm sure an improved design could incorporate your ideas.

    Thanks for your input! It keeps me alive and thinking...


    10 months ago

    Interesting project from all angles

    1 reply

    Thank you!

    As I mentioned in the introduction, at the time I designed this I was testing different air engine designs for a balloon powered vehicle. Tthis design was too heavy and required too large airflow, but I did like the rotor concept.

    Thanks again!

    You could make exhaust holes that are blocked by the Piston until it reaches the bottom, letting out the high pressure right before it begins opposing it's return. I believe this is how a Stirling engine works, but don't quote me on that. Love it

    1 reply

    I'm truly glad you enjoyed this design!

    If you look carefully at the lower right side of the block assembly in the video, you'll see a small (5mm) hole that I placed in the original design. After testing, I determined that the hole was not necessary so I removed it in the final design.

    Thanks again!

    This is awesome! Definitely giving this one a try, I’ll let you know how it goes. For what it’s worth, this will be the first 3d printed project where I’ll be making someone else’s design. Kudos to you!

    1 reply

    Thanks, I'm sincerely glad that you enjoyed it!

    If you do decide to run it, please proceed with caution!

    Thanks again!

    That looks really cool. Any plans on trying to expand it and make it larger?