Run by just blowing air into it. Demonstration:

Exploded view in 3d viewer: https://skfb.ly/6LQBR

If you like how it works, you can support me by purchasing the Pneumobile Chassis.

Extra parts needed: 

• M3x8 Bolts (x15) 
• M3 Nuts (x15) 
• Thin Nitrile Glove (thumbs from M sized gloves worked the best) or Silicon (worse) or PET (not tested) ones

Optional parts: 

• M3x12 or longer bolts (2x)(for the gears) 
• M3 Nuts (x2)(for the gears) 
• 6x4mm Silicon Pipe (can be found in aquarium shops) 
• M5x10mm Washers (x2) (instead of printed ones) 
• 105ZZ Bearings (x2) (noticed no difference in performance)
• Printed well with 0.6 nozzle at 0.2 layer height. Some parts (Cylider_Back_Lid and Valve_Lid_6mm) need supports. After printing cut away the brims from Cylinder_Ring parts.

Printed well in PLA  with 0.6 nozzle at 0.2 layer height. Parts with "_support" in their names will print a bit prettier with supports. Parts with "X_" in their names are optional.

Assembly: 

• Use 3d diagram as a guide: https://skfb.ly/6LQBR 
• Cut off a glove finger tip (thumbs from M sized glove worked well) 
• Pull it onto a Cylinder_Ring (Use Helper_Finger to leave enough height) 
• Connect a Piston to it with two Washers 
• Clamp the Cylinder_Ring between Cylinder_Plate and and Cylinder_Lid 
• Clip them with Helper_Clamp so they won't fall apart at once 
• Bolt them together to Plate_Top 
• Repeat for the other 2 cylinders 
• Insert th Crank with 2 Washers
• Fix it in place with Frame_Bottom 
• Bolt all the parts together 
• Put the Valve_Inner in place
• Bolt the Valve_Lid into place 
• The engine should be running!

Troubleshooting: 

• Rotated freely by hand, but struggling when working - the inner valve may have too much friction against the frame (sanding the valve and the frame may help for a while). 
• Jams on a certain cylinder - this glove finger may need a bit more slack. 
• Isn't rotated freely by hand - gears bolted too tightly to frame (should have a bit of play), some gear teeth didn't print very well, or the whole gear printed with "elephant foot" and runs too tight with another gear.