Our school needed a simple air engine for some balloon rocket cars that was easy to print, did not weigh much and was suitable for refilling without sharing an inlet nozzle between students. 

The balloon engine can use a plastic straw (inserted in the nozzle outlet) to refill the balloon to minimise sharing of fluids between students. The design is lightweight and has flattened sides for easy mounting. A whole or partial straw maybe left inside the outlet to lengthen the distance between the balloon and the end of the car. 

When you are printing it, use a heavy infill (60% or more) and around 4 or 5 layers for the walls to minimise the chance of air leaking through a wall at seams. We have found randomising the seams made little difference in sealing the walls to air leakage.