Reciprocating air engines have been around for a long time, here is the most basic piece that is an easy build and popular with students. It can be varied according building skills and available materials. It's operation is as simple as inhaling and exhaling. The same basic principles that drive this motor are used in today's modern engine where expanding gas is used to drive a piston and a crank is used to convert the movement into rotary motion. The idea of a flywheel is used to overcome the positions where little or no torque is applied to the crank, the flywheel keeps it moving due to it's moment of Inertia. This lung powered engine is used as a demonstration of these principles and does require a little practice to get the rhythm and timing right.
This video is a reposting on simple air engines that has been posted previously. It was previously flagged for mature audiences only. After reviewing it several times I have not been able to determine why nor was explained. However, I have had students in middle school make these motors and found it to be a valuable project in understanding the basis for reciprocating motors.