A LEGO small engine I made recently (a few months ago).

The cog gears are 1965 LEGO Samsonite gears. Used them because the yellow gear has half the teeth as the red gear, so you get a realistic 2:1 ratio just like a 4-cycle engine.

Also made it with a 6x6 piston. my usual size is 4x4. Lots of detail and a flywheel made of plates and bricks.