Here you see a primitive & crude model of a two-stroke axial/swashplate piston engine. This is the easiest type of piston engine to build in KSP until I build my plugin for a working ignition system. Build time: one hour. Worked with the second try. Efficiency & TWR: not to be mentioned, you can’t drive anything with it in this state. It is reliable though.
To start: press space.
Built in the SPH in KSP version 1.0.5.