The spring serves two purposes: to translate (carry) the power performed by the piston compression to the main rotor and to return the piston to it's rest position for re-firing.
The fact that it always rotates in one direction will minimize harmonics. In fact, the very act of the object spinning when a piston is compressed is itself a sort of harmonic balancer. My expanded design and explanation below with three pistons is much more balanced, my initial drawing was a quickie...
Indeed, it is it's own flywheel. When a friend of mine saw it for the first time, that was his first comment.
Incredulous, Thanks very much for your real drawing. I have no CAD software. Just rulers and compasses and ideas!!
Here is am improved, expanded drawing:
The square item surrounding the rotor is the engine housing and actuators.
In this case, there are six actuators, three magnetic and three internal combustion type.
IF the pistons were permanent magnets oriented N-S AND the coils (EC1, EC2, EC3) were oriented S-N, then you can see a burst of charge through a coil when the piston was directly underneath, would force the piston into the cylinder, and the rotor would turn.
I added 3 Hemi heads in this drawing. HH1, HH2, and HH3. With a proper fuel mix in the heads, and even a cheapo spark plug, the explosion would force the piston down into the cylinder, and once again, the unit would spin.
What we have here is basically a single unit that could run on gas, ethanol, or Eveready batteries!
Now, I've thought about this idea for a long time, and consider questions like "What do you do with the exhaust" and "What about thermal expansion" to be somewhat minor design and materials questions. I am not an expert in large tooling machines or material properties, although I know a few who are, and all of them say this is very workable.
The simplicity of a passive rotor is amazing!
BTW, in my drawing I used the term "crankshaft" hastily. I am very familiar with a crankshaft. I should have called it "drive shaft".