How the engine piston is driven back?

i had recently read a readhowyouwant.com article, you can refer to it for a thorough understanding of the question, i wanted to know how an engine piston, once the fuel has ignited and has pushed it down, comes back? even if momentum is there and other pistons are working on the crankshaft, what happens in a single piston engine if the load is too much, in that case momentum wont do.... so wont the engine stop when the piston cant come back to re-take the fuel, because there is no force to bring the piston back to release the burnt gases and re-take the fuel....