This is probably really stupid sounding, but while I was sketching a normal 4 stroke internal combustion engine I'm going to try to build (unfortunately I'm not allowed to use a welding torch without being "supervised by an experienced professional" and I don't know anyone that fits those qualifications), I thought of this random engine design:
The piston fits almost airtight in the walls of the combustion chamber, and beneath the piston head there is a near vacuum. Some kind of flexible rubber seal with bearings makes connects the connecting rod to the chamber walls, making the chamber beneath the piston head almost air tight. An electromagnet sits where the fuel/exhaust valves would normally go, and when the piston is attracted to it when the electromagnet is turned on. Perhaps some sort of switch connected to the drive shaft controls the electromagnet. Basically, the piston is sucked down due to the vacuum beneath it, then is attracted upwards by the electromagnet.
How ridiculous/far fetched does this sound?
You might make it work, but it sounds a hell of a lot less efficient that an ordinary rotating electric motor.