The linear combustion engine (LCE) utilizes a piston with magnets on it that moves back and forth next to some coiled wire, generating electricity. It moves back and forth by combustion occurring alternately on each end; when it gets to one end, it forces it to the other end and so on. The link below explains it in much more detail and better than I can, so I'll just let you look at it for yourself.

I do have one idea for it, however. If you replaced the combustion with permanent magnets, perhaps it would allow it to bounce back and forth on its own. Theoretically, you could put magnets on the ends of the piston and at put magnets on the inside of the housing the piston is in, allowing the piston to be repelled first from one side and then from the other. Some would argue, though, that this would be "free energy" and therefore impossible; I think the resistance may cause it to slow down eventually, requiring an extra boost of energy somewhere to keep it going. I'd like to try this sometime once I get more experience with electronics; I'll update this if I ever get around to it.