Recently Mazda patented a six-stroke engine using a decomposer to convert unburnt exhaust into carbon and hydrogen then using the latter for the fifth stroke (re-expansion) of the cycle.
My first though after reading the article was "If you can readily separate hydrogen from gaseous hydrocarbons, why not using it in a fuel cell ? It would be more efficient" then the second was "You can also sent the carbon into another specialized fuel cell and get energy out of it.". So I set into designing a setup where both elements from the fuel would be fully exploited while having less moving parts than an internal combustion engine.