system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we harness the vaporized fossil fuel from engine exhaust and use it again?
gpt
The exhaust from a standard combustion engine isn't "vaporized fossil fuel"; the fuel doesn't vaporize, it combusts, which converts the fuel into energy and then chemical byproducts. Those byproducts are what is exhausted. 

None of those byproducts are particularly energy dense like gasoline is, so recapturing them wouldn't be of much use. Carbon Monoxide and Dioxide, for example, are both fairly inert; there are no simple processes we could do to them that would release useful energy.