Huh. So I assume the lack of casting cost is intentional. I really like this idea, so I hate to point out that Fuel will lose its ablity as soon as it becomes hidden information as a card in your hand again. Maybe something like "Until end of turn, if this card is in your hand, it remains revealed"? That would probably solve that problem.

Maybe easier as something like splice? I mean, you're not wanting to do it EXCEPT when casting that instant, so why not:

When you cast a red instant you may discard this. If you do, when that spell resolves return it to your hand instead of putting it in the graveyard.

To make this easier to understand maybe make this cost and have "As an additional cost to cast fuel, exile a card from your hand". Add to your mana pool then exile fuel.