Similar to how Gauntlets function in Arena, Fuel is used to play a Penny's Pursuit level. 5 of them are required to play and are consumed as soon as the player enters a level.
A unit of Fuel can be obtained every 24 minutes. Additionally, 15 of them can be bought in the store for 30 gems. 5 of them can also be obtained by watching an ad. Fuel has maximum capacity of 15. If you watch an ad for more Fuel, it will go over 15.