I don't know if this is a feature or a bug, when I made the design for the super smelter, the minecart hopper for fuel is always filled with materials to be melted down, when I checked, it came from the minecart which is on the diagonal of the minecart hopper, or is it a minecart hopper or not chest minecart. if i use chest minecart it's not fast enough to fill from the hopper.