Actually the cube uses any organic matter as fuel. Also power shards or even slugs.
What happens is, the power shard will be consumed (hence no fuel is used). It's not unlimited! The "power shard fuel" runs off too at some time. You can also use berrys, nuts, mushrooms, mycelia or even creature leftovers to fuel the vehicle. So be carefull what you put in there (famous words, you tell your pubescent son).