At first I was confused by how to throw the coal into the engine. I kept picking it up and bringing it to the engine thinking it was taking it and after a couple minutes I figured out I had to press E to throw it, probably me just being dumb. But it made me think is there a point in allowing the player to walk around with the coal, why not just make it so pressing E while on the coal throws it into the engine?
Other than that I think the game is great. Love the pixel art, the music, awesome shooting mechanic and the helicopters were a nice challenge. Great job! :)