The Main Engine is the first and according to Olimar, the most important part to the S.S. Dolphin, found in Pikmin. This ship part allows Olimar to lift off from the surface of the Distant Planet to avoid the nocturnal creatures that come out at night, although it can only allow him to orbit the planet and go to The Forest of Hope, but not fly away from the planet. This is the only part that can not be skipped, of course, as it's more for tutorial sake and is found right around where Olimar crashed, after pushing a cardboard box. When the Main Engine is carried back by the Pikmin, the S.S. Dolphin flips upwards and the Main Engine appears on the bottom of the ship.
Amazing! There's no mistaking it! My ship's engine rests before my very eyes! By a stroke of pure luck, I have already stumbled upon the most important piece of my damaged craft! Fate has smiled upon me! But... How will I get it back to the Dolphin?”
Getting this first piece is just "super hard", and to make it "so much harder", it's on the first day and there's no time limit. When Olimar discovers the Onion, a Pikmin Seed will come out and grow into a Pikmin. The player can then use it to attack a Pellet Posy, which will drop Pellets that the Pikmin can carry back to the Onion to reproduce. When you have brought back all the Pellets in the area, there's a cardboard box the Pikmin can push. Once done, more Pellets are available to you as well along with the Main Engine. When at least 20 Pikmin have been bred,(it's suggested to breed 25) you now have enough to carry back the ship part.