I like novel puzzle game ideas, but I've found this one to be extremely hard to built up intuition about, and had to resort to a walkthrough a few times.
This would arguably be more fun if there were less upgrades, particularly having to invest in the engine takes away a lot from the fun. It would also be fun if the game encouraged/demanded wacky designs more. Say, controllable pistons or other "transforming" parts. Not sure what kind of levels would need that, though.
Surprisingly cool. At first it looked pretty boring, but then I noticed you can hide inside your own trail, which at higher levels is a necessity to survive. While it does in the end get pretty impossible, it's a really cool game. 4/5