I use oven cleaner and power washer, I also found this great product at the home depot that is for stripping decks/washing.
It comes in a 4 litre jug that is purple, I dont have the name right now its out in the shop but i just brush it on with gloves and glasses and let it sit for a while like 30 mins, then hit it with power washer.
Comes out great. It also says that is biodegradable, i dont think after the engine dirt it is but the chemical is.