There are a bunch of engine-cleaning sprays & stuff at your standard McParts stores. Spray it on, let it soak, hose it off. Problem is that it makes a mess, and the "Gunk" type stuff ain't very good for the environment, nor is washing the oil/grease down the sewer. Many people use Simple Green or other similar household cleaners, which are pretty good and much better for the environment than the engine sprays.

Best way is to go to a local autowash that has "engine degreaser" as one of the sprays. Let the engine cool down, spray that stuff all over, let it soak, then do a standard soap and rinse spray. You have to make sure the engine is cool--you could muck things up spraying cold water on a hot engine. If you use the long "wand" type sprayer, it's a lot easier to get to the underside of the engine and trans which are often the most gunked up. You also don't want to get the degreaser on your paint--at the minimum it'll remove all the wax.

For a diesel, the only thing you need to be careful about is the alternator--don't do a direct hit on that with a high-pressure spray. Nothing else on the engine will be hurt by the degreaser. Also be careful about the battery and the fuse box on the fire-wall--a little overspray won't hurt them, but you don't want to blast them with water.

The car wash should have a re-cycler and will filter out all the bad stuff and the grease & oil.

Good luck.