Regular Simple Green at full strength works really well at cleaning dirty engines, grease, oil, etc.
When detailing an engine, after covering the obvious components; I spray the Simple Green on pretty heavy, wait for a good 5 minutes, and rinse with a garden hose with a "Firehose" nozzle.
Dirt just sheets off.
Helps to have the engine a little warm at the time to completely remove all the junk.
Only down side is that Simple Green is not completely Aluminum Friendly and will dis-color it.
If you have a Car Quest Store in your area, their house brand Purple Cleaner also works quite well.