I usually use wheel cleaners for under hood cleaning. The kind that is acid-free, good for "all wheels" and foams up when you spray. Not stuff like Sonus which is stronger and possibly react with metals under the hood.
Just spray on and hose off. Remember to cover electrical connections. Much easier than APC because you don't need to scrub. Totally safe for all the automotive surfaces under the hood. Won't dry out rubber, plastics etc. Doesn't quite degrease as well as APC but if your engine is maintained well there isn't much grease under the hood anyway.
