Like anything else, engines work better when they're clean. But cleaning
an engine is not the same as cleaning the rest of your car. With the
right procedure, you can keep your engine in tip-top shape. No need to
hire a professional; you won't need anything you can't find lying around
the house.
Warm up the engine slightly. Do not get the engine to operating temperature, but allow it to run for a minute or so if it is heavily soiled.