It certainly isn't the glamorous part of detailing, but keeping your engine bay clean is an important part of vehicle maintenance. Aside from looking nice and keeping flammable debris away from a hot engine bay, having a clean engine bay makes it much easier to identify any leaks or damage.
Modern motors and electrical equipment are pretty well protected from weather and water, but care should still be taken around these components. Additional care should be taken around things like the air intake, alternator and any valleys, seals and belts. It's best to use tin foil or bags to mask these areas off and only wipe them with a towel loaded with cleaner.
The first thing to consider when tackling an engine detail is level of effort. We look at engine cleaning as either cosmetic or intensive. Cosmetic cleaning focuses on visible surfaces whereas intensive cleaning focuses on all the nooks and crannies you may never see without a mirror and flashlight.
For the average owner, a cosmetic cleaning is most appropriate. It can be done in a relatively short amount of time and requires only a few tools and products. We focus on the underside of the hood, the wiper cowl, the top of the firewall and the top of the engine. It is best to break up the engine bay into manageable sections so that cleans don't have time to dry. Start with the underside of the hood and work your way down. Then consider which direction water flows in your engine bay and start at the high point and work your way to the lower areas, this usually means starting at the back and working forward. Begin by spraying an all purpose cleaner or degreaser on the surface and allow it time to dwell and start to break down soiling. Next, utilize a soft bristled brush or towel to agitate the cleaner . Then, rinse any cleaners and residues with a gentle water stream, I recommend getting a garden sprayer with a wand for this task, you can rinse the entire engine with well under a gallon of water, minimizing the potential for water issues. Next, wipe dry any remaining water. Once the entire engine bay is clean, you may want to start the motor and allow it to warm up slightly to aid in drying any water you can't reach, another alternative is to use a leaf blower or compressed air to blow dry any remaining water. Finally, apply a dressing to surfaces and allow it to penetrate before buffing to an even finish. This type of cleaning is best done every 3-6 months depending on the use and environment. Keep in mind to never spray anything on a hot engine, the air intake or alternator.
For show cars and special interest vehicles that may be judged or otherwise scrutinized, an intensive engine cleaning may be appropriate. For this type of work we make every effort to remove soiling from every crevice possible. This requires many different tools and often a little ingenuity to figure out how to get into a tight spot. We follow a similar to process as noted above, working top down and in the direction water flows, but instead of stopping in the areas we can see, we continue further down to get areas less visible. To get to these areas we may use special brushes or other tools. If it's an area we can't reach, we may consider the use of very strong chemicals, extra dwell time or removing parts from the car. Ideally this type of cleaning only needs to occur once every few years and in the case of a show car, probably only once or twice in a lifetime.
Aside from the brute force of cleaners and brushes, there are a few tools that can make engine cleaning much faster and easier. A steam machine will output superheated steam (well above water's boiling point) at higher pressures. The steam works to emulsify dirt and grease and the pressure works to flush it away. Steam machines also have the advantage of using very little water and what water is there, evaporates quickly. A further advantage is that often no cleaners are required at all, the engine can be cleaned with just water, which eliminates some risk.
Another option is the Tornador cleaning gun. This tool has a mixing valve allowing it to spray solution from a tank, or just air. It also has a rotating tip that creates a vortex effect causing the solution to agitate any soiling on the surface. The Tornador requires a substantial air compressor, but allows for the use of a very small amount of liquid to clean an engine, in the neighborhood or 12 ounces.