We have Tyre/Engine cleaner, High Pressure Soap and High Pressure Rinse that are good for washing engines. While washing your engine usually won’t cause issues or damage, Cruisers Car Wash does not accept any responsibility for customers washing their engines.
Spray the whole engine, engine bay and the underside of the bonnet with the low pressure Tyre/Engine cleaner to loosen heavy grime, dirt or oil first, then wash with High Pressure Soap. Finally, rinse everything down with High Pressure Rinse. When using the high pressure water, spray from a distance using a wide fan spray rather than up close with a concentrated jet to avoid forcing water into electrical connectors or seals.
It's not a bad idea to wash the engine while it is off for safety reasons - there are moving parts and electrical connections in the engine bay, but washing an engine while it is running can be ok too.
For ongoing care, washing your engine bay every twelve months is usually enough.