Right, the engine should be steam-cleaned only when it is removed for major work.
However, the smell when riding inside a car, if the engine is in the front, is much of the smell downwind of the engine compartment: why does a new car smell so nice? So then, what if one deems it important to make a good impression on a passenger in one’s car?
Here is my strategy for evaluation. Start with a clean engine - my car was used when I got it, but was quite clean - and then keep it clean. Here is what I do. With a spray bottle using a solution of preferred liquid cleaner (I like that green stuff) spray the diluted solution of cleaner over everything under the hood - including the underside of the hood. Let stand - overnight even - then spray with water from hose with nozzle. One could be careful with the intensity of the spray - however I have used a carwash sprayer. Don’t forget the underside of the hood. Then follow with a spray bottle of a dilution of that common brand of rubber/plastic protectorant/cleaner spray liberally on the entire engine compartment. This should preserve all the plastic and rubber components, lubricate linkages, and inhibit the build-up of grime; so that the compartment could be easily kept clean in the same way. When driving you will notice a clean smell (oh clean sheets). A clean engine is valuable when it comes to inspection and diagnosis of potential or actual problems. I have yet to give a ride to someone I wish to impress, but it is the thought. How much risk am I taking toward causing damage to something vital? Is this shortening the life of the alternator? Once every 6 months is enough.
A new car smells nice for the same reason a new cellphone smells nice: new plastic in an enclosed space. You are absolutely without a doubt definitely shortening the life of the alternator.
I’ve heard a lot of engineers from the car service our office cleaning service uses saying that it’s not good for the engine to be washed with water. Your solution for keeping it clean is quite ingenious and I guess it can’t be harmful in any way if it’s only done once every six months. The clean, fresh smell it brings to the inside of your car is totally worth it.