I've never been to one but I'm curious. I have a gas powered pressure washer at home that delivers 2200PSI with the standard 4 or 5 tips.

You been around the washers enough to know if I would "hurt" my engine? I've used my pressure washer on another guys engine bay, held the head back about 2 feet, didn't hurt anything.

Do you even have to be careful with the car wash ones?

Does your washer have an "idle" setting in addition to the pressure setting? The commercial car washes usually deliver 1000-1500psi with pressure, and maybe 300-500 without. I only use it without pressure, just on the idle setting. Before that, I spray any dirty areas with foaming engine brite... all in all, it works well and I haven't had any problems. You can also just use an engine cleaner and a garden hose. The degreaser does a lot of the work, but the pressure makes a big difference. Once every couple months I wash it at home, and maybe every 6 months or so, I do a more thorough cleaning.

It's probably possible to hurt some of the electrical connections with 2200psi and direct application to those areas/connections. So using a low pressure setting is a good idea in my book.

No idle setting, just full bore or nothing. There is a non-calibrated, no dial pressure regulator.

I have always done the Gunk or Simply Green soak with a garden hose rinse routine. Then go drink a beer and wipre with paper towels in the garage at night. I'm sure a little pressure would help.

I might look into rigging something up. I now a little more pressure than a garden hose with a "fan" spray wouuld be a nice "tool". Especially in some areas like under the trunk lid hinges.

If so, does it have an engine speed adjustment? If you could lower the engine speed, that should lower the pump output and PSI.

lowering the revolutions on the pump will decrease the volume but not the pressure. I'm probably wrong, I can only relate in terms of 3 point hitches, log splitters, pressure relief valves and front loaders.

Then there is the theory and the actual practice. I remember my Basic Hydraulics teacher looking in horror as I reached out and stopped a hydraulic motor by grabbing the shaft with my hand. (It was only turning at about 3 RPM, I'm not crazy, just stuupid, or is it the other way around?)

I am going to work right now, I'll grab my hydraulics book.

I don't know a ton about how pressure washers work, just that as with a water/trash pump, when you reduce the RPM the flow is reduced... lower flow means lower pressure. I don't see how the 2200psi can be kept up if the flow is cut by 2/3. It would be pretty simple to figure out, IMO. Just turn the engine speed down and see how the washer goes with it!

I picked up my car from Audi daler diteling and found, that engine is clean and not greased.

I assumed, that engine must be covered with protection against water and elemnts.