I bought the cheapo one a while back (from Amazon) and it works pretty well on my farm equipment. On the vehicles, it still involves some hand work with a sponge
I use the packaged car wash soap from autozone in the gallon jugs. I put maybe an ounce in the bottle and fill with water. I bet it would work well as a fire extinguishing tool too.
Don't know it the foam loosens the dirt but I do know my 3000 psi pressure washer with a fairly narrow spray tip does a good job of eliminating the dirt with the foam over top.