The high-pressure washer is a system of rotating water jets that spray concentrated streams of water onto the car. The nozzles of each water jet are typically arranged like a pinwheel, with each nozzle angled slightly away from the center.
The force of the water shooting from the nozzles causes the water jet to spin rapidly. This means that the stream of water moves in a circular pattern as it hits the car. The strength of the stream and the circular motion combine to provide a powerful scrubbing action on the surface of the car. The force of the water is incredible, with some systems rated at 1,000 pounds per square inch (psi), enough to easily knock a person off his or her feet!
High-pressure systems use a lot of water -- perhaps 300 to 400 gallons (1,100 to 1,500 liters) per car. In order to provide so much water in a rapid manner, a car wash usually has a special pressure tank nearby that holds the water for this specific system. In most systems, almost all of the water is recaptured and recycled back to the pressure tank after each use.
A lot of car washes, particularly those in areas where winter means lots of snow, have a device called an undercarriage wash applicator. This system is located at ground level and has several nozzles pointed upward to wash dirt, mud and salt from the bottom of the car.