How Car Washes Work

A look inside a typical automated car wash.
A look inside a typical automated car wash.

­Let's face it. Most of us have a car or truck and we enjoy driving it, especially when it's shiny and clean. For this reason, car washes have remained popular ever since two Detroit men opened the first one, the Automated Laundry, in 1914. A lot of people wash their own cars at home, but the convenience of an automated car wash and relatively low cost can be hard to beat.

Car washes fall into five categories:

  1. Self service - An open bay (the area that the car sits inside) is typically used in these systems. Self-service systems have a pressure sprayer, and sometimes a foaming brush, that is connected to a large central pump. The sprayer has a coin-operated dial system to select the option you want, such as "soap," "rinse" and "wax." A timer shuts the water off after a certain period of time, at which point you must put in more coins if you want more water.
  2. Exterior rollover - A system that is growing in popularity, exterior rollover car washes are automated systems where you drive your car inside the bay. Once your car is in the correct position, a signal informs you to stop. At that point, the car-wash equipment moves over your car on a track, performing a specific function, such as applying soap or rinsing, with each pass. Exterior rollover systems are very common at gas stations, where the price is often discounted in conjunction with buying a tank of gas.
  3. Exterior only - This automated system is popular in the northeastern part of the United States, but can be found all over the world. You drive your car into the entrance of a long, tunnel-like bay. The front tire, usually on the driver's side, is positioned on a special conveyor belt, and you put the car in neutral. The conveyor belt guides the car through the bay, where the car goes past several pieces of equipment, each with a specific purpose.
  4. Full service - A modification of the exterior-only system, full service uses the same conveyor-belt-based automated system. The difference is that the interior is manually cleaned by attendants, and some exterior services, such as hand-drying and wheel-cleaning, are available.
  5. Detail shop - A detail shop may hand wash or use an automated system to wash the car. Then, attendants completely clean and polish the car, normally applying wax and using a tool called a buffer to remove the wax and polish the car. Detail shops are often able to remove dull paint and small scratches, steam clean carpets and seats, brighten chrome, remove tar and perform a variety of other services.

In this ar­ticle, we will focus on the conveyor-driven systems used in exterior-only and full-service systems. You will learn about each step of the process and see the machinery that makes it happen. So sit back, make sure that your windows are closed and that your antenna is down. Let's enter the tunnel.­ 

Drive In

The correlator in this system is a set of long rollers.
The correlator in this system is a set of long rollers.

­­Car washes are normally either touchless or cloth friction wash. A touchless car wash relies on high-powe­red jets of water and strong detergents to clean the car. Only the water and cleaning solutions actually come in physical contact with the car.

Cloth friction wash systems use soft cloth that is moved around against the surface of the car. The system that we will discuss uses cloth friction wash technology, but quite a few of the same components are used in touchless car washes.

First, the car is placed on the conveyor track. At the beginning of the conveyor is a device called a correlator. This is simply a series of wheels or rollers that allow the wheel of the car to slide sideways until it is aligned with the conveyor.

The car is turned off and placed in neutral. Most conveyor systems have small rollers that pop up behind the wheel once it is on the conveyor. The roller pushes the wheel forward, causing the car to roll along through the tunnel, which is the term used to describe the long bay used for exterior-only and full-service systems. There are two standard types of conveyor systems:

  • Front-wheel pull (FWP) - Engages the front left wheel
  • Rear-wheel push (RWP) - Engages the rear left wheel
This car wash uses RWP to pull the car along the conveyor system.

Once the car enters the tunnel, it passes through an infrared beam between two sensors, called eyes.

The eye on one side emits infrared light that is picked up by the eye on the other side.

As soon as the beam is interrupted, the eyes send a signal to the digital control system (DCS), the computer that runs the automated portion of the car wash. By measuring the amount of time that the signal is interrupted, the DCS determines the length of the vehicle and adjusts the system accordingly.


Soap Up

In this photo, you can see the tire applicators, the mitter curtain and part of the pre-soak arch.
In this photo, you can see the tire applicators, the mitter curtain and part of the pre-soak arch.

­ Immediately after the eyes, most car washes have a pre-soak. This is an arch that conta­ins several small nozzles that spray a special solution all over the car. This solution does a couple of things:

  • Wets the car down before the application of any detergents
  • Contains chemicals that begin loosening the dirt on the car

A lot of car washes also have a set of nozzles arranged near the ground that are called tire applicators. These nozzles spray the tires with a solution designed specifically for removing brake dust and brightening the black rubber of the tire.

In this car wash, the car then passes through a mitter curtain. This is a series of long, soft strips of cloth that hang from a frame near the top of the tunnel. The frame is connected to a motorized shaft that moves the frame up and down in a circular pattern. This makes the cloth strips rub back and forth across the horizontal surfaces of the car.

The mitter curtain cleans the hood, roof and trunk of the car by swishing back and forth over the surface.

The next item in our car wash is the foam applicator. The foam applicator applies a detergent to the car that becomes a deep-cleaning foam on contact. The nozzles on the foam applicator, as well as most other spray systems in a car wash, can be adjusted to change the angle of the spray and the size of the opening. The foam is created by mixing a chemical cleaner, which varies between car washes, with water and air. There are usually separate adjustment controls for determining the exact mix of the three components. The chemical typically contains some coloring agent to make the foam more eye-pleasing and obvious.

You can see the foam created by detergent from the foam applicator.

With a good coat of foam on the car, let's move on to the scrubbers.


Most car washes have multiple pairs of scrubbers.
Most car washes have multiple pairs of scrubbers.

­­Scrubbers are large vertical cylinders with hundreds of small cloth strips attached to them. The scrubbers rotate rapidly, anywhere from 100 to 500 rpm, spinning the cloth strips until they are perpendicular to the cylinder. Although the cloth strips are quite soft, it would feel like a whip if you got hit by them. Scrubbers normally have hydraulic motors that spin them. There is at least one scrubber on each side, and there may be two or more. As the car moves past the scrubbers, the cloth strips brush along the vertical surfaces of the car.

Some car washes also have wrap-around washers. These are scrubbers on short booms that can move around to the front and rear of the vehicle, scrubbing those vertical surfaces as well. Like most of the mechanical equipment in the car wash, the washers are run by a combination of electric motors and hydraulics. Normally, a single, large hydraulic power unit is connected to all of the various hydraulic pumps throughout the car wash.

Wrap-around washers clean the front and back of the car.

The cloth used in the scrubbers is very soft and regularly cleaned to ensure that there is nothing caught up in them that could scratch the cars. They are replaced once they become worn or too soiled to clean effectively.

The scrubbers remove the dirt that the foam and pre-soak has loosened up.

In addition to the mitter curtain and scrubbers, a lot of car washes have a high-pressure washer.


The nozzles of the water jet are reminiscent of a pinwheel.
The nozzles of the water jet are reminiscent of a pinwheel.

­ ­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!

The powerful water jets remove most of the detergent and grime from the car.

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.

The pressure tank for a high-pressure washer

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.


The rinse arch removes almost all of the residue left from the cleaning systems.
The rinse arch removes almost all of the residue left from the cleaning systems.

­ N­ext, the car goes through a rinse arch. This is a series of nozzles arranged on an arch that use clean water to remove whatever residue is left after the high-pressure washer, scrubbers and mitter curtain have done their respective jobs.

In an average car wash, there are multiple rinse arches, usually after each major cleaning station. A typical car wash may have the following stations:

  1. Pre-soak
  2. Mitter curtain
  3. Rinse arch
  4. Foam applicator
  5. Scrubbers
  6. High-pressure washer
  7. Undercarriage wash applicator
  8. Rinse arch
  9. Wax applicator
  10. Mitter curtain
  11. Scrubbers
  12. Rinse arch
  13. Dryer

As you can see, the example above has three rinse arches. It also has two mitter curtains and two sets of scrubbers, which is also common in most installations. In fact, some car washes have even more of each type of station!

Most car washes have two or more mitter curtains along the tunnel.

The last rinse arch in the tunnel, aptly called the final rinse, should always use clean, non-recycled water to ensure that all residue is removed from the surface of the car.

Going through the final rinse

The majority of car washes also provide some type of protectant that can be applied to the car.


This wax arch has a triple-foam applicator. Each wax protectant is a different color of foam.
This wax arch has a triple-foam applicator. Each wax protectant is a different color of foam.

­ ­A standard feature of the car wash is the wax arch. The wax that is used in a car wash, which forms a water-resistant coating, is quite different from the wax you would apply by hand. One of the key differences is that car-wash wax is formulated to work on glass, chrome and rubber, as well as the painted plastic and metal surfaces of the car. Also, it leaves a clear, thin film that does not have to be polished first. However, car-wash wax does not provide the same level of protection, nor help to remove or cover up tiny scratches, as standard wax does.

The wax arch uses one of two methods to apply wax. The first type of wax arch uses a system of foam applicators, the most common being a triple-foam applicator, to apply a foam wax.

The wax foam is applied to the car in a heavy coating.

The second type uses nozzles, similar to those of the rinse arch, to apply a liquid wax. In this case, the next step is usually to go through a rinse arch. But when wax foam has been applied, the car usually goes through another set of scrubbers and another mitter curtain before going through a rinse arch.


On this dryer arch, you can see the silencer and nozzles on the left side.
On this dryer arch, you can see the silencer and nozzles on the left side.

­ ­Af­ter the car is completely washed, the final step in the automated process is the dryer. Much like a giant ­hair dryer, the dryer in a car wash heats large amounts of air and forces it out through a series of nozzles. These heated blasts of air rapidly dry the surface of the car.

The dryer has a large, flat, round section just before the nozzle opening. This section is called the silencer. Like a muffler or the silencer on a gun, the dryer's silencer deadens the noise created by the air being forced through the system.

A dryer in a full-service car wash does not completely dry the car because attendants will go over the car with towels once it leaves the tunnel.

Some car washes apply a special chemical after the final rinse, before the dryer, that speeds up the drying process. The temperature and force of the dryer can be set. Most full-service car washes set the dryer lower than exterior-only car washes. This is because a full-service car wash usually has attendants who hand-dry the car with towels to remove all of the water.

Touch Up

The end of the line
The end of the line

As the car comes out of the tunnel, it is pushed off of the conveyor track.

In an exterior-only system, you most likely remain in the car. When it comes out of the tunnel, you put it in park, start the engine and leave. In a full-service car wash, an attendant drives the car over to the finishing station. Here, attendants clean the interior of the car, removing trash and vacuuming. They usually clean the windows, wipe down the dashboard and doors, add some air freshener and hand-dry the exterior. They may also clean and polish the wheels and polish any chrome, depending on the service options available.

Attendants at Bunkey's Car Wash hand-dry a car.

The vacuum system at a car wash is a lot different from your typical home vacuum. It normally has a large central vacuum with multiple hoses connected to it. The hoses are usually either stretched overhead to each vacuuming station or buried underground.

This vacuum is about 6 feet (1.8 m) tall and over 2 feet (0.6 m) in diameter.

The air pump on this vacuum is very powerful, which is necessary to support all the hoses and handle the distance that each hose must cover.

The Controls

These boxes contain the fuses for each station in this car wash.
These boxes contain the fuses for each station in this car wash.

­ A­ll of the equipment in an automated car wash requires a heavy-duty power source. Each station has its own fuse-protected circuit. Most car washes are designed so that the car wash can continue to operate even if one of the stations completely fails.

The digital control system (DCS) is the brains of the car wash.

The digital control system

From the moment that the eye tells the DCS that a car has entered the system, the DCS controls every aspect of the car wash. It knows exactly where the car is at all times and turns on the appropriate stations as they are needed.

The Operation

A typical car wash uses less than half the amount of water you would use to wash your car at home.
A typical car wash uses less than half the amount of water you would use to wash your car at home.

­ ­Operating a car wash is a competitive and often stressful business. Because the revenue per car is very low, you have to wash a lot of cars to make a reasonable profit. Probably the biggest factor in this business is the weather. If it is raining, people do not go to the car wash. In fact, a frustrating problem for car-wash owners is the forecast of bad weather. Whenever the forecast calls for rain or some other precipitation, people don't get their cars washed because they figure that the bad weather will make it a wasted effort.

An interesting fact is that most car washes use substantially less water to wash your car than you would use if you were washing it yourself at home. For example, one report says that washing your car at home typically uses between 80 and 140 gallons (304 and 532 L) of water, while a car-wash facility (without a high-pressure wash) averages less than 45 gallons (171 L) per car. In addition, all of the chemicals and detergents are washed into the sewer when you wash your car at home, but a car-wash facility must dispose of the waste in accordance with local regulations. This means that washing your car at a car-wash facility is usually better for the environment.

This pit captures the water that runs off the cars and recycles it.

To cut down on the amount of water used, a lot of car washes recycle water. The recycled water is normally used in the early rinses and to mix with the detergents. It may also be used in the high-pressure washer. It should never be used in the final rinse.

There you have it! Next time you pull up to the car wash, you will know just what each machine is doing. Be sure to check out the links on the next page for additional information.

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