Tire-inflation systems have three general goals:
- Detect when the air pressure in a particular tire has dropped - This means they have to constantly (or intermittently) monitor the air pressure in each tire.
- Notify the driver of the problem
- Inflate that tire back to the proper level - This means there has to be an air supply as well as a check valve that opens only when needed.
While the available tire inflation systems vary in design, they share some common elements.
- They all use some type of valve to isolate individual tires to prevent airflow from all tires when one is being checked or inflated.
- They have a method for sensing the tire pressures. This is addressed in most cases with central sensors that relay information to an electronic control unit and then to the driver.
- They have an air source, which is usually an existing onboard source such as braking or pneumatic systems. When using an existing system, however, they have to ensure that they don't jeopardize its original function. For this reason, there are safety checks to ensure that there is enough air pressure for the source's primary use before pulling air for tire inflation.
- There has to be a way to get the air from the air source to the tires, which is usually through the axle. Systems either use a sealed-hub axle with a hose from the hub to the tire valve or else they run tubes through the axle with the axle acting as a conduit.
- There has to be a pressure relief vent to vent air from the tire without risking damage to the hub or rear-axle seals.
Now, let's look at how each of the main self-inflating tire systems on the market uses these elements to make the system work, starting with the Central Tire Inflation System (CTIS).