Tire-inflation System Basics

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).

Pressure-monitoring Systems and Self-inflating vs. Run-flat

There are lots of tire-pressure technologies currently on the market and some soon to be on the market. Because of the TREAD Act, all vehicles will soon have to have a tire-pressure monitoring system so all drivers know when their car's tires are losing air pressure.

These types of pressure-monitoring systems have been around for decades and are already standard in some car models. They simply monitor the air pressure in each of the car's tires and notify the driver if a tire drops below a preestablished, optimum pressure point.

Our tires aren't what carry the weight of our cars and trucks -- it's the air inside the tires. Run-flat tires use a strong sidewall material that supports the car even if there is no air in one or more of the tires. This makes it possible to get where you're going even if a tire is punctured and deflated. Run-flat tires are constructed using alternating layers of heat-resistant cord and rubber and usually crescent-shaped wedges of weight-supporting material, strengthening the sidewalls to prevent them from folding over when there is no air pressure.

Self-inflating tires, on the other hand, are designed to constantly maintain tire pressure at the proper level. Self-inflating systems are designed more for slow leaks and for optimizing performance and safety than for keeping a vehicle moving on a tire that will no longer hold air.