TTI’s technical editor explains why maintaining tire pressure remains a persistent problem in pneumatic tire design.
The persistent problem of the pneumatic tire is the never-ending lowering of internal pressure in otherwise robust components of cord and rubber. Inflation pressure drops continuously by a chemical process known as permeability while a temperature drop reduces pressure without loss of air mass (gas laws of physics). Relatedly, self-inflating tires (which stabilize pressure during roadway service) achieved some recent notoriety with a US$65m jury award to Coda Development against Goodyear in September 2022. The case centered on patents, trade secrets and non-disclosure accords; the Prague-based plaintiff alleged Goodyear stole its ideas for self-inflating tire technology. It’s a brief three-mile drive from the federal courtroom in Akron to Goodyear’s global headquarters – seemingly a ‘home team’ advantage for the USA-based defendant in front of a Northern Ohio jury. But that was not to be.
For context, central tire inflation systems (CTIS) have been reliably used for decades, for example, in military vehicles to provide operator-controlled changes in tire pressure to meet service demands. CTIS, while not self-inflating, requires an onboard compressor and air lines connected to a rotating wheel. The cost and complexity of such systems are market detriments for automobile fitments; however, CTIS is attractive for improving the ride quality of trucks and buses featuring air-spring suspensions, regulating the tire pressure of agricultural tractors when operating on soft soil and cutting the fuel costs of long-haul truck tires.
In contrast to CTIS, harvesting of energy generated by rolling tires seems appealing for both light- and heavy-duty vehicles. Tires in motion produce and then dissipate mechanical strain energy internally (hysteresis) and externally via kinetic energy (friction at tire-road interface). For example, four fuel-efficient tires on a mid-size sedan consume about 3hp (2.2kW) in overcoming rolling resistance at 97km/h – while generating heat dissipated as energy flow or power loss. The same sedan braking on a dry road at 97km/h at peak tire-road friction generates twice the energy flow, which can be partially harvested with a regenerative system. This approach is attractive for heavier vehicles and EVs because it captures otherwise lost energy (converted to power) to charge batteries. Analogously, piezoelectric energy transducers can transform tire deformational energy to low-power electrical wattage that can be used to monitor, without regulating, tire pressure and temperature. The patent literature is replete with such concepts.
Returning to self-inflating tires, technical elements of the recent Coda versus Goodyear litigation involved principles of peristaltic pumping. Peristaltic pumps are ubiquitous in the world of animate and inanimate objects. The human heart serves as a pump using peristaltic contractions to circulate blood. Squeezing the last bit of toothpaste from a tube starting at the end and working forward with your fingers is a peristaltic action. Similarly, the Coda concept integrates a tube in the sidewall region. The tube chamber, a peristaltic pump open to atmospheric air, is kept closed at ground contact. During tire rotation, the air in the chamber is pushed along the curved tube while compressing and pumping more air into the tire with each wheel revolution to help maintain the designated pressure. Although prior art tire patents covering peristaltic devices first appeared over 100 years ago, none have been successfully commercialized. However, Coda did receive the Tire Technology of the Year Award for Innovation and Excellence in 2009 in Hamburg, Germany. Coincidentally, three years later, Goodyear introduced a very similar Air Maintenance Technology (AMT) for truck tires at the IAA Commercial Vehicle Show, held in Hannover, Germany; Goodyear then abandoned AMT development without achieving commercial success.
Technologies and global regulations addressing tire pressure maintenance received heightened attention as a result of the Ford-Firestone rollover fiasco of 1992-2000. Highway accident investigators determined that many Ford Explorer rollovers occurred on severely underinflated tires. Coda’s development is a recent advance in long-standing efforts to control tire inflation pressure. However, widespread commercial adoption of any energy harvesting method, including peristaltic action, to maintain tire pressure has yet to be achieved. Will material scientists ever be able to develop an impermeable tire innerliner?