Do Changes in Temperature and Inflation Pressure Affect Rolling Resistance During Road and Track Testing for Fuel Economy of Class 8 Tractor-Trailers?

Data from air cavity thermistors, tire pressure monitoring systems (TPMS), and SAE J1269 rolling resistance tests were analyzed to evaluate the significance of changes in tire pressure on rolling resistance during fuel economy tests of class 8 tractor trailers. Thermistor data show that air cavity temperatures vary, with the main increase happening during the warm-up run, and measurable cooling during the fuel measurement breaks between runs. Inflation pressure also increases by 50 - 70 kPa during the warm-up run, but once the tire has warmed up, the pressure is more stable, rarely varying by more than 20 kPa during a test run. Results of SAE J1269 rolling resistance tests allow estimation of rolling resistance force for any specified load and inflation pressure. Using the test weight of the truck, rolling resistance force was estimated for inflation pressures ranging from 550 to 860 kPa. The relationship between the inflation pressure and rolling resistance was roughly linear. The relationship was then used to estimate changes in fuel consumption due to changes in inflation pressure normalized to the cold inflation pressure. For each change of relative inflation pressure of 5%, rolling resistance would change by about 1%. Using a common return factor of a 1% change in fuel consumption for every 5% change in rolling resistance, a change in relative inflation pressure of 5% would result in a change of fuel consumption of about 0.2%. The precision of the J1321 fuel economy tests was measured to be plus or minus about 1%. This suggests that the warm-up run provided for in the test method stabilizes the tire pressure and rolling resistance, and that interference due to changes in rolling resistance during a test run or between runs is a concern only for tests that measure small changes in fuel consumption. While the results obtained here are used to assess the effect of inflation pressure on the SAE J1321 test and only apply to the particular tires tested, the method of analysis may be useful in the assessment of the effect of over- or underinflated tires on fuel consumption in the wider long-haul trucking fleet.