Bryan A. Grisso

Systematic formulation of efficient and durable axle lubricants for light trucks and sport utility vehicles

Consumer demand for size, weight and horsepower has dictated a prominent role for sport utility vehicles and light trucks in the product lines of major North American automobile manufacturers. Inherently less efficient than passenger cars, these vehicles will be facing more stringent light duty CAFE (Corporate Average Fuel Economy) standards beginning in 2005 when mileage targets will be elevated to 21 mpg; this figure will be further increased to 22.2 mpg by 2007. In order to accommodate both public demand and CAFE requirements, vehicle manufacturers are seeking ways to improve fuel economy through design and material modifications as well as through improvements in lubrication. The axle lubricant may have an important impact on fuel economy, and axle lubricants can be tailored to deliver higher levels of operating efficiency over a wide range of conditions. Improvements in city-highway axle efficiency can be gained through the lubricant when appropriate rheological properties are coupled with lighter (SAE 75W-90) viscosity grades to minimize frictional churning losses. Light trucks and sport utility vehicles (SUVs), often functioning under high duress and heavy loading, demand lubricants which are capable of controlling operating temperatures; historically, higher viscosities (SAE 75W-140) coupled with special rheological characteristics have served this purpose. The penalty for high load durability and longer vehicle life is often a loss in city-highway efficiency; at the same time, high city-highway efficiency ratings are not generally consistent with vehicle durability under highly stressful operation. The challenge is bridging the efficiency-durability gap. Laboratory test rigs which simulate the FTP-75 and various trailer-tow duty cycles, recently described by these authors, are valuable tools which enable the formulator to design versatile and balanced axle lubricating fluids. Both rigs operate with a high level of control over testing conditions and produce repeatable fluid rankings. Herein, we will describe a fluid development program utilizing key physical and chemical screening methods, efficiency and durability rig testing, bearing life testing, and field validation testing. Of particular interest, certain SAE 75W-140 grade candidate fluids emerging from this process have demonstrated traditional high torque durability (and bearing life) but also significantly improved operating efficiency.