Fuel economy in heavy duty diesel engines. Part 1: measurement of oil film thickness on an operating engine

Ever increasing pressure to develop engine oils to enhance vehicle fuel economy has resulted in a trend of decreasing finished fluid viscosity. Impressive fuel economy improvements have been yielded through the application of these lower viscosity fluids without compromising component wear, principally due a combination of improvements in engine design, manufacture and additive packages. However, component assemblies such as journal bearings rely on fluid film separation to function effectively. If the viscosity is reduced below a critical value, the component will no longer reside in the fluid film lubrication regimes and will inevitably operate in the regimes of contact. Without radical redesign of the assembly, or advanced lubricant formulation, this will inevitably result in catastrophic engine failure. This article describes the development of an ultrasonic system to measure film thickness in main engine bearings under firing conditions. This unobtrusive method of measuring film thickness in operating contacts has been applied to the main bearings of operating diesel engines. Preliminary measurements were taken during dynamic engine ramping to identify the thinnest operating film condition. Further film measurements were taken at this condition with the engine running on engine oils with varying bulk oil viscosity.