Our expectations in lubrication systems applied in modern powertrains are continually rising. Beginning with the basic function of cooling and lubricating tribological contacts such as cylinder liner and piston, bearings, gears, chain drives and various valve train contacts, the oil in the lubrication system is now increasingly being used as a hydraulic fluid. Examples are (fully) variable valve trains, variable compression ratio systems or complicated transmission hydraulics. Driven by the general trend to minimize mechanical losses in order to increase the overall powertrain efficiency, the introduction of variable capacity oil pumps is commonly seen in latest engine designs. The potential to decrease oil pressure levels and volumetric oil flow in order to minimize mechanical losses on the one hand significantly complicates the reliable fulfillment of the aforementioned tasks of the lube systems on the other hand. In this context one of the core parameters for a reliable operation of modern lubrication systems in engines and transmissions is the gas content of the oil. Even though this circumstance is still partially underestimated during engine development, it should today more than ever be taken into account, that only careful consideration of oil aeration performance will allow to finally implement lower oil pressures and oil flows. This results in a decrease of overall friction of those subsystems without compromising its functions or durability. The physical background of this issue is, that undissolved gas has a significant impact on various physical parameters of a lubricant, such as density, thermal conductivity, viscosity and compressibility. Especially the two properties named last are crucial for a reliable function of new hydraulic systems.
To address this issue, FEV has improved its static volumetric measurement apparatus for determining the gas content in an oil sample. Furthermore, a new method has been developed utilizing an all new continuous volumetric measurement. This technique enables direct, accurate and reproducible measurements of the gas content in dynamic testing scenarios.
The first part of this paper describes the fundamental necessity for gas content measurements during powertrain system development. This is followed by a discussion of the volumetric measurement principle and its advantages. Both, the static and continuous measurement approaches for precise measurements of the gas content by means of the volumetric principle are presented and their individual characteristics are compared.