Leakage of hydraulic oils causes long-term damage to the environment. Every year, a significant amount of hydraulic oil is released into the environment through accidents and leakages. Biohydraulic oils minimize the risk of environmental damage due to their biodegradability and lack of toxicity. Mobile machines offer great substitution potential for biohydraulic oils. The majority of machines are operated with mineral hydraulic oils. Furthermore, in view of decreasing energy resources and rising CO₂ emissions, there is a strong interest in reducing the fuel consumption of mobile machines. Triggered by this trend, the power density of hydraulic systems and their components continue to increase. Resulting, the used hydraulic fluid is subject to greater thermal stress and the system thermal of the used fluids are becoming increasingly important. Biohydraulic oils offer a great potential due to their thermal properties, regarding viscosity temperature dependency. Therefore, the content of this article is to consider the influence of hydraulic oil’s viscosity on the efficiency of a crawler excavator. For this purpose, experimental tests are discussed by regarding viscosity related energy losses of hydraulic systems of crawler excavators by using biohydraulic oil. A hypothesis about the dependency of rheological properties of biohydraulic oils on the hydraulic system’s efficiency is formulated.

Hydraulic fluids ensure power transmission and lubrication in hydraulic systems. The performance of hydraulic fluids primarily depends on their physical properties. The development of environmentally friendly hydraulic fluids has become more important. Depending on their chemical structure, these fluids have different physical properties, which influence the formation of load-bearing lubricating films in tribological contacts. In this paper, the influence of pressure-viscosity behavior on the formation of lubricating films in tribological contacts will be discussed by means of tribometric tests. For this purpose, hydraulic oils with different base oils are subjected to two test procedures. The tests are done for mineral oil based hydraulic oils (HLP) and biohydraulic oils based on synthetic esters (HEES). To determine the hydraulic fluids friction behavior, they are tested using a disc-ondisc tribometer. This test allows investigating the formation of load-bearing lubricating films of the oils. A comparison of the lubricant film formation at increasing rotational speeds is made. This allows evaluating the friction conditions and the influence of the oil's pressureviscosity behavior based on Stribeck curves. To obtain a detailed statement on the formation of the lubricating film, tests are also carried out on a high frequency reciprocating rig (HFRR). The HFRR is an oscillating friction wear test, which provides information about the lubricity of fuels and lubricants.

The selection of pressure fluids plays an important role in hydraulic systems' efficiency. Depending on physical properties, especially the viscosity, different volumetric and hydraulic-mechanical losses occur in the system. This paper investigates the influence of hydraulic fluids with different physical properties on a crawler excavator's energy efficiency. For this purpose, the overall efficiency of the excavator is examined for different mineral oils and an alternative water-based fluid with deviating viscosity classes and indexes. The study results show that reducing the energy demand over a wide temperature range by lowering the viscosity grade or increasing the viscosity index is not generally feasible. To reduce losses by adapting the hydraulic fluid in a hydraulic system further physical and chemical properties such as pressure viscosity behaviour as well as the density must be considered.