## 1 Introduction  
One of basic elements of machine hydraulic systems is the working fluid. The main task of the working fluid is to transfer energy between the driving part and the driven part of a machine's drive system. In modern hydraulic systems, the working fluid also performs several different important functions. Among these functions are the lubrication of moving parts, heat dissipation, the discharge of solid contaminants from the system, sealing the system and corrosion protection. The most important and basic physical attributes of working fluids are, among others, kinematic viscosity, density, the boiling point, the freezing point and specific heat. Working fluids performing in hydraulic systems should also have anti-wear and anti-corrosive properties, as well as resistance to oxidation and to thermal degradation. Kinematic viscosity is a basic physical property of a working fluid; the amount of flow losses within the hydrodynamic clutch's working space depends on this property. In order to minimize the flow losses, it is advised to use working fluids with a very low viscosity. The viscosity coefficient value depends on the temperature. A low boiling point contributes to the occurrences of cavitation phenomena. A significant increase in the working temperature of the hydrodynamic clutch can cause a decrease in the quality of the working fluid as a result of its oxidation and thermal destruction. This is why it is necessary for the working fluid to have antioxidative properties which will ensure the required durability. The working fluid of hydrodynamic clutches should also have good anti-foaming and anti-corrosive properties, and it should not decompose. Foaming in the working space poses a risk of leaks of the hydraulic fluid through the sealing. The working fluid cannot contain any substances that would react chemically with construction materials. Additives which improve the antioxidative, anti-corrosion or anti-wear properties of the working fluid should not decompose in order to prevent precipitation. In hydrodynamic clutches, the most commonly used working fluids are oils which are mixtures of various hydrocarbons obtained from crude oil distillation. Crude oil is one of the basic raw materials in the power industry and the chemical industry. However, over the last several dozen years, there has been a debate (both among scientists and in society) concerning the time of depletion of non-renewable resources of the raw material. Another aspect of the debate concerns the possible development of industry sectors which are now completely dependent on these resources. One of the possible solutions to these problems is to replace crude oil with natural and biodegradable materials. Such solutions are favoured due to growing consumer awareness concerning the negative influence of industrial production based on the processing of crude oil. In the case of hydraulic working fluids which are produced from oils obtained from crude oil processing, their producers make efforts to develop products based on raw materials with lower negative impacts on the environment [1; 2; 3; 4; 5]. There is a lot of ongoing research aiming to use water as a working fluid in hydraulic systems [6; 7]. Using water in hydrokinetic systems is more advantageous than using oils for a few reasons. The viscosity of water is significantly lower than the viscosity of oils, it does not depend on temperature and the thermal conductivity of water is five times higher than the thermal conductivity of oil. Water poses no fire or explosion risk. The main disadvantages limiting the usage of water in hydraulic systems are a narrow range of working temperatures, causing the corrosion of steel (which usually makes up the elements of hydraulic systems) and low lubricity. However, the lower limits of the operating temperature can be shifted by using additives, lowering the freezing point of water. Corrosion can be eliminated by using stainless steel or anti-corrosion coatings for regular steel.