ABSTRACT
The environment of Arab countries suffers from the relatively high concentration of dust in air, due to the lack of rain and the vast area of desert, which causes an increase in wear rate of engines. Consequently, the period of lubricating oil change has been reduced to get rid of the contaminated oil to avoid further wear. This solution is severely polluting the environment because some people used to get rid of the used oil by burning causing hazardous toxic fumes due to the presence of the chemical additives. Besides, used oil can leak to the water resources and pollute them. In addition to that, lubricating oil contains antiwear additives which need longer time to react and give up the required lubricating properties.
 
To overcome those severe environmental impacts, it was planned in the present work to investigate the change of the lubricating properties of the engine oils in use. Experiments have been carried out to investigate the lubricating properties of the used oils. Oil samples taken from three passenger cars, running in City of Al-Taif, Kingdom of Saudi Arabia, were tested using a test rig designed especially for that purpose.  It was found that both the friction coefficient and wear increased with increasing running distance up to 2000 km then decreased to values lower than that displayed by fresh oils.  It was found that antiwear additives (zinc dialkyl dithiophosphate ZDDP) decompose yielding products that react with the metal surface to form a film that is highly wear resistant. This observations confirm the necessity to extend the operating time of the used oil up to 5000 km running distance to make full use of the improvement of the lubricating properties of used oil during usage. Besides, considering the measurements of both kinematic viscosity and total base number, it can be concluded that the tested used oils were still efficient and their use can be extended for longer running distance. The application of a by-pass oil cleaners containing a super fine filtering materials which absorb contaminants down to the size of 0.1 µm is necessary to remove most of the solid and liquid contaminants in the lubricating oil. The proposed oil cleaners can trap sludge and carbon soots. This helps to prevent wear of engine and reduce the cost of lubricating oil, which occupies a great portion of the maintenance cost.