Estudo da variação do atrito cinético em mancais de deslizamento devido à contaminação do fluido lubrificante por biodiesel

Abstract

During a combustion engine's operation, it’s common that a small portion of the combustion gases surpass the seal provided by the piston rings and get into the crankcase, contaminating the engine oil, in a process named dilution. This phenomenon changes the viscosity of the lubricating oil, directly influencing the whole lubrication system of the moving parts, such as the crankshaft bearings. This study looks for information about how the dilution of soybean oil biodiesel in a SAE 15W40 lubricant affects the sliding friction coefficient on a sliding bearing. To reach this objective, friction and Stribeck tests were performed on the Sliding Friction Machine (MAC), property of the Contact Surfaces Laboratory – LASC of the Federal Technological University of Paraná – UTFPR. The friction tests are performed using tribological pairs, each formed by a bushing made from AISI 304 stainless steel internally covered with a babbitt alloy, and by a shaft made from SAE 1020 carbon steel. During 1 hour and 30 minutes, the sliding bearing, properly lubricated by pure oil and also under contaminated lubricants – with concentrations of 2.5%, 5% and 10% of biodiesel B100 – was submitted to the MAC’s axis rotation. The data analysis focused the sliding friction coefficient’s achievement considering the last 15 minutes of each test. After 20 tests – five tribological pairs for each of the four blends composed by oil and biodiesel – the average values obtained for the sliding friction coefficient were plotted against the level of dilution. Then, it was noticed a downward trend of friction up to the dilution level of 5%, with a further increase to levels higher than this one. The Stribeck tests were looking for information about the lubricant regime - limit, thin-film and thick-film – by the shaft speed’s variation, about the test’s sliding friction coefficient. For this purpose, 3 hour duration tests were performed varying the shaft speed from 3000 rpm to 250 rpm, and submitting the sliding bearing under constant load, aiming to reach those three lubricant regimes. The results showed, however, that for the adopted load conditions, the graphic curves obtained were not close to theoretical models. The difficulty of controlling the test temperature, the presence of mechanical vibrations and the bushings loose manufacturing tolerances have hindered the achievement of the expected Stribeck’s curves. Lastly, it can be said that the dilution of soybean oil biodiesel is not beneficial, since it can cause mechanical damage to the engine’s moving parts. Then, replacing the contaminated oil is needed, always respecting the exchange periods set by the engine manufacturer.