Submitted to: Surfactants in Tribology
Publication Type: Book / chapter
Publication Acceptance Date: 5/15/2010
Publication Date: 4/15/2011
Citation: Rudnick, L.R., Mohamed, A., Gul, O., Biresaw, G. 2011. Vaporization and carbonization tendency of vegetable oils as a function of chemical composition: morphology of carbon deposits on steel surfaces at elevated temperatures. In: Biresaw, G., Mittal, K.L., editors. Surfactants in Tribology. Vol 2. Boca Raton, FL: Taylor & Francis. p. 409-433. Interpretive Summary: In this work,we showed that the chemical composition of vegetable oils is directly related to the amounts of un-degraded (residue) materials remaining on the surface of the spark ignition engine. Higher residues remained un-burned when vegetable oil is high in palmitic acid as well as total saturated fatty acids. Stearic acid was found to have no influence on the amounts of the remaining residue. Fatty acid composition was found to directly relate to the morphology of the residue. Coconuts oil showed sponge-like morphology because it is solid at room temperature due to the high saturated fatty acid content. In this work, vegetable oils with different fatty acid profile and degree of saturation were tested for their lubrication performance and oxidative stability. Volatility was measured using thermogravimetric analysis (TGA), and the oxidative stability was performed using DSC, where samples were heated form 25ºC to 250ºC in the TGA and up to 320º in the DSC. The temperature regime of these studies was in the range of the temperatures in a conventional and high performance engines.
Technical Abstract: The objective of this study was to investigate the volatility of a series of vegetable oils and to relate the results to the vegetable oil fatty acid profile and deposit forming tendency. Since the amount of maximum deposit is related to what remains to carbonize, volatility is a contributing factor in the overall result. Volatility was measured using thermogravimetric analysis (TGA). In the thermal treatment experiments the vegetable oils were thermally and oxidatively treated at high temperatures (250 'C). The temperature regime of these studies was in the range of crown head and upper piston temperatures in a conventional and high performance spark ignition (SI) engine. Differences in deposit forming tendencies of the oils studied were significant. These large differences in deposit-forming tendencies under the same conditions can be attributed to the differences in the chemical composition of the vegetable oils. The amount of carbonaceous deposit left on a stainless steel (SS) 304 strip was found to be directly proportional to the amount of palmitic and total saturated acids present in the vegetable oil. Carbon deposit was found to be independent of the amount of stearic acid present in the vegetable oil. Oxidative stability was found to be dependent on the total unsaturated fatty acid amount. Different deposit morphologies or surface coverages were observed from vegetable oils having different fatty acid compositions. In general, the SS 304 surface after thermo-oxidative treatment was covered more uniformly with higher oleic acid content vegetable oils, whereas a layered surface coverage was seen with vegetable oil containing higher palmitic acid content. A sponge-like deposit morphology was observed with coconut oil, a vegetable oil that is solid at room temperature having 90% total saturates.