| The sulfur content in diesel fuel has been strictly limited to meet the demand of regulations about emissions. Because the polar materials, which have good lubricity, were easily removed during the hydrodesulfurization process, the low-sulfur diesel fuel has a bad lubricity. It is necessary to find out some additives to improve the lubricity of low-sulfur diesel fuel. Biodiesel, which is a mixture of saturated and unsaturated fatty acid esters, has a good lubricity. In order to have a better understanding of the principle and influence factor of fatty acid ester as lubrication additives, the lubricity of fatty acid esters have been done by four-ball machine. And the adsorption behaviors of fatty acid esters on iron surface were investigated by Materials Studio molecular simulation software to discuss the effects of fatty acid ester’s carbon chain length, carbon-carbon double bond and hydroxyl group on its lubricity. The main research contents and conclusions are as follows:(1) The methyl myristic(C14:0), methyl palmitic(C16:0), methyl stearate(C18:0), methyl oleate(C18:1), methyl linoleate(C18:2), methyl linolenate(C18:3), methyl ricinoleate(C18:1 OH), castor oil methyl ester and soybean oil methyl ester were added to low sulfur diesel fuel by 1.0%, 2.0%, 3.0% and 5.0%, respectively. And then, the samples were tested on a four-ball machine. The results show that the wear scar diameter decreased regularly with an increasing concentration of biodiesel fuels and fatty acid methyl esters. The biodiesel fuels have a better improvement than the pure fatty acid methyl esters. As for the saturated fatty acid methyl esters, the lubricity can be ordered as follows: C18:0>C16:0>C14:0, namely, the longer carbon chain length, the better lubricity. For the C18 fatty acid methyl esters series, the lubricity can be ordered as follows: C18:1 OH>C18:3>C18:2>C18:1>C18:0, namely, an enhancement in lubricity with the increase in the unsaturation degree of esters and esters with hydroxyl group has a better lubricity than double bond.(2) The adsorption behaviors of C14:0 molecular system on Fe(100)、Fe(110) and Fe(111) surfaces were studied by Materials Studio molecular simulation software, respectively. The simulation results show that Fe(110) surface can adsorb fatty acid methyl ester more easily than other esters because it has a highest adsorption energy per unit area.(3) The Monte Carlo simulation for single molecular and MD simulation for molecular system of saturated fatty acid methyl ester, C14:0, C16:0 and C18:0 on Fe(110) surface have been done, respectively. The Monte Carlo simulation results show that the increase of carbon chain length can effectively increase the single molecule’s adsorption energy on iron surface. The MD simulation results show that the saturated fatty acid methyl esters have similar adsorption energy. The van der Waals interaction energy, which is one part of cohesive energy of adsorption film, is obviously enhanced with the increase of carbon chain length. And those simulation results of adsorption behaviors are agreed with the lubricity of saturated fatty acid methyl esters.(4) The Monte Carlo simulation for single molecular and MD simulation for molecular system of unsaturated fatty acid methyl ester, C18:1, C18:2, C18:3 and C18:1 OH on Fe(110) surface have been done, respectively. The Monte Carlo simulation results show that the single molecular C18:1, C18:2 and C18:3 have similar adsorption energy on Fe(110) surface. C18:1 OH has higher single molecular adsorption energy than other unsaturated fatty acid methyl esters because the hydroxyl group has a bigger polarity. The MD simulation results show that the unsaturated fatty acid methyl esters have similar adsorption energy on Fe(110) surface. The van der Waals interaction energy of unsaturated fatty acid methyl esters is similar because the relative molecular mass is close. But the cohesive energy has a great change because of the difference of electrostatic energy, which is closely related to polarity. The cohesive energy of adsorption film of different unsaturated esters can be ordered as follows: C18:1 OH>C18:3>C18:2> C18:1, which is agreed with the lubricity of unsaturated fatty acid methyl esters. |
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