# Study on the Coupling Relationship of Low Temperature Fluidity and Oxidation Stability of Biodiesel
## 3 Experiment Results and Discussion
### Oxidation Stability Experiment Results
#### 3.3.2 Biodiesel Fatty Acid Methyl Ester Data Acquisition
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **RME** & **TME** & **SME** & **OME** & **PNME** & **JCME** & **CSME** & **CTKME** & **CME** & **PME** \\ \hline CFPP/\({}^{\circ}\)C & \(-\)9 & \(-\)9 & \(-\)9 & \(-\)5 & 8 & 12 & \(-\)3 & 6 & \(-\)14 & \(-\)5 & 8 \\ OIT/h & 3.72 & 3.57 & 2.46 & 3.68 & 2.79 & 5.07 & 3.22 & 1.47 & 2.48 & 7.38 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition of biodiesel fatty acid methyl ester. It can be seen from Figure 10 that the species that meet or approach the biodiesel oxidation stability are far away from the apex of polyunsaturated fatty acids (lower left apex). Besides, the species that meet or approach the biodiesel CFPP are far from the apex of saturated fatty acids (lower right apex). Therefore, considering the biodiesel CFPP and oxidation stability requirements, the area of the gray part of the figure is delineated. The fatty acid methyl ester composition in this region generally contains a small amount of saturated fatty acids (\(<\)20%), a large amount of unsaturated fatty acids (\(>\)20%), and a certain amount of polyunsaturated fatty acids (\(<\)70%). Biodiesel in this area is JCME, SME, CME, PNME, RME, OME and TME. The main feature of these biodiesel is the high monounsaturated fatty acid methyl ester content. The only special case is PNME. The reason is that the high content of long chain saturated fatty acid methyl esters in PNME results in a high CFPP. Therefore, in the present study, not only the fatty acid methyl ester composition but also the structure and chain length of the fatty acid methyl ester should be considered. 3.4 Effects of Blending Ratio on the Coupling Relationship between Low Temperature Fluidity and Oxidation Stability of Biodiesel  
From Figure 11, it can be concluded that the fatty acid methyl ester composition generally contains a small amount of saturated fatty acids (\(<\)20%), a large amount of unsaturated fatty acids (\(>\)20%), and a certain amount of polyunsaturated fatty acids (\(<\)70%). To confirm this conclusion, PME with better oxidation stability was blended with RME with better low temperature fluidity. The composition of fatty acid methyl esters of blended fuel is shown in Table 4, and the effects of blending ratio on the coupling relationship between low temperature fluidity and oxidation stability of biodiesel can be found in Figure 11.  
Figure 10: Relationship between low temperature fluidity and oxidation stability of biodiesel and fatty acid methyl esters. It can be seen from Figure 11 that the CFPP of the blended fuel increases with the increase of the PME addition ratio, and the low temperature fluidity gradually deteriorates. The OIT of the blended fuel increases with the increasing of the PME addition ratio, and the oxidation stability gradually becomes better. When the blending ratio of PME and RME is R60P40, the CFPP is 0 \({}^{\circ}\)C, and the OIT is 5.9 h. Obviously, the method for improving the low temperature fluidity and oxidation stability of biodiesel by adjusting the composition and content of biodiesel fatty acid is feasible.