# Study on the Coupling Relationship of Low Temperature Fluidity and Oxidation Stability of Biodiesel
## 1 Introduction  
With the rapid development of the economy, the economic development based on petrochemical energy is increasingly constrained by the petrochemical resources shortage and environmental pollution. As a substitute for petrochemical diesel, biodiesel has been extensively concerned by countries globally for its environmental friendliness and renewable advantages with a very broad market prospect [1; 2]. The higher the content of saturated fatty acid methyl ester in biodiesel, the easier it is to crystallize and precipitate at a low temperature, affecting the storage and transportation of biodiesel at a low temperature, and making it easier to block the pipes and filters of diesel engines, while in turn influencing their normal oil supply [3; 4; 5; 6; 7]. The higher the content of unsaturated fatty acid methyl ester in biodiesel, the worse the oxidation stability is. Meanwhile, it is more likely to oxidize during the storage period, and it is not suitable for long-term storage. Moreover, the biodiesel contains unstable double bonds and will be polymerized in the oil pipeline for long-term use. The formation of macromolecular gelatinous substances, such as methyl dimer acid, may cause problems such as fuel system gelation, filter and nozzle clogging [8; 9; 10]. Obviously, the low temperature fluidity andoxidation stability of biodiesel not only affect its performance, but also affect the operation of various systems of motor vehicles and the storage of biodiesel [11; 12; 13]. As a result, the low temperature fluidity and oxidation stability of biodiesel are investigated. The performance of biodiesel is objectively evaluated, and it provides certain guiding significance for screening biodiesel additives. Additionally, it can promote the application of biodiesel on diesel engines and alleviate energy crises, thus improving the ecological environment. At present, the research on low temperature fluidity mainly focuses on the influencing factors of low temperature fluidity of biodiesel, the development of cold fluidity improver and the methods of improving low temperature fluidity. The research on oxidation stability has mainly focused on the factors affecting the oxidation stability of biodiesel, the oxidation mechanism and the development and screening of biodiesel antioxidants. However, the low temperature fluidity and oxidation stability of biodiesel are associated with each other. Few people have comprehensively studied the low temperature fluidity and oxidation stability of biodiesel; that is, the coupling relationship between the two. However, there is a mutual coupling relationship between low temperature fluidity and oxidation stability of biodiesel. How to simultaneously improve the low temperature fluidity and oxidation stability of biodiesel remains a difficult problem in biodiesel research. [PERSON]. [14] studied the low temperature fluidity of biodiesel blending oil, finding that the biodiesel fatty acid composition will exert no effect on the low temperature fluidity of low-proportion biodiesel blending oil unless biodiesel the content of medium and long carbon chain saturated fatty methyl ester is \(>\)48%. [PERSON]. [15] and other scholars believed that biodiesel fatty acid methyl ester composition is one of the factors which can affect its low temperature fluidity. The low temperature fluidity of biodiesel deteriorates with the increasing of saturated fatty acid methyl ester content and carbon chain length. [PERSON] et al. [16] considered that the oxidation stability of biodiesel is related to raw materials. Vegetable oil is abundant in polyunsaturated linoleic acid and linolenic acid, making the biodiesel produced by it generally poor in oxidation stability. [PERSON] et al. [17] concluded that the oxidation stability of biodiesel relates to the unsaturation of biodiesel fatty acid methyl ester, and the antioxidant used are caffeic acid and ferulic acid.