# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 7 Compatibility
### Oxidation Stability  
The oxidation stability of a fuel indicates the tendency of the fuel to react with oxides at normal temperatures and its relative sensitivity to degradation due to oxidation. Because of the differences in composition, biodiesel is often more prone to oxidative deterioration than fossil diesel. Fossil diesel is a complex hydrocarbon blend containing alkanes, olefins, and aromatics obtained through a series of processes from petroleum crude oil such as fractionation. Unlike fossil diesel, long-chain fatty acid esters undergo a transesterification reaction with alcohols under the action of a catalyst to produce fatty acid monoalkyl esters as the main component of biodiesel [142]. The special structure of esters in biodiesel is very sensitive to its oxidation and thermal degradation [143], which are affected by the degree of unsaturation in the fuel. As shown in Figure 19, the influence of the compound structure of fatty ester and the number of double bonds is very important for fuel degradation, and there is a strong inverse relationship between oxidation stability and unsaturated fatty acid content. The auto-oxidation of biodiesel is a free radical chain reaction, which usually starts from the allyl site and includes three steps: initiation, propagation, and termination. The free radicals generated during the initiation phase interact with oxygen and form peroxides during the propagation phase. The peroxide then extracts hydrogen from the methylene group to form hydroperoxide and carbon radicals to promote chain reactions. Carbon radicals will continue to react with oxygen until termination [144]. The peroxides accumulate and eventually decompose to produce secondary oxides such as alcohols, aldehydes, acids, and oligomers, which will unfavorably affect fuel quality.  
The oxygen concentration, temperature, light and other factors affect the degree of oxidation, which is generally measured by indexes such as iodine value, peroxide value, acid value, and fluidity. One study [145] found that most of the biodiesel from different feedstocks will increase in acid value, peroxide value, density and viscosity due to its oxidative degradation under permissible oxidation conditions, especially at higher temperatures. The problem of poor oxidation stability can be solved by controlling the material in which the biodiesel is stored, the ambient temperature and  
Figure 18: Effect of biodiesel blends on WS1.4 (wear scar 1.4) and viscosity of different concentrations [140].  
light, and also by adding the appropriate antioxidants. [PERSON] et al. simulated the effect of storage containers on fuel oxidation by immersing copper sheets in biodiesel [146]. It was found that the induction time of biodiesel with antioxidant tert-butylhydroquinone was extended from 6.5 h to 24 h, which improved the oxidation stability of the fuel. Similarly, pyrogallol, gallic acid, propyl gallate, catechol, nordihydroquaiaretic acid, butylated hydroxyanisole and other antioxidants have also been found to have nice effects on the stability of biofuels [147].