## 3 Results and Discussion
### Surface Analysis  
Figure 5 shows the SEM images of the wear zones in the cylinder liners of different oil samples. Figure 5a presents the initial surface morphologies of the cylinder liners. The preparation process produced numerous caves, as indicated by the white arrows in the  
Figure 4: The surface diagrams of cylinder liners immersed in FFEO and EBF oils at 50 \({}^{\circ}\)C for 5 days. (**a**) Initial, (**b**) FFEO, (**c**) EBF. Figure 3: Variations in mass losses of the cylinder liner and piston ring with different oils. SEM images. Figure 5b illustrates the surface morphology of the cylinder liner after it was tested with pure EBF. Figure 5: SEM images of the wear zones in cylinder liners. (**a**) Initial, (**b**) EBF, (**c**) FFEO, (**d**) 20 vol% EBF, (**e**) 40 vol% EBF, (**f**) 60 vol% EBF, (**g**) 80 vol% EBF. Machining marks, corrosion pits, and furrows were observed on the surfaces of the cylinder liners. The corrosion pits were attributed to the acid and oxidised matter in the EBF. Figure 5c shows that the smooth surface was produced by the FFEO lubrication. The machining marks on the surface of the wear zone also confirmed that the FFEO provided better lubrication than the EBF. The peeling of debris could have been due to the fatigue wear with 20 vol% EBF in the FFEO. However, Figure 5d also indicates a smooth surface. After the EBF content was increased to 40 vol%, plenty of debris was found on the surface of the wear zone of the cylinder liner, which was produced by the EBF's corrosiveness, as illustrated in Figure 5e. On the one hand, the metal surface softened because of the corrosiveness of the EBF component. On the other hand, the soft-state matrices were easily peeled off during the rubbing process [24]. Figure 5f illustrates the corrosion phenomenon. Both the machining marks and furrows became lighter, and numerous pits disappeared compared with the case using 20 vol% and 40 vol% EBF in FFEO. This phenomenon was attributed to the corrosiveness of EBF. More wear debris and crackles were found on the surface of the cylinder liner exposed to 80 vol% EBF, as shown in Figure 5g. These observations would explain the corrosiveness of EBF, which was consistent with the results presented in Figure 4c. Figure 6 shows the variations in contour profile and surface roughness of cylinder liners lubricated by different oils. There are many parameters to describe a surface contour profile, such as _Ra_, _Rsm_, _Rsr_, and _Rz_, which depend on different definitions of surface contour profile. We selected _Ra_, the arithmetic mean of the absolute value of contour offset within the sampling length, to characterize the surface roughness of cylinder liners lubricated by different oils. In actual measurements, the more measuring points, the more accurate _Ra_ is. Figure 6a-c indicates that the cylinder liner lubricated with pure FFEO had higher surface roughness than the liners lubricated with initial and pure EBF. This was due to the properties of the different oil samples. Generally, oil with high kinematic viscosity results in a thick layer of the better-lubricating oil film. The initial crackles were protected by the FFEO oil film during the rubbing process. The low surface roughness of pure EBF was attributed to the corrosiveness of the biomass oil and the thinner oil film compared with that of FFEO, as shown in Figure 6b [25]. The surface roughness increased and then decreased when 20, 40, and 60 vol% EBF were added to the FFEO, as shown in Figure 6d-f, because of the negative effect of EBF on the film thickness of FFEO. The surface roughness (\(R_{a}\) = 0.419) of the sample lubricated with 80 vol% EBF in the engine lubricating oil, as shown in Figure 6g, was close to that of the sample lubricated with pure EBF (\(R_{a}\) = 0.417). This result, which is also indicated in Figure 5g, confirmed the corrosion function of EBF. X-ray dispersive energy (EDS) was employed to investigate the elements and their contents in wear zones lubricated with 20 or 80 vol% EBF in the FFEO.