## 3 Results and Discussion  
### Friction Reduction and Wear Resistance  
Figure 2 reports the variations in friction coefficient among the oil samples at a load of 210 N and a reciprocating frequency of 10 Hz. The friction coefficients of all oil samples initially decreased for 45 min because of the running-in effect.  
Figure 1: Schematic diagram of the tribometer simulating the friction pair of cylinder liner–piston ring. 1—cylinder liner, 2—piston ring, 3—fuel tank, 4—temperature sensor, 5—pressure sensor, 6—load.  
The friction coefficients of the oil samples increased simultaneously with the EBF content, mainly because of the dilution function of the EBF. The thickness of the oil film generally increased along with the kinematic viscosity of oil. The trend of kinematic viscosity is discussed in Table 2. An increase in EBF content promoted corrosive wear. These results were verified by the degradation of the FFEO additives and decreases in EBF anti-friction components [21].  
The friction coefficient of pure EBF was lower than that of pure FFEO during the 140 min initial rubbing process. Opposite results were observed after the rubbing process because of the easy absorption of active EBF components on the wear surface, forming a lubrication film during the initial rubbing process [14]. The low friction coefficient of FFEO was attributed to the antifriction and anti-wear additives in engine lubricating oil, whose mechanism will be discussed later.  
Figure 3 presents the variations in the mass losses of the cylinder liner and piston ring with the different oil samples lubricated at a load of 210 N and at a reciprocating frequency of 10 Hz. The mass loss of the pure FFEO barely revealed the anti-wear property of the additives. The mass losses of the cylinder liner and piston ring increased after EBF content in the FFEO was increased. For the FFEO with low EBF contents (20 vol% and 40 vol%), the increases in mass loss suggest reduced efficiency of engine oil additives. With regard to the FFEO with high contents of EBF (60 vol% and 80 vol%), the increase in wear mass losses was also caused by the EBF's corrosiveness apart from the aforementioned reason. This phenomenon was explained by observing the surface of cylinder liners immersed in pure FFEO and EBF at 50 \({}^{\circ}\)C for 5 days. Obvious pit corrosion caves, as shown in Figure 4c, were found on the surfaces of cylinder liners immersed in EBF [22].  
Figure 2: Variations in friction coefficient for various oil samples at a load of 210 N and a reciprocating frequency of 10 Hz.  
The mass losses of the cylinder liner and piston ring in pure EBF were less than those of samples in EBF-contaminated oil, but were higher than those of samples in pure FFFO. These results indicate that the anti-wear property of EBF is worse than that of FFEO, and provide a plausible explanation for why the anti-wear property was degraded as the EBF content in the FFEO increased. The lubrication component of EBF was consumed by its interaction with additives in the FFEO [23]. With 80 vol% EBF, the maximum mass losses of the cylinder liner and piston ring were as much as 69.9 and 2.9 mg. This phenomenon was caused by the interaction of EBF with FFEO and the corrosive wear of EBF.