Effects of fuel diluting low-viscosity oil on lubrication property of cylinder liner piston ring in diesel engine

The cylinder liner-piston ring is one of the most significant friction pairs in diesel engines. This study aims to explore the effects of fuel diluting low-viscosity oil on the lubrication property. Taking a farm diesel engine as the research object, the typical wall wetting was utilized to clarify the variation in the lubricant viscosity of diluting oil. The results indicated that the lubricant viscosity first dropped rapidly, and then remained stable, as the diluting ratio increased. Specifically, the lubricant viscosity dramatically declined by 44.9%, when the diluting ratio of oil rose from 0 to 10%, whereas, the dynamic viscosity decreased by 38.8% when the diluting ratio of oil was continuously elevated from 10% to 30%. It infers that there was a great effect of a little fuel diluting in the lubricating oil on the lubricant viscosity, while this effect became weaker than before when the diluting ratio reached a critical value. A mixed lubrication model was then built to investigate the tribological properties of the cylinder liner-piston ring, where the percentage of fuel diluting lubricant was changing. The simulated data showed that the minimum thickness ratio of fuel film decreased significantly, as the diluting ratio rose, indicating that the specific thickness of fuel film decreased between the cylinder liner-piston rings in a diesel engine. The area of hydrodynamic lubrication also decreased on the friction pair of the cylinder liner-piston rings, while the mixed lubrication area expanded with the increase of the fuel dilution ratio. Furthermore, the thickness of the oil film was too thin between the friction pair of cylinder liner and piston rings, especially at the diluting ratio of 30%. It was found that the whole process was in a mixed lubrication state, where the fuel lubrication was not enough. The hydrodynamic lubrication was therefore dominated by the mixed lubrication between cylinder liner and piston ring when the external load of the piston ring was relatively small. Even if the viscosity of lubricant was very low, a certain thickness of the oil film was also enough to bear the external load. Correspondingly, the external load of the piston ring was extremely large, while the oil film thickness was very thin in the compression stroke and expansion stroke. The mixed lubrication between cylinder liner and piston ring was dominant in the whole process, where the pressure of the oil film cannot fully bear the external load eventually. Additionally, the area of mixed lubrication was extended to the threshold, as the region of hydrodynamic lubrication shrank constantly, where the film pressure declined, while the asperity friction force increased significantly. Therefore, the average asperity pressure increased in turn to nearly double times at a 30% diluting ratio, compared with that without dilution. As such, the probability and pressure of direct contact surface were higher between the cylinder liner-piston rings near the top dead center of reciprocating stroke, when much more diesel fuel was mixed in the lubricating oil. In this case, there was a negative impact on the surface wear of friction pair with the growing diluting ratio. The asperity friction force increased in turn to a large friction, as the dilution ratio increased from 0 to 30%, indicating there was a significant effect on the total friction of cylinder liner and piston ring. The viscosity decreased gradually with the increase of diluting ratio, which made the fluid friction decrease. The asperity pressure also contributed to the asperity friction, thereby to the total friction. In addition, the cycle friction loss on the ring/liner pair decreased at first and then increased, finally reached the lowest point at a 10% diluting ratio. An engine test bench was built to carry out the motored friction test. The testing data verified that the variations of motored torque were consistent with the simulated ones under different diluting ratios. Consequently, potential guidance was that the diluting ratio should be controlled under 20% to keep the lubrication effects in the application of low-viscosity lubricating oil.