燃油稀释低黏度机油对柴油机缸套活塞环润滑性能的影响

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

为研究燃油稀释低黏度润滑油对缸套活塞环润滑性能的影响,该研究以农用柴油机为研究对象,基于燃油湿壁现象考察了燃油稀释低黏度润滑油对混合液的黏度变化影响。试验结果显示,随着燃油稀释率增大,混合液的动力黏度呈现先急剧下降后缓慢下降的变化规律,稀释率从0增加到10%,动力黏度降幅达44.9%,表明少量柴油稀释低黏度润滑油将导致黏度迅速降低。通过构建缸套-活塞环润滑摩擦理论模型,采用数值计算方法探究了燃油湿壁效应对缸套-活塞环摩擦性能的影响机理。模拟计算结果显示,随稀释率的增加,缸套-活塞环之间的油膜厚度变薄,流体动压润滑区间不断缩小,而混合润滑区间不断扩大,导致摩擦副表面微凸体接触增多,特别是在压缩行程上止点附近,缸套-活塞环的摩擦力随着稀释率增加而增大;而缸套-活塞环摩擦副的循环摩擦损失随稀释率增大呈现先降低后升高的趋势,稀释率为10%时摩擦损失最小。通过搭建发动机测试台架进行倒拖试验发现,当稀释率从0增大到30%时倒拖转矩呈现先减小后增大的趋势,且当稀释率为10%时倒拖转矩最小,验证了不同燃油稀释率下倒拖转矩变化与模拟计算结果的一致性。在发动机中应用低黏度润滑油,应控制其稀释率低于20%,以保持必要的润滑作用。研究结果可为为低黏度润滑油的推广应用提供指导。

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.