| The tribology characteristics of friction pair in internal combustion engine have significant influence on engine performance and reliability. The low friction technique is selected as the core problem of tribology in internal combustion engine. It is the basic premise for low friction technique to make sure of lubrication state and frictional power loss of each friction pair. The useful power is outputted by piston-connecting rod-crankshaft mechanism with high speed motion. The power mechanism is the main source of frictional power loss, so, the power mechanism is chosen as the study friction pairs. Piston assembly-cylinder liner causes the largest proportion of frictional power loss. So, in order to predict engine frictional power loss more accurately, following research works are carried out:1. Thermal boundary of piston assembly-cylinder liner lubricationThe temperature and deformation of cylinder liner and piston are the thermal boundary conditions of the friction pair. The thermal boundary conditions have decisive effect on the friction and lubrication of piston assembly-cylinder liner friction pair. To get exact thermal boundary conditions, a new in-cylinder heat transfer model is built by considering both total and local heat transfer; boiling heat transfer in water-jacket and oscillating heat transfer in piston oil channel are taken into account; coupling simulation is implemented between gas side,water side, lube oil side and structure temperature field. If boiling crisis occurs in water-jacket, the wall temperature will go up rapidly, it will cause lube oil coking, lubrication failure and material damage, so, the boiling state should be evaluated.Accuracy of the boiling heat transfer model is validated according to the rectangular cross-section experiment channel data, five kinds of regions are defined by the possible heat transfer state and characteristics:forced convection (FC), partially developed nucleate boiling (PDNB), fully developed nucleate boiling (FDNB), safety allowance for CHF (SAC), transition and film boiling (TFB). Based on the consideration for model error and heat transfer safety allowance, the design criterion for water-jacket is proposed.Based on the dimensional formula h(φ,i)=C(φ)d-0.2p0.8w0.8T-0.8 μe-0.47λe0.67cp0.33,a local heat transfer model was developed. By ensuring that the total quantity of heat transfer from local heat transfer model is always equal to Woschni correlation at every crankangle, the undetermined coefficient C(φ) was obtained. The improved model was applied to a single-cylinder water-cooled natural aspirated four-stroke direct injection diesel engine. The simulation results are satisfactorily in agreement with the experiment, the feasibility of the in-cylinder model is validated.Based on in-cylinder heat transfer model, heat transfer boundary condition of gas side is obtained; based on homogeneous flow boiling heat transfer model, heat transfer boundary condition of water side is obtained, oil side heat transfer boundary condition of piston oil channel is got by VOF model. Based on inverse distance weighting and point cloud fast registration method, coupling simulation is implemented between gas side, water side, oil side and structure temperature field. Take a heavy duty commercial vehicle diesel engine as example, the simulation results are satisfactorily in agreement with the experiments of cylinder head and piston. Based on the temperature field result, the deformation of cylinder liner and piston are obtained. Results show that:cooling water is in partially developed nucleate boiling state near bridge zone and exhaust port; boiling phenomenon is outstanding between adjacent two cylinder because of the thermal load from both of the two cylinder, it has reached the fully developed nucleate boiling state.2. Numerical analysis of piston assembly-cylinder liner lubrication and frictionDue to the high speed motion of piston relative to cylinder liner, after the thermal boundary conditions are gotten, the transient heat transfer model between cylinder liner and piston assembly is proposed to get the heat transfer of oil film and solid boundary layer, as well as the effect on frictional power loss. Based on the temperature field and deformation of cylinder liner and piston skirt, the lubrication calculation model is built for piston skirt with consideration of multi-body dynamics, lubrication and friction heat transfer, asperity contact, cavitation and micro-elastohydrodynamic conditions. The model is applied to this diesel engine, results show that:piston skirt is in hydrodynamic lubrication state; thermal deformation and lube temperature have great influence of frictional power loss; when the temperature of lube oil is lower, deviation of lube temperature will cause large absolute and small relative change of frictional power loss; compared with the condition of transient heat transfer, when the lube temperature is constant, the frictional power loss is smaller at the power stroke.Based on the temperature field and deformation of cylinder line, the mixed lubrication model of piston ring pack is built by simultaneous solution of the dynamics, blow-by, lubrication, cavitation and micro-elastohydrodynamic conditions. The model is applied to this diesel engine, results show that:the asperity contact takes place at all of the compression rings and oil ring; the top ring make the maximum contribution to the total frictional power loss, flowed by oil ring and second ring; the top ring has the maximum asperity frictional power loss, and the peak of transient total frictional power loss is caused by the asperity friction of top ring.3. Numerical analysis of connecting rod bearing lubrication and frictionBased on multi-body dynamics, the lubrication calculation model is built for connecting rod big end bearing with consideration of mixed thermo-elastohydrodynamic, cavitation and micro-elastohydrodynamic conditions. The method to find cavitation erosion region is proposed, the lubrication state is analyzed, besides, heat dissipation of frictional power loss is calculated. The feasibility of the model is validated by comparison between sliding bearing experimental and calculated results. Take the connecting rod bearing of heavy duty commercial vehicle diesel engine as example, results show that:the bearing is in mixed lubrication state, the asperity contact takes place at both edges of upper bearing top; cavitation erosion position is recognized by axis orbit, lube oil fill ratio, fill ratio rate of change and hydrodynamic pressure rate of change; mean frictional power is 0.44 kW; maximum transient asperity frictional power is only 111.1 mW, monitoring of transient frictional power will not accurately recognize the local lubrication state; heat conduction is the main way of energy dissipation of big end bearing lubrication.4. Sub-system frictional power loss experiment and numerical analysisIn order to get the frictional power loss of subsystems of diesel engine, the disassembly motored experiment is conducted, the power loss of one subsystem is got by the difference between the power before and after disassembly. The power loss of lube oil pump is got according to theory calculation. The working process model is built at fuel cutoff and non-supercharged condition, and the pumping loss due to motored condition is acquired with that the power loss of valve timing mechanism is corrected. The frictional power loss of main bearing at rated condition is gained by simultaneous solution of multi-body dynamics and mixed thermo-elasto-hydrodynamic lubrication model.To verify the feasibility of the calculation model and mothed of main bearing, connecting rod bearing, piston skirt and piston ring pack, the simulation models of disassembly motored experiment are built, by that the frictional power losses are obtained. Compared with experimental frictional power loss of main bearing, the error of the calculated results is 0.8%; for piston and connecting rod assembly, the error is 1.3%. So, the feasibility of the models and methods used for piston assembly and radial sliding bearings is validated.The differences are compared between disassembly motored experiment and actual working condition, as well as between simulation and experiment results, results show that:under the disassembly condition, as there is none pressure in cylinder, the frictional power loss of main bearing is the synthetic result of in-cylinder pressure and lube temperature reduction. Under the disassembly experiment condition, the frictional power losses of piston ring pack and connecting rod bearing are overestimated, and the piston skirt is underestimated. While the total frictional power losses of piston ring pack is overestimated, the asperity frictional power loss is underestimated, at the same time, each piston ring’s contribution to the total power loss has been changed. Among three piston rings, the total, asperity and hydrodynamic frictional power loss are very close, while the total and asperity frictional power loss of top ring are the highest under the rated condition. |
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