Research On The Multibody Dynamics Based Prediction Model And Measurement Technology For The Piston-liner Friction

The internal combustion engine is still the main powerplant for passenger cars,commercial vehicles,and ships in the near future.With the improvement of fuel economy requirement and the development of compact,lightweight and high power density design concepts for engines,the low friction and reliability designs for the key friction pairs in the engines become the public concern.As the core power subsystem of an engine,the pistonliner system consumes most of the friction losses in an engine,and it is also the determinant of the engine service life due to scuffing and seizure issues.An insight into the tribodynamics coupling characteristics from numerical and experimental aspects for the pistonliner system has great theoretical and practical significances to its designs.However,on the one hand,in previous studies on friction prediction models for the piston-liner system,most of them only focused on a single friction pair and ignored the coupling effect between multiple friction pairs.On the other hand,due to the restrictions of engine running conditions,it is very difficult to experimentally measure and evaluate the piston-liner friction,which limits the accurate and in-depth understanding of this system.Therefore,the above two issues will be intensively studied and solved in this thesis.Firstly,this work constructed tribo-dynamics models for the piston skirt-liner system and full floating piston pin bearing under the framework of piston-connecting rod-crankshaft multibody dynamics system.Based on the models,some deep predictive analyses in terms of tribology and dynamics for the system was carried out.Secondly,a technique was developed for the measurement of piston-assembly friction based on the instantaneous IMEP method and wireless telemetry technology.The piston-assembly friction was successfully measured for a gasoline engine under the motored condition and the above numerical model was validated.The theoretical and experimental research in this dissertation will provide better support for the low-friction designs of the piston-liner system in engines.The main contents of the dissertation are as follows:(1)Considering the coupling between fluid lubrication,solid contact and system dynamics of the friction pair,a multibody dynamics modeling method and a high efficiency computation framework were developed for predicting the friction performance of the piston-liner system with piston pin bearings.In this work,the multibody dynamics model for the complex mechanical system was built using the Lagrange approach,whilst the mixed lubrication models for tribo-pairs were governed by the Average Reynolds equation with consideration of lubricant rheology properties and surface roughness.Coupling with lubrication equations,the motion equations in the multibody system is turned into nonlinear ODEs.The MEBDF,a particularly powerful class of high order A-stable method,was adopted to the time integration of multibody dynamics motion equations,which greatly enhances the computational efficiency of the whole model.(2)A piston lubrication and dynamics analysis model was established by coupling the skirt-liner mixed lubrication model with the piston-rod-crank multibody dynamics model.The lubrication performance and secondary dynamics characteristics were studied systemically from the perspectives of piston structural parameter and working condition.First,a deterministic analysis for the common machining micro-grooves on skirt was conducted using the efficient model.The distinction of effects of the micro-grooves and piston to bore clearance,the influence the groove parameters including depth,density and shape on the hydrodynamic lubrication were studied and disclosed.Then,a cycle-by-cycle investigation into the piston tribo-dynamics characteristics during the engine cold-and warm-start was conducted.Particularly,the piston lubrication states and movement mechanisms at the FTDC of the first ignition cycle,which is the poorest working condition during the start-up,were insightfully studied.In the light of the above analysis,a piston profile optimization scheme was proposed and its influence on piston scuffing resistance and energy efficiency enhancement is assessed.(3)In order to consider the effect of the piston pin bearing on the tribological performance of the piston-liner system,a numerical model for the lubrication and dynamics analysis of full-floating pin bearing and a pin wear prediction model were established.Then,integrating it into the above piston-liner lubrication and dynamics model,a coupling analysis model with multiple lubricated tribo-pairs for the piston-liner system was futher constructed.Firstly,compared with the semi-floating piston pin bearing,the friction reduction and wear resistance mechanisms of the full-floating piston pin bearings were analyzed.Also,the motion characteristics of piston pin under different engine speed were studied and revealed.Secondly,the coupling effects of multiple lubricated tribo-pairs and some relevant design parameters for the piston-liner system were investigated so as to reveal their effects on its performance in terms of tribology and dynamics.(4)A measurement technique for the piston-assembly friction was developed based on the instantaneous IMEP method.The friction force was obtained indirectly by measuring the cylinder pressure,the connecting rod force and the crank angle position,in which the challenge is how to measure the connecting rod force reliably and accurately.Wireless telemetry technology was employed to measure the connecting rod force,which greatly minimizes the engine modifications and enhances the measurement reliability.Many engineering problems including sampling trigger synchronization for multi-channel signal,wireless data transmission and power supply etc.were addressed in the development of this technology.A comparative analysis was conducted between the measurements and the simulations from the above prediction model.The good agreement demonstrates that both the prediction model and measurement system have high accuracy,which can provide powerful prediction,analysis and evaluation tools for the low-friction designs of the pistonliner system.