Dynamics Study Of A Power-split Hybrid Electric Vehicle During The Engine Start Transition

Intelligent engine start-stop technology is a significant method to reduce fuel consumptions and emissions in hybrid electric vehicles,which reasonably controls the working state of the engine to shorten the inefficient working time and to improve the efficiency of the hybrid system.However,frequent engine starts and stops will not only cause vibrations and noises to deteriorate the comfort,but also cause the driving torque interrupts and impacts to deteriorate the drivability.Therefore,it is significant to improve the vehicle drivability and comfort on the premise of the optimal system efficiency and emissions.Compared with conventional vehicles and other hybrid vehicles,the powersplit hybrid system studied in this thesis has no clutch to disengage the engine and driveline,resulting that the engine excitation during engine stats will affect the car body not only through the mounting system but also through the driveline path.Therefore,the vehicle vibration caused by engine starts is a transient dynamics problem of a multidimensional coupled system.In addition,the engine stopping process can be viewed as the inverse action of the engine starting process,so only the vehicle dynamics of the engine starting process is mainly studied in this thesis.The power-split hybrid system studied in this thesis employs a compound planetary gear train as the power splitting device.On the one hand,the hybrid system can optimize the working efficiency of the engine to save energy and reduce emissions by controlling speeds and torques of two motors;on the other hand,the hybrid system can smoothly switch the operation modes by coordinating torques of two motors and the engine.Hence,the vehicle control system is an important foundation of the dynamics analysis.In order to study the dynamics behaviors and vibration attenuation of the power-split hybrid vehicle during the engine starts,the mechanical structure and control architecture are introduced firstly.Secondly,four different operation modes are analyzed,and the law of mode shifts is briefly summarized.Thirdly,the dynamic torque coordination controller is built,meanwhile,two types of engine starting processes are discussed in detail.Finally,the “source-transferreceiver” method is adopted to qualitatively analyze the vehicle vibration behavior during the engine starts,and the results show that the car body vibrations are generated by multi-source and transferred through multi-path.Based on the qualitative analysis,the transient dynamic responses during the engine starts are measured and analyzed.The results show that there are severe longitudinal and vertical vibrations of the car body during the cranking phase and the initial ignition phase,which seriously affects the vehicle drivability and comfort.In order to improve the vehicle drivability and comfort,the numerical simulation method is adopted to predict the system dynamics response,analyze the vibration mechanism and research the vibration attenuation methods.Firstly,an engine dynamic model affected by the cylinder pressures,inertia forces and frictions is built to predict the high-frequency excitations during the engine starts.Due to the large difference of cylinder pressures in the crankingphase and igniting-phase,the cylinder pressures of these two cases are accurately obtained by bench tests.The engine dynamic model can predict the excitations under the ignition frequency.Secondly,the dynamics models of the driveline,powertrain mounting system and suspension system are also established by using MATLAB and SIMULINK software platform,and the natural characteristics of these linearized models are analyzed.Finally,dynamics responses of the whole vehicle during the engine starting process are analyzed by the joint simulation of the dynamic torque coordination controller,the engine dynamic model and the nonlinear dynamics model.At the same time,the multidimensional system dynamics model is verified by comparing with the test data.The numerical simulation results show that during the engine cranking phase the sever longitudinal vibrations of car body are mainly stimulated by the driveline resonances,because the engine excitations have a characteristics of sweep-frequency;while the vertical vibrations of car body are mainly caused by the mounting system resonances under the action of engine inertia force.According to the vibration mechanism,the influences of the initial position of crankshaft,the start time of engine,the stiffnesses and dampings of elastic components on the car body vibrations are studied.Simulation results show that minimizing engine excitations,shortening the engine start-up time and optimizing the stiffness and damping of elastic components are very limited to reduce the longitudinal jerk and vibrations of the car body,because the hybrid driveline is a weak damping system.In the end,using the fast response characteristics of electric motors and the existing sensors,the active damping control strategy is studied to reduce the driveline torsional vibrations.The results show that the elasticity of torsional damper is the main influence factor of the active damping control strategy,and the active damping control strategy can effectively restrain the driveline resonances and the longitudinal vibration of the car body during the engine starts,thus the drivability and ride comfort can also be improved.