基于磁流变液的汽车液力缓速器制动稳定机理研究

The hydraulic retarder is a significant auxiliary braking device for the automobile road safety, which dissipates the mechanical energy through the fluid damping in the working wheels. In order to ensure the braking stability, the liquid filling rate in working wheels should response to the different vehicle driving conditions dynamically. However, the hysteresis effect of the liquid charging and discharging progress in working wheels is the bottleneck for the enhancement of the braking stability. In the research, the magnetorheological fluid with low concentration is used as the working medium for the force transmitting, and the relationship among the external magnetic field intensity, the distribution and motion characteristics of the magnetic field as well as the braking torque of the magnetorheological fluid could be determined. Then, the temperature and the velocity compensation law of the external magnetic field for the magnetorheological fluid could be obtained according to the analysis of the relationship among the viscosity, the wheel velocity and the temperature. Furthermore, the close-loop heat transfer model is built for the two-phase thermal management system so as to analyze the influence of the stator temperature and heat flux to the magnetic force of the magnetorheological fluid. Finally, the external magnetic field control law and the management methods of the phase-change cooling heat transfer are explored for fulfilling the braking stability in different braking conditions. All above establish the theoretical foundation for the stability enhancement of the braking torque for the hydraulic retarder, thus the vehicle constant-speed braking stability could be improved and the deceleration braking safety would be ensured.

液力缓速器是保证重型汽车行车安全的重要辅助制动装置。该装置通过其工作轮内部传动工质的阻尼作用实现机械能的耗散。为保证制动稳定性，缓速器工作轮腔内的充液率需要动态响应整车行驶工况的变化。然而充、放液过程存在的迟滞效应制约了缓速制动稳定性的提高。本项目以磁流变液作为液力缓速器的传力工质，首先研究外加磁场的强度、分布以及运动特征对缓速器制动力矩稳定特性的影响；然后分析磁流变液的粘度随温度及缓速器动轮转速的变化特点，得到磁流变液外加磁场的温度及速度补偿规律；通过建立冷却工质与磁流变液之间的双向传热模型，明晰缓速器定轮壁温及热流密度对工作轮腔内磁流变液磁性作用力的影响；最后探索恒速辅助制动和减速辅助制动工况下，实现制动力矩稳定所需的外加磁场的控制规律及相变冷却传热的控制方法,为提高液力缓速器制动力矩的稳定特性提供理论支撑，从而提高我国重型汽车恒速制动的稳定性，保障减速制动的安全性。

本课题提出基于磁流变液传动的汽车液力缓速器方案，以磁流变液代替传统液力缓速器传动工质，通过磁流变液对磁场快速响应的特性，增加缓速器制动力矩的控制精度和稳定性。通过分析磁流变液的特性和制动机理，确定了磁场的布置方式。研究了磁场对磁流变液缓速器制动力矩的影响，其制动力矩随着周向磁场的增加而减小，随着轴向磁场的增加而增加，然后，进一步比较了不同励磁方式下磁场对力矩的控制能力，周向磁场对制动力矩的调节范围占最大制动力矩值的70%，仅通过磁场调节就可以达到磁流变液缓速器的力矩调控需求，而轴向磁场仅为15%，适用于制动力矩的微调，周向磁场对制动力矩的调控能力远优于轴向磁场。根据磁流变液的温度敏感性，得出磁流变液缓速器制动力矩与温度之间的关系，得出制动力矩随着温度的升高而增加，但其增长率反而随温度的升高而变小；同时，原有磁感应强度也会影响温度升高时带来的力矩变化量，力矩波动有随着原有磁场增大而增大的趋势，即高磁场时温度变化引起的力矩波动更需要补偿；对于温度引起的力矩波动，温度偏离基准温度越远时，所需补偿磁场越大，在低磁场区域，磁场补偿量随着原有磁场线性增长，而在高磁场区域，补偿磁场有维持稳定趋势。基于朗肯循环，建立了液力缓速器的温控模型，探明不同工况下液压缓速器的发热性能和油外循环性能。通过比较ORC系统与传统水冷系统的仿真结果，对液压缓速器的温度控制稳定性，系统能耗和制动力矩特性进行了对比。实验证明了使用ORC系统控制油温的有效性。结果表明，在相同的工作条件下，ORC系统将油温变化降低了87％。此外，可以减少最多150Nm的制动扭矩波动。