| Subframe, an important chassis component which has gradually evolved from D-classluxury sedans to many A-Class household cars, draws more and more consumers’attentions forits significant influences on engine vibration isolation and full vehicle performance. A set ofparameters of subframe system may have effect on its characteristics, like the subframestructural flexibility, the mounting stiffness and damping coefficient. In order to establish aprecise research model, this paper firstly built a dynamic engine model in Adams/Car, whichcould output the fluctuant excite force and torques along with the720degree’s crankshaftrotation. And then, a full vehicle model including the dynamic engine and flexible subframe andmounting systems have been established. After finishing the vehicle model, frequency domainanalysis have been conducted for studying on the subframe isolation performance. Finally, therelated conclusions have been confirmed by full vehicle time domain simulation via ride andhandling performance cases.Here are the research highlights:1. Building of dynamic engine model. For the purpose of obtaining accurate engineexcitation, this paper improved the Adams/Car powertrain template and embedded the dynamicengine model. Instead of using the engine MAP diagram, dynamic engine model gets the insidecylinder pressure by the interpolation algorithm of indicator diagram. The variables needed forthe interpolation are throttle, crankshaft speed and crankshaft angle. The Piston connecting Rodsystem has also been built, which is for the calculation of engine torque. In addition, the engineexcitation force applying on the mounting system results from the constraint force on the Pistonconnecting Rod system. The method of embedding dynamic engine model into Adams/Car isAdams/Solver subroutines, which has also been introduced in details in the paper. At last, thepaper provided the engine model validation results.2. Building of the flexible subframe dynamics model. In order to study the influence offlexible subframe on the isolation performance, this paper realized the deformable subframe modeling. Firstly, the subframe Finite Element model has been built. And then analyze themodal characteristics of the subframe. Finally transfer the FEA model into Modal Neutral File,which could be imported into Adams/Car to conduct calculation and simulation.3. Frequency domain analysis of the subframe isolation performance. The naturalcharacteristic of the vehicle vibration model has been analyzed using Adams/vibration. Theinput channels were vertical excitation force and engine torque of the crankshaft axis of rotation,and the output channels were the subframe mounting axial force and the vehicle center verticalvibration acceleration. The frequency response performance of the subframe mounting force aswell as the influence of subframe flexibility, subframe mounting stiffness and damping havebeen studied. The results show that the subframe design should increase the first order modalfrequency to reduce possible resonance with engine excitation. And the subframe mass andmounting stiffness both influence the subframe rigid body vibration mode. Subframe mountingdamping should meet the low frequency and large damping, high frequency and low dampingcharacteristics.4. The influence of subframe on full vehicle performance. The full vehicle simulationswere conducted under accelerated resonance case, ride comfort case and handling and stabilitycase. The effects of various parameters on the subframe vibration isolation performance havebeen verified. In conclusion, subframe could effectively isolate engine vibration transmitted tothe body and improve vehicle comfort. However, the subframe mounting flexibility will reducethe stiffness of the front suspension system; thereby influence the vehicle handlingperformance. |
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