Study On Dynamical Model Of High Quality Commercial Vehicles

Since there is a vigorous development of high-way network and goods communication by road, the commercial vehicles are required to transfer goods as much as possible and enhance the transfer speed as well in order to prompt the efficiency per day. So far, the load is close to legislation limits and difficult to improve. Whereas the speed is relatively low and could be prompted. So, commercial vehicle manufactures in the world spare no effort to enhance the vehicle speed. The top foreign heavy commercial vehicle speed has been over 100 km/h, while domestic commercial vehicle speed is far less than that. There are 2 ways to enhance the speed, one is a equipting powerful engine and another one is improving quality of the vehicle platform.When the commercial vehicle speeds up, there are some negative phenomenons during the dynamic process, such as the decrease of driving stability, the increase of vehicle vibration, et al, appear to counteract the promotion of driving speed. The main reason is that under middle and high frequency the response of assemblies are coupled and influenced by others, whereas under low frequency the influence could be neglected. High speed and the structure of the vehicle itself could result in the increase of both vehicle and environment excitation frequency. The approach aiming to improve single performance of vehicle dynamics which suitable for the low speed could not impede the influence of the transient factors mentioned above. The total vehicle dynamics performance should be considered in order to get optimum subsystem characteristics.During pre-development stage, the vehicle dynamics model based on assembly characteristic is a powerful tool for assembly characteristic matching and full vehicle performance evaluation. Current commercial vehicle model based on assembly characteristic depends on the simulation of steady state performance, and the results are in high coherence with the tests at steady state. But it can't simulate the exact state when there is negative dynamic phenomenon mentioned above.Then if we want to further explore the potential of vehicle model based on assembly characteristic during pre-development stage and make the digital simulation accuraty, we need a model that not only can predict exact steady-state performance, but also can simulate the dynamic process phenomenon.After a detail review on domestic and foreign commercial vehicle dynamics and vehicle model development, considering current commercial vehicle models that are mainly focused on steady-state simulation and lack of description of medium and high frequency coupling, some key issues on the commercial vehicle model based on assembly characteristic which could be capable of dynamic process simulation are addressed. From the view of difficult points of this research, we have done the following work:First, a primary commercial vehicle dynamics model based on assembly characteristic is set up. Based on analysis of the key points of vehicle stability and simulation of dynamic process, the motivation and the key issues of modeling the assembly characteristic are introduced and analyzed. The vehicle model is validated by proving ground data.Second, chassis torsion characteristic of commercial vehicle is modeled and simulated. Compared to passenger cars, commercial vehicle chassis torsional stiffness is smaller. The excitation force would result in relative position changes of key assembling hard points and coupling response at medium and high frequency as well. The current model based on discrete multi-chassis structure mass includes torsion spring as analogue of the chassis torsion characteristic. Torsional stiffness is from static test, which can show the steady-state torsion characteristic, but can't show the dynamic phenomenon. A transfer function which is an equivalent method is introduced in this paper to identify the mass moment of inertia and torsional stiffness. This method could be used to describe dynamic chassis torsion characteristic in a specified frequency range.Furthermore, leaf spring solid axle suspension is modeled and simulated. The K&C and load supporting characteristic of suspension assembly play an important role on vehicle handling and ride preformance directly and thus are an important characteristic at commercial vehicle pre-development stage. At present, suspension K&C characteristic is mainly from K&C rig test, but the relevant equipments are not available in China. Suspension load supporting characteristic are mainly described by SAE 3-link model and Fancher model. The 3-link model can describe the non-linear nominal stiffness characteristic of suspension, but couldn't simulate hysteresis phenomenon. Fancher model can simulate hysteresis phenomenon, but it couldn't describe the movement of envelope curve and the change of transition curve. In this paper, a multibody dynamics suspension model based on structure is established by adding stick-slip friction force element to get suspension K & C characteristics; through adding static and dynamic friction effect of the force element to the existing SAE three-link model of the hinge point to achieve hysteresis simulation; by virtue of suspension deformation correction to improve Fancher suspension and achieve load supporting character boundary move and delay coefficient simulation.Finally, powertrain system is modeled and simulated. Powertrain is an important excitation source of the commercial vehicle, non-steady-state characteristic model could efficiently enlarge the analysis frequency range of the vehicle model. Engine torque is expressed by engine steady-state universal characteristic in the existing model, because of the lack of non-steady-state characteristic and excitation of chassis part, could not be simulated the torsion fluctuation and phenomenon of excitation. Combustion pressure characteristic of engine cylinders at typical condition is acquired by simulating, then the instantaneous pressure is obtained by three-dimensional interpolation, and the engine torsion fluctuation and vibration phenomenon can be simulated finally. The secondary torque excitation model of universal joint is also established and used in the chassis system, so that the excitation effect of powertrain to the chassis can be simulated.Above all, several key issues of commercial vehicle dynamics model based on assembly characteristic are researched in this paper. They are mainly about factors affecting the promotion of analysis frequency and dynamic process description. Five main innovation are as follows:Firstly, according to the deficiency of present method to get the chassis torsion characteristic, a method based on transfer function equivalence theory is brought out to identify the torsion spring moment of inertia and stiffness characteristic, so that the dynamic effect of chassis torsion stiffness characteristic can be simulated.Secondly, to solve the problem that the present Fancher model could not describe envelopes movement and transition curves changes, a method is brought out to revise them by suspension displacement, so the suspension load hysteresis characteristic is simulated exactly.Thirdly, since the SAE3-link model could not describe the hysteresis characteristic of suspension, a method of building stick-slip friction force element in the joints is used to resolve this problem.Finally, because it is kind of shortcoming to use the steady-state torque characteristic in the present engine model, a non-steady-state engine dynamics model based on cylinder combustion pressure characteristic is brought out. This model can simulate engine torque fluctuation and engine vibration.