# Finite Element Analysis and Optimization of Hydrogen Fuel Cell City Bus Body Frame Structure
## 5 Conclusions  
The hydrogen fuel cell city bus has the advantages of environmental protection and long mileage, and has entered a rapid development stage in the public transportation market. In this paper, to ensure the driving safety of the hydrogen fuel cell city bus, the finite element model of the hydrogen fuel cell city bus body frame is established firstly. Then, the static and dynamic mechanical performance of the hydrogen fuel cell city bus are studied using the finite element simulation method, and the results of the static structural simulation test and dynamic modal simulation test are analyzed. The frequency of the bus body frame vibration is obtained by the dynamic modal simulation test, which allows the investigation of potential body resonance behavior. The maximum stress value and maximum displacement value are obtained by static structural simulation test, and it is found that there are two kinds of dangerous operating conditions. Based on this, the size optimization method is applied to optimize the bus body frame structure, finally achieving the driving safety of the hydrogen fuel cell city bus. The following conclusions are obtained:  
1. The static structural simulation results indicate that the maximum stresses under horizontal bending and ultimate braking conditions are 220.7 MPa and 305.6 MPa, respectively, which meet the strength design requirements. But, in the condition of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Object** & **Initial Value (kg)** & **Optimized Value (kg)** & **Change** \\ \hline mass & 2107 & 2001 & \(-\)5.03\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of bus body frame mass before and after optimization. emergency cornering, the maximum stress experienced by the hydrogen fuel cell city bus body frame reaches 328.9 MPa, which approaches the material yield strength of 345 MPa. Under the condition of ultimate torsion, the maximum stress of the body frame is 348.6 MPa, exceeding the material yield strength of 345 MPa, and does not meet the strength design requirements. This suggests that the bus body frame is at risk of dangerous operation under these two driving conditions. 2. The results of dynamic modal simulation tests indicate that the modal frequency range of the hydrogen fuel cell city bus body frame is 5.97-19.41 Hz, which is higher than the excitation frequency of the urban road surface. Additionally, it does not fall within the frequency vibration range of the bus's drivetrain and drive motor. Therefore, the bus body frame will not experience the resonance phenomenon. 3. The optimization results demonstrate that, after structural size optimization, the maximum stress of the hydrogen fuel cell city bus body frame is reduced from 328.9 MPa to 262.7 MPa in the emergency cornering condition and from 348.6 MPa to 300.6 MPa in the ultimate torsion condition; the maximum reduction is 20.13%. At the same time, the mass of the body frame is reduced by 106 kg. Furthermore, through verification, the maximum stress in the other two operating conditions is further reduced, and all these values are below the material's yield strength of 345 MPa. Additionally, the maximum displacements in all four operating conditions are reduced to varying degrees, and the modal frequencies are appropriately increased. These findings indicate that the optimized bus body frame meets the design requirements for strength and stiffness, and it is more distant from the road surface excitation frequencies, thus ensuring the safety of the bus during operation. Based on the above conclusions, the effectiveness of the optimization method applied to the bus body frame has been demonstrated in this paper. Moreover, while enhancing the strength and stiffness performance of the hydrogen fuel cell city bus body frame, weight reduction has been achieved, further increasing the bus's range capability. Overall, the optimization has yielded favorable results and provides valuable insights for the company during the bus body frame design phase. Conceptualization, R.Y.