## 2 Driving Simulation
### Hydrogen Tube Trailer Modeling  
To perform the driving simulation of a hydrogen tube trailer, we constructed a full car model with a suspension system, steering system, and powertrain (driving shaft, wheels, and tires). The truck tractor shown in Figure 2 consists of the front suspension (leaf spring and cab suspension), rear suspension (tandem drive axle, drive wheel, and tires), powertrain, and steering. The truck trailer in Figure 2 includes a trailer suspension (trailer axle, trailer wheel, and tire), and a trailer body (tube skid trailer). The suspension and steering systems of the hydrogen tube trailer are typically used in common commercial trucks and the corresponding templates in MSC/Car Truck are utilized to describe them.  
The tube skid trailer was modeled as a flexible body to calculate the acceleration received by the structure during driving.  
The model is composed of a vehicle (trailer) and a container loaded with skids by referring to the specifications of an actual in Table 1 to improve the accuracy.  
The entire skid structure is made of ASTM A36 steel. The container-type skid structure is designed and manufactured according to ISO/TC 104 and Convention for Safe Containers (CSC) certification. As shown in Figure 3a, the container is a box-type structure (length 12,196 mm, width 2436 mm, and height 2591 mm) with V-shaped reinforcements for each section on the left and right sides of the longitudinal direction and X-shaped reinforcements in the front and back. The base frame is composed of rectangular tube-type bottom side rails and cross members which are welded together as sub-assembly and the roof frame is joined by three roof bows, a fixed frame, and a corner reinforcement plate. The skid structure is loaded with 64 hydrogen tubes and a pressure control valve and pipeline to transport compressed hydrogen. The partitions that hold the hydrogen tubes are composed of 4 extruded plates assembled with bolts, and two partitions are paired to fix the front and rear adapters of the tubes through a holder.  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Specification** \\ \hline Weight of truck tractor (kg) & 4000 \\ Maximum payload (kg) & 27,000 \\ Length (mm) & 12,915 \\ Width (mm) & 2480 \\ Height (mm) & 1470 \\ Wheelbase (mm) & 10,100 \\ Track (tread) (mm) & 1840 \\ Tire type & 12R22.5-16 PR(D) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main specification of the tube skid trailer.  
Figure 3: Simplified tube skid design and dimensional information, (**a**) Skid with hydrogen tubes; (**b**) skid without hydrogen tubes.  
Figure 2: Full car model of the hydrogen tube truck.  
The hydrogen tubes are classified into Type 1 to Type 4 depending on the material, composition, and strength. Type 1 and 2 containers can store up to 150-300 bar, and Type 3 and 4 containers can store up to 350-700 bar. The hydrogen tubes loaded on the skid, in this study, were capable of storing and charging 700 bar using a Type 4 container.  
As shown in Figure 2(b), the modeling process was simplified by removing the pipelines and valves to facilitate the analysis process. All 64 hydrogen tubes were also removed to implement a method of applying the load to each bracket. The mesh quality was also improved by removing the rounded edge of each pipe that forms the skid structure.  
The FE model used a 10-node tetrahedral element with a 25 mm size to perform the mesh. The total number of elements is 177,790 and the number of nodes is 633,825. The number of degrees of freedom is 1,901,475. The transient structural analysis according to dynamic load (acceleration) was carried out using an implicit time integration scheme. It took about 12 h of analysis time based on i7 Intel(r) CPU (3.8 GHz) 4 core (Intel Corp., Santa Clara, CA, USA) for each driving course.