Passenger airbag virtual design and simulation for C-NCAP frontal test of an electric vehicle*

Cheng-yue JIANG，Hong-yun LI，Yuan-zhi HU，Xi LIU

1Chongqing University of Technology，Chongqing 400050，China;

2China Automotive Technology and Research Center，Tianjin 300300，China

1.Introduction

The updated China New Car Assessment Program(C-NCAP)thereafter July 2012 increased its 40%load case velocity from 56 km/h to 64 km/h，while keep the full frontal crash test at 50 km/h［1］．The offset frontal test specification is similar to the 64 km/h test in Euro-NCAP［2］．This modification increases the difficulty for the restraint system development．The summary of the test specifications and dummy set-ups for C-NCAP(2012 edition)was shown in Table 1．

Compared to the Internal-Combustion Engine(ICE)vehicle，the mass of Electric Vehicle(EV)increases due to the high-voltage battery compared with the fuel tank，considering maintain of cruising distance and cabin space［3］．The energy absorption by body structures during the crash increases due to the increased mass of the battery．Therefore，it is essential to control the cabin deformation and the body deceleration for occupant protection in the frontal impact．The pulse of the EV is different with that of the ICE vehicle similar in size and platform，which contributes to the increases of the chest deflections for both driver and passenger side dummies［4］．Besides，research indicated the increased mass would introduce more compartment intrusion，which makes it hard to achieve good occupant protection performance［5］．

In the development of this EV，the main work of vehicle structure was to ensure the safety of high voltage batteries，as well as the protection for the occupants by keep the cabin deformation．In this study，the main focus was on the development of a passenger airbag for the EV(based on a B-class platform)，which contributed to the vehicle’s passive safety per-formance and C-NCAP test final score．Due to increased mass and structure change，there were following changes for the airbag design and simulation of the EV，as shown in Table 2．

Table 1．C-NCAP 2012 edition frontal test load cases and instructions

Table 2．Input changes for the airbag design of the EV

2.Airbag virtual design and simulation

For the airbag design and following analysis，a computer aided engineering(CAE)model containing instrument panel，windscreen，and knee bolster was built-up．Considering the passenger airbag shape design，two airbag models were developed from the original airbag model(equipped in the vehicle of the same platform)with computer aided design(CAD)tool．To reduce the calculation time，the average temperature solution［6］ was used to achieve the mass flow of inflator gas，see Equation 1．

where msis mass flow rate，Ptankis the pressure，cvis the constant volume heat capacity，Ttankis inflator gas temperature，MWSis the molecular weight of species，cpis the constant pressure heat capacity，Tsis the average inflator temperature．

The calculated mass flow and mass temperature were used for the airbag deployment simulations．Figure 1 shows one airbag concept design(case 1)and another design with optimized lower part(case 2)．

Figure 1．Comparison of the two concept airbags:case 1(left)and case 2(right)

Compared with case 2，case 1 airbag simulation showed faster inflation process and the head-airbag contact surface was more appropriate than that of case 2．Thus，a manual sample airbag of case 1 concept was made and folded for the following in-vehicle deployment test，see Figure 2．

Figure 2．Airbag sample(left)and its folding(right)

The sample of case 1 passenger airbag was made by manual and deployed in the vehicle’s compartment(see Figure 3 left)．The CAE analysis work was conducted to make sure the deployment kinematic of the CAE model correlate well with that of the test(see Figure 3 right)．

Figure 3．In-vehicle airbag deployment test(left)and deployment simulation(right)

3.C-NCAP frontal test simulations

After defining the airbag dimensions and its folding process，the main focus of the following work was on the detailed parameter optimizations in the CAE circumstance．For the C-NACP front test simulations，parameters including the seat-belt elongation，seat-form stiffness were calculated based on the component test results．The elongation and stiffness data was used as the input of the passenger side simulation model，see Figure 4 below．

Figure 4．Component tests for passenger side simulation

The vehicle acceleration pulse(see Figure 5 left)and compartment intrusions from the structure crashworthiness simulation were used as the inputs for the occupant kinematics and passenger airbag simulations．As shown in Figure 5，the peak value of the EV is less than that of ICE，and the duration of the crash pulse is about 90 ms in this case．

Figure 5．Comparison of B-pillar crash pulses(left)and velocities(right)

Based on the passenger side simulation analysis，the optimal parameters for the following frontal test were list in Table 3．

Table 3．Optimal simulation parameters for the frontal test

4.C-NCAP frontal test results

The passenger airbag module’s performance was evaluated in the real world full frontal test，and it protected well at the dummy’s head，neck and chest regions．Table 4 shows the passenger side score proportion and head acceleration result．

Table 4．Passenger injury result and head acceleration

Besides，when comparing the occupant kinematics in both simulation and test，as shown in Figure 6，they correlate well with each other．Thus this method is beneficial in predicting occupant injury index and occupant kinematics with certain accuracy．

Figure 6．Comparison of occupant kinematics in both simulation(left)and test(right)

5.Conclusion

In this paper，passenger airbag virtual design method and simulation process were presented，which resulted in good test results．Conclusions are drawn as follows:

1)Airbag design and optimization through CAE analysis is efficient for the vehicle passive safety development，which can help identify the vehicle’s passive safety performance at early stage before test．

2)In-vehicle airbag deployment test and simulation is essential for the airbag concept design and module kinematics analysis．

3)For this electric vehicle，more attention should be paid on the reduction of the compartment intrusions which are critical for the occupant kinematics and final injury results．

［1］ China Automotive Technology and Research Center［Z］．C-NCAP test protocol，2012，http://www．c-ncap．org．cn/c-ncap_en/ep/2012english．pdf．

［2］ Euro NCAP．Frontal Impact Working Group-draft protocol for full width test 2015［Z］，2014．

［3］ Hayata Uwai，Atsushi Isoda，et al．Development of body structure for crash safety of the newly developed electric vehicle［C］．Proceeding of the 22ndInternational technical conference on the enhanced safety of vehicles，Washington，2011．

［4］ Hua Yang．Front impact simulation analysis of electric vehicle［D］．Wuhan:Wuhan University of Technology，2008．

［5］ Fadi Tahan，Chung-Kyu Park，et al．The effect of reduced mass on frontal crashworthiness［C］．The proceedings of the IRCOBI 2013 Conference，2013．

［6］ TNO．MADYMO Model Manual version 7．4［Z］，2012．