Pedestrians are a high-risk group in vehicle impacts, especially in urban areas. In Europe pedestrians account for around 20 % of all traffic fatalities. In the rest of the world this frequency varies from 14 % in USA up to 47% in Thailand. The majority of pedestrian fatalities are due to head impacts.
Today’s cars are very densely packed under the bonnet. Certain stiff parts, such as the spring tower and the top of the engine, are very close to the bonnet. There is often not enough space for bonnet deformation by an impacting head. The consequence is often a severe or fatal head injury.
Therefore, a protection system has been developed to decrease the severity of head-to-bonnet impacts. The system is activated at the impact by a sensor located in the bumper, at speeds above 20 km/h. The sensor is able to discriminate objects with a different geometry (another car versus a leg), as well as with a different stiffness (a pole versus a leg). Two actuators lift the rear part of the bonnet approximately 100 mm. The actuators were tuned to have lifted the bonnet at 60–70 milliseconds after the leg-to-bumper impact, but before the head impact. The actuators/lifting elements were also tuned to stay up during the upper torso impact, but still be energy-absorbing to keep the head loading down if the head impact is on top of the lifting elements.
The system has been tested by a headform impacting the bonnet at various locations and speeds up to 50 km/h, as well as with a complete car front on a sled impacting a pedestrian dummy. The dummy tests were performed to check the timing of the system, but also to check that the lifting elements were strong enough to keep the bonnet in a lifted position during the upper torso impact until the head impacted the bonnet. The kinematics of the pedestrian dummy was compared to that of a validated pedestrian mathematical model. In headform tests in 40 km/h the system decreased the HIC values to acceptable levels (<1000) in all test points for the lifting bonnet, including the headform contact locations above where the bonnet was lifted. In the 50 km/h headform test above the bonnet’s stiffest point, a large reduction of the HIC value was achieved. It was reduced over 90 % to a value of 1213, with the active bonnet system compared to the standard bonnet.