TRW has developed a novel type of actuator to pop up the bonnet of a car when it senses an impact with a pedestrian, writes Mark Wilkinson. The bonnet, because it can be raised in only a few milliseconds, thus offers a crumple zone to decelerate the head before it hits the rigid engine block underneath, greatly reducing head injuries in an impact.

TRW has now developed a spring system that is able to actuate the bonnet mechanism in a sufficiently rapid and controlled manner. Previously, only pyrotechnic actuators (such as those used in airbags) were thought to be fast and cheap enough for the job. The advantage of a spring mechanism is that it can be re-set after use - pyrotechnics are single-use only and need to be replaced after firing. TRW and its competitors are also working on pyrotechnic solutions, however, where more cost-effective solutions are called for.

The first phase of the new European pedestrian protection legislation comes into force from October 2005 and will affect all new cars and CDVs introduced after that date. However, not all cars will need active bonnet systems. It depends on the energy absorption capabilities of the car's nose - some vehicles will be able to make do with passive, impact-absorbing structures and a higher bonnet line.