As regulations on pedestrian protection have become more stringent in the EU, Magna Steyr, Rühl Puromer and Hennecke have developed an engine bonnet with a honeycomb core. It not only passed the pedestrian protection tests, but is said to be lighter and to provide better thermal and acoustic insulation than conventional bonnets made from steel or aluminium. Volume production of the bonnet is now imminent.
In 2009, the EU published Global Technical Regulation (GTR) no 9 on pedestrian protection, the implementation of which is being worked on by automotive manufacturers and suppliers around the world. Pedestrian protection is also assuming considerably greater importance in the redefinition of the Euro NCAP ratings, a non-manufacturer-specific crash test programme. In order to continue receiving the sought-after 4-star or 5-star ratings for their vehicles, the manufacturers must make greater efforts to ensure pedestrian protection in future.
Focus on bonnet design
The engine bonnet is a critical point for pedestrian protection. It should be designed in such a way that it absorbs as much energy as possible in the event of a collision with the body. This requires free space underneath the bonnet. In current motor vehicles, however, every last cubic centimetre of space in the engine compartment is filled. As a result, various manufacturers have developed quick-acting systems that raise the bonnet in the event of a collision, in some cases fitted with additional airbags, in order to achieve the prescribed values. The disadvantage of these systems is the high technical and economic expense resulting from the use of sensors, control systems and pyrotechnic triggers, as well as the increased maintenance requirements. Furthermore, these systems in turn require additional space in the front of the vehicle. This inevitably means that compromises have to be made in the design.
Magna Steyr, Rühl Puromer and Hennecke co-produced a bonnet based on the idea of utilising one of the most fundamental mechanical properties of the interlaced paper honeycomb structure: crash protection.
RIM ensures surface quality
A particular challenge was the paintable class-A surface of the engine bonnet, required for exterior parts. In addition to the puropreg system for manufacturing honeycomb sandwich support structures, adapted specially for the application, Rühl Puromer have developed a purorim system based on polyurethane (PU) that uses the reaction injection moulding (RIM) process to ensure the required surface quality and hence successfully solves the task.
Honeycomb – glass fibre sandwich structure
When selecting the appropriate mixing and metering systems for PU processing, the decision was made in favour of Hennecke and its PU CSM processing systems. After the development period, volume production of the bonnet is now imminent. In principle, the sandwich structure of the bonnet resembles the design of a PREG load floor for luggage compartment systems. A honeycomb core is used for the production of the bonnet, with glass fibre reinforcement applied from above and below. In order to achieve the different compression hardnesses and stability required in the bonnet, for example for hinges, the honeycomb core is designed as a reinforced structure at the relevant points. Afterwards, the PU matrix is applied using the PU-CSM spraying process. In this patented spray-up approach, the semi-finished fibre products are wetted on both sides with a thermally activated PU system. This makes it possible to apply a thicker layer of material in specific, targeted areas of the component. The part is then compressed and cured inside the mould. In the next stage, the RIM process is used on the outside and around the outer edge of the bonnet to produce the paintable class-A surface.
Design freedom for front section
The PU spray coat creates an optimal and durable connection between the glass fibre and the honeycomb core. This is said to ensure high stability and torsional rigidity. The thickness and structure of the honeycomb core allows the crash performance of the bonnet to be adjusted in a defined manner. If the bonnet is designed in such a way that it extends down to the radiator grille at the front, this area then also corresponds to the regulations for pedestrian protection, and allows the designers almost total freedom to design the front section of a vehicle.
The structural design also includes other positive effects. In addition to the substantially lower weight, the bonnet has good insulating properties thanks to its honeycomb core. For example, it requires no noise-absorbent mat, which is almost inevitable in conventional bonnets. But it not only exhibits insulating properties in terms of noise, it is also said to provide thermal insulation. This generates advantages when it comes to CO2 emissions, because the motor cools down more slowly, and under ideal circumstances cold starts can be avoided more often.
Fulfils pedestrian protection regulations
The engine bonnet has passed all of the comprehensive approval tests for use in the automotive industry – and naturally also the new requirements for pedestrian protection. This new design will be extended to many other exterior parts in future, such as tailgates, doors and roofs.