Mercedes-Benz Atego - Commercial Auto Component Development

Increasingly large engines in trucks, growing restrictions on noise emissions and weight and cost requirements are all conflicting factors in the development of noise encapsulation systems. DaimlerChrysler commissioned a new generation of noise encapsulation systems from the supplier Carcoustics, which chose engineering services provider Bertrandt as its partner for the development work. Together they broke new ground in the course of developing a new noise encapsulation system for lightweight DaimlerChrysler Atego trucks.

New blow moulding technology used

The basis for the evaluation of noise emissions from commercial vehicles is having the vehicle drive past under acceleration. DaimlerChrysler’s objective was to replace the existing noise encapsulation system with one that produced at least the same levels of noise reduction but was significantly lighter. Another requirement was for the system to remain undamaged if the vehicle was subjected to a ground impact in the front axle area. The initial stage of the project involved of around 400 relevant records in order to produce a geometric basis for the development of the component. In the next stages of the development process a blow moulding technique for the creation of large flat components, which had been used several times by Carcoustics, was to be employed. This technique allows the production from one material, and even in one component, of cells for the engine side and flat panels for the road side of the vehicle that have acoustically effective resonance absorption. The process, which had not been used before for truck encapsulation systems, received the approval of the DaimlerChrysler managers and was chosen for use in the lightweight Atego trucks.

Concept produced for new system

The next stage was to develop a blow moulding concept for a capsule of this size, and in particular height (1 000mm x 800mm x 300mm). The engineers considered the option of fixing the side walls on with film hinges, which would allow them to be folded down into the tool position with no undercut. Weakening the product in this way would help to meet the customer’s requirement that a ground impact in the sump area would not cause any damage to the noise encapsulation system. It was calculated that, as a result, the side walls would bend outwards and the floor would flex until it touched the sump. The capsule, which is attached to the vehicle chassis, would therefore form a parallelogram which in the case of lateral acceleration tends to result in a pendulum effect occurring. In order to resolve this problem, the capsule was divided into two parts. The back part acts as a support for the side walls. The two folded side walls are fitted in the correct position first using lugs and then attached to the back part of the capsule using locking nuts. One additional benefit of the process is the fact that components such as the sump drain plug are easily accessible for maintenance without the need for the manufacture of any additional parts. The maintenance cover is attached to the capsule using a film hinge.

Capsule safety ensured using FEM simulation
The multiple “weak points” in the capsule prompted Bertrandt to carry out an FEM simulation to test the safety of the component. Both mechanical properties and high temperatures were taken into account. Despite the complex fixed support the results of the FEM simulation only highlighted a small number of critical areas. As a result only a few slight modifications were needed before the data was submitted on schedule to the prototype tool release stage. Components produced in one step, without additional processing, were thoroughly tested by DaimlerChrysler on a test bench which simulated poor road conditions with the addition of weights (ice/snow/sand). The capsule passed all the tests without problems and was ready for volume production.