Automotive Engineer

The steering wheel driver airbag is an integral part of the restraint system. The market is developing worldwide, but the focus in Europe is on reducing the cost and weight and making packaging simpler. At the same time OEMs are requesting more functionality, with both single and dual-stage systems.

But the basic functionality of the airbag has remained the same. The system must deploy within 40ms of an accident, and work no matter what the external environmental conditions are.

Materials are changing as Tier One suppliers look to reduce the weight and cost of systems. In Europe there has been greater use of plastics, and many companies are developing these materials to meet the high durability requirements of the safety technology.

Standardisation is also helping. Historically airbags have had different pyrotechnic inflators depending on their position in the vehicle cabin. TRW among other companies is trying to standardise this component, so it could be used in more areas – reducing cost.

The lightweighting trend is also having an effect. OEMs’ demand for reduced mass in the driver airbag is growing – although this has to be balanced by which vehicle segment the airbag will be used in.

Traditionally German OEMs have used the largest 60-65 litre airbags, while French OEMs have used smaller 45-55 litre airbags. 

Size standardisation would help to reduce costs, but it requires OEMs to accept a generic technology rather than having such tight control over specifications.

1. Steering wheel armature
Material choice is dependent on integrity issues as well as natural frequency requirements. It is typically made from either magnesium or aluminium, but hybrid construction of either steel and magnesium or steel and aluminium is also used.

2. Carrier
Material usage in Europe is moving away from metals to greater use of plastics. It typically weighs 100g, helping to meet OEM weight reduction targets. In the Asian market the main trend is to use steel, as the price is much lower than in Europe. The main challenge is integrity at –35°C and +85°C.

3. Retainer
The metal retainer is used to clamp the bag between the retainer and the carrier, tightly securing the bag in place. It is also used to divert the flow of the gas, sending heat from the inflator into the retainer’s walls instead of the airbag.

4. Bag
Standard material is polyester 470 decitex or 580 decitex. The number of yarn in one direction and weight vary depending on OEM requirements. The bag must be robust enough to withstand the high temperatures of 700-900°C when the airbag is fired. Bags typically vary in size from 40 litres to 65 litres.

5. Driver airbag cover
Durability is the key challenge for this plastic component. It has to work within a temperature range of –35°C to +85°C. In a worst-case scenario, full separation of the cover can occur if it can’t meet these temperature ranges. Should the airbag need to be deployed, the component also has to open in 4ms with the flap opening at speeds reaching 400km/h.

6. Inflator
The 60mm diameter inflator can be either a single- or dual-stage device. The pyrotechnic inflator is now being designed so it can be used in both the driver and passenger airbag.
New propellants are being developed to reduce the size further. Typical chemicals used in inflator propellants include: sodium azide, potassium perchlorate and 5-amino terazole, among others.

7. Sleeves
The interface with the driver airbag. The sleeves are locked either directly into the steering wheel armature or into a foam base. Using a foam base helps to reduce vibrations, improving NVH and the positioning of the airbag.