2. Airbag system architecture

An airbag system is not a single device but a distributed restraint network. Each airbag is packaged as a module — a self-contained assembly of cushion, inflator, housing, and mounting hardware — that is triggered by the same crash-sensing ECU but tuned to protect a specific occupant region. For CAE correlation work, the architecture matters because every module boundary defines where a model must be split, where contact must be defined, and where gas-flow or fabric boundary conditions change. This section maps the five common airbag families, the inflator technologies that drive them, how modules are constructed, and the interfaces that govern deployment.

Airbag families and their protection zones

Driver airbag (DAB) is mounted in the steering wheel hub and deploys rearward toward the driver's chest and head. It is typically a single-chamber or dual-chamber cushion with a tethered center to control the inflated depth. Because the steering wheel also carries the horn and switches, the module housing must fit within a tight packaging envelope, often 60–90 mm deep before deployment.

Passenger airbag (PAB) sits behind the instrument panel on the passenger side and is the largest frontal cushion, commonly 60–120 L inflated volume. It deploys upward and rearward through a tear seam in the IP skin. PAB modules frequently use a tethered or dual-stage cushion to manage out-of-position (OOP) occupants, and the housing is integrated into the IP substrate rather than a separate can.

Side airbag (SAB) is mounted in the seat backrest or door trim and protects the thorax and pelvis. Seat-mounted SABs deploy forward along the seat bolster; door-mounted versions deploy inward. Inflated volume is small (8–20 L) and deployment time is short, often 10–20 ms to full inflation.

Curtain airbag (CAB) is packaged along the roof rail and A/B/C pillars and deploys downward to cover the side windows. It is a long, flat cushion, typically 1.5–2.5 m in length, and is often divided into chambers by internal tethers or seams to control pressure distribution. CABs are frequently paired with a side-impact sensor to trigger on rollover or side pole events.

Knee airbag (KAB) is mounted below the steering column or in the lower IP and deploys rearward toward the occupant's knees and lower legs. Its purpose is to control femur and tibia loading and to keep the occupant in position for the primary frontal restraint. Volume is small (10–25 L) and the cushion is usually a simple single-chamber design.

Inflator types and their CAE implications

The inflator is the gas source and the most model-sensitive component. Three families dominate:

• Pyrotechnic inflators burn a solid propellant to generate gas. They are compact and fast but produce hot, particulate-laden gas. In CAE, the mass-flow rate curve and gas temperature must be prescribed as a time-dependent boundary condition.
• Hybrid inflators combine a small pyrotechnic charge with a stored compressed gas (usually argon or helium). The pyrotechnic heats and releases the stored gas, giving a cooler, cleaner fill. The model requires both the combustion mass flow and the stored-gas release profile.
• Cold-gas inflators release only compressed gas, often through a pyrotechnic-activated valve. They are used where low temperature and low particulate output are required, such as some curtain and side modules.

For correlation, the inflator is represented by a mass flow rate vs. time curve and a gas temperature vs. time curve, both measured in a tank test. These curves are the primary inputs to the cushion inflation model, and any mismatch between tank-test data and the CAE inlet condition will propagate directly into cushion pressure and deployment kinematics.

Module construction and deployment interfaces

A module is more than cushion plus inflator. The housing provides structural support and directs the gas; the diffuser distributes flow into the cushion; the retainer clamps the cushion to the housing; and the cover or tear seam controls the opening path. Each of these is a potential contact or boundary-condition interface in the model.

Deployment-relevant interfaces include:

• Module-to-vehicle mounting — bolt pattern, bracket stiffness, and reaction surface. In CAE, these are represented as fixed constraints or spring elements; incorrect stiffness changes the reaction force on the occupant.
• Cushion-to-trim contact — the IP skin, seat foam, or roof rail trim that the cushion pushes against during deployment. Friction and tear-seam failure must be modeled if the trim opens or deflects.
• Occupant-to-cushion contact — the interaction that determines injury metrics. The cushion's inflated shape, pressure, and venting behavior are all outputs of the inflation model.
• Sensor-to-ECU interface — the crash pulse and trigger timing. The time-to-fire (TTF) and the deployment time window are inputs that set the initial conditions for the simulation.

Visualizing the architecture and deployment chain

The diagram below shows the five airbag families, their typical mounting locations, and the deployment chain from crash sensing through inflator firing to cushion-occupant interaction. It also highlights the key CAE interfaces at each stage.

From architecture to correlation

When building a CAE model, the architecture dictates the model decomposition. A frontal model may include DAB, PAB, and KAB as separate subassemblies, each with its own inflator curve and cushion material. A side-impact model adds SAB and CAB, which require different fabric properties and often a different gas composition. The interfaces listed above — mounting stiffness, trim contact, tear-seam failure, and occupant contact — are the calibration knobs. If the model over-predicts cushion pressure, check the inflator mass flow first; if it under-predicts occupant excursion, check the venting and fabric permeability; if the deployment path is wrong, check the tear-seam and trim contact definitions.

In the next section, we will look at how these modules are triggered and sequenced by the ECU, and how trigger timing affects the correlation window.