Airbags serve as a Supplemental Restraint System (SRS), which is secondary to the seatbelt system.

Sensing and evaluating a crash

  • What happens in a crash

    When a crash happens,

    sensors 1

    send data to the

    ACU 2

    (Airbag Control Unit), which decides to deploy the airbags. Once the decision is made, the ACU instructs the

    inflator 3

    to initiate. Inflators either burn fuel to create gas, or heat expandable gas, which fills the

    bag 4


    The deployment process usually happens in under 0.03 seconds. That's about ten times faster than the blink of an average human eye!

Sensing and evaluating a crash

Crash sensors located throughout the vehicle collect information and send it to the ACU. Most modern crash sensors are microelectromechanical systems (MEMS), which are tiny devices that detect mechanical stress and convert it into an electrical signal.

  • Pressure sensors

    The pressure sensors use a force collector, such as a spring or diaphragm, to determine the amount of pressure being applied and convert the pressure into an electrical signal.

    2 Front crash sensors


    3 Side crash sensors

    are similar in design and function, and detect external forces.


    4 Seat occupancy sensor

    uses a silicone bladder on top of a pressure sensor to sense the weight of the passenger. If a passenger is detected, the sensor sends a signal to the ACU, and the corresponding airbag is deployed.
  • 1 Wheel speed sensors

    Wheel speed sensors use a toothed wheel attached to the vehicle’s wheel and a magnet surrounded by a coil to determine wheel speed. As the teeth pass by the magnet, the magnetic field fluxes and creates a current.

  • 5 ACU

    The ACU consists of a processor core, memory, inputs, and outputs. It uses a logic gate programmed with algorithms to "decide" whether to deploy the airbags. If the resulting velocity and acceleration of a crash surpasses a set threshold, the ACU sends a signal to the inflation unit of the airbags. The ACU also performs regular diagnostics and, in most modern cars, contains a "black box" or Event Data Recorder (EDR) that saves crash information.

  • 6 Accelerometer

    The accelerometer detects changes in acceleration by measuring the displacement of a small mass, and then converting the movement into an electrical signal. Frontal crashes cause rapid deceleration, while side crashes cause rapid acceleration.

  • 7 Gyroscope

    A vibrating gyroscope senses angular velocity (change in rotational angle over time) by measuring the directional change of vibration (as the vehicle’s orientation rotates) in the mechanical structures and converting the movement into an electrical signal.

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Airbag inflation

The inflation unit inflates the airbags, which then slightly deflate to cushion the occupants of the vehicle. Pyrotechnical and heated gas inflators are the main types that are used. On average, the entire inflation process takes only 10-15 milliseconds (0.01 – 0.015 seconds).

  • Pyrotechnical inflator

    • 1 Initiator

      The initiator is an electric match, which is a bridgewire (an electrical conductor) wrapped in combustible material. Electrical currents heat up the bridgewire, which ignites the combustible material.

    • 2 Propellant

      The initiator heats a solid propellant that rapidly decomposes and produces nitrogen gas. The most common propellant chemicals used are phase-stabilized ammonium nitrate and nitroguanidine.

      Nitroguanidine, for example, could break down something like this: CH4N4O2 (nitroguanidine) 2N2 (nitrogen gas) + CO2 (carbon dioxide) + 2H2 (hydrogen gas). The gas that fills the bag (nitrogen), nitrogen, and the byproducts exit through metal mesh filters (which cool and remove particles from the gas) and into the airbag.

  • Heated gas inflator (HGI)

    • 3 Initiator

      Similar to pyrotechnical inflators, the initiator in HGIs is also an electric match.

    • 4 Compressed gas cylinder

      The compressed gas consists of hydrogen, oxygen, and nitrogen.

    • 5 First burst disk

      The initiator ruptures the first burst disk and heats the compressed gas.

    • 6 Second burst disk

      The internal pressure increases enough to rupture the second burst disk.

    • 7 Diffuser

      The heated gas exits the cylinder through the diffuser and into the airbag.

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  • 1 Woven nylon fabric

  • 2 Powder

    The airbag assembly usually contains talcum powder or cornstarch as a lubricant so the airbag doesn’t get stuck on deployment.

  • 3 Storage

    Airbags are folded and stored in a compartment inside the vehicle, which has designated seams that tear open during deployment.

  • 4 Vents

    Airbags have vents in the back that allow them to slightly deflate and cushion the vehicle’s occupant as they make contact with it.

Types of airbags

The location and purpose of airbags determine their shapes, sizes, firmness, and how quickly they inflate.

  • 1 Front airbags

    Front airbags inflate at a rate determined by seat occupancy and crash severity.

  • 2 Side airbags

    Side torso airbags have a lower, firmer chamber to support the pelvis and an upper, softer chamber to cushion the ribcage.

    Side curtain airbags stay inflated for several seconds to protect occupants if the vehicle rolls over.

  • 3 Knee airbags

    Knee airbags specifically prevent knees from shattering on impact.

  • 4 Center airbags

    Center airbags prevent impact between the driver and passenger or between the rear passengers.

  • 5 Seatbelt airbags

    Inflatable seatbelts increase the surface area against an occupant’s body to reduce injury.

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