An airbag is a vehicle occupant-restraint system using a bag designed to inflate extremely quickly, then quickly deflate during a collision. It consists of the airbag cushion, a flexible fabric bag, an inflation module, and an impact sensor. The purpose of the airbag is to provide a vehicle occupant a soft cushioning and restraint during a crash event. It can reduce injuries between the flailing occupant and the interior of the vehicle.

The airbag provides an energy-absorbing surface between the vehicle’s occupants and a steering wheel, instrument panel, body pillar, headliner, and windshield. Modern vehicles may contain up to 10 airbag modules in various configurations, including: driver, passenger, side-curtain, seat-mounted, door-mounted, B and C-pillar mounted side-impact, knee bolster, inflatable seat belt, and pedestrian airbag modules.

During a crash, the vehicle’s crash sensors provide crucial information to the airbag electronic controller unit (ECU), including collision type, angle, and severity of impact. Using this information, the airbag ECU’s crash algorithm determines if the crash event meets the criteria for deployment and triggers various firing circuits to deploy one or more airbag modules within the vehicle. Working as a supplemental restraint system to the vehicle’s seat-belt systems, airbag module deployments are triggered through a pyrotechnic process that is designed to be used once. Newer side-impact airbag modules consist of compressed-air cylinders that are triggered in the event of a side-on vehicle impact.

The first commercial designs were introduced in passenger automobiles during the 1970s, with limited success, and actually caused some fatalities. Broad commercial adoption of airbags occurred in many markets during the late 1980s and early 1990s with a driver airbag, and a front-passenger airbag, as well, on some cars, and many modern vehicles now include six or more units.

As a supplemental restraint system

Frontal airbag

The auto industry and research and regulatory communities have moved away from their initial view of the airbag as a seat-belt replacement, and the bags are now nominally designated as supplemental restraint systems (SRS) or supplemental inflatable restraints. From around 2000, side-impact airbags became commonplace on even low- to mid-range vehicles, such as the smaller-engined versions of the Ford Fiesta and Peugeot 206, and curtain airbags were The emergence of the airbag has contributed to a sharp decline in the number of deaths and serious injuries on the roads of Europe since 1990, and by 2010, the number of cars on European roads lacking an airbag represented a very small percentage of cars, mostly the remaining cars dating from the mid-1990s or earlier. Many new cars in Latin America, including the Kia Picanto, Kia Rio, Hyundai Accent and Suzuki Alto, are still regularly sold without Airbags, as neither Airbags nor Automatic braking systems in new cars are compulsory in Latin America.

Side airbag

Essentially, two types of side airbags are commonly used today – the side-torso airbag and the side-curtain airbag. Most vehicles equipped with side-curtain airbags also include side-torso airbags. However, some, such as the Chevrolet Cobalt, 2007–09 model Chevrolet Silverado/GMC Sierra, and 2009–12 Dodge Ram do not feature the side-torso airbag.

Knee airbag

The second driver-side and separate knee airbag was used in the Kia Sportage (a Korean SUV launched in 1995) and has been standard equipment since then. The airbag is located beneath the steering wheel

The Toyota Caldina introduced the first driver-side SRS knee airbag on the Japanese market in 2002 Toyota Avensis became the first vehicle sold in Europe equipped with a driver’s knee airbag. The EuroNCAP reported on the 2003 Avensis, “There has been much effort to protect the driver’s knees and legs and a knee airbag worked well.”Since then certain models have also included front-passenger knee airbags, which deploy near or over the glove compartment in a crash. Knee airbags are designed to reduce leg injury. The knee airbag has become increasingly common from 2000.


An ACU from a Geo Storm

The airbags in the vehicle are controlled by a central airbag control unit (ACU), a specific type of ECU. The ACU monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors,wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors. The bag itself and its inflation mechanism is concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors which are designed to tear open under the force of the bag inflating. Once the requisite threshold has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag’s volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant’s body, compared to a seat belt alone.

The signals from the various sensors are fed into the airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and “curtain” airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 and 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60–80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.

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