How a Car Airbag Senses a Crash and Inflates in Time
An airbag must decide, fire and fill within about 50 milliseconds. Here is how crash sensors, a control module and a gas inflator do that, and what airbags can and cannot do.
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A front airbag is a fabric cushion that fills with gas during a crash, and the whole sequence from impact to a full bag takes about 50 milliseconds, a twentieth of a second. In that time, sensors must decide whether the crash is serious, a small chemical charge must make gas, and the bag must unfold before the occupant moves forward into it. This article walks through those steps using material from the Insurance Institute for Highway Safety (IIHS) and a University of British Columbia engineering text.
Sensors and the control module
The brain of the system is the airbag control module. According to the UBC text, it draws on accelerometers, impact sensors, brake pressure sensors and seat occupancy sensors. Some sensors sit near the module and act as a safeguard against missing a real deployment in a violent crash. Others sit near the likely impact area and help avoid false deployments in minor bumps.
The module samples constantly and estimates how severe a crash is from the acceleration and from how quickly that acceleration changes. The UBC text says the decision must be made within roughly 45 to 50 milliseconds of a frontal collision. More advanced systems also consider occupant weight and position, and can switch off deployment for a small occupant or a child.
When a front airbag deploys
IIHS says front airbags inflate in moderate-to-severe frontal crashes. For unbelted occupants, deployment happens in a crash equal to hitting a rigid wall at about 10 to 12 miles per hour (mph). For belted occupants the threshold is about 16 mph. IIHS adds that newer systems use a belt sensor and a computer program to decide whether to deploy. The UBC text gives a range of roughly 16 to 18 mph and notes that US federal safety standards do not regulate the threshold, so manufacturers set it.
How the gas is made
When the module signals a crash, an electrical igniter starts a chemical reaction inside the inflator. The UBC text explains that the main gas source in this type of inflator is sodium azide, which breaks down when heated into nitrogen gas. The pellets also contain potassium nitrate and silicon dioxide, which react with the leftover sodium and form a harmless-looking glassy residue.
The same source says one gram of the compound gives roughly 0.56 litres of nitrogen at standard conditions. That adds up to about 28 to 45 litres for a driver bag and about 140 litres for a passenger bag. It also notes that sodium azide is toxic, which is a concern when airbags are recycled.
Filling, restraining and venting
The UBC text describes six phases: the trigger, the first gas, the bag beginning to unfold, complete unfolding, full inflation and then restraint of the occupant. Inflation has to be slow enough not to injure the person and fast enough to meet them before they reach the steering wheel or dashboard. Small vents let gas out as the person presses into the bag, so it works as a cushion. The text shows deflation happening around 150 milliseconds in one example.
Side and curtain airbags
Side crashes leave little room between the door and the occupant, so side airbags must be quicker. IIHS says they have to inflate within the first 10 to 20 milliseconds, with thresholds as low as 8 mph for narrow objects such as poles and trees and 18 mph for vehicle-to-vehicle side impacts. Curtain airbags can also deploy in rollovers and may stay inflated for 10 seconds or more to cover several rolls.
How much difference they make
IIHS reports that front airbags reduce driver deaths in frontal crashes by 29 per cent and deaths of front-seat passengers aged 13 and over by 32 per cent. It states that an airbag combined with a lap-and-shoulder belt reduces the risk of death by 61 per cent, compared with 50 per cent for belts alone and 34 per cent for airbags alone. Airbags are meant to work with seat belts, not replace them. IIHS also says airbags alone are not enough in side crashes, because the vehicle structure must resist intrusion into the cabin.
Risks and the Takata recall
IIHS notes that from 1990 to 2008, NHTSA estimated more than 290 deaths from frontal airbag inflation in low-speed crashes, nearly 90 per cent in vehicles built before 1998, and over 90 per cent of the passenger deaths were children and infants, many unbelted or in rear-facing seats near the bag. Airbag force was reduced from 1998 models, and IIHS says serious injuries from properly working airbags are now rare.
The Takata inflator recall, which began in 2015, is described by IIHS as the largest in US history. About 67 million vehicles had been recalled by 2022, with 6 million still needing replacement, and it has been linked to 23 confirmed US deaths and more than 250 injuries. Owners who are unsure about their vehicle can follow the manufacturer's and regulator's recall notices. This article is general information, not a safety instruction.
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