Torque shop: When does an airbag go boom?
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Airbags are meant to supplement the vehicle's seatbelts to protect occupants.
PHOTO: HYUNDAI
Shreejit Changaroth
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How does an airbag deploy and what is the car’s travelling speed to activate the airbag?
Airbags are designed to inflate within 40 milliseconds of impact to mitigate bodily injury in the event of an accident.
To perform effectively in the shortest possible time, a chemical reaction is triggered to produce nitrogen gas that expands rapidly to fill the airbag.
A network of sensors feeds critical digital information to the crash-detection electronic control unit (ECU) of the airbag system. The severity of a collision is primarily a function of the time taken for the vehicle to decelerate from its travelling speed to 0kmh.
Accelerometers measure the rate at which the speed drops in order to calculate a deceleration value. The higher the speed or shorter the time interval, the higher will be the deceleration.
A car travelling at 30kmh colliding into the rear of another vehicle will decelerate to 0kmh in about 80 milliseconds. Exact figures vary slightly depending on factors like how the car’s body is engineered to crumple on impact to absorb the energy.
Based on this “time-to-zero”, a deceleration value – which can be described as the force experienced by a body (human or otherwise) during the event – can be derived.
This critical information, plus data on vehicle speed and braking force, is sent to the ECU, which immediately activates the pyrotechnic gas inflators. Based on years of research data, airbag systems are designed to fully inflate in less time than it takes the driver’s head to impact the steering wheel in a collision.
With strategic sensor locations and dynamic feedback signals, the ECU can analyse if a sudden jolt is due to a collision or merely a pothole in the road to decide whether to trigger the airbag.
Airbags are part of a passenger vehicle’s Supplemental Restraint System (SRS), while the seat belts serve as the primary occupant restraint system.
SRS airbags will not be triggered if an impact is due to collision at parking speeds or at travelling speeds usually below 20kmh. At these speeds, the seat belts provide sufficient restraint to prevent injury to the occupants, provided of course they are strapped-in. Damage to vehicles will also be minimal.
