Application of Airbags in Automobiles | Working Principle of Airbags in Car/Automobile

An Overview of Airbags in Automobiles

Sikander Anwer
Department in Textile Engineering
University of Management & Technology, Lahore, Pakistan
Cell: 0322-4875571

The airbag specified for automobile use traces it origins to air-filled bladders as early as 1941. John W. Hetrick, an industrial engineer and member of the United States Navy, designed the original safety cushion commonly referred to as an airbag. It was designed based on his experiences with compressed air from torpedoes during his service in the navy, as well as a need to provide protection for his family in their automobile during accidents. Hetrick worked with the major American automobile corporations at the time, but they chose not to invest in it.

In 1967, a breakthrough occurred in the development of airbag crash sensors when Allen K. Breed invented a mechanically-based ball-in-tube component for crash detection, an electromechanical sensor with a steel ball attached to a tube by a magnet that would inflate an airbag under a 30 milli-second window. Sodium azide instead of compressed air was also used for the first time during inflation. Breed Corporation then marketed this innovation first to Chrysler.

An airbag is an elastic bag or cushion like makeup which inflates and deflates quickly at some stage in certain types of car accidents.

It is a safety device aimed at preventing or minimizing injury to passengers when such an accident occurs.

Types of Airbags:
  1. Frontal Airbags
  2. Shaped airbag
  3. Side Airbag 
  4. Side Torso Airbag
  5. Curtain airbag 
  6. Knee Airbag
  7. Rear Curtian Airbag 
  8. Centre Airbag
Working Principle of Airbags in Car:
The design is conceptually simple; a central "Airbag control unit" (ACU) (a specific type of ECU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors, wheel speed sensors, gyro scopes, brake pressure sensors, and seat occupancy sensors. When 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.
Working Principle of Airbags
Sensors Signals Working:
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 to 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.

Reaction Sequence:
Inside the airbag is a gas generator containing a mixture of NaNO3, KNO3, and SiO2 . The signal from the deceleration sensor ignites the gas generator mixture by an electrical impulse when head-on collision, creating the high temperature conditions necessary for sodium asides to decompose at 300˚C . This causes a relatively slow kind of detonation (Deflagration) that liberates a pre-calculated volume of N2 gas through series of chemical reaction, which fills the air bag.

(1) 2 NaN3 → 2 Na + 3 N2 (g)

The first reaction is the decomposition of NaN3 under high temperature conditions using an electric impulse. This impulse generates to 300°C temperatures required for the decomposition of the NaN3 which produces Na metal and N2 gas. Since Na metal is highly reactive, the KNO3 and SiO2 react and remove it, in turn producing more N2 gas.

(2) 10 Na + 2 KNO3 → K2O + 5 Na2O + N2 (g)

The second reaction shows just that. The reason that KNO3 is used rather than something like NaNO3 is because it is less hygroscopic. It is very important that the materials used in this reaction are not hygroscopic because absorbed moisture can de-sensitize the system and cause the reaction to fail.

(3) K2O + Na2O + 2 SiO2 → K2O3Si + Na2O3Si (silicate glass)

The final reaction is used to eliminate the K2O and Na2O produced in the previous reactions because the first-period metal oxides are highly reactive. These products react with SiO2 to produce a silicate glass which is a harmless and stable compound

Air Bag Production:
The air bag is manufactured by a complete process that has been mentioned in a flow chart:
Production flow chart of airbag
Properties of Airbags:
  1. High Tensile strength 
  2. Good heat stability
  3. High Tear strength
  4. Low Air permeability 
  5. Free of knots, splices, spots and broken ends.
  6. Good Heat capacity
  7. Good Folding behavior
  8. Better Energy absorption
  9. Good Coating adhesion
  10. Functionality at extreme hot and cold conditions
  11. Package ability
  12. Reduced skin abrasion (softness)
Fabric Construction For Airbags:
Mostly used raw material for the airbag fabric is nylon 66 yarns in the deniers ranging from 420 to 840. The side impact airbags used 1880 D nylon-6.6. These fabrics are generally woven with the constriction of:
  • 840 X 840 D, 98 X 98 /dm plain weave, 60” width.
  • 420 X 420 D, 193 X 193 /dm plain weave, 60” width 
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Airbags for Automobiles | Materials and Properties of Airbags | Production Process of Airbag
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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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