Airbags

Newton’s first law of motion states that “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.” This means that when we (as passengers) are travelling in a car at a particular speed, then we will continue to move forward at that same speed even after brakes are applied due to the momentum our body gains. The momentum is higher if there is a collision and hence our body is thrown in front and we get seriously injured. Air bags provide a cushion to our body in the event of a collision to prevent any serious injury.

Air bag is a safety device used in vehicles that consists of a flexible fabric bag that inflates instantaneously in the event of a collision to protect the driver and passengers from serious injury to their head, face and chest. Air bags are like a soft pillow to land against in the event of a crash. The purpose of an air bag is to slow the forward motion of the passenger in a fraction of a second.  

Air bags were first introduced commercially for automobiles in 1980. Looking at the reduction in fatalities, U.S made it mandatory for all cars to be installed in cars since 1998.

Where is air bag installed?

Typically air bags are installed in the steering wheel for driver and in the instrument panel for the front passenger. Apart from that, in high end luxury models, door mounted side air bags and seat mounted air bags are also installed.

Working of an air bag:

The air bag consists of a thin nylon bag which is folded inside the steering wheel or dashboard. There is a crash sensor that detects any crash when the vehicle collides. The crash sensor signals the inflator to inflate the air bag when necessary.

Air bags will inflate only if the vehicle achieves a minimum speed of 16 to 14 kmph. When crash occurs, accelerometer sends the information to the crash sensor. The crash sensor then actuates the inflation system. The inflation system in older air bags sets off a chemical reaction between sodium azide (NaN₃), potassium nitrate (KNO₃) and silicion dioxide (SiO₂) to form nitrogen gas. The hot blast of nitrogen gas inflates the nylon bag. The bag is covered with talcum powder for its lubrication and to help it inflate smoothly.

Solid propellant inflators:

Solid propellant inflation systems are used to rapidly produce nitrogen gas in huge amount to inflate the nylon bag. The airbag circuit passes the current to the heating element. The heating element heats up sodium azide (NaN₃) which acts as an explosive.

2 NaN₃     →    2 Na + 3 N₂

10 Na + 2 KNO₃     →     K₂O + 5 Na₂O + N₂

K₂O + Na₂O + SiO₂     →     Na₂K₂SiO₄ (alkaline silicate glass)

Sodium azide decomposes at 300°C to form sodium and nitrogen gas.  The highly reactive Na metal is removed by KNO₃ and SiO₂ and in turn produces nitrogen gas. The K₂O and Na₂O formed suring second reaction is highly reactive and reacts with SiO₂ in the third reaction to form silicate glass which is harmless and stable.

The gas travels at more than 300 kmph and inflates the bag completely in 60 to 80 milliseconds from point of collision. The bag has small pores which lets the nitrogen gas to escape after few seconds of inflation.

Safety Concerns with Airbags:

Airbags best work as a supplement to seat belts. Hence, it is also known as Supplement Restraint System (SRS). The force of an airbag can also hurt if the driver is sitting too close to the steering wheel. It is recommended to be at least 10 inches away from the air bag or steering wheel.

Learn about Seat belts by going to this link:

• Seat Belts