Chapter 9
Air bags and Seat belt
tensioners
Air bags
An airbag is a vehicle occupant-restraint system using a bag designed to
inflate extremely quickly, then quickly deflate during a collision. The purpose
of the airbag is to provide a vehicle occupant with a soft cushioning and
restraint during a collision. It can reduce injuries between the flailing
occupant and the interior of the vehicle.
Air bags: Components and circuits
The main components of a basic airbag system are as
follows.
• Driver and passenger airbags.
• Warning light.
• Passenger seat switches.
• Pyrotechnic inflaters (squibs).
• Igniter.
• Crash sensor(s).
• Electronic control unit.
Driver & passenger air bags
The airbags are usually made of nylon or polyamide.
Prior to inflation the airbag is folded up under suitable padding that has specially
designed break lines built-in.
Holes are provided in the side of the airbag to allow rapid deflation after
deployment.
Warning bulb
A warning light is used as part of the system monitoring circuit. This gives an indication of
a potential malfunction and is an important part of the circuit. Some manufacturers use
two bulbs for added reliability
Passenger seat switches
Consideration is being given to the use of a seat switch on the passenger side to
prevent deployment when not occupied. This may be more appropriate to side-impact
airbags mentioned in the next section.
Crash sensor(s)
The crash sensor can take a number of forms; these can be described as mechanical or
electronic.
Mass-type sensor
Roller type
The other main type of crash sensor can be described as an accelerometer.
Piezoelectric crystal accelerometer
Pyrotechnic inflators (squibs) & Ignitor
• Airbags take roughly 50
milliseconds to be deployed
 It contains a number of fuel tablets
in a combustion chamber. The
igniter
consists
of
charged
capacitors, which produce the
ignition spark.
 In case of a collision, the ACM sends an electric spark to the inflator canister, which
packs the solid propellant, setting off a chemical reaction that produces nitrogen
gas. The gas expands, inflating the airbag and protecting the driver/passenger.
 The airbag's inflation system reacts sodium azide (NaN3) with potassium nitrate
(KNO3) to produce nitrogen gas. Hot blasts of the nitrogen inflate the airbag
Air bag Control Module
Diagnosis : If a failure is detected in any part of the circuit then the warning light will
be operated. Up to five or more faults can be stored in the ECU memory, which can
be accessed by blink code or serial fault readers.
The role of this airbag module is to communicate with the airbags so that they can be
smoothly deployed in the event of a collision or an accident.
Seat-Belt Tensioners
Taking the ‘slack’ out of a seat-belt in the event
of an impact is a good contribution to vehicle
passenger safety
The two main types of tensioners are:
• Spring tension.
• Pyrotechnic.
The mechanism used by one type of seat-belt
tensioner is shown in Figure 15.47. When the
explosive charge is fired, the cable pulls a lever
on the seat-belt reel, which in turn tightens
the belt. The unit must be replaced once
deployed.
Mechanism: Pyrotechnic seat belt tensioners