Page 341 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Electrical systems 323

Figure 3.131 Passive safety is used only in an emergency. (Source: Saab Media)

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Figure 3.132 Air bag unit: 1, container; 2, gas generator; 3, airbag; 4, cover

40 km/h, the seat belt alone is no longer adequate. Research after a number of
accidents has determined that in 68% of cases an air bag provides a signifi cant
improvement. It is suggested that if all cars in the world were fi tted with an air
bag then the number of fatalities annually would be reduced by well over 50 000.
Some air bag safety issues have been apparent in the USA, where air bags are
larger and more powerful. This is because in many areas the wearing of seat
belts is less frequent.
The method becoming most popular for an air bag system is that of building
most of the required components into one unit ( Fig. 3.132 ). This reduces the
amount of wiring and connections, thus improving reliability. An important aspect
is that some form of system monitoring must be built in, as the operation cannot
be tested; it only ever works once.
The sequence of events in the case of a frontal impact at about 35 km/h is as
follows ( Fig. 3.133 ):
1 Driver in normal seating position prior to impact.
2 About 15 ms after the impact the vehicle is strongly decelerated and the
threshold for triggering the air bag is reached. The igniter ignites the fuel
tablets in the infl ater.
3 After about 30 ms, the air bag unfolds and the driver will have moved forwards
as the vehicle’s crumple zones collapse. The seat belt will have locked or been
tensioned, depending on the system.