THE first airbag patents were filed back in 1951 and published two years later by engineers Walter Linderer (from Germany) and John Hetrick (from the US). The world had to wait until the 1970s before this safety feature debuted on production cars. Today, we have front, knee, side, roof, seatbelt and even external airbags for pedestrian protection.
Why are they important?
The function of an airbag is to distribute the impact pressure on the human body during an accident over as large an area as possible. This lessens the force-per-unit area and lowers the risk of bodily harm. In the event of a frontal crash with severe deceleration, the occupants’ bodies continue in the direction of travel at roughly the speed of the vehicle before impact. The initial retardation of the torso is the responsibility of the primary safety systems (seatbelts with pretensioners), but other parts such as the head, arms and legs still move forward. The job of the airbags is to prevent these parts from impacting hard areas in the cabin such as the dashboard and steering wheel. The compressible air cushion also provides a longer impact distance that, in turn, lessens the peak deceleration experienced by the human body.
When do they deploy?
We recently received a number of emails from readers (who drive different makes of vehicles) complaining that the airbags in their vehicles did not deploy during accidents (see TECHmail on page 130 of the June issue of CAR 2014). One such email involved a Ford EcoSport (where the driver sustained injuries) and we asked the engineers at Ford to explain the system components, development and trigger conditions for the vehicle’s six-airbag system. We used their response as a basis to explain how airbags work.
Airbag System Components
The EcoSport has six airbags, two side-impact sensors fitted at the bottom of the B-pillars, seatbelt and occupancy-detection switches, a front-impact severity sensor, the instrument cluster (which displays information on the airbag system) and the restraint-control module (RCM).
The RCM contains the algorithms that predict accident severity and it then decides if and which of the airbags should deploy. It’s connected to the switches and accelerometers via the controlled area network (CAN) bus and uses the signals as an input to the deployment algorithms.
Once a crash has been identified by the RCM, the crash signal is sent to the other control units that can switch on the hazard lights, unlock the vehicle, deactivate the fuel-supply system (or disconnect the battery of electric vehicles) and even make an emergency call if Internet connectivity is available.
The airbag consists of a thin nylon bag folded up, with corn starch, French chalk or talcum powder acting as the lubricant during deployment (the white powder is evident after deployment). Initial designs used compressed air (explaining the origin of the name), but it soon became clear that the inflation process took too long. In later designs, inflation pressure was provided by a pyrotechnical chemical reaction between sodium azide (NaN3), potassium nitrate (KNO3) and silicon dioxide (SiO2) to produce nitrogen gas, which is triggered in the solid-state propellant by a detonator. Currently, there is a drive to find alternative compounds that deliver the same result but with fewer toxic by-products. An airbag has small perforations that allow it to deflate after deployment to allow occupants to exit the vehicle quickly after a severe crash.
In order to be sold in a stringently regulated market such as Europe, vehicles must meet economic commission for Europe (ECE) type-approval regulations, including crash requirements. EuroNCAP has raised these requirements to an even higher level, and conducts its own independent crash testing of new vehicles. Owing to the widespread publicity EuroNCAP tests garner, manufacturers have been forced to increase the safety features of their vehicles to score a higher crash rating (see the technical feature in August 2012). NCAP tests the following:
• Frontal car-to-car impact: head-on collision at 64 km/h at 40% overlap.
• Side car-to-car impact: side-on with impact speed at 50 km/h.
• Side-pole impact: a solid pole hits the vehicle on its B-pillar at 29 km/h.
• Pedestrian protection: this assesses the bumper/bonnet’s performance when colliding with a pedestrian.
• Child protection: restraint system, including Isofix, is checked.
• Whiplash testing: what’s the vehicle’s level of seat support when it sustains a rear-end collision.
• Safety assists: NCAP testing offers additional points for seatbelt reminders, speed warnings, etc.
Engineers use these crash tests to determine the “must-deploy” thresholds for the airbags. Similar crashes at slightly higher and lower speeds are also conducted to validate the calibration thresholds.
The problem facing the engineers who work on the calibration of crash algorithms is finding a balance between a false detection (premature deployment) or not deploying in the event of a severe crash. In order to minimise unwanted (incorrect) deployments, the calibration considers data from other types of non-critical events that the vehicle may see under abusive conditions, including dynamic driving and operation on rough roads, curb hits and gravel roads. Ford, for example, also uses tests that include door slams, ball strikes, shopping-trolley hits and bicycle accidents.
It is important to note that it is impossible to calibrate the system for all types of road accidents because they are so varied. However, a vehicle that scored a positive NCAP rating provides good occupant protection. The EcoSport managed to score a four-star overall rating and 93% in the adult-occupant-protection tests. It failed to score a fifth because the engineers had concerns regarding its pedestrian protection and the fact that speed-limitation assistance is not fitted.
Operation and Trigger Conditions
The restraint-control module analyses the signals from the accelerometers and crash switches on a real-time basis to establish whether an airbag-deployment threshold has been breached. Both the magnitude of the acceleration and the duration thereof are important in the decision-making process. As speed is of utmost importance, the task rate of the crash signals are faster than 8 ms and the total time it takes from the crash to a fully inflated frontal airbag is about 40 ms.
The Future of Airbags
The safety benefits of airbags have undoubtedly been proven, as many serious injuries have been prevented and countless lives saved. But these systems still undergo constant refinement. Lately, the inflation pressure of some airbags can be controlled according to the severity of the crash, as well as the seatbelt-restraint reaction to avoid injuries owing to excessive airbag inflation. The number of airbags in modern vehicles will keep growing, to the point where the occupants are enveloped by a cushion of air in a severe accident.
Of course, prevention is better than cure, and many active-safety technologies such as ABS, ESC, lane-keeping warnings and automatic braking have lowered the number of accidents. The automotive world might even reach a point where airbags won’t need to deploy simply because autonomous vehicles will have removed the weakest link – the driver – and so be able to avoid accidents completely.
Can airbags kill?
Yes. Small children and infants are particularly vulnerable to injury or even death due to the extreme forces exerted by a deploying airbag. That’s why cars offer the option to switch off the front passenger airbag when a child seat is installed. It’s also important to sit far enough away from the steering wheel or dashboard to avoid injuries when the airbag activates. That said, the life-saving capabilities of airbags far outweigh the risks of sustaining injury.