Editing Airbag (section)

== Operation ==
{{more citations needed section|date=May 2016}}
[[File:2008-04-14 Airbag control unit.jpg|thumb|An ACU from a [[Geo Storm]] ]]
The airbags in the vehicle are controlled by a central airbag control unit<ref>{{cite web| url= http://www.audi.com/audi/com/en2/tools/glossary/safety/airbag_control_unit.html |title=Airbag control unit |work=Audi |archive-url= https://web.archive.org/web/20081205193106/http://www.audi.com/audi/com/en2/tools/glossary/safety/airbag_control_unit.html |archive-date=5 December 2008 }}</ref> (ACU), a specific type of [[Electronic control unit|ECU]]. The ACU monitors several related sensors within the vehicle, including [[accelerometers]], impact sensors, side (door) pressure sensors,<ref>{{cite press release |url= http://mediacenter.conti-online.com/internet/generator/MAM/index,templateId=Folder_2FrenderDefault.jsp.html?method=show&action=/details.do&oid=2109316 |title= Media Center | archive-url= https://archive.today/20120710003210/http://mediacenter.conti-online.com/internet/generator/MAM/index,templateId=Folder_2FrenderDefault.jsp.html?method=show&action=/details.do&oid=2109316 |archive-date=10 July 2012 |website=continental.com}}</ref><ref>{{cite journal|last=Garcia |first=P. |title=A Methodology for the Deployment of Sensor Networks |journal=IEEE Transactions on Knowledge and Data Engineering |volume=11 |issue=4 |date=December 2011}}</ref> [[wheel speed sensor]]s, [[gyroscope]]s, [[pressure sensor|brake pressure sensors]], and seat occupancy sensors. Oftentimes, ACUs log this—and other—sensor data in a circular buffer and record it to onboard non-volatile memory, to provide a snapshot of the crash event for investigators. As such, an ACU frequently functions as the vehicle's [[event data recorder]]; not all EDRs are ACUs, and not all ACUs include EDR features.{{r|TRB-report|p=15}} An ACU typically includes capacitors within its circuitry, so that the module remains powered and able to deploy the airbags if the vehicle's battery connection to the ACU is severed during a crash.{{r|TRB-report|p=102}}{{r|MCFRS|p=3}}

The bag itself and its inflation mechanism are concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors that are designed to tear open under the force of the bag inflating. Once 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.

The signals from the various sensors are fed into the airbag control unit, which determines from them the angle of impact, the severity, or the 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 and 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 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.

Older airbag systems contained a mixture of [[sodium azide]] (NaN<sub>3</sub>), KNO<sub>3</sub>, and SiO<sub>2</sub>. A typical driver-side airbag contains approximately 50–80 g of NaN<sub>3</sub>, with the larger passenger-side airbag containing about 250 g. Within about 40 milliseconds of impact, all these components react in three separate reactions that produce nitrogen gas. The reactions, in order, are as follows.

# 2 NaN<sub>3</sub> → 2 Na + 3 N<sub>2</sub> (g)
# 10 Na + 2 KNO<sub>3</sub> → K<sub>2</sub>O + 5 Na<sub>2</sub>O + N<sub>2</sub> (g)
# K<sub>2</sub>O + Na<sub>2</sub>O + 2 SiO<sub>2</sub> → K<sub>2</sub>SiO<sub>3</sub> + Na<sub>2</sub>SiO<sub>3</sub>

The first two reactions create 4 molar equivalents of nitrogen gas, and the third converts the remaining reactants to relatively inert [[potassium silicate]] and [[sodium silicate]]. The reason that KNO<sub>3</sub> is used rather than something like NaNO<sub>3</sub> is because it is less hygroscopic. The materials used in this reaction must not be hygroscopic because absorbed moisture can de-sensitize the system and cause the reaction to fail.

The particle size of the initial reactants is important to reliable operation.<ref>{{cite patent| country = US |number=5089069 |title=Gas generating composition for air bags |pubdate=18 February 1992 |fdate=22 June 1990 |invent1= Coodly P. Ramaswamy |invent2=Francis Souriraja |url= https://www.google.com/patents/US5089069}}</ref> The NaN<sub>3</sub> and KNO<sub>3</sub> must be between 10 and 20&nbsp;[[µm]], while the SiO<sub>2</sub> must be between 5 and 10&nbsp;µm.

There are ongoing efforts to find alternative compounds so that airbags have less toxic reactants.<ref>{{Cite web|url= http://www.gwp.eu/fileadmin/seiten/DEMO-Berichte/DEMO_Bericht_20050322_airbag_services_Rev05.pdf |archiveurl= https://web.archive.org/web/20140202143520/http://www.gwp.eu/fileadmin/seiten/DEMO-Berichte/DEMO_Bericht_20050322_airbag_services_Rev05.pdf |url-status=dead|title=Tests on Airbags: Analyses of Gases, Dusts, Structures and Squibs |archivedate=2 February 2014}}</ref> The reaction of the Sr complex nitrate, (Sr(NH<sub>2</sub>NHCONHNH<sub>2</sub>)∙(NO<sub>3</sub>)<sub>2</sub>) of carbohydrazide (SrCDH) with various oxidizing agents resultS in the evolution of N<sub>2</sub> and CO<sub>2</sub> gases. Using KBrO<sub>3</sub> as the oxidizing agent resulted in the most vigorous reaction as well as the lowest initial temperature of the reaction. The N<sub>2</sub> and CO<sub>2</sub> gases evolved made up 99% of all gases evolved.<ref>{{cite journal|doi=10.1002/1521-4087(200011)25:5<224::AID-PREP224>3.0.CO;2-O |title=The Strontium Complex Nitrates of Carbohydrazide as a Non-Azide Gas Generator for Safer Driving-the Thermal Behavior of the Sr Complex with Various Oxidizing Agents |journal=Propellants, Explosives, Pyrotechnics |volume=25 |issue=5 |pages=224–229 |year=2000 |last1=Akiyoshi |first1=Miyako |last2=Nakamura |first2=Hidetsugu |last3=Hara |first3=Yasutake|doi-access=free }}</ref> Nearly all the starting materials will not decompose until reaching temperatures of 500&nbsp;°C or higher, so this could be a viable option as an airbag gas generator.

In a patent containing another plausible alternative to NaN<sub>3</sub> driven airbags, the gas-generating materials involved the use of [[guanidine nitrate]], [[5-aminotetrazole]], bitetrazole dihydrate, [[nitroimidazole]], and basic [[copper nitrate]]. It was found that these non-azide reagents allowed for a less toxic, lower combustion temperature reaction, and more easily disposable airbag inflation system.<ref>{{US patent reference|number=6958101 |issue date=11 April 2003|inventor=Ivan V. Mendenhall, Robert D. Taylor |title=Substituted basic metal nitrates in gas generation}}</ref>

Front airbags normally do not protect the occupants during side, rear, or rollover collisions.<ref>{{cite web |title=Air Bags |url= https://www.chop.edu/pages/air-bags |publisher=The Children's Hospital of Philadelphia |date=30 March 2014 |access-date=18 July 2022}}</ref> Since airbags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the airbag's benefits and they help restrain occupants during the initial and any following collisions.

In vehicles equipped with a rollover sensing system, accelerometers, and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, [[Side curtain airbag|side-curtain]] airbags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.

=== Triggering conditions ===
[[File:Peugeot 206 1999 Hatchback 1.1 TU1JP(HFZ) 05.JPG|thumb|Some cars provide the option to turn off the passenger airbag]]
Airbags are designed to deploy in frontal and near-frontal collisions more severe than a threshold defined by the regulations governing vehicle construction in whatever particular market the vehicle is intended for: United States regulations require deployment in crashes at least equivalent in deceleration to a {{convert|23|km/h|mph|abbr=on}} barrier collision, or similarly, striking a parked car of similar size across the full front of each vehicle at about twice the speed.<ref>{{cite web|url= http://www.nhtsa.gov/people/injury/airbags/airbags03/page3.html |title=What You Need to Know About Air Bags, DOT HS 809 575 |website=nhtsa.gov |access-date=17 October 2010 |url-status=dead |archive-url= https://web.archive.org/web/20100528074720/http://www.nhtsa.gov/people/injury/airbags/airbags03/page3.html |archive-date=28 May 2010}}</ref> International regulations are performance-based, rather than technology-based, so airbag deployment threshold is a function of overall vehicle design.

Unlike [[crash test]]s into barriers, real-world crashes typically occur at angles other than directly into the front of the vehicle, and the crash forces usually are not evenly distributed across the front of the vehicle. Consequently, the relative speed between a striking and struck vehicle required to deploy the airbag in a real-world crash can be much higher than an equivalent barrier crash. Because airbag sensors measure deceleration, the vehicle speed is not a good indicator of whether an airbag should be deployed. Airbags can deploy due to the vehicle's undercarriage striking a low object protruding above the roadway due to the resulting deceleration.

The airbag sensor is a [[Microelectromechanical systems|MEMS]] [[accelerometer]], which is a small [[integrated circuit]] with integrated micromechanical elements. The microscopic mechanical element moves in response to rapid deceleration, and this motion causes a change in capacitance, which is detected by the electronics on the chip that then sends a signal to fire the airbag. The most common MEMS accelerometer in use is the ADXL-50 by [[Analog Devices]], but there are other MEMS manufacturers as well.

Initial attempts using [[mercury switch]]es did not work well. Before MEMS, the primary system used to deploy airbags was called a "[[rolamite]]". A rolamite is a mechanical device, consisting of a roller suspended within a tensioned band. As a result of the particular geometry and material properties used, the roller is free to translate with little [[friction]] or [[hysteresis]]. This device was developed at [[Sandia National Laboratories]]. Rolamite and similar macro-mechanical devices were used in airbags until the mid-1990s after which they were universally replaced with MEMS.

Nearly all airbags are designed to automatically deploy in the event of a vehicle fire when temperatures reach {{convert|150|–|200|C|F}}.<ref>{{cite web|url= http://www.nhtsa.dot.gov/people/injury/airbags/airbags03/page3.html |title=What You Need to Know About Air Bags, DOT HS 809 575 |website=nhtsa.dot.gov |archive-url= https://web.archive.org/web/20091126011343/http://www.nhtsa.dot.gov/people/injury/airbags/airbags03/page3.html|archive-date=26 November 2009 |access-date=16 March 2014}}</ref> This safety feature, often termed auto-ignition, helps to ensure that such temperatures do not cause an explosion of the entire airbag module.

Today{{when|date=January 2023}}, airbag triggering [[algorithms]] are much more complex, being able to adapt the deployment speed to the crash conditions, and prevent unnecessary deployments. The algorithms are considered valuable [[intellectual property]]. Experimental algorithms may take into account such factors as the weight of the occupant, the seat location, and seat belt use, as well as even attempt to determine if a [[baby seat]] is present.

==== Inflation ====
{{one source | section|date=May 2016}}
When the frontal airbags are to deploy, a signal is sent to the [[gas generator|inflator unit]] within the airbag control unit. An igniter starts a rapid [[chemical reaction]] generating primarily [[nitrogen]] gas (N<sub>2</sub>) to fill the airbag making it deploy through the module cover. Some airbag technologies use compressed [[nitrogen]] or [[argon]] gas with a pyrotechnic operated valve ("hybrid gas generator"), while other technologies use various energetic [[propellant]]s. Although propellants containing the highly toxic [[sodium azide]] (NaN<sub>3</sub>) were common in early inflator designs, little to no toxic sodium azide has been found on used airbags.

The azide-containing pyrotechnic gas generators contain a substantial amount of the propellant. The driver-side airbag would contain a canister containing about 50 grams of sodium azide. The passenger side container holds about 200&nbsp;grams of sodium azide.<ref>{{cite web|url= http://www.sdearthtimes.com/et0800/et0800s9.html |title=ET 08/00: Sodium azide in car airbags poses a growing environmental hazard |website=sdearthtimes.com |access-date=16 March 2014 |url-status=live |archive-url= https://web.archive.org/web/20140917155736/http://www.sdearthtimes.com/et0800/et0800s9.html |archive-date=17 September 2014 }}</ref>{{better source needed|date=May 2016}}{{better source needed|date=May 2016}}

The alternative propellants may incorporate, for example, a combination of [[nitroguanidine]], phase-stabilized [[ammonium nitrate]] (NH<sub>4</sub>NO<sub>3</sub>) or another nonmetallic oxidizer, and a nitrogen-rich fuel different from azide (e.g. [[tetrazole]]s, [[triazole]]s, and their salts). The burn rate modifiers in the mixture may be an alkaline metal [[nitrate]] (NO<sub>3</sub>-) or [[nitrite]] (NO<sub>2</sub>-), [[dicyanamide]] or its salts, [[sodium borohydride]] (NaBH<sub>4</sub>), etc. The coolants and [[slag]] formers may be e.g. [[clay]], [[silica]], [[alumina]], glass, etc.<ref>{{cite web |url= http://www.freepatentsonline.com/6306232.html |title=Thermally stable nonazide automotive airbag propellants – Patent 6306232 |website=freepatentsonline.com |access-date=16 March 2014 |url-status=live |archive-url= https://web.archive.org/web/20140316213602/http://www.freepatentsonline.com/6306232.html |archive-date=16 March 2014}}{{primary source inline|date=May 2016}}</ref>{{better source needed|date=May 2016}}{{primary source inline|date=May 2016}}{{Original research inline|date=May 2016}} Other alternatives are e.g. [[nitrocellulose]] based propellants (which have high gas yield but bad storage stability, and their [[oxygen balance]] requires secondary oxidation of the reaction products to avoid buildup of carbon monoxide), or high-oxygen nitrogen-free organic compounds with inorganic oxidizers (e.g., di or tri[[carboxylic acid]]s with [[chlorate]]s (ClO<sub>3</sub>-) or [[perchlorate]]s (ClO<sub>4</sub>-) and eventually metallic oxides; the nitrogen-free formulation avoids formation of toxic [[nitrogen oxide]]s).

From the onset of the crash, the entire deployment and inflation process is about 0.04 seconds. Because vehicles change speed so quickly in a crash, airbags must inflate rapidly to reduce the risk of the occupant hitting the vehicle's interior.<ref>{{Cite web |title=Airbags |url=https://www.iihs.org/topics/airbags |access-date=2025-04-24 |website=IIHS-HLDI crash testing and highway safety |language=en}}</ref><ref>{{Cite web |title=How do air bags work? |url=https://www.scientificamerican.com/article/how-do-air-bags-work/ |access-date=2025-04-24 |website=Scientific American |language=en}}</ref>

==== Variable-force deployment ====
Advanced airbag technologies are being developed to tailor airbag deployment to the severity of the crash, the size, and posture of the vehicle occupant, belt usage, and how close that person is to the actual airbag. Many of these systems use multi-stage inflators that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the airbag control unit know if someone is occupying a seat adjacent to an airbag, the mass/weight of the person, whether a seat belt or child restraint is being used, and whether the person is forward in the seat and close to the airbag. Based on this information and crash severity information, the airbag is deployed at either a high force level, a less forceful level, or not at all.

Adaptive airbag systems may utilize multi-stage airbags to adjust the pressure within the airbag. The greater the pressure within the airbag, the more force the airbag will exert on the occupants as they come in contact with it. These adjustments allow the system to deploy the airbag with a moderate force for most collisions; reserving the maximum force airbag only for the severest of collisions. Additional sensors to determine the location, weight or relative size of the occupants may also be used. Information regarding the occupants and the severity of the crash are used by the airbag control unit, to determine whether airbags should be suppressed or deployed, and if so, at various output levels.
[[File:Airbag SEAT Ibiza.jpg|thumb|Post-deployment view of a [[SEAT Ibiza]] airbag]]

==== Post-deployment ====
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A chemical reaction produces a burst of nitrogen to inflate the bag. Once an airbag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it is sometimes called, the cushion) and cools. Deployment is frequently accompanied by the release of dust-like particles, and gases in the vehicle's interior (called effluent). Most of this dust consists of [[cornstarch]], [[french chalk]], or [[talcum powder]], which are used to lubricate the airbag during deployment.

Newer designs produce effluent primarily consisting of harmless talcum powder/cornstarch and nitrogen gas. In older designs using an azide-based propellant (usually NaN<sub>3</sub>), varying amounts of [[sodium hydroxide]] nearly always are initially present. In small amounts this chemical can cause minor irritation to the eyes and/or open wounds; however, with exposure to air, it quickly turns into [[sodium bicarbonate]] (baking soda). However, this transformation is not 100% complete, and invariably leaves residual amounts of hydroxide ions from NaOH. Depending on the type of airbag system, [[potassium chloride]] may also be present.

For most people, the only effect the dust may produce is some minor irritation of the throat and eyes. Generally, minor irritations only occur when the occupant remains in the vehicle for many minutes with the windows closed and no ventilation. However, some people with [[asthma]] may develop a potentially lethal asthmatic attack from inhaling the dust.

Because of the airbag exit flap design of the steering wheel boss and dashboard panel, these items are not designed to be recoverable if an airbag deploys, meaning that they have to be replaced if the vehicle has not been [[total loss|written off]] in a collision. Moreover, the dust-like particles and gases can cause irreparable cosmetic damage to the dashboard and upholstery, meaning that minor collisions that result in the deployment of airbags can be costly, even if there are no injuries and there is only minor damage to the vehicle structure.