At the moment of a serious collision, the passenger and driver have a split second to react, but the human body is not physically capable of such actions. This is where it comes into play passive safety system, designed by engineers to minimize the effects of impact. An airbag, or Airbag, is not just a bag of air, but a highly complex engineering complex that deploys faster than you can blink.
Many drivers mistakenly believe that the airbag is inflated with air from the passenger compartment or a compressor, but this is not the case. Within the system there is an instantaneous chemical reaction, generating huge volumes of inert gas in a matter of milliseconds. Understanding this process helps you understand why you should not neglect your seat belt, even if the car is equipped with dozens of airbags.
In this article we will analyze in detail the design of the module, the logic of the electronic control unit and the reasons why the system may not work or, conversely, fire falsely. You will learn what substances are used for filling and why there may be a specific smell in the cabin after an accident.
Design and components of the SRS system
Abbreviation SRS (Supplemental Restraint System) translates as “additional restraint system.” The key word here is “additional”. It does not work on its own, but only complements the main seat belt. The basis of the system is the inflatable element itself, which, when at rest, is rolled into a tight roll and hidden under a plastic cover in the steering wheel, dashboard or body pillars.
The main actuator is squib (gas generator). It is a small device containing solid fuel that releases gas when ignited. It is the squib that starts the filling chain reaction. The second most important element is a network of impact sensors located in the front of the car, in the doors and sometimes even in the rear bumper.
All these peripherals are connected to the electronic control unit (ECU). This computer constantly polls all sensors, analyzing the acceleration and deceleration of the body. If you accidentally hit a snowdrift with your bumper or hit a door with a hammer during repairs, the ECU will analyze the nature of the vibrations and understand that this is not an accident, but external noise.
Never attempt to open or repair an airbag yourself - there is an explosive inside that is unstable if not handled correctly.
Modern cars use new generation sensors that are capable of determining not only the force of an impact, but also its vector. This allows the system to decide which airbags need to be activated: only the front airbags, the side airbags, or also the curtain airbags.
Triggering algorithm: from impact to opening
The system activation process takes a negligible amount of time, measured in milliseconds. The entire cycle can be divided into several critical stages, each of which is fine-tuned down to microscopic fractions of a second. First, the sensor detects a sharp change in vehicle speed.
As soon as the signal from the sensor exceeds the threshold value, the ECU sends an electrical impulse to the squib. The current must be sufficient to heat the nichrome thread inside the cartridge. The temperature of the filament instantly reaches the values necessary to ignite the solid fuel.
As a result of combustion of a special composition (often based on sodium azide), gas is released. The pressure inside the module increases sharply, and the cushion expands, tearing the plastic cover. This entire process, from the moment of impact to full deployment, takes on average 20 to 50 milliseconds.
☑️ Airbag trigger factors
It is important to note that the airbag deployment speed is about 300 km/h. If the person's face is too close to the module at this moment, the injury from being hit by the airbag itself may be more serious than from a collision. That's why two finger rule (chest to handlebar distance) is critical.
Chemistry of salvation: what's inside the pillow?
Contrary to popular belief, there is no compressed air in a cylinder inside the pillow. Gas is generated at the time of the accident. The main component of solid fuel in most modern systems is sodium azide (NaN3). This substance is stable under normal conditions, but decomposes instantly when heated.
When the squib is activated, a chemical decomposition reaction occurs, resulting in the release of pure nitrogen. It is this inert gas that fills the shell. Nitrogen is safe to breathe, although in high concentrations in a confined space it can cause mild dizziness.
However, a byproduct of the reaction is sodium metal, which is extremely dangerous because it causes a strong alkaline reaction when in contact with water (or wet sweat on the skin). To neutralize sodium, potassium nitrate and silicon dioxide are added to the fuel pellet. As a result of a secondary reaction, sodium is converted into a harmless glassy substance - sodium silicate.
⚠️ Attention: After the airbags are deployed, fine dust (talc or starch) may form in the cabin, which covers the shell for adhesion, as well as residues of chemical compounds. Do not inhale this dust deeply and ventilate the car immediately.
In new developments, manufacturers are phasing out sodium azide due to its toxicity in its original state. They are being replaced by less aggressive substances, such as nitroguanidine, which produce less heat when burned and do not form dangerous by-products.
Typology of cushions and protection zones
A modern car can be equipped with a whole complex of inflatable elements, each of which has its own specific design and response time. Frontal airbags located in the steering wheel and dashboard are designed to protect against frontal impact and have the largest volume.
Side airbags are built into the seat backs or door panels. They operate faster than the front ones, since the distance to the passenger in a side impact is minimal, and the space for deformation of the body is limited. Their task is to protect the chest and pelvis.
Inflatable curtains (Curtain Airbag) are located along the roof above the side windows. They open from top to bottom, covering the windows and protecting the passengers' heads from fragments and impacts on the pillars. Unlike other types, the curtains can remain inflated for several seconds after an impact, providing protection if the vehicle rolls over.
| Pillow type | Location | Response time | Main function |
|---|---|---|---|
| Front (Driver) | Steering wheel | 30-50 ms | Driver head and chest protection |
| Front (Passenger) | Instrument panel | 30-50 ms | Front passenger protection |
| Side | Seat back | 10-20 ms | Abdominal organ protection |
| Curtain | Roof pillar | 10-20 ms | Side impact head protection |
| Knee | Bottom of dashboard | 20-30 ms | Preventing belt diving |
Why are knee pads less common?
Knee airbags are installed mainly in premium cars or in expensive trim levels. Their task is to prevent the driver’s legs from hitting rigid structural elements or “diving” under the seat belt, which could lead to severe spinal injuries.
False positives and blocking conditions
The electronic control unit is programmed to ignore many external influences. You can slam a door hard, drive through a deep rut, or even drop a heavy object on the hood - the system will not work. The algorithm analyzes not just the impact force, but the nature of the change in the acceleration vector (G-force).
There are situations when the pillow shouldn't operate even upon impact. For example, in a rear-end collision, the inertia throws the passengers forward, but the front airbags in this case can only do harm, sharply hitting the person who is already moving forward. The system may also not activate when hitting soft obstacles such as a snowdrift or tall grass.
A common cause of failure is a malfunction in the power circuit. Signal lamp SRS or Airbag lights up on the instrument panel every time the ignition is turned on for self-testing. If it is constantly on or flashing, this is a signal that the system is deactivated and will not work in the event of an accident.
⚠️ Attention: If the security lamp does not light up when the ignition is turned on, this is also a malfunction. Perhaps the indicator itself or the diagnostic circuit has burned out, and you will not know about the real breakdown of the system.
In addition, there are special switches for disabling the passenger airbag. They are necessary to install a child seat in the front seat. If you install the cradle rear-facing, disabling the front airbag necessarily, otherwise the impact of an open curtain can be fatal for a child.
Maintenance and replacement of modules
There is a myth that airbags do not require maintenance and last the entire life of the car. While the tissues and gas generators themselves can remain stable for decades, the electronics and contacts are susceptible to aging. Oxidation of connectors in doorways (where the wires pass to the side airbags) is a common cause of errors in the system.
After any, even the most insignificant operation, the module must be completely replaced. Restoring the squib or sewing up the shell “for show” is strictly prohibited and deadly. The impact sensor must also be replaced if it has been activated, and often the seat belt itself, since its pretensioner mechanism has already been used.
When purchasing a used car, be sure to check the accident history and condition of the interior components. The absence of seams on the steering wheel, unnaturally even gaps in the dashboard, or the presence of a new carpet in the trunk (where the ECU may be located) may indicate that the airbags have already been fired and were replaced makeshiftly or were not replaced at all.
The lifespan of an airbag is not limited in time, but is limited by operating conditions: vibrations, temperature changes and humidity can eventually damage the electronics.
Professional diagnostics of the SRS system requires a special scanner capable of reading error codes from the control unit. A regular multimeter tester will not help here and may even cause accidental operation if current is applied to the squib while testing the circuit.
Is it possible to drive if the Airbag light is on?
Technically, the car will drive and the engine will not lose power. However, you are driving without one of the main safety systems. In the event of an accident, the airbag will not deploy and the seat belt pretensioners may not engage. Operation is only possible for transportation to a repair site.
Will the airbags deploy if the car rolls over?
This depends on the design and availability of tilt accelerometers (gyros). In modern vehicles equipped with Rollover Protection, the curtains and side airbags will deploy to support the occupants' heads inside the cabin.
Why does the pillow smell burnt after the shot?
The smell appears due to the combustion products of solid fuel in the gas generator and fine dust (talc) with which the fabric is treated. This odor is not toxic in the long term, but can be irritating to mucous membranes immediately after an accident.
Will airbags be replaced by insurance after an accident?
Yes, if you have a CASCO policy, the insurance company is obliged to cover the costs of restoring the passive safety system, as this affects the safe operation of the vehicle. With compulsory motor liability insurance, payment depends on guilt and the terms of the contract.
Can airbags be installed in an old car?
Theoretically possible, but economically and technically infeasible. It will be necessary to replace the steering wheel, dashboard, install a wiring harness, sensors and, most importantly, integrate the airbag ECU into the car's standard wiring, which requires deep knowledge of auto electrics.