How does an airbag work step by step?
Airbags deploy in milliseconds to cushion occupants and reduce injuries in crashes.
In this explainer, we map the sequence from crash detection to airbag deflation, detailing how sensors, inflators, and seatbelts coordinate protection across driver, passenger, side, and curtain airbags.
Deployment sequence: step by step
Trigger and decision
The deployment starts when the car’s safety network detects a crash that meets predefined criteria and the airbag control unit (ACU) decides whether inflation is appropriate for the given situation.
- Crash sensing: Accelerometers and other sensors detect rapid deceleration or impact and relay data to the ACU.
- Decision and timing: The ACU evaluates crash severity, occupant position, seat occupancy, and other factors to determine if deployment should occur and which inflators to use.
- Electrical trigger: If deployment is warranted, the ACU sends an electric signal to the inflator’s squib, initiating the inflation process.
- Gas generation: A solid propellant inside the inflator burns rapidly, producing nitrogen gas (and sometimes other inert gases) to fill the bag.
- Bag deployment: The airbag unfurls from its housing (steering wheel, dash, or door/side modules) and inflates within roughly 20–40 milliseconds.
- Cushioning and venting: The bag provides a protective cushion while vents or porous fabric allow controlled deflation to reduce resistance and rebound.
- Seatbelts and pretensioners: Seatbelts tighten through pretensioners to position the occupant optimally for airbag interaction and to reduce forward movement.
- Additional airbags: Side, knee, and curtain airbags deploy as needed to protect various body regions based on crash dynamics and occupant location.
- Post-crash and servicing: After deployment, the system requires inspection and replacement of airbags and inflators before the vehicle is put back into service.
These steps unfold in fractions of a second, and the exact sequence can vary by vehicle design, occupant size, seating position, and crash characteristics.
Variants and safety features
Configurations and adaptations
Airbags come in multiple configurations and work alongside other restraints to tailor protection. The following points summarize common variants and enhancements.
- Multi-stage/dual-stage inflators adjust inflation level based on crash severity and occupant weight, providing a gentler or stronger fill as needed.
- Side thorax airbags and curtain airbags protect the torso and head during side impacts or rollovers, while knee airbags help reduce leg injuries.
- Occupant sensing and weight-based deployment determine whether to deploy the passenger airbag, helping to avoid injury to small children or mispositioned occupants.
- Seatbelt pretensioners and load limiters coordinate with airbags to position and restrain occupants effectively while minimizing chest and neck injuries.
These features form part of an integrated restraint system designed to maximize protection while reducing the risk of secondary injuries during and after a crash.
Summary
Airbags deploy within milliseconds after crash detection, using inflators to generate gas that fills a folded fabric bag and cushions occupants. The system coordinates with seatbelts and occupant sensors, and it includes variants such as side and curtain airbags to protect different body regions. After deployment, airbags and related components must be replaced, and the vehicle’s safety network is reset for future use. Ongoing research aims to refine timing, deployment location, and adaptability across a wide range of occupants and crash scenarios.
What is the 5 10 20 rule for airbags?
A simple rule to remember is the 5-10-20 Rule which depicts the clearance from a deployed or undeployed airbag. 5 inch clearance from side impact airbags, 10 inch clearance from a steering column airbag and 20 inch clearance from a passenger side dashboard airbag.
At what speed do airbags deploy in an accident?
Airbags typically deploy in moderate to severe frontal crashes, equivalent to hitting a solid barrier at 8 to 14 mph or faster. This speed can be higher for belted occupants and varies based on the crash's specifics, with the system detecting impact force to determine if deployment is necessary to prevent severe injury from hitting the steering wheel or dashboard.
Deployment speeds and factors
- For unbelted occupants: Frontal airbags usually deploy at the lower end of the range, around 10 to 12 mph, as the car's sensors detect a higher risk of injury.
- For belted occupants: The deployment threshold is higher, often around 16 mph, because seat belts are already providing a significant level of protection.
- For side airbags: Deployment speed varies based on the type of impact. In a narrow object crash, like hitting a pole, side airbags can deploy at about 8 mph. In a more general side impact, they might deploy at around 18 mph.
- Other factors: Airbags are designed to deploy based on the combination of speed and the force of impact, not just speed alone. The system uses a variety of sensors to make a decision.
How to interpret "speed"
- The speed is not a simple speed limit, but rather a measure of the force of the impact.
- A collision at 10-14 mph with a solid object can be severe enough to cause injury from hitting the steering wheel or dashboard.
- Hitting a parked car is different from hitting a solid wall. Hitting a parked car of similar size is roughly equivalent to hitting a solid wall at half the speed, according to Spada Law Group LLC.
- The deployment threshold can vary by vehicle model and the specific type of airbag.
- Even if a car is stationary, airbags can still deploy if another vehicle hits it at a sufficient speed, as long as the engine is running, notes this YouTube video.
What triggers an airbag to deploy?
Airbags deploy when crash sensors detect rapid deceleration, a sudden drop in speed, in a moderate to severe collision. If the impact is severe enough, the sensors send a signal to the airbag control unit, which then triggers a chemical reaction to instantly inflate the airbag with gas.
This video explains how airbags work in detail: 1mSabin Civil EngineeringYouTube · Nov 26, 2021
How it works
- Sensors detect a crash: A network of sensors, including front sensors and "safing" sensors, are positioned throughout the vehicle. They monitor for a sudden change in speed or deceleration.
- The control unit analyzes the impact: The sensors send data to the airbag control unit (also called the airbag control module), which is the brain of the system. The control unit analyzes the data, along with information like impact force, angle, and even whether seatbelts are used, to determine if an airbag is needed.
- Airbags deploy: If the impact is severe enough, the control unit sends a signal to an igniter in the airbag module. This triggers a chemical reaction (often using sodium azide) that produces a large amount of nitrogen gas in a fraction of a second, inflating the bag to cushion the occupant.
Factors that influence deployment
- Impact severity: Airbags are designed for moderate to severe crashes. Minor bumps or fender benders usually don't have enough force to trigger them.
- Collision type: Frontal and near-frontal collisions are the most common triggers for frontal airbags. Side airbags are designed to deploy in side impacts or serious frontal crashes. Rear-end collisions generally do not trigger airbags.
- Speed: While there isn't a single speed, frontal airbags typically deploy at speeds of about 8-14 mph or higher, especially when hitting a solid object.
- Other factors: Modern systems use advanced sensors that can also consider factors like seat occupant weight and position to prevent deployment when it's not necessary or potentially harmful, such as with a child in a car seat.
How does an airbag work in a car step by step?
Area. It not only inflates. But also deflates to slow you down during impact while deflating It gives more time to travel. You can see two vent holes in airbags. The air vents out from these holes.
