How do Airbags work?

What is an airbag?

The airbag is a part of the supplemental restraint system. It was invented for protecting the human body during high-speed accidents. It's a big engineering challenge to bring the human body to rest from a high speed in a time duration of 100 milliseconds without causing much harm to the body. Let’s learn about this life saving technology by understanding the physics behind the accident.

What is the physics behind the accident?

During the accident, the seatbelt exerts a huge force on the chest area and causes internal organ injury. To avoid this huge force on the chest, modern seat belts slightly release the seatbelt with the help of a torsion bar (refer fig 1a). Because of this the upper body slightly moves forward. But once the torsion bar is done with the belt release the upper body’s movement is arrested here. But the neck and head are not arrested so you can easily guess here, now what will happen? The head goes in a perfect pendulum movement in a high-speed accident. The devastation can be horrific even with the seatbelt (refer fig 1b).

Fig 1a : A torsion bar is helpful for slightly releasing the seatbelt.
Fig 1b : The devastation can be horrific even with a seatbelt

This is why engineers came up with the idea of airbags. Airbags have a cushioning effect, and at the same time, prevent you from hitting the dashboard. Now I will explain to you about the first airbag design.

First airbag design

The first airbag design consists of an inflatable bag connected to a tank of compressed air. To inflate the airbag using a compressed air arrangement was a failure. When a car is crashed, the spring detects the crash and goes forward allowing compressed air to inflate the airbag and protect the driver.

But this design had two main issues, first is the spring was not able to determine the crash accurately(refer fig 2), and the second is the compressed air was not able to fill the airbag fast enough to prevent damage from the crash as shown in fig 2. Because of these two reasons, airbags were not commercially used. Now we will discuss in the next section, how these two problems have been solved.

Fig 2 : First airbag design having compressed air and spring mechanism

Ball in-tube sensor

The Great founder of Joyson safety systems, Mr. Allen K. Breed, came up with some groundbreaking inventions to solve the above issues.

1) First, he improved the accuracy of the sensor using a ball-in-tube sensor. In this sensor, a steel ball is held in position with the help of a magnet as shown in fig 3a below.

2) And the second contribution is the use of sodium azide chemical explosives. Explosion of sodium azide generates nitrogen gas which is used to inflate the airbag. This is a quick and reliable reaction which removes the necessity of storing the compressed air onboard. The sodium azide is stored in an airtight cylinder inside the steering wheel you can see in fig 3b.

Fig 3a : Ball-in-tube sensor to accurate crash detection
Fig 3b : Sodium azide is used in Allen k invention instead of the compressed air systemb

How does a ball in-tube sensor work in an airbag?

When a collision occurs, the car decelerates very quickly, and the ball gets separated from the magnet due to its inertia. The ball moves forward to close the circuit and sends a signal through a pyrotechnic device. It's a thin resistance wire. Now the current is passing through the wire as shown in the fig 4a. It generates a temperature of more than 300 degrees celsius. This causes the sodium azide to explode and generate nitrogen gas very quickly. This is able to inflate the bag within 30 milliseconds.

Fig 4a : After the collision occurs, the ball moves forward
to close the circuit and sends a signal to the inflator
Fig 4b : Explosion of sodium azide, generate nitrogen gas very quickly and inflate the bag

That’s all about Allen k Breed’s invention. This patented design of Breed corporation was released by Chrysler in their Dodge Daytona Model cars in 1988. This was a massive success, and all other car manufacturers started to implement similar airbag technologies.

However, this design has some major drawbacks.

1. First, the gas produced after the sodium azide explosion, is toxic. The sodium metal of the exhaust causes trouble. Scientists were able to neutralise this issue by adding potassium nitrate and silicon dioxide(refer fig 5).

2. Due to a notorious property of sodium azide ― they absorb moisture content easily. If there is a leakage in the design stage or manufacturing, sodium azide will absorb moisture. After the moisture absorption, when triggered, the chemical will undergo violent explosions, causing the rupturing of airbags and shrapnel flying to the passengers(refer fig 5b). This is exactly what happened with Takata’s airbags. To avoid such unfortunate incidents, the addition of a drying agent will be helpful(refer fig 5b), and strict quality control measures are required for components like airbags.

Fig 5a : Potassium nitrate and silicon dioxide are added to solve the problem of toxic gas
Fig 5b : Sodium azide absorb moisture content easily causing the
rupturing of airbags and shrapnel flying to the passengers

Nowadays there are many improvements has done in the airbag. Sodium azide is replaced by guanidine nitrate as a gas generator, it is less toxic and less explosive than sodium azide.

Fig 5c : Sodium azide is replaced by guanidine nitrate as a gas generator

You thought, all problems are solved now, but not yet.

The issue was there with the ball and tube type sensor. They would occasionally get activated even when they fell in a pothole. It can’t distinguish between a pothole and a crash. The reason ― the electrical switches do not provide information about the rate of deceleration which is necessary to determine the severity of the crash. That means it can’t distinguish between a pothole and a crash.

Therefore nowadays, to detect the crash more accurately, MEMS sensors are used. Let's discuss here, how MEMS sensors are more effective?

How do MEMS sensors work in airbags?

MEMS sensors are used with an advanced Electronic Control Unit(ECU). This is a capacitance-based method, and the severity of the crash can easily be determined. ECU also takes input from the wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors as shown in fig 6a.

MEMS sensors have an algorithm that determines when to trigger the gas generator and how much to inflate the airbag based on the severity of the crash. The igniter ignites the explosive within 2 milliseconds and fills the bag within 20-30 milliseconds. The airbag remains fully open for 100 milliseconds(refer fig 6b).

Fig 6a : MEMS sensors are used to determine the severity of the crash
Fig 6b : The airbag remains fully open for 100 milliseconds

Now the airbag is ready to cushion you. The airbag spreads the impact force over a larger 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 the airbags. The air vents out from these holes, and the airbag deflates to slow down your body(refer fig 7).

Fig 7 : Airbag has holes to vent out the air while deflating

I hope you have a good understanding of airbags and enjoyed this article. Thank you for reading the article.


Amar Pattanshetti

Amar is working as a product developer at Lesics Engineers Pvt. Ltd. His areas of interest are fluid dynamics, vehicle dynamics and exploring Tesla's inventions. He has done projects such as a Tesla valve, Tesla turbine, an airbag, hill start assist, working of cryogenic engine, etc. Check out this link for more information about the author.