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About this Article
Written by: Jesse Patterson
Written on: September 1st, 2000
Tags: chemical engineering, mechanical engineering, physics
Thumbnail by: Zoltan/SXC
About the Author
Jesse Patterson, Jr. was an undergraduate student at the University of Southern California in 2000.
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Volume I Issue I > The Engineering Behind Automotive Airbags
Thousands of lives are saved by airbags during high-speed car accidents. Exactly how does the airbag deploy at such a high speed, and what trends in airbag technology should we look forward to in future developments? The development of the automotive industry has lead to enormous developments in safety, specifically airbag design and technology. This article details the development of airbags in cars, the design of airbags (chemically and mechanically), as well as the future of airbag safety.

Introduction

Henry Ford revolutionized the world in the 1920s by using an assembly line technique to produce cars that everyone could afford. His "Model T" prototype spawned a new era in which personal freedom and independence were expressed through the ownership and usage of an automobile. The Model T was simple in design and function by today's standards; it neither accelerated nor braked quickly and did not handle very well. It had a permanent convertible-style top that offered no shelter from weather, and it did not have safety belts or other constraining devices. Shortly after its creation, Ford stated, "There is no need for further development in motorcars, for I have perfected the modern automobile." Little did Ford realize that he had pioneered a machine that would undergo decades of technological safety revisions.
Zoltan/SXC
Figure 1: Passenger bags have become an integral component of automotive safety.
Years after the Model T was assembled, the seat belt was implemented in some cars as the only form of restraint. People were reluctant to use the belts at first, claiming that they were uncomfortable and restricted their movement. However, statistics have proven that seat belts are responsible for saving lives in automobile accidents. Most regions in the United States and many other countries now enforce mandatory safety belt laws.
Although airbags have never replaced seatbelts, they were designed to provide maximum safety when used in combination with seatbelts. It is important that they be used at the same time as safety belts because most airbags only work in front end collisions faster than 10 miles per hour. Regardless, airbag technology is currently undergoing innovative revisions and improvements at a phenomenal rate, while seatbelt design has remained stable throughout the years. This article will explore the historical development of airbags, explain the engineering behind the airbag's deployment technology, and speculate on safety concerns and the future of airbags.

Historical Development of Airbags

Airbags have been under development for many years. Their purpose being to restrain automobile drivers and passengers in a collision, whether or not they are wearing seat belts. Early airbags were mainly used as inflatable crash landing devices for airplanes. The first patent on an airbag however, was filed during World War II, and the automobile industry started researching airbag technologies in the late 1950s.
The early airbag designs were impractical and expensive. The main concern for design engineers was the storing and releasing of a compressed gas. Issues were raised such as where to store the container for gas in the car, how to develop a mechanism that allows the gas to expand quickly, and how to make sure that the stored gas would have a shelf life at least equal to that of the car. In addition, the automobile industry found that in order to be effective, an airbag must deploy and inflate within 40 milliseconds. The airbag systems must also be able to differentiate between major and minor collisions.
These issues were addressed in the 1970s with the invention of small propellant "inflators," devices that initiate a chemical reaction that releases hot nitrogen gas into the airbag. This device was a major stepping-stone in the development of airbag technology, as it has enabled the common use of commercial airbag systems that have been available since the late 1980s.

Physics Behind the Airbag

One should review some basic physics concepts to better understand the engineering of an airbag. According to Newton's first law of motion, every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. Newton's third law of motion says that for every action there is an equal and opposite reaction. Moving objects have momentum, which is the product of the mass and velocity of an object. Unless an outside force acts upon the object, it will continue to move at its initial velocity (both speed and direction). Cars consist of several objects, including the vehicle itself, loose objects in the car, and the passenger(s). Unless they are restrained, the car's contents will continue moving at the car's velocity (Newton's first law), even if the car is stopped by a collision (Newton's third law).
A constant force in the opposite direction of the car's motion is required to safely stop the momentum of objects inside a car. When a car crashes at a high velocity, a very large force is needed to stop the objects inside because the car's momentum has changed instantly while the passenger's has not. Thus, the goal of seatbelts and airbag systems is to help stop passengers with as little damage to the passenger as possible.