Ask Joe Mechanic - Side, Curtain and other Air Bags

Side airbags utilize a different type of system for deployment. They are usually packaged into the backrest of the front seats and deploy by splitting the seam of the covering.  Though a few manufacturers place the bag into the center post between the doors. The system is set up with a pressure sensor in each door and each door has a sealed covering inside the inner door trim to maintain it as a sealed cavity. When the door gets hit, the change in pressure from the outer panel crushing sends the signal to the ACU, which then sends the trigger signal to the appropriate airbag(s).  The airbag will then deploy in a manner similar to how the front airbags do.

Some manufacturers, such as Saab, have developed a two-stage side airbag where the lower or torso section of the bag deploys first, and then a split second later the upper section deploys. This is intended to prevent the whipping motion that would cause the head to strike the side door.

            Certain vehicles use a head curtain airbag in place of or sometimes in conjunction with the seat-mounted side airbag. Head curtain airbags deploy from the same signal as a side airbag from the pressure sensor in the door.  They also deploy in a similar method, but deploy downward from the inside of the roof-rail. Head curtain airbags can offer an additional protection if the vehicle is equipped with a rollover sensing system. If so equipped, the head curtain airbag can deploy in the event of a rollover to prevent injury, and can possibly even prevent ejection from the vehicle.

Gyroscopes and accelerometers that determine the angle of inclination, the speed and direction of travel, and send their signals to the ACU actuate rollover systems.  This in turn, analyzes the signals and deploys the airbag or bags if needed. These sensors are the same ones, which actuate a fuel shutoff in the event of a rollover. Usually on four-wheel drive vehicles there is a manual switch to shut off the rollover system if the operator is going off-roading.

            There are also some vehicles that are equipped with either knee or torso frontal airbags. These systems work similar to and read the same signals as the front airbags.

            Seat-belt pretensioners are an important part of most airbag systems. When the trigger signal is sent to the airbag, the same signal goes to the seat-belt pretensioners for the front seat occupants. Each seatbelt unit has a small tube, usually about six inches long with an actuating charge at the lower end, attached to the recoil mechanism on the seat belt retractor. When the unit receives a trigger signal, the charge goes off, locking the belt and propelling the lock up the tube, which pulls in three to four inches of slack from the seatbelt and locking it in place to restrain the person occupying that seat.

When locked, the seat belt may be pulled so tight that it might have to be cut it if the latch cannot be released. While working as a Saab technician, I had to remove and replace a seat belt unit under warranty. The defective unit was to be returned after replacement, but we were not allowed to ship it charged, so I detonated it for shipment. I have to say that it was about as loud as setting off an M80 firecracker.  I also detonated an airbag for the same reason. 

To detonate an airbag, Saab supplied an actuator button which had two leads attached to it, one to connect to a car battery, and the other about fifteen feet long to connect to the airbag. We placed the airbag backside down on the pavement outside the building, attached the leads and detonated it. The bag actually jumped about four to five feet into the air and the sound was extremely loud. My only thought was, ‘did I really want something like that going off in my face?’  However, considering the possible alternatives in the event of a serious crash, I guess I would appreciate it after all.

            Next week we will review anti-whiplash head restraints. Some information for this article was obtained from www.wikipedia.org.

Ask Joe Mechanic - Air Bags Part 2

Many people do not understand the operation and inherent dangers of airbags. While airbags are one of the best safety features introduced in vehicles since the seatbelt, they do contain certain dangers due to their designing and operation.  This is especially true if the vehicle’s occupants are taking certain care as recommended.

            In design, the airbag system is fairly simple. An Airbag Control Unit (ACU) is a specifically designed Electronic Control Unit (ECU) or computer which receives signals from a number of sensors in the vehicle including impact sensors, side door pressure sensors, wheel speed sensors, brake pressure sensors, accelerometers, gyroscopes, and seat occupancy sensors. Upon receipt of the required signals (information) from the sensors, the ACU will send out an electronic signal to the trigger in the required airbag or airbags and other related items such as seat belt pretensioners.  The trigger then sets off a chemical reaction, which creates a gas that rapidly inflates the airbag.

            To explain this in a little more detail, after the ACU receives an electronic signal and sends out a trigger signal, it reaches a pyrotechnic device called an initiator or an electric match. An electric match is generally an electrical conductor wrapped in a combustible material, which will burn with just one to three amps of charge in less than two milliseconds. This ignites a gas generator, which in the case of an airbag is a mixture of a number of chemicals. In the most commonly used mixture, this sets off a series of three separate chemical reactions to create nitrogen gas. The time lapse from impact to receipt of the signal is normally 15 to 30 milliseconds, with a further 20 to 40 milliseconds to full inflation.

            The one chemical used in most airbags is sodium azide, which is highly reactive and gives off toxic byproducts, which are neutralized in the third step of the process. This is the reason many people complain of burning of their skin or in their throat after an airbag deployment.  Sometimes the byproducts do not get 100% neutralized, resulting in some irritation, but there are no known permanent effects.  There are replacement chemicals, which have been tested, and many manufacturers are using in the latest generation airbags. Though, many of the older airbags will be around for a long time. There is also testing on another chemical ongoing, which holds even more promise as a suitable replacement. The reason that many of the original chemicals were chosen is that the chemicals cannot be hygroscopic, in other words, they cannot absorb water, which would affect the operation of the airbag. The need to totally isolate the effects of water from the system is the reason for many of the connectors having a thin gold plating to prevent corrosion.

            After the third stage of the reaction is completed, the bags have fully inflated. There are small bleed holes in the bag to allow a controlled deflation as the body comes forward to contact the bag. This cushions the body and head and the controlled deflation allows a slowed forward movement rather than a sudden stop. Another advantage of the newer chemicals is that the operation temperature of the reaction is lower, so there is less chance of getting any burns from the airbag.

            Unlike crash tests into barriers, real world crashes occur at all sorts of angles, so a crash sensor must be designed to recognize this and to send the proper signals. The presently used sensors are called a MEMS accelerometer. It contains a small integrated circuit with microprocessors and a micro-mechanical element that moves quickly upon deceleration. This changes the built-in resistance of the unit, which triggers the electronics to send the signal to the ACU.

            At present, the federal requirement is that an airbag must deploy at a 14 miles per hour barrier collision or the equivalent deceleration. Another requirement is in the event of a fire in the vehicle, when a threshold temperature of 300 to 400 degrees F is reached, the airbags will automatically deploy. This is called auto ignition and is to prevent a total explosion of the entire airbag module.

            The newest airbag technologies are being designed to adjust to factors such as the severity of the crash: the size, seating position and posture of the occupants, seatbelt usage, and how close the person is seated to the airbag. They use multi-stage inflators with specific sensors to adjust the force of the deployment.

            The dust released during deployment has long been a source of complaint from people who experienced an airbag deployment. It is usually cornstarch, French chalk, or talcum powder, which is used to lubricate the bag while it deploys.  It is not harmful.

It may cause some minor irritation of the throat and eyes.  However, that usually occurs only if the person remains in the vehicle for a period of time after the crash, such as during an entrapment.

            Some automakers such as Mercedes Benz call for the replacement of any un-deployed airbags after a period of years in order to guarantee their effectiveness in the event of a crash in an older airbag equipped vehicle. Most manufacturers who do recommend replacement consider a lifespan to be about fourteen years. On the other hand, Volvo, a very safety-oriented company has said, “airbags do not require replacement during the lifetime of the vehicle except after deployment.”

Next week we’ll look at side airbags, seat belt pretensioners and other airbag systems. Some information for this article was obtained from Wikipedia.org.