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.

Ask Joe Mechanic - Air Bags Part 1

Even though airbags have been around for quite a number of years, they have undergone a number of changes in recent years.  Some other safety systems are dependent on the airbag system for their operation.  Therefore it is important to understand how they operate. It is also imperative for your own safety to know how they operate and to possibly dispel some of the myths about airbags.

            American John Hetrick and German Walter Linderer first patented airbags in 1951. Linderer’s system was deemed ineffective as it depended on compressed air either released by bumper contact or by manual release by the driver, but it inflated too slowly to provide any margin of safety. Hetrick however, was an industrial engineer and had also worked with torpedoes in World War II.  He put this knowledge to use in designing and building an operable airbag. He actually worked for one of the “Big Three” automakers at the time.  Even though airbags are now required standard safety equipment on every car, when he showed his invention to each of the Big Three, there was no interest in purchasing or using it in their cars. At that time, the automakers could not see any way that it would make their cars more saleable or increase their profits. Saftety was not considered that important then as it is today. After his patent expired in 1971, Ford experimented with it in a few cars.

            The first major breakthrough in airbag technology came in 1967 when Allen Breed developed an electromechanical airbag crash sensor, which would detonate a small charge of sodium azide. Ford, Chrysler and an Italian company were soon experimenting with variations of this. In the early 1970s, both Ford and GM offered airbags on government fleet vehicles. Oldsmobile offered a drivers airbag on their Toronado model, but it was dropped in 1977 for lack of interest. Part of the problem of acceptance at that time was the fact that seatbelt usage was approximately ten percent of the driving public. During this era, airbag manufacturers required that the drivers use their seatbelt at all times, so this meant that most people wanted nothing to do with it.

            In 1981, Mercedes Benz introduced an airbag system that integrated a seatbelt with a pretensioner that would tighten the belt to limit forward movement in the event of a frontal collision. This essentially became the first supplemental restraint system (SRS). In 1987, Porsche became the first manufacturer to offer standard driver and passenger airbags on the 944 Turbo and as an option on the other 944 models. In 1988, Chrysler became the first US automaker to offer standard driver’s airbags on its most popular models, followed a year later by its availability across its entire product line.

During the 1990s, airbags became more common in many different vehicles; especially after the US Transportation Efficiency Act of 1991 mandated that all passenger cars and light trucks built after September 1, 1998 must have driver and passenger front airbags as standard equipment. During the 2000s, side impact airbags became commonplace, first on high-end luxury cars, later on many medium-priced cars and then even on some entry-level models. In recent years, head curtain, knee and torso airbags have been installed on some vehicles.

            The early 2000s brought some changes in the front airbag systems. The first change was the second-generation airbags, which deployed with less force than the earlier bags, especially in lower speed collisions. The next innovation was integrating a sensor into the system, which determined when there was no one occupying the passenger seat or someone who weigh less than a predetermined weight and deactivated the passenger bag. This also resulted in lower repair costs as the passenger airbag and the dashboard cover did not have to be replaced.  There are some inherent limitations to airbags. First is the fact that airbags will not protect someone in the event of multiple impacts. Once an airbag deploys, it deflates quickly and then is rendered useless.

            A second even more dangerous and more common situation is called an underride collision. This takes place when an auto strikes the rear of a large delivery truck or tractor-trailer. The ICC bumper which is standard on most North American tractor trailers, will often break away when struck from behind. This means that the vehicle will go under the back of the truck, sometimes as far as the windshield or on rare occasion even further, resulting in serious injuries or even death by decapitation. The problem here is that the impact is not sufficient enough to set off the airbag until the vehicle could potentially be way under the truck, without the airbag having deployed. European trucks require a much stronger rear bumper, and many newer North American trucks have a better-built reinforcement system.  However, there is no requirement to retrofit the thousands of trucks on the road that are not reinforced.

            A third problem has recently surfaced and received a lot of press coverage due to a major recall of GM vehicles and now some other automobiles too. This is due to if the power to the airbag system is interrupted because of defective ignition switch or other factors, the airbags will not deploy in the event of a crash.  This loss of ignition also affects many other systems such as power steering and brakes and there is a high likelihood of a crash if the switch cuts out while the car is in motion.

            The final limitation or hazard of airbags is the risk of injury from airbag deployment if the front seat occupants are not wearing seatbelts and move toward the airbag at time of impact. Also, children or adults of small stature have a risk if seated too close to the airbag.  The people who place a rear-facing child seat in the front place the child at high risk of serious injury or death if the airbag deploys.  However, when you consider that between 1990 and 2000, the US National Highway Traffic Safety Administration estimates that airbags are directly responsible for saving approximately 6,400 lives as opposed to 175 fatalities caused by airbags (104 of those being children).

            Next week, we will examine how airbags operate. Some information for this column was sourced from Wikipedia.org.