Airbags are described as explosives or feather pillows, depending on who is doing the describing. Here in The High Road, we always assume you have an above average knowledge, and we'll start from there. If you are an idiot, they have a magazine for you. It is called Consumer Reports.
A 1990 to 1997 Miata has four crash sensors. The airbag system can be identified by its blue connectors in the wiring harness. Three D sensors are mounted near the front of the car. One is in front of the upper header tank on the radiator, next to the hood release. The other two are on the outside of either "frame rail", behind the bumper, adjacent to the front tie down loops. The remaining S sensor is under the dashboard on the inside of the firewall, behind where the VIN number is stamped. Sensors are all made of a gold plated steel ball, about 3/8 diameter, stuck to a small magnet. This ball is poised behind two gold plated contacts. An impact will cause the ball to fly forward and complete a circuit through the sensor.
The airbag control computer is a standard issue Ford part with a Motorola microprocessor. It is in a blue box mounted under the instrument panel near the steering column. If it senses continuity in the S sensor and one or more of the D sensors, it sends a signal to inflate the airbag. If the car battery or wiring is destroyed in the first milliseconds of an accident event, a backup "battery" will inflate the airbag(s). The backup "battery", also in a blue box and located next to the airbag computer, is really a 2200 uf capacitor. It can store enough energy to ignite the inflator. The dashboard airbag light backup is a buzzer mounted in the airbag computer, it is not the seat belt buzzer.
The gas generator in a Miata is filled with sodium azide as most inflators. Airbag inflators are a spin-off of military and rocket propellant industries. One of the major suppliers of inflators to the car industry is rocket fuel manufacturer, Morton Thiokol (They also make the space shuttle boosters). To ignite the inflator, it is provided with a 12 volt input from the airbag control computer. This heats a resistive wire element which initiates the exothermic chemical reaction which decomposes the sodium azide (NaN3) in a three step process. The chemical deflagration includes potassium nitrate (KNO3) and silicon dioxide (SiO2). The reaction proceeds as follows:
This reaction forms sodium and the nitrogen which inflates the airbag.
2 NaN3 -> 2 Na + 3 N2
The sodium byproduct of the first reaction and the potassium nitrate generate additional nitrogen in this reaction.
10 Na + 2 KNO3 -> K2O + 5 Na2O + N2
And finally the previous two reactions leave potassium oxide and sodium oxide to react with the third component of the mixture, silicon dioxide, forming alkaline silicate "glass".
K2O + Na2O + SiO2 -> alkaline silicate
As you can see, the reactions release nitrogen in steps 1 and 2. It is this hot nitrogen that fills the airbag (or airbags for 1994 to 1997). The inflating airbag tears through the this plastic cover on the steering wheel hub or dashboard. Harmful sodium created in step 1 combines with potassium nitrate in step 2 to produce more nitrogen, potassium oxide, and sodium oxide. But the final result is nitrogen gas and alkaline silicate.
It is said that the airbag is packed with some sort of talcum powder to prevent it from sticking together. If the gas generation involves the inflation of an airbag (our test did not), a cloud of talcum powder may be created.
It is important to note there is one more part of the occupant restraint system that needs to be modified when the airbag is removed. The seatbelts have an area where the webbing is overlapped and stitched together. This area is covered by a cloth sleeve near the door side anchor point. The stitching tears out in and accident, reducing the seatbelt loads on the occupant's hips and chest by allowing another few inches of deceleration space to "ride down" the collision. By the way, the passenger bag is intended to bounce off the window to control its inflation. This usually breaks the window. While the stretchable belt reduces torso injuries, it is only possible because there is an airbag to keep the victim's face out of the steering wheel and windshield. To complete our conversion it was necessary to discard the cloth sleeve and use a razor blade to carefully remove the stitching and the related warning label. There is a plastic stop on the webbing that prevents the latch from sliding down to the floor when the belt is stowed. It will need to be relocated.
It is time to put our tools away and clean up. With the inflator in our hands, the next step is to detonate it. This is a guy thing. When we said we're going to light this off, a woman asked, "Why?". All of the guys present looked at each other and shrugged. It hadn't occurred to us NOT to set it off! Chris Pilla, you're the man! At this point, you expect us to caution you that we are professionals and you should not do this yourself. You would be wrong. They have magazines for you: Popular Sissy and Car & Lawyer. We say, go ahead and try it! Some guys might wear goggles, gloves, aprons or other typical lab safety gear. Whatever.
Here at the Peak To Peak Miata Club labs, the inflator was detonated on our test range (driveway). We had removed the airbag assembly, detached the inflator, and retained the steering wheel with its folded airbag still intact for later use. The bare inflator is a squat metal cylinder about 4 inches in diameter and 1½ inches high. There are numerous holes around the circumference of the cylinder on one end to allow the expanding gas into the airbag. Fortunately the orientation of the holes means that no net thrust is developed by the unit and restraint was not necessary.
We attached a car battery to the leads on the inflator through a 25 foot twin conductor wire. BOOM! If you can imagine launching a dozen Estes model rockets at once, you've seen this. Be aware that 20 minutes after the ignition, the metal case of the inflator was still too hot to touch and, in fact, would boil water! We attempted to cool the unit by submersion in water. This is not recommended either. As you can see, there is sodium (Na) in the intermediate reaction. In our case, there was unreacted excess sodium present. The reaction of sodium and water is very violent, on the order a second detonation! Fortunately, that little speck of molten metal didn't permanently scar the sink porcelain.
For 1999, the Miata airbag system sees a major re-design. The crash sensors and back-up battery have been relocated to the airbag control computer under the instrument panel. Of course, a passenger side shut-off has been provided as well as ugly safety labels all over the place. The driver side airbag has been repackaged in the smaller hub of the Momo wheel. It is the same azide chemistry of previous Miatas. On the passenger side the air bag is now inflated by hot argon gas. The detonator uses a pyrotechnic device to heat the argon which expands to fill the bag.
Also, the new seatbelts have a couple of changes. The load limiting function of the tear-out section of belt is replaced with a collapsible spool in the retractor mechanism. Instead of an exposed warning label on the belt webbing an overstressed belt will not roll smoothly from the retractor.
In order to accept the new "depowered" airbags being demanded by the public, the standard 30 mph crash into a fixed barrier is being scrapped. The revised test mounts the car on a sled that is fired in a reverse direction by compressed air. I do not know if the 1999 Miata is tested with this new procedure, but it might explain the elimination of the behind-the-seat cockpit brace.
This sled test provides a smoother more predictable g load than that pesky concrete wall. The acceleration of the sled to 30 mph is adjusted to achieve some justifiable equivalence to the old test. Manufacturers like the test because it is not necessary to use an entire car. The passenger compartment alone can be attached to the sled. Also, the car is not destroyed by the test, so tests are faster and cheaper.
However, with the new protocol, there is no test of sensors, trigger system, and crumple behavior of car body and frame. It is not as good a test but allows government to claim substantial equivalence due to similar speed and g load energy even though peak load is different and unpredictable with the old test. The government doesn't want to appear to have been wrong about airbags or "soft on safety" with this revised test. Depowered bags might be tested by running into the concrete wall at, say, 25mph. NHSTA couldn't decrease the speed. That would look bad. If you like airbags, you would like to keep the old style test (which could be modified for depowered bags). If, like me, you don't like airbags then who cares if the test is any good?
'90 to '93 (1.6 NA)
Driver airbag only, computer and back-up battery over steering column, three D sensors in nose, one S sensor inside firewall. Airbag computer known to interchange with similar vintage Ford Taurus.
'94 (1.8 NA)
Dual airbags. Same sensors. Back-up battery incorporated into airbag computer which is still over steering column.
'95 to '97
All sensors and back-up battery integrated into airbag computer which is relocated to where the S sensor was. '95 airbag computer recalled for spurious inflation.
Same, with passenger shut-off switch added. Airbag computer functionality changed, but not location. Passenger airbag changed from sodium azide nitrogen generator to pressurized argon canister.
Crush sensor added to nose (remote crash sensor reappears).
'06 and subsequent (NC)
Dual stage airbags, side airbags, door crush sensors, belt pretensioners, system now smart enough not to fire un-needed airbags.
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