Air Bag Resistance In Passenger Car

Air Bag Resistance in Passenger Car

Chapter-1

INTRODUCTION Car occupants protection is number one goal in any car development Current automobiles are equipped with an 'active' restraint system of lap and shoulder belts. A system is termed as passive which automatically protects the occupant. One of the most advanced system is the 'Air Cushion' system commonly called the "Air Bag". In accidents, occupant can get in contact with the interior of the car. By putting an Airbag a direct contact can be avoided. The large area and low contact pressure (about 8 bar) of "Air Bag" results in low contact forces occurs avoiding greater head rotation and neck forces are lowered. It is seen that a "Three point Passenger safety belt in combination with 'Air Bag' system gives the best protection that is obtainable.

This paper gives general information on: a) Nature and value of air bag devices. b) Air bag development procedure. c) Effectiveness of air bag system. d) Functions of air bag components. e) Gas generating compositions. f) Advantages and dis-advantages of 'Air Bag' system

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Air Bag Resistance in Passenger Car

Chapter- 2

SYSTEM FUNCTIONING 'Development threshold' and 'BEV are the two terms which are used in conjunction with the Air bag development. The level of frontal crash is called development threshold. Where as BEV is the 'Barrier Equivalent Velocity'. These two are already set. There are generally three deployment zones, they are: i) Deployment - The gas fills in the air bag and the air bag deploy when it is required. ii) No Deployment - The gas does not fill in the air bag as the 'development threshold' is less than that which is set. iii) May deploy - This is the zone of the development threshold in which it is not certain whether the bag will deploy or not. The air bag system consists of a following main devices. 1) Sensors 2) Diagnostic Unit. 3) Coil/ Slip rings. 4) Initiating devices. 5) Gas generator. 6) Bag. 7) Cover.

8) Warning lamp.

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Air Bag Resistance in Passenger Car

The air bag development procedure can be written in brief as follows. 1) Sensor picks up the impact energy and give it to an initiating device. 2) Initiating device operates booster composition. 3) Gas generating composition gets activated by the booster composition and produces non toxic gas that fills the air bag. When following the crash signal path, we first come to the sensors that registers the acceleration time history of an accident. The diagnostic or control unit has the task of checking entire air gas system during operation of the car on one hand and of triggering the air bags. The trigger in signal is transferred to airbag modules by an a signal transmission device i.e. slip rings or contact coil for the driver side air bag and a wiring harness for the passenger side system. The airbag models consist of a gas generator, bags, mounting plates and covers. Depending upon the sensor systems, there are two basic configurations of air bag system as shown in fig. Air bag system with central sensor or with external sensor, electromechanical systems are use for external sensors. While electronical systems for central sensors.

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Air Bag Resistance in Passenger Car

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Air Bag Resistance in Passenger Car

Driver side EUROBAG system

Parts of a passenger side EUROBAG system

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Air Bag Resistance in Passenger Car

Chapter- 3

WORKING OF AIR BAG COMPONENTS 3.1) SENSOR SYSTEM:

The sensor system must detect the driving situation and give the signal to activate air bags. 3.1.1) ALL MECHANICAL SENSORS (A.M.S.)

These sensors are mounted directly on gas generators allowing to ignite the initiator mechanically by releasing firing pin. One of the most accepted system is the "ball in tube" developed by Breed. No electrical energy wiring harness is needed. They are located in the steering wheel or dashboard, far away the first contact.

3.1.2.) ELECTRO-MECHANICAL SENSORS (E.M.S.)

It consists of a mass held by a spring or magnet during crash the mass moves against the spring or the and closes a contact in the EMS system and activates the initiating device.

3.1.3.) ELECTRONIC SENSORS

These sensors provide an electrical signal which is in fixed relation to the acceleration of car. These sensors use piezoelectrical capacitive or strain gauge effects to measure acceleration.

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Air Bag Resistance in Passenger Car

The future trend goes to Central (electronical) Sensor system because of less wiring installation expenditure, less number of parts, less weight and hence lower cost. Additionally with low algorithms, it is possible to detect low speed and /or angled or offset crash situations also in the central position in the passenger compartment.

3.2) DIAGNOSTIC AND CONTROL UNITS

These units are used in all electrical airbag systems. They are the links between the sensors and the airbag modules.

3.2.1) DIAGNOSTIC UNIT

These units are used in the system which have external crash sensors. Their main task is to check the entire airbag system during car operation. If failures are indicated, the unit signals the driver that he has to go to a repair shop. The diagnostic unit often has a memory to store the faults of the systems. It also has a non - erasable memory to store the data about the functioning of the airbag system during crash. Some systems have a time delay electronic to trigger the second gas generator at the passenger side, if a dual gas generating model is used. Most systems have voltage constancy and an energy reserve device to make it sure that the system will work even if the battery or some wirings are destroyed. They also have a safety or alarming sensor.

3.2.2.) CONTROL UNITS

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Air Bag Resistance in Passenger Car

These are the main components of an air bag system with central (electronic sensor). The sensor signal is an input information to an algorithm that calculates the triggering time for the system. In most of the cases, the analog sensor signal has to be converted into a digital signal by an A-D converter. Some control units use parallel computers to calculate the triggering signals. All additional features that are found in the diagnostic unit are also parts of most control units as safety sensors, energy reserve, time delay electronic, memories, voltage constancy and so on. These units are important because the bag should not delay under severe conditions and minor impact should not activate the system.

3.3) SLIP RING/COIL

Since the gas generator and air-bag are stored in the steering wheel, it is important to have facility to transfer through this area Naturally, reliable, high quality permanent contact is required. Slip rings are used where entire system is integrated into the steering wheel and a simple power connections are required when the sensor diagnostic system is remote and a hard wire connection is required.

3.4) INITIATING DEVICES

In EMS the initiating device consists of two electrodes insulated from the another and connected by a bridge wire which is embedded in layers of

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pyrotechnic

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Air Bag Resistance in Passenger Car

composition. When activated, it gives a flash (heat) of sufficient intensity to set off the booster composition. In AMS the initiating devices uses a fixing pin which is designed to fire at certain energy. 3.5) GAS GENERATORS

Gas generators have the task to produce gas to fill the airbag in sufficient time. In a frontal crash of about 50 Kmph the bag filling time has to be about 3 minutes. Generally pyrotechnic gas generators are used to produce such an amount of gas in a short time.

3.5.1) GAS GENERATORS FOR DRIVER AND PASSENGERS

The bags at the driver side have volumes between 50 lit to 80 lit. The gas is produced by burning a pyrotechnic propellant which generally consists of sodium oxide. This material is ignited by an initiator which is activated by either electrical or mechanical or mechanical way. The propellant at a very high temperature (about 1500 0c) should be cooled by filtering device which also holds back some burning products. The mass of the driver inflator should be as low as possible. Hence aluminium is generally used. Gas generators for passenger side has an air-bag ranging between 100 lit. to 240 lit. 3.5.2) HYBRID GAS GENERATOR

Most propellants for gas generators are not easy to recycle. Today, the common possibility of disposal it to ignite inflators. The idea in this is to substitute most of the propellant by pressed gas such inflators are called hybrid gas generators. 3.5.3) AUGMENT-GAS GENERATOR C.O.E. & T.,Akola

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Air Bag Resistance in Passenger Car

They use argon at about 200 bar. An electrically activated ignitor drives a hollow pin forward to break a burst disc to let the compressed Argon to flow. The pin travels further and ignites the propellant which heats the Argon, this reduces icing of nozzles and increasing gas volume. All these generators with compressed gas need pressure switch to assure the pressure over the entire lifetime of car. Compared with the standard gas generators hybrid gas generators have a lot of tow environmental risk, low recycling problems, avoiding of misuse, no smoke, low temperature of the gas and the inflator.

3.6) AIR BAG The bag itself is normally constructed from nylon. It is usually coated or may be silicon impregnated. In some cases, the bag is woven in a single piece thus avoiding the problem of stitching.

3.7) COVER The cover is usually an injection moulded unit, frequently double skin. This cover is not only the lid which conceals the system but also as integral part of the steering wheel. The cover is required to open when the bag is deploying.

3.8)WARNING LAMP This is in most cases usually a LED which is incorporated into the system, steering wheel or instrument panel. It is connected to the diagnostic electronics with the function of providing continuous monitoring of the state or readiness of system.

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Air Bag Resistance in Passenger Car

Chapter- 4

GAS GENERATING COMPOSITIONS 4.1) BOOSTER It is a mixture of boron and potassium nitrate which is a proven first composition. It has a high calorific value (1800 cal/gm) and high temperature stability (ignition temperature > 4500 c) It is kept in sealed cup in the system. 4.2) GAS GENERATING COMPOSITION Earlier used compositions were giving, CO2 which is abandoned due to possibility of formation of CO, as it is toxic, All propellants are based on the use of alkali metal oxides as oxidants. Other additives are added to slag out the product of reaction. Sodium and potassium azides are there, but sodium axide is the preferred source N2. Oxidisers suggested are potassium chlorate, perchlorates of sodium, Potassium and ammonium, nitrates of sodium, potassium and calcium Oxides of Mn, Fe, Ni, Cu and Copper chromite. Among the deslagging agents, fumed silicas of different densities and clays like kaoline and Bantonite have been suggested. Propellant is housed in a sealed

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Air Bag Resistance in Passenger Car

container and is surrounded by carefully designed combinations of filters to withhold the slag formed.

Chapter- 5

TESTING 5.1) TANK TEST In this type the explosive are arranged in a train as in completed airbag, with a steel tank substituting fabric bag. it has probes to measure the tank substituting fabric bag. It has probes to measure the tank pressure, exit gas pressure and temperature. It also has ports for sampling the generated gas. Data from the test provides pressure time profile and ( dp/dt) at different time intervals.

5.2) BAG TESTING In this type the explosive are arranged in a train as in completed airbag with a steel tank substituting fabric bag used in real system. It gives information on bag development bag pressure bag integrity.

5.3) SLED TEST This test is exactly simulates a crash using airbag systems and dummy in place of a driver, passenger it can simulate crash conditions upto 30 C angle on either C.O.E. & T.,Akola

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Air Bag Resistance in Passenger Car

side using vehicle made up of interior of the car but with mechanical changes to enable it to use it in multiple tests and ability to accelerate it as in collision it gives p-t profile. This data coupled with the tank test and mathematical modeling studies forms the basis of designing airbag system.

Driver with serial Belt system Serial steering wheel with air bag

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Serial Steering wheel

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Air Bag Resistance in Passenger Car

A steering wheel airbag where loads act on the upper body but not on the lower extremity The combination of lap belt and airbag giving polvic and upper body restraint The energy absorbing steering system and knee bolster and passenger restrained by C.O.E. & T.,Akola

A lap belt in the rear seat A lap-shoulder belt in the right-front seating position.

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Air Bag Resistance in Passenger Car

Chapter- 6

EFFECTIVENESS OF AIRBAG SYSTEMS The general hypothesis in the consideration of crash surviability is the survival prospects are highest when two conditions are met. * That the occupant has the closest possible coupling to vehicle shell. * That the striking surface within the passenger compartment. In first case close coupling is ensured by the use of seat belts which ensure close coupling to the occupants seat and also avoids chest injuries. In the second case, the main problem is striking of head This is usually on the steering wheel in case of the driver. Such striking gives rise to unacceptably high HIC (Head Injury Criteria ) values in the crash testing Facial injuries even of are not threatening to life present a major physiological problem to victim. The incident of the head injury can be avoided by the use of an airbag. The inflated bag cushions the head. The forward movements is arrested in a controlled manner thus avoiding striking injury. The back absorbs the energy and in this way any bounce back with resultant neck injury is avoided. Thus airbag meets the requirement which are acceptable by the end user.

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Air Bag Resistance in Passenger Car

Chapter- 7

FAULTS IN THE AIRBAG SYSTEM No restraint system including air-bags can prevent all injuries in every crash. Some crashes are too severe that safe devices cannot effectively prevent a fatal injury. Also sometimes it is possible that the car occupant wants the bag to deploy and the bag doesnot deploy due to high development threshold. In some cases the occupant do not want the airbag to deploy and the deployment of the airbag occurs in what they judged to be a minor crash. In fact relative minor injuries such as abrasions from contact with the deploying airbag can occur to a properly scented occupant even in a crash of moderate severity, Also some smoke ( visible particular matter of powder) is produced in all the airbag developments- like minor side effect of the countermeasure. Some abrasions which are 'rug burns' from the airbag fabric abrasions on the skin may be there. The powder residue may be slight irritating to skin. It is toxic if

it

contains corn starch or talcum powder as lubricant and sodium compounds like baking soda accompanied by minute deposits of sodium hydroxide. The powder residue cause slight irritation but poses no long term health hazard. C.O.E. & T.,Akola

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Air Bag Resistance in Passenger Car

If you are not seated in a normal riding position with your back against seat back, the air bag may not protect you properly and may possibly hurt you as it inflates.

Chapter - 8 FUTURE TRENDS IN AIRBAGS 1)

Development of a non azide propellant replacing the toxic sodium azide.

2)

Reducing the size of airbag system by combination with automatic restraint belt

which is explained later. 3)

Engineering development to cater to impact form the side and the behind portion.

4)

Reduction of smoke and leakage from airbags.

5)

Reduction in the weight of the airbag system.

6)

Reduction in the volume of air bag.

8.1) EUROBAG SYSTEMS. The head contact of the occupant is possible in very severe accident to avoid that the contact an air bag system is best solution. Due to the seat belt usage, the lower part of the body is well protected and the airbags are necessary only to protect the head from contact with that knowledge smaller airbags can be used. Where the airbags has to absorb entire energy of the occupants upper body. These are called facebages or headbags.

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Air Bag Resistance in Passenger Car

In principle Eurobags can be downsized passive systems with big volume of 35 lit to 40 lit. The remaining components are same .

Chapter- 9

CONCLUSION Airbag systems are found today in many cars in the upper price category specially in U.S.A. Airbags are found as the standard equipment in many cars. The number of cars with airbags is steadily growing and therefore the prices will decrease and will soon be found in low priced cars. With the development of so called Eurobags the airbag has got a new quality. The observations show that the safety and air-bags can provide the greatest overall reduction in the risk of injury and death in traffic erases but only if these devices are use properly. While the usage of the airbag is continuously increasing there is obviously room for the further progress. But the critical need to promote the proper use of these devices continues.

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Air Bag Resistance in Passenger Car

Chapter-10

REFERENCES 1. V.E.WEZEL - Airbags Systems - Actual Status and Outlook for the Future Safety TheVehicle and The Road Vol.2, FISITA 92 FISITA No. 925217, page no. 1 2. T.D. McMaster Airbags a Survival Strategy International Conference on worldclass manufacturing technologies, Des. 1993, Session 3, page No.l 3. Auto India ( Magazine, July, 1991) 4. Joseph C. Marsh - Supplement Airbag Restraint Systems, Consumer Education and Experience'. Frontal impact protection seat belts and airbags, 1993, SAE - SP - 947 Page No. 95 5. David C.Viano - Effectiveness of Safety Belts and Airbags in preventing Fatal Injury, Frontal Crash Technologies of 90 's Feb 1991 SAE SP 852 page no 159 6. H.A. Lupker and W.E.M.Bruijis - Gas jet Model for Airbag inflator's Frontal impact Protection Seat and airbag's March 1993 SAE SP 947, SAE 930645, Page No. 85.

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