Imagine you're in your car driving home. The weather isn't particularly bad, the road conditions are fine, and you're singing along with the radio. Then out of nowhere, a car makes an illegal turn, winds up in front of you, and you don't have time to stop. You hear the screech of metal as your car collides with the other, and brace yourself to go flying through the windshield. Instead, you are pushed backward into your seat, held there for a split second, and then the pressure subsides. Your seatbelt was the first line of defense but you are upright in the driver's seat because your car is equipped with a vehicle airbag.
Manufacturers have many different names for their systems:
*Supplementary Restraint System (SRS) *Air Cushion Restraint System (ACRS)
*Supplemental Inflatable Restraint (SIR)
But, they all have the same purpose and operation.
Airbags are supplemental restraints. Supplemental means they help another system, or they are secondary to another system. In this case it is the seat belt. Though later airbags are required to be able to, airbags are not meant to be used without a seat belt; in fact they can be very dangerous or even deadly without a seat belt.Contrary to what a lot of people think air bags are not made to stop you from lunging forward during a crash. In a crash, let’s say 30 mph, the car is moving 30 mph and your body is moving 30 mph along with it, if the car hits a wall and stops, not being part of the car your body is still moving at 30 mph, causing you to lunge forward.
This is the job of the seat belt, to hold you in place.
Have you ever wondered how race car drivers have such horrible crashes and never get hurt?
Let’s look at a race car, with a five point harness the driver is a stationary part of the seat. He can not move forward or backward, there is no slack in the harness, he is actually a part of the car. Then look at the 200 mph crashes they walk away from without a scratch. Why? Because his body stopped at the same time as the car did. Now look at a passenger vehicle: Who wants to be restrained that tight in a car on the city streets, or on a long trip? Our seat belts have some slack in them; therefore not being a stationary part of the vehicle, we are going to lunge forward some what in a crash.
This is where the air bag, or supplement, comes in.
The air bag deploys at 200-300 mph, depending on the manufacture. From the time of impact to the time of full airbag deployment is from 21 to 27 milli-seconds. This means it is already fully deployed before yourbody ever lunges forward.
The idea is for the air bag to be deployed, so fast that it is fully inflated, before your body is thrown forward. Then as you fall into the bag, it should have already started to deflate. The bag then lowers you down at a slower speed and cushions you.
The design is conceptually simple; a central "Airbag control unit"(ACU) (a specific type of ECU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors,wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors. The bag itself and its inflation mechanism is concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors which are designed to "tear open" under the force of the bag inflating. Once the requisite 'threshold' has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag's volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant's body, compared to a seat belt alone.
The signals from the various sensors are fed into the Airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and "curtain" airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.
Front airbags normally do not protect the occupants during side, rear, or rollover accidents.Since airbags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the airbag's benefits and they help restrain occupants during the initial and any following collisions.
In vehicles equipped with a rollover sensing system, accelerometers and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, side-curtain airbags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.
Chemistry Behind Air Bags:
Inside the airbag is a gas generator containing a mixture of NaN3, KNO3, and SiO2. When the car undergoes a head-on collision, a series of three chemical reactions inside the gas generator produce gas (N2) to fill the airbag and convert NaN3, which is highly toxic (The maximum concentration of NaN3 allowed in the workplace is 0.2 mg/m3 air.), to harmless glass (Table 1). Sodium azide (NaN3) can decompose at 300oC to produce sodium metal (Na) and nitrogen gas (N2). The signal from the deceleration sensor ignites the gas-generator mixture by an electrical impulse, creating the high-temperature condition necessary for NaN3 to decompose. The nitrogen gas that is generated then fills the airbag. The purpose of the KNO3 and SiO2 is to remove the sodium metal (which is highly reactive and potentially explosive, as you recall from the Periodic Properties Experiment) by converting it to a harmless material. First, the sodium reacts with potassium nitrate (KNO3) to produce potassium oxide (K2O), sodium oxide (Na2O), and additional N2 gas. The N2 generated in this second reaction also fills the airbag, and the metal oxides react with silicon dioxide (SiO2) in a final reaction to produce silicate glass, which is harmless and stable. (First-period metal oxides, such as Na2O and K2O, are highly reactive, so it would be unsafe to allow them to be the end product of the airbag detonation).
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Published by Ravindra(Mechanical Engineering)
Manufacturers have many different names for their systems:
*Supplementary Restraint System (SRS) *Air Cushion Restraint System (ACRS)
*Supplemental Inflatable Restraint (SIR)
But, they all have the same purpose and operation.
Airbags are supplemental restraints. Supplemental means they help another system, or they are secondary to another system. In this case it is the seat belt. Though later airbags are required to be able to, airbags are not meant to be used without a seat belt; in fact they can be very dangerous or even deadly without a seat belt.Contrary to what a lot of people think air bags are not made to stop you from lunging forward during a crash. In a crash, let’s say 30 mph, the car is moving 30 mph and your body is moving 30 mph along with it, if the car hits a wall and stops, not being part of the car your body is still moving at 30 mph, causing you to lunge forward.
This is the job of the seat belt, to hold you in place.
Have you ever wondered how race car drivers have such horrible crashes and never get hurt?
Let’s look at a race car, with a five point harness the driver is a stationary part of the seat. He can not move forward or backward, there is no slack in the harness, he is actually a part of the car. Then look at the 200 mph crashes they walk away from without a scratch. Why? Because his body stopped at the same time as the car did. Now look at a passenger vehicle: Who wants to be restrained that tight in a car on the city streets, or on a long trip? Our seat belts have some slack in them; therefore not being a stationary part of the vehicle, we are going to lunge forward some what in a crash.
This is where the air bag, or supplement, comes in.
The air bag deploys at 200-300 mph, depending on the manufacture. From the time of impact to the time of full airbag deployment is from 21 to 27 milli-seconds. This means it is already fully deployed before yourbody ever lunges forward.
The idea is for the air bag to be deployed, so fast that it is fully inflated, before your body is thrown forward. Then as you fall into the bag, it should have already started to deflate. The bag then lowers you down at a slower speed and cushions you.
The design is conceptually simple; a central "Airbag control unit"(ACU) (a specific type of ECU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors,wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors. The bag itself and its inflation mechanism is concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors which are designed to "tear open" under the force of the bag inflating. Once the requisite 'threshold' has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag's volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant's body, compared to a seat belt alone.
The signals from the various sensors are fed into the Airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and "curtain" airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.
Front airbags normally do not protect the occupants during side, rear, or rollover accidents.Since airbags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the airbag's benefits and they help restrain occupants during the initial and any following collisions.
In vehicles equipped with a rollover sensing system, accelerometers and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, side-curtain airbags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.
Chemistry Behind Air Bags:
Inside the airbag is a gas generator containing a mixture of NaN3, KNO3, and SiO2. When the car undergoes a head-on collision, a series of three chemical reactions inside the gas generator produce gas (N2) to fill the airbag and convert NaN3, which is highly toxic (The maximum concentration of NaN3 allowed in the workplace is 0.2 mg/m3 air.), to harmless glass (Table 1). Sodium azide (NaN3) can decompose at 300oC to produce sodium metal (Na) and nitrogen gas (N2). The signal from the deceleration sensor ignites the gas-generator mixture by an electrical impulse, creating the high-temperature condition necessary for NaN3 to decompose. The nitrogen gas that is generated then fills the airbag. The purpose of the KNO3 and SiO2 is to remove the sodium metal (which is highly reactive and potentially explosive, as you recall from the Periodic Properties Experiment) by converting it to a harmless material. First, the sodium reacts with potassium nitrate (KNO3) to produce potassium oxide (K2O), sodium oxide (Na2O), and additional N2 gas. The N2 generated in this second reaction also fills the airbag, and the metal oxides react with silicon dioxide (SiO2) in a final reaction to produce silicate glass, which is harmless and stable. (First-period metal oxides, such as Na2O and K2O, are highly reactive, so it would be unsafe to allow them to be the end product of the airbag detonation).
Published by Ravindra(Mechanical Engineering)
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