Thursday, 27 July 2017

Hot Bags of Gas, For Safety of Course

No Comments

Despite the fact that many people die in automobile accidents every year, the modern auto is much safer now than it has ever been. In a study released by the U.S. Department of Transportation, the last few years have seen some of the lowest death rates on U.S. highways since Harry Truman was in office. The Department of Transportation first began to compile such data in 1949 and the number of deaths per 100 million miles traveled is far lower now than it was back then, and it’s even much lower now than it was 20 years ago. These stats are especially significant if you consider that every year Americans travel more and more miles, in more and more vehicles.

The reasons for this good news are varied. People pay more attention to safety than they used to. Seat belt use is the norm. The parent who fails to buckle their kid into a car seat is viewed as neglectful, and kids are no longer the seat surfers that they once were. People think about things such as distracted driving, drunk driving, and drowsy driving much more than they used to. Perhaps the biggest reason for this decline in traffic deaths is the abundant technology that is now incorporated into the modern automobile for the purpose of making them safe.

Consider the following video. This was produced by the Insurance Institute for Highway Safety. They are the people who do all of the crash testing and safety scoring of each vehicle sold in the U.S. In this video they run a 1959 Chevrolet Bel Air into a 2009 Chevrolet Malibu. The results are startling.

A popular misconception is that the old vehicles were safe because they were heavy, and solid, and made of steel, not plastic. This is not true as this crash test clearly shows. People who used to get in a wreck while riding in an old car used to get decapitated on the steering wheel, and get their bodies smashed to bits slamming into a steel dash.

Modern vehicles have things like crumple zones. These are areas of the car body and chassis that are meant to absorb the impact instead of passing it through the vehicle into the passenger compartment. New cars are built so that if a head-on collision is serious enough to break the engine loose from its mounts, it will actually dive under the vehicle instead of ending up in the laps of the front seat passengers.

Modern cars have all sorts of electronic safety measures built-in. Anti-lock brakes allow the driver to steer under maximum braking. Anti-skid control is now so advanced that it can take accelerator and brake control away from the driver in order to counteract improper steering inputs away from the driver, and get the vehicle going straight down the road again. Cars are even smart enough to tell if you are wandering out of your lane. They can also see a sudden stop of the vehicle in front of you and begin to apply the brakes before you do.

srs airbag

SRS stands for supplemental restraint system.

All of these things are passive; they require no action on the part of the driver and will just do their thing whenever necessary. Of all of the passive safety systems, the one that is perhaps the most controversial, but maybe the most effective, is the supplemental restraint system (SRS). This is the system that controls the airbags. The controversy centers on the fact that an airbag going off in your face is a scary thought. Many people have been in a wreck, had an airbag deploy in there face, and came away with a bloody or broken nose, or minor burns and abrasions. This is nothing compared to a fractured skull and massive concussion from hitting the dash. Recently, there has been a lot of controversy around airbags made by the Takata company for defects. Anything defective on a car is a problem and these defective airbags should not detract from the fact that airbags save lives. In a few incidents, children have been hurt by airbags, but in every case the child was in the front seat, not restrained at all, or sitting in rear facing child seat. These are sadly the fault of the adult driving the car, and in some cases the child probably would have been launched through the windshield anyway.

So what makes the airbag work? The controls and actuators essentially boil down to a computer controlled pyrotechnic device that ignites in any collision over a given speed. The burning propellant inflates a bag with nitrogen gas. This small explosion is hot and powerful and very fast. Some types of airbags will use a pyrotechnic charge to release a canister of highly compressed gas to inflate the airbag. The typical inflation time for an airbag is about 30 milliseconds (1000 milliseconds in one second). In less than a tenth of a second, the airbag deploys and is already beginning to deflate. White smoke can usually be seen after airbag deployment, this smoke is essentially sodium carbonate which is totally harmless. This powder protects the airbag while it sits packed into the assembly over the years.

Generally, the airbag must be triggered by two different sensors in order to deploy. Typically these sensors are mounted in the front corners of the vehicle to deploy the driver and passenger airbag, and in the B pillar to deploy side, and side curtain airbags. The SRS control unit which is usually mounted in the center of the vehicle near the floor also may contain an accelerometer type sensor that can pick up the rapid deceleration which occurs with a major impact.

Besides the driver and passenger airbag, which became mandatory starting in the 1994 model year, vehicles now must also have side airbags that come out of the seats or the doors to protect occupants in the event of a side impact. New cars also have side curtain air bags which pop out of the roof liner to protect the occupants head and shoulders from slamming into the windows, doors, and side pillars. Some vehicles also have knee airbags that come out of the lower part of the dash, center airbags for rear seat passengers, or even shoulder strap airbags that come out of the seat belt itself.

airbag light

The airbag light comes on and stays on when there is a problem with the airbag system.

SRS systems on the most modern cars are more intelligent that the airbags that were mandated back in the mid 90’s. Before an airbag will deploy, it will look at things such as the position of the seat to determine how the airbag should be deployed. If the driver is sitting very close to the steering wheel like your grandma does, then it will deploy the airbag with less force. If there is a short person sitting the passenger seat it will deploy the airbag with less force, because it can sense how tall the person is. If a small child is sitting in the passenger seat and seat belt is buckled it will deploy the air bag with less force. If there is a child in the passenger seat and the seat belt is not buckled it will automatically turn off the airbag.

This system that is so intelligent is known as the occupant classification system (OCS). The system uses strain gauges built into the seat mounts, or something similar, to weigh the passenger. It also uses sensors on the seat track to see how close the seat is to the dash. The system may also incorporate special antennae in the seat cushion and seat back to classify the height of the passenger, and determine the position of the passenger in the seat. If the passenger is leaning against the window taking a snooze the system will see this and it will disable the side impact airbag. Indicator lights on the dash will tell the occupants of the vehicle which airbags might be disabled in certain situations. Pickup trucks have an off switch on the dash that allows the passenger airbag to be completely turned off. This allows kids to be buckled safely into the passenger seat and it also allows the use a rear facing child seats since many pick ups do not have a back seat.

Devices known as seat belt pretensioners are also controlled by the airbag system. These devices are found in the shoulder strap retractors or in the seat belt buckles. They are there to tighten the seat belts against the occupants to more firmly restrain them in their seats. These pretensioners use a much smaller explosive charge to accomplish this task very quickly.

The SRS system is continuously monitored by the computer that controls everything. When there is a failure or break-down in the system that might cause the airbags to fail to deploy, the control unit will illuminate the airbag or SRS light. This alerts the driver that the system has a fault and ought to be checked out by a qualified technician. The technician will be able to interface with the SRS control unit using a scan tool and find out what the problem might be.

Cars are probably not as safe as they can be, but they are much safer than what the used to be. Supplemental restraint systems are a large part of the reason for the increased safety. Despite the controversy, airbags have proven to be safe when they are used correctly. The future of auto safety is hard to predict. The devices of the future are likely just things that we haven’t thought of yet. Considering the downward trend of deaths from traffic accidents, and that cars are likely to only get safer, someday traffic fatalities might become a thing of the past.

Leave a Reply

Your email address will not be published. Required fields are marked *

Latest in Alternative Fuels

Latest in How’s that Work

Latest in Outrageous Opinion