Increasing road safety with anti-submarining airbags

The designers of modern cars are focusing more and more on road safety. Accidents are simulated again and again with different dummies, exactly modeled on different people, in order to develop improvements to the vehicles. This increases the safety of driving for each individual. Different accident situations require differentiated safety precautions in order to minimize the number of injuries and fatalities in road traffic. Airbags were created in this course a few years ago. At first only in the front area by the driver, later also by the front passenger, they are intended to protect the occupants in the event of a rear-end collision. The airbag is part of the occupant restraint system in the vehicle and, in the event of an accident, opens within 20 to 50 milliseconds with a bang between vehicle interior parts and occupants. Somewhat later, other airbags were developed to protect the head and sides of the vehicle. Nowadays, in every new car you will find at least the two front airbags, in mid-range cars also side airbags in addition and in the higher class of course all types of airbags, whereby, depending on the angle of the collision and vehicle deceleration, only certain airbags open. The airbag serves as additional safety, but does not replace the safety belt. According to statistics, the airbags increase the chance of survival of the belted occupants by 30%.

New development in the history of airbags

One of the latest developments in airbags are the seat cushion airbags or anti-submarining airbags. They are located on the front seats in the front area and prevent slipping through the seat belt, thus eliminating the risk of sliding down into the front of the vehicle. This new airbag was developed by Renault in 2002 and is made of metal, which is mounted in the seat frame under the seat of the driver or front passenger. This is the world's first airbag that does not come into direct contact with the occupants as it is integrated into the steel frame. 0.3 mm thick sheets are welded together to form a closed container. Presented for the first time as a world first in the new Megane, it functions in the same way as conventional other airbags. In the event of a collision, two stages are activated simultaneously. The metal airbag is inflated with the help of a gas generator, which then presses against the seat cushion, this forms itself according to the pool impression. An exit port provides pressure equalization inside the vehicle. A pressure against the seat belt is created and the forces on the lumbar region and abdomen that occur in an accident are reduced. Pelvic movement and stretching is also reduced. First developed for the five-door Megane, a system was quickly released for the three-door Laguna. The third generation programmed restraint system, also called PRS III, was thus optimized, because the lateral, double belt tensioner on the outside of the seat only made it unnecessarily difficult for the rear occupants to get into the seat. This belt system is only suitable for the five-door models.

Anti-submarining ramps vs. Anti-submarining airbags

Anti-submarining means "do not slide forward", and that is how the security features are designed. In the event of a crash, the seat belt prevents the occupant from slipping through, because the occupant is often pressed down into the seat and thus slips through the lap belt. Serious injuries, mostly in the pelvic area, can be the result. For this reason, most car manufacturers have ramps installed in the seat to prevent it from being pushed down into the seat in the event of an accident. Only the active anti-submarining system uses an airbag to prevent slippage and ensure better positioning of the occupant. This system is especially good when the driver or passenger constantly change and do not always adjust the seat belt to fit the body. Due to the difference in body length, most passengers are too short for the standard seat belt, which is set rather high, and slip through at the bottom. Also, in the event of an accident, the different body weight is positioned differently, for this reason the active anti-submarining system is optimal.

What happens in the event of an accident?

The inertia of the mass ensures that the parts not connected to the vehicle move in the direction of travel in the event of a rear-end collision. In the event of a collision or heavy braking "flying" these parts forward. Steering wheel, dashboard or windshield slow down this flight and convert the resulting energy into deformation energy. The three-point restraint belts also slow down movement and various airbag systems dissipate energy so that the risk of injury is minimized. Contact points and material are also chosen with this in mind. During a collision, such enormous forces are generated that modern one- or two-stage belt tensioning systems with belt force limiters, in an interplay of restraint force and a type of slipping clutch, achieve optimum efficiency without additional risk of injury. Injuries in the knee, abdominal or chest area are, however, repeatedly found, despite belt tensioners and optimal belt height, which is why anti-submarining ramps are installed for the constructive design of the seat frame or anti-submarining airbags to deform the seat and thus adapt it to the body position. In the course of weight reduction and the new CO2 requirements, a container made of a textile fabric has now been produced instead of the metal one, thus achieving a weight reduction of approx. 60 %. Whether an active or a passive anti-submarining system is installed is up to the designers of the various models. The only important thing is to improve road safety and limit the risk of injury as much as possible.

Various airbags

Different airbags provide more safety on the road, which means fewer bad accidents. For many decades, only the three-point safety belts served as safety against serious injuries as a result of accidents, until the experiments with dummies showed the first successes of the airbags. Since then, development has continued and there are now models equipped with a complete safety package. These include:

• Front airbags for driver and front passenger – located in the center of the steering wheel and above the glove box. For front seat passengers, the front airbag can often be manually or automatically deactivated due to rear-facing baby seats.
• Side airbag in the door – protects the thorax, but is no longer installed
• Side airbags integrated in the front seat – different versions protect the thorax, head as well as pelvis. These airbags for the driver's and front passenger's seats are mostly standard in new vehicles and consist of one or two chambers.
• Side airbags for rear occupants – same as side airbags in front seats, but only available as optional equipment
• Knee airbags – usually installed only for the driver, they prevent the knees from hitting the dashboard, because that is where they are placed
• Rear seat airbag – mass-produced airbag in the large center console that inflates between both front seats in Japan
• Curtain airbag – this covers the entire window front on the side and thus prevents injuries to the head on the B-pillar, window front or objects there. It also prevents injury to the cervical spine due to overstretching. Because they remain inflated for several seconds, they are also optimal in rollovers and multiple collisions. Installed above the roof or in the door panel of convertibles.
• Head airbags – integrated into the seat, they protect the head and, in some cases, the thorax as well
• Anti-submarining airbags – prevent slipping through the 3-point belt by raising the seat. Firmly integrated in the linkage of the seat, a metal chamber is inflated and deforms the seat in the front area, the occupant does not come into contact with this seat cushion airbag.
• Rear airbag – is installed in the roof of the rear windows and the triggered curtain protects the rear passengers between the rear window and headrests, ideal protection in the event of rear impact
• Seatbelt airbag – the complete 3-point seatbelt is inflatable and prevents the pressure on the chest in the event of a collision, especially suitable for the belts for child seats
• Pedestrian airbag – this airbag deploys externally over the radiator, A-pillar and the lower part of the windshield and prevents severe injuries in the event of a collision with pedestrians.

How the airbags work

The airbag system consisting of airbag module, crash sensor and control unit is always triggered centrally by the airbag control unit. The module, which contains the gas generator and the airbag bag, is controlled by the crash sensor, which is usually located in the control unit. Often satellite sensors are also distributed in the vehicle, which work with acceleration sensors. If these sensors (usually two independent ones are installed for safety) report a delay, the airbags are triggered independently of each other. In addition to these sensors, seat occupancy sensors for the rear and front passenger seats, seat belt contact switches and position sensors are fitted to the driver's seat. Thus, data is constantly compared with the control unit and then decides on the activation of the various airbags. Internal capacitors ensure continued operation even without a power source. Two different generators provide for the inflation of the nylon bag. On the one hand, the hybrid gas generator, which creates a connection between pressure accumulator with gas preloaded up to 250 bar and pyrotechnic generators, whereby the gas flows in the form of a pyrotechnic charge into the curtain airbags, because this system is used for this purpose. Secondly, the gas generator, which activates the ignition of the solid propellant with the help of solid propellant and ignition unit by controlling with current pulse. The fuel is usually in tablet form and, after ignition, flows through a grid from the gas generator into the airbag. The ca. 1350°C hot gas cools down by expansion to approx. 150°C in the nylon bag.

The effectiveness of the anti-submarining system has been proven by numerous evaluations of accident injuries and crash tests, which is why it can only be advantageous to install it in all current new passenger cars. Implementation is also possible from an energy-saving point of view, because the passive system does not significantly increase the weight of the seat frame, and the active system still allows a reasonable increase in weight. The conversion of an older vehicle is possible with both systems, because only the old seats must be replaced with new seats that have one of the two systems integrated. The anti-submarining system is useful not only for the driver's seat, but also for the front passenger's seat, because the large footwell gives the passenger plenty of room to slide through. Such seats are offered by many manufacturers as special accessories. However, in the case of the active system, the cost must not be forgotten, because the seat must be renewed after an accident. It would be good to have a system that could do without seat renewal to reduce the cost of repairing the vehicle. In addition, the sensors used to trigger the various airbags need to be constantly improved to avoid malfunctions, which every now and then lead to recalls in the various models. Advancing technologies and the time factor play a major role in the further development of modern cars, so road safety can be improved more and more.