Front Impacts
The front airbags, the front seat belt pre-tensioners and BST are deployed in the event of a frontal impact of sufficient severity that exceeds the MRS control unit impact trigger threshold.
When the accelerometer and safing sensor in the MRS control unit senses the impact, the control unit triggers the front airbag modules by firing an igniter:
- Driver's front airbag - the igniter in turn burns tablets of sodium azide, which generate a large amount of nitrogen gas causing airbag inflation.
- Passenger front airbag - the igniter causes pressurized gas to be released from integral gas canisters for airbag inflation.
The front airbags are fully inflated as the occupants move into contact with the airbag. After full inflation has been reached, the airbags then immediately discharge the gas from vent holes to provide progressive deceleration for the moving occupant and thus reduce the risk of injuries caused by the force of the airbag acting against the occupant.
The control unit simultaneously triggers the front seat belt pre-tensioner operation. An igniter in the seat mounted buckle assembly causes propellant to burn producing nitrogen gas.
The expanding nitrogen gas drives a piston along the piston tube, the rear of the piston is attached to a steel cable, and the other end of the steel cable is connected to the seat belt buckle.
The tension produced through the steel cable under the influence of the moving piston causes the buckle to be drawn down towards the buckle anchorage. The resulting shortened seat buckle stalk removes any slack in the seat belt in time to restrain the seat occupant during impact and airbag deployment.
The seat belt pre-tensioners have a lower activation threshold than the front airbags, therefore the pre-tensioner will deploy earlier. Depending on the severity of the crash it is possible that only the seat belt pre-tensioner will deploy and the front airbags will not (minor crashes).
The MRS control unit is able to distinguish between rough road conditions and a frontal collision. If the control unit's main sensor detects a frontal collision of sufficient severity and it is confirmed by the safing sensor, the control unit sends a fire signal to the relevant airbag module and seat belt pre-tensioner initiators.