Gas producer for filling a gas cushion restraining device

A gas producer is disclosed for filling a gas cushion restraining device for occupants of vehicles. The gas producer includes a combustion chamber filled with fuel and surrounded by a wall which exhibits apertures for the discharge of the gas produced, said apertures being sealed hermetically by a destructible cover at the start of the burning process of the fuel up to a specific pressure rise in the combustion chamber. In order to obtain a planned pressure rise irrespective of the initial temperature of the gas producer, it is provided according to the invention that the total cross-sectional area of the discharge apertures is automatically enlargeable as a function of the rising combustion chamber pressure.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to a gas producer for filling a gas cushion 
restraining device for occupants of vehicles, having a combustion chamber 
filled with fuel and surrounded by a wall which exhibits apertures for the 
discharge of the gas produced, said apertures being sealed hermetically by 
a destructible cover at the start of the burning process of the fuel up to 
a specific pressure rise in the combustion chamber. 
The efficiency of such gas producers is dependent on temperature, so that 
the gas cushion is inflated more rapidly at high temperatures of the fuel 
than at low temperatures. The reason for this is the dependence of the 
burning behaviour of the fuel upon temperature for a constant 
cross-sectional area of the nozzle of the combustion chamber. However, the 
strengths of the entire system, comprising producer, gas cushion, 
retaining means and, for example, steering wheel have to be dimensioned 
for the maximum efficiency of the producer at high ambient temperatures 
for safety reasons. 
This dependence of the burning behaviour of the producer upon the 
temperature is also disadvantageous in another respect. Only a certain 
time, of 30 msec for example, is available for the filling of the gas 
cushion. This time is also required in full by a producer, the temperature 
of which is -30.degree. C. for example at the time of tripping or 
detonation. However, if the producer temperature at the time of the 
accident is +80.degree. C. for example, then the gas cushion will have 
been filled after only 24 msec, for example, which results in an 
unnecessary increase in the acoustic pressure with its negative effects 
upon the vehicle occupants. 
It is therefore an object of the present invention to avoid these described 
disadvantages and to develop a gas producer so that a more uniform 
pressure curve is obtained irrespective of the inherent temperature of the 
producer at the time of tripping. 
This object is achieved according to the invention in a generic gas 
producer, in that the total cross-sectional area of the discharge 
apertures is automatically enlargeable as a function of the rising 
combustion chamber pressure.

DETAILED DESCRIPTION OF THE DRAWINGS 
The gas producer illustrated partially in FIG. 1 of the drawing will be 
explained here only insofar as this is necessary for an understanding of 
the present invention. 
The gas producer exhibits a combustion chamber 2 accommodating the fuel 1, 
which is connected through apertures 3 to a diffuser chamber 4 radially 
externally adjacent thereto, which in turn communicates through apertures 
5 with the interior of the air bag. In the design assembled state the 
apertures 3 between combustion chamber 2 and diffuser chamber 4 are sealed 
hermetically by a cover 7, in the form of a metal foil, for example, 
destructible upon corresponding pressure evolution, and placed against the 
wall 6 on the inside. 
In the exemplary embodiment of the invention according to FIG. 2 apertures 
3 of equal size are provided in the wall 6. The tear-open cover 7 located 
behind in the drawing exhibits in the region of each of the apertures 3 a 
readily destructible central region 8, starting from which radial breaking 
lines 9 extend which tear open only at relatively higher pressure. 
In the exemplary embodiment illustrated in FIG. 3, apertures 3a, 3b, 3c of 
different sizes are illustrated, which are cleared in the sequence of 
their size by the destruction of a customary foil cover 11 located behind 
them. If desired the tearing strength of the foil cover can be varied in 
the region of different bore sizes by varying their effective wall 
thickness. 
Lastly, FIG. 4 shows an embodiment of the invention in which a fuse 10, 
which is inserted into an aperture 3d, burns due to the hot gases flowing 
through and thereby enlarges the aperture 3d continuously in conformity 
with the pressure rise in the combustion chamber 2. 
In conclusion, the background of the invention will be explained again in 
summary form. 
As already explained, the burning speed of the fuel is a function of its 
initial temperature with corresponding pressure in the combustion chamber. 
Therefore if the cross-sectional area of the discharge apertures is 
dimensioned for a mean fuel temperature of +20 C. for example, very rapid 
burning occurs at this ambient temperature, and does not exhaust the 
maximum permissible time of, for example, 30 msec. The maximum, 
permissible time would be exhausted, for example, at an initial 
temperature of the fuel of -30.degree. C., whereas at an initial 
temperature of +60.degree. C., for example, an extremely rapid buring of 
the fuel and hence filling of the air bag would occur, which would result 
in a highly undesirable rapid pressure rise in the passenger compartment 
too. 
These differences in the burning time can be largely compensated by the 
invention. 
Thus at -30.degree. C. a relatively low initial pressure would exist in the 
combustion chamber, which causes the opening of only a relatively small 
cross-sectional area of the discharge apertures. This in turn causes a 
further pressure build-up in the combustion chamber and hence an increase 
in the burning speed. 
At +60.degree. C. on the other hand a high initial pressure in the 
combustion chamber will be established very rapidly, leading to the 
opening of the greatest possible discharge area, whereby a further 
pressure rise in the combustion chamber is at least flattened and the 
burning speed thereby reduced. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only, and is not to be taken by way of 
limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.