Pressure sensor for pressure detection in combustion chamber of internal combustion engine

A pressure sensor for pressure detection in a combustion chamber of internal combustion engines of motor vehicles has a housing provided with an opening, a measuring element, a plunger arranged in the opening of the housing and having one end acting on the measuring element so that a measuring signal is produced proportionally to a pressure to be determined, the plunger directly abutting against the measuring element and having a material with a yielding point which is smaller than a breaking limit of a material of the measuring element, the end of the plunger which acts on the measuring element is spherical and abuts against the measuring element with such a contact surface that occurring stresses do not exceed a predetermined nominal value.

BACKGROUND OF THE INVENTION 
The present invention relates to a pressure sensor for pressure detection 
in a combustion chamber of internal combustion engines. 
With the known pressure sensors of this type, a housing is provided with an 
opening in which a plunger is arranged. The opening is closed from one 
side by a diaphragm, and the end of the plunger abuts against the 
diaphragm. The other end of the plunger is in an operative connection with 
a measuring element. Also, a semi-ball is glued to this end of the plunger 
for a pressure transmission. In another embodiment, different elements are 
glued under one another. A glueing of different elements has, however the 
disadvantage that such connections in a mass production are difficult and 
expensive to treat. The connections can change there shape in time. 
Thereby is absolute measuring pressure sensors zero point changes can 
occur. It is especially disadvantageous when these changes occur in 
operation. The temperature resistance of the glueing connections is also 
not optimal. Also, the stamped layers used between the measuring element 
and the plunger are subjected over the lifetime of the pressure sensor to 
creep, effect, which can lead to uncontrolled zero point changes and 
thereby to measuring value errors. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a pressure 
sensor for pressure detection in combustion chambers of internal 
combustion engines, which avoids the disadvantages of the prior art. 
In keeping with these objects and with others which will become apparent 
hereinafter, one feature of the present invention resides, briefly stated 
in a pressure sensor, in which the plunger abuts directly against the 
measuring element, the yielding point or limit of the material of the 
plunger is smaller than the breaking limit of the material of the 
measuring element, the end of the plunger is spherical, and the spherical 
end of the plunger abuts against the measuring element over such a contact 
surface of such a size that the occurring stresses do not exceed a 
predetermined nominal value. 
When the pressure sensor is designed in accordance with the present 
invention, no zero point drift can occur during the service life of the 
pressure sensor. Simultaneously the pressure sensor can be produced in a 
simple and cost-favorable manner. No changes of the connections between 
the individual elements and no reduction of the pretensioning of the 
plunger occur anymore. Due to the utilized elasticity modulus of the 
materials, a high linearity of the measuring curve is provided. Since no 
changes of the geometry of individual elements is possible, a high 
stability of the sensitivity of this measuring curve is guaranteed. 
Furthermore, no dampening layers or elements are provided in the pressure 
sensor so that the measuring curve has a low hysteresis. 
In accordance with further features of the present invention, the 
difference of the roughness of the surface of the end of the plunger and 
the roughness of the surface of the measuring element is smaller than or 
equal to two micrometer, preferably smaller than or equal to one 
micrometer. 
The measuring element can be formed as a silicon chip in which the 
resistors connected in Wheatstone bridge are arranged. 
The plunger can be guided in the opening of the housing. One of the opening 
can be closed with a diaphragm while the other end of the plunger can abut 
against the diaphragm. 
Finally, the contact region of the plunger and the measuring element can be 
provided without a passivating layer covering the resistors. On the other 
hand, in a gap between the surface of the measuring element and the 
surface of the plunger, a gel-like mass can be arranged on the passivating 
layer. 
The novel features which are considered as characteristic for the present 
invention are set forth in particular in the appended claims. The 
invention itself, however, both as to its construction and its method of 
operation, together with additional objects and advantages thereof, will 
be best understood from the following description of specific embodiments 
when read in connection with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A pressure sensor for detecting of a pressure in a combustion chamber of an 
internal combustion engine is identified in FIG. 1 as a whole with 
reference numeral 11. It has a housing 10 provided with a central, 
throughgoing, stepped opening 12. An opening portion 13 of the opening 12 
of the housing, which faces the combustion chamber, is closed with a 
diaphragm 14. The diaphragm 14 is welded to the housing 10 in the region 
of the edge at the end side 15 of the housing. A plunger 18 abuts with its 
one end against a central region of the diaphragm 14, or in other words in 
a bending region of the diaphragm. The other end of the plunger 18 
cooperates with a measuring element 19. The diaphragm 14 is not absolutely 
necessary, and the pressure can be applied also directly onto the plunger. 
The measuring element can be formed, for example, as a piezo-resistive 
element or piezo-electrical element. The piezo-resistive elements are 
elements which change their resistance value under the action of pressure. 
For this purpose for example thick layer resistors can be utilized. The 
materials for the measuring element can be such as cermet, contactive 
plastic, or metal. 
The measuring element 19 is composed of resistors 25 which are arranged on 
a support 26 of silicon, a so called silicon-chip, by etching or printing. 
The measuring element 19 abuts with the support 26 against a 
countersupport 20 which is inserted in the opening 12 and fixedly 
connected with the housing 10. Not shown electrical components of an 
evaluating circuit can be mounted on the support 26 of the measuring 
element 19. Conductors of the evaluating circuit or the measuring element 
19 can extend outwardly of the housing 10 of the pressure sensor 11 
through not shown openings in the countersupport 20 so as to reach a not 
shown evaluating circuit and a control device of the internal combustion 
engine. A recess 21 is formed in the region of the opening 13, so that the 
opening 13 has a stepped shape. 
A narrow gap 23 is provided between the wall of the opening 12 and the 
plunger 18. The gap 23 must be maintained as small as possible to provide 
a good heat transmission between the housing 10 and the plunger 18. 
Furthermore, a lateral guidance of the plunger 18 in the opening 12 must 
be unobjectionable. 
The abutment region of the plunger 18 against the measuring element 19 is 
shown in detail in FIG. 2. The end of the plunger 18 is formed as a 
hemisphere 24. The radius of the hemisphere 24 has such a value that the 
contact region 29 between the hemisphere 24 and the measuring element 19 
which forms the abutment surface, with consideration of the elasticity of 
the material of the measuring element 19, or in other words a support 26 
(the silicon chip), reaches only a predetermined mechanical stress. It is 
to be understood that no impurities are allowed in this contact region 29, 
since they can cause voltage peaks and thereby damages to the components. 
The impurities usually can be both the impurities in the material of the 
plunger 18 and in the material of the measuring element 19, as well as 
particles located in the gap between the plunger 18 and the measuring 
element 19. 
Furthermore, it should be mentioned that the roughness of the surface of 
the measuring element 19 approximately corresponds to the roughness of the 
surface of the hemisphere 24 of the plunger 18. In other words, the 
difference of the roughness of the surface of the hemisphere 24 and the 
surface of the measuring element 19 must be smaller than one micrometer. 
Also, the yielding point or limit of the material of the plunger 18 must 
be smaller than the breaking limit of the measuring element 19. If for 
example the radius of the hemisphere 24 of the plunger 18 is 20 mm, then a 
diameter of the contact region 29 is approximately 0.3 mm. The resulting 
stresses are approximately 300 N/mm.sup.2. Furthermore, the roughness of 
the silicon or in other words the material of the support 26 of the 
measuring element 19 is in the region of several nanometer. The roughness 
of the surface of the hemisphere 24 of the plunger 18 composed for example 
of metal or ceramic is 100 nanometer. Silicon further has substantially 
higher breakage strength than the yielding point of the plunger material, 
or approximately 500-600 N/mm.sup.2. 
The resistors 25 of the measuring element 19 are arranged in form of a 
Wheatstone bridge circuit, squarely on the support 26 of the measuring 
element 19. The resistors 25 are located outside the contact region 29. 
Further, the resistors 25 are located under a passivating layer 27 
composed of Si.sub.3 N.sub.4. Since this passivating layer 27 has a low 
strength, the passivating layer is removed in the contact region. Silicon, 
or in the case of a layer structure of the support. 26, silicon and 
silicon oxide (SiO.sub.2) have a higher strength value than the 
passivating layer 27. Further, the end of the plunger 18 is surrounded by 
a gel 28 so that no dirt can penetrate into the gap 30 between the 
hemisphere 24 and the measuring element 19. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in 
pressure sensor for pressure detection in combustion chamber of internal 
combustion engine, it is not intended to be limited to the details shown, 
since various modifications and structural changes may be made without 
departing in any way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.