Abstract:
A pressure sensor that is mounted on an intake tube of an internal combustion engine which prevents icing of the pressure connection. The pressure sensor of the invention has a stublike pressure connection, which extends from an inner wall of an intake tube to approximately the middle of the intake tube or beyond the middle, and as a result the risk of icing of the pressure sensor is considerably reduced. A pressure sensor is intended especially for mixture-compressing internal combustion engines with externally supplied ignition.

Description:
BACKGROUND OF THE INVENTION 
     The invention is based on a pressure sensor for an internal combustion engine having an intake tube. A known pressure sensor (German Offenlegungsschrift 43 17 312) has a plastic housing with a stublike pressure connection, and in the state in which it has been installed on the intake tube it is essentially flush with an inner wall of the engine intake tube, or protrudes from it by only a few millimeters. In such pressure sensors mounted directly on the intake tube, however, there is the risk of icing up of the pressure connection, so that in the worst case, failure of the pressure sensor can occur. The risk of icing is especially high in engines with exhaust gas recirculation and crankcase venting into the intake tube, since the flowing medium has a high proportion of water dissolved in gaseous form, which at low temperature can precipitate out in the form of ice, predominantly in the interior of a thin metal sensor tubule of a pressure measuring instrument of the pressure sensor. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The pressure sensor of the invention has the advantage over the prior art that icing up of the pressure sensor is precluded with very great certainty. Advantageously, the deposit of liquid components of the medium flowing within the intake tube on the pressure sensor is also prevented, thus further reducing the danger of icing of the pressure sensor. 
     By means of the provisions recited herein, advantageous further features of and improvements to the pressure sensor are possible. It is especially advantageous that existing pressure sensors can be retrofitted in a simple way without having to make major structural changes, so that the production costs rise only slightly. Moreover, the pressure sensor of the invention advantageously requires no additional space at the installation site of the pressure sensor. 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE of the drawing shows an exemplary embodiment of the invention in simplified form. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawing, a pressure sensor  1  is shown whose essential component is a pressure measuring instrument, embodied for instance in the form of a sensor cell  3 , which is accommodated in a housing  6  of the pressure sensor  1 . The housing  6  is in two parts and is composed of a stublike pressure connection  7  and a caplike housing part  8 . The pressure connection  7  and the housing part  8  are preferably made of plastic, for instance by plastic injection molding. The sensor cell  3  is made of metal and is accommodated in a tublike indentation  9  in the housing part  8  into which it is inserted with positive engagement. The sensor cell  3  is held in the tublike indentation  9  by an adhesive, for instance, so that except for the entrance of a thin metal sensor tubule  4  mounted on the sensor cell  3 , the sensor cell  3  is hermetically sealed off from its surroundings. The known purpose of the pressure sensor  1  is to measure the internal pressure of a medium flowing in an intake tube  5  of the engine, in particular the air aspirated by the engine. By means of the internal pressure of the intake tube and the engine rpm, it is possible to determine the quantity of air aspirated by the engine. The flow direction of the medium in the intake tube  5  is indicated in the drawing by arrows  30  and extends from left to right. 
     The sensor cell  3  comprises a sensor chip made of silicon, into which a pressure membrane is etched. A change in pressure causes expansion of the pressure membrane, which is detected via changes in resistance (this is known as the piezoresistive effect). The evaluation circuit, including calibration, is integrated into the sensor chip. The pressure to be measured is carried via the sensor tubule  4  and a glass base provided in the interior of the sensor cell to a back side of the pressure membrane, which communicates with a reference vacuum, so as to enable measuring the absolute pressure. The output signal of the evaluation circuit is carried to a plug contour  15  embodied on the housing part  8  via electrical conductors  11  which are connected with a printed circuit board  12  and a bent metal pin  14  mounted on the printed circuit board  12 . The plug contour  15  serves to receive an insertable plug, so that via the plug the electrical signals of the pressure measuring instrument, that is, of the evaluation circuit, of the sensor chip can be carried to an electronic control unit. 
     The sensor tubule  4  of the censor cell  3  discharges into a through opening  17 , provided in the pressure connection  7 , that extends from the sensor tubule  4  to an end  18  of the pressure connection  7  that is open toward the flowing medium, so that there is communication from the interior of the sensor cell  3  to the flowing medium via the sensor tubule  4  and the pressure connection  7 . A sealing ring  23 , embodied for instance as an O-ring, seals off the sensor tubule  4  in the pressure connection  7 . The pressure connection  7  is mounted on side walls  20  of the housing part  8  and is retained, for instance in grooves  21 , with the aid of an adhesive  22  placed there. The side of the housing part  8  has a formed-on feature  24  with an opening  25  into which a screw  26  can be inserted, so that the pressure sensor  1  can for instance be secured firmly by a bolt threaded into the female thread  27  on the intake tube  5 . On installation, the pressure sensor  1  is introduced by its stublike pressure connection  7  into a through opening  16  of the intake tube  5 , whereupon with a connection part  19  it protrudes from an inner wall  29  of the intake tube  5  into the intake tube. A seal  28 , for instance in the form of an O-ring, provided between the inner wall  29  and an outer wall  33  of the intake tube  5  on the pressure connection  7 , seals off the atmosphere from the flowing medium. 
     According to the invention, the stublike pressure connection  7  of the pressure sensor  1  extends relatively far from the inner wall  29  of the intake tube  5  into the flowing medium, so that the open end  18  of the pressure connection  7  extends approximately as far as a middle of the intake tube  5 , identified by a longitudinal axis  31 , or even beyond the middle of the intake tube  5 . The pressure connection  7 , which is embodied with a relatively long length, prevents the possibility of icing of the pressure connection  7  at low temperature, especially on an inside surface  34  of the through opening  17  of the pressure connection  7  and on the sensor tubule  4 , so that a free through opening  17 , or communication from the intake tube  5  to the sensor cell  3 , is always available. 
     As shown in the drawing, the pressure connection  7  may also have a shorter length, however. For instance, it is essentially possible to use commercially available pressure sensors with a shorter pressure connection  7 , and to lengthen the pressure connection by means of a hose  35 , which can be slipped over the pressure connection  7  of the connection part  19 , for instance, and at least partly surrounds the pressure connection  7 , in such a way that the through opening  17 , via an inner opening  32  of the hose  35 , extends with an open end  36  of the hose likewise approximately as far as the middle of the intake tube  5  or even beyond the middle of the intake tube  5 . The cross section of the internal opening  32  of the hose  35  changes, for instance at the end  18  of the pressure connection  7 , via a conically converging part, into a somewhat larger opening cross section than the through opening  17 , for instance. The cross section of the internal opening  32  of the hose  35  is chosen to be small enough that the internal opening  32  cannot become stopped up or plugged by dirt and contaminants, yet there is adequate hindrance to diffusion. 
     The hose  35  comprises fuel-resistant plastic and is made from fluorocaoutchouc, for instance. The hose  35  is embodied flexibly and, as shown in the drawing, can be bent more or less in the flow direction  30  of the medium by the flowing medium. The flexing motion of the hose  35  occurring during engine operation also makes it possible for any freezable liquid ingredients of the medium that might have collected in the hose  35  to be shaken off and carried away by the medium, thus still further reducing the risk of icing of the pressure sensor  1 . The flexibly embodied hose  35  also has the effect that resonance occurring in the intake tube  5  can reach as far as the pressure measuring instrument of the sensor cell  3  only in extremely attenuated form, and as a result a precise pressure measurement is made. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.