Abstract:
A glow plug including a tubular body with elements for fixing it into a bore, an intermediate tubular part arranged inside the tubular body, a finger disposed inside the intermediate tubular part with a heating electrode, and a pressure sensor connected to the intermediate tubular part, wherein a face of the sensor facing the finger is pressed thereto.

Description:
The present invention concerns a glow plug comprising a pressure sensor 
     BACKGROUND OF THE INVENTION 
     In an internal combustion engine, in particular a Diesel engine, it has been noted that knowledge of the value of the pressure within each cylinder makes it possible to better control the progress of the combustion in the engine. This information is then used to regulate the injection of fuel in each of the cylinders. The polluting emissions of the engine may thus be reduced and the consumption optimized. 
     In an engine of Diesel type, each cylinder comprises a glow plug which makes it possible to heat the inside of the corresponding combustion chamber, in particular on starting the engine. This glow plug is disposed in a threaded bore which passes through the cylinder head of the engine. IT is already known to the person skilled in the art to integrate a pressure sensor into a glow plug. In this way, the bore formed in the cylinder head to receive the glow plug also serves to accommodate a device for measuring the pressure within the combustion chamber. Forming an additional bore in the cylinder head is thus avoided. Forming such a bore would create numerous problems due in particular to the lack of space as well as problems of cost, fluid-tightness, accessibility, etc. 
     A glow plug generally comprises a tubular body having on its outer surface a threaded portion enabling it to be fixed into a corresponding bore formed in the cylinder head. One portion of that body is then located within the combustion chamber and the other portion outside it. The portion within the combustion chamber bears a finger within which is located a heater electrode. This electrode is supplied by a core which passes through the tubular body of the glow plug. The portion outside the combustion chamber is also termed glow plug head. At that head, the core is connected to a source of electrical energy. It is also known to situate a pressure sensor at that head. This sensor then measures the pressure exerted on the finger of the glow plug. The principle is to measure the force exerted on the finger by the ambient pressure in the combustion chamber by a measurement of the stresses between the body of the glow plug and the finger.  FIG. 1  appended hereto represents an embodiment of a glow plug of the prior art provided with a pressure sensor. 
     In this embodiment of the prior art, described in more detail below, the pressure sensor is disposed between the body of the glow plug on which it bears and a nut fastened to the upper end of the core supplying the pre-heating electrode with electrical energy. In this embodiment, the core passes through the sensor. 
     Such a glow plug of the prior art makes it possible to measure the pressure exerted within the combustion chamber on the glow plug finger. However, other signals become superposed on the signal corresponding to the pressure measured. To make a correct measurement of the ambient pressure within the combustion chamber of the cylinder considered, the background noise perturbing the pressure measurement should be excluded. This noise arises for example from the deformations of the cylinder head in which the glow plug is mounted. 
     SUMMARY OF THE INVENTION 
     The present invention is thus directed to providing a glow plug having a pressure sensor for which the signal corresponding to the pressure measured is not perturbed by parasite signals. 
     To that end, it provides a glow plug comprising:
         a tubular body having means enabling it to be fixed into a bore,   a finger arranged inside the body of the glow plug and receiving a heater electrode, and   a pressure sensor.       

     According to the invention this glow plug further comprises an intermediate tubular part; the finger is disposed within the intermediate tubular part; the intermediate tubular part is mounted in the tubular body; the sensor is connected by its face that faces away from the finger to the intermediate tubular part, and the face of the sensor facing the finger bears on the latter. 
     The fact of providing the glow plug with an intermediate tubular part mounted as stated above makes it possible to decouple the finger and the body of the glow plug. The pressure sensor may then measure the stresses arising from the thrust exerted by the ambient pressure in the corresponding combustion chamber without being hindered by the stresses existing at the bore in which the glow plug is fixed and which are partly transferred to the body of that glow plug. 
     The invention also provides for the intermediate tubular part to be fastened to the tubular body for example in the vicinity of the end of the tubular body adapted to be located in the combustion chamber of the corresponding engine. This zone of the tubular body is not in general subject to external stresses and the intermediate tubular part thus does not undergone such stresses. 
     In a preferred embodiment, the intermediate part is fixed to the tubular body by an interference fit. In this way, the fixing to the tubular body is excellent. Fixing by welding may also be envisaged here. Similarly, the finger may be fixed to the intermediate part by an interference fit or by welding. 
     For better sensitivity for the measurement of pressure at the pressure sensor, the intermediate tubular part has a deformation zone between a first fixing zone in which the finger is fastened to the intermediate tubular part and a second fixing zone in which the intermediate tubular part is fastened to the tubular body. This deformation zone gives the freedom of movement to the finger and avoids creating tension between the intermediate tubular part and the finger which would be detrimental to the measurement of pressure between those two parts. 
     To transmit the forces exerted by the pressure of the combustion chamber on the finger of the glow plug, one embodiment provides that a spacer be disposed between the finger and the corresponding face of the sensor within the intermediate tubular part. This spacer must be rigid in order to be able to pass on high stresses. Furthermore, it must not perturb the measurement of the pressure by its vibrations. It thus has its own vibration frequency which is very different from the bandwidth of the pressure sensor. To satisfy these demands, the spacer is for example formed from a ceramic material. 
     The spacer referred to above may take the form of an elongate tubular bush of which the outer diameter is less than the inner diameter of the intermediate tubular part and terminates at the end adapted to come into contact with the pressure sensor with a frusto-conical portion so as to present a bearing surface adjacent to the sensor substantially corresponding to the surface of the sensor. In this embodiment, a single part is disposed between the finger and the sensor. This is of course favorable to good transmission of the forces but also enables easier assembly of the glow plug. 
     In the glow plug according to the invention, the sensor is preferably placed in a housing provided for that purpose in the intermediate tubular part, adjacent to the opposite end of that part from the end adapted to receive the finger and the sensor is for example connected to the intermediate tubular part by a nut having a screw thread on its outer periphery engaged in a tapping made in the inner end of the intermediate tubular part. In the case in which a spacer is provided between the finger and the sensor, the nut may clamp the sensor against that spacer. 
     The present invention also concerns an internal combustion engine, in particular an engine of Diesel type, characterized in that it comprises a glow plug as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details and advantages of the present invention will appear more clearly from the following description, made with respect to the accompanying drawings in which: 
         FIG. 1  represents a glow plug of the prior art, 
         FIG. 2  represents a side view of a glow plug according to the invention, 
         FIG. 3  represents the glow plug of  FIG. 2  in longitudinal cross-section, and 
         FIG. 4  is an exploded perspective view of the glow plug represented in  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  represents a glow plug of the prior art in longitudinal cross-section comprising a pressure sensor so as to be able to measure the pressure in an engine cylinder. This glow plug  2  is mounted in a conventional manner in an engine  4 , of Diesel type, and more particularly in a cylinder head of that engine. It comprises a body  8 , a finger  10 , a core  12  and a pressure sensor  14 . 
     The body  8  is adapted to be fixed to the engine by screwing To that end, the cylinder head  6  comprises a threaded bore passing through it and opening into a combustion chamber of said engine  4 . Concerning the body  8 , this has a screw thread  16  on its other surface corresponding to the bore formed in the cylinder head  6 . When the screw thread  16  cooperates with the threaded bore of the cylinder head  6 , the glow plug  2  being in its mounted position in the engine  4 , a portion of the body  8  extends inwardly of the engine  4 , that is to say towards the combustion chamber, whereas another portion extends outwardly the engine. 
     The body  8  is a tubular body of steel within which are housed in particular the finger  10  and the core  12 . The finger  10  projects from the tubular body  8  inside the combustion chamber. It is mounted with an interference fit inside the body  8  and receives a pre-heating electrode (not shown). The core  12  transmits electrical energy to the electrode situated in the finger  10  and, thereby, is in contact with that electrode and is connected to the finger  10  which it extends within the body  8 . At the opposite end of the finger  10 , the free end of the core  12  enables it to be electrically connected to a power conductor. 
     The pressure sensor is a piezo-electric sensor. It comprises a piezo-electric element  18  disposed between two contact elements  20  of electrically conductive material, the sensor itself being electrically insulated from the rest of the glow plug  2  by electrically insulating members  22 . The pressure sensor  14  comes to bear on the body  8  via a bearing part  24 . The other face of the sensor comes to bear against a nut  26  fastened to the upper end of the core  12 . The part of the glow plug  2  where the pressure sensor  14  is to be found and which is situated outside the engine is also termed glow plug head. At this head, the body  8  has a gripping zone  28  used for the mounting/de-mounting of the glow plug  2  with respect to the cylinder head  6 . Usually, this gripping zone  28  is cylindrical and of hexagonal cross-section. In this way, using a spanner, and obtaining access from the exterior of the cylinder head  6 , the mounting and the demounting of the glow plug  2  are possible. The diameter of this gripping zone  28  is, in a conventional manner, greater than that of the rest of the body  8 . 
       FIG. 2  represents an external view of a glow plug according to the invention. With respect to the glow plug of  FIG. 1 , there is once again a body  8  having a screw thread  16  on its other surface. A gripping zone  28  can also be recognized on the head of the body  8 . At the other end of the glow plug is a finger  10 ′. In the embodiment represented in the drawing it is a ceramic finger  10 ′. A first advantage of such a finger is to have a longer life than the fingers generally encountered for glow plugs of the prior art. It is also more compact in form. Thus, the finger  10 ′ is very appreciably smaller than the finger  10  of  FIG. 1 . 
     The glow plug of  FIGS. 2 to 4  also comprises a pressure sensor  14 . Here too this is a piezo-electric sensor which may be of the same type as the sensor of the glow plug represented in  FIG. 1 . 
     Whereas in  FIG. 1  the finger  10  fits into the body  8 , in the embodiment of  FIGS. 2 to 4 , in accordance with the invention, an intermediate tubular part  30  is provided between the body  8  and the finger  10 ′. This intermediate tubular part  30  is intended to isolate the finger  10 ′ from the body  8 . 
     An important difference between the sensors of the prior art and the sensor according to the invention described here is that the pressure sensor is no longer disposed between the body  8  and the finger  10  but is disposed between the finger  10 ′ and the intermediate tubular part  30 . This is apparent in particular in the cross-section of  FIG. 3 . 
     In the following portion of the description, it will be considered that the members intended to be located at the combustion chamber side when the glow plug is fixed into the engine are at a lower level than the members located outside the combustion chamber at the glow plug head side. 
     The finger  10 ′ has an interference fit with the lower end of the intermediate tubular part  30 . In a conventional manner, only the upper portion of the finger  10 ′ is within the intermediate tubular part  30 , the lower portion of the finger  10 ′ projecting outside the part which carries it so as to be situated within the combustion chamber and to heat the gaseous mixture which is located therein. 
     The core  12  supplying the finger  10 ′ with electrical energy passes through the entire intermediate tubular part  30 . 
     The intermediate tubular part  30  is itself fixed with an interference fit to the lower end of the body  8  at a fixing zone  32 . The end of the glow plug is also sometimes referred to as glow plug cone. Above the fixing zone  32  of the intermediate tubular part  30  on the body  8 , the intermediate tubular part  30  extends freely within the body  8 . In its upper portion, the intermediate tubular part  30  flares out and has a zone of greater diameter serving as a housing  34  for the sensor  14 . In the glow plug, that sensor  14  is thus located at the head of the glow plug, that is to say in the upper portion thereof. 
     The upper portion of this housing  34  is tapped so as to be able to receive a nut having an outer screw thread. This nut  36  is provided to press on the upper face of the sensor  14 . The lower face of the sensor  14  rests on a spacer  38 . The latter links the finger  10 ′ to the sensor  14 . 
     The sensor  14  is for example a sensor of the type described with reference to  FIG. 1 . It is not useful to repeat the description of the sensor here. The important different between the sensor  14  of  FIG. 1  and that of  FIGS. 2 to 4  is that in  FIG. 1  the sensor  14  works in tension whereas in  FIGS. 2 to 4  is works in compression. This does not however modify its structure and in particular there is once again a piezo-electric element disposed between two contact elements to which electric wires are connected. 
     The spacer  38  is a tubular part to enable in particular the passage of the core  12  through its center. The outer diameter of the spacer  38  is adapted to the inner diameter of the intermediate tubular part  30  so as to extend freely therein. In its lower portion, the diameter of the spacer  38  is adapted to the diameter of the finger  10 ′. In its upper portion, the diameter of the spacer  38  is adapted to the diameter lower face of the sensor  14 . Thus, as may be noted on the drawing, the spacer has a frusto-conical zone  40  in its upper portion making it possible to progressively pass from the diameter corresponding to the finger  10 ′ to that of the sensor  14 . 
     The pressure sensor  14  solely measures here the stresses arising from the ambient pressure in the corresponding combustion chamber. The variants stresses exerted on the cylinder head  6  in which the glow plug is mounted, in particular those which are not proportional to the ambient pressure in the combustion chamber, have no influence on the pressure measured by the sensor  14 . This is because the intermediate tubular part  30  is held in the body  8  at a zone of the body which does not undergo the stresses linked to the deformation of the cylinder head  6  into which the glow plug  2  is screwed. More particularly, these deformations are rather to be found at the screw thread  16 . 
     The sensor  14  is mounted between the intermediate tubular part  30  and the finger  10 ′. To obtain a correct measurement of the pressure exerted on the finger  10 ′, care is taken in the design of the glow plug to provide a deformation zone  42  of the intermediate tubular part  30 . This deformation zone  42  is located between the zone in which the finger  10 ′ has an interference fit inside the intermediate tubular part  30 , and the zone in which the intermediate tubular part  30  fits into the body  8 . This deformation zone  42  favors the transmission of the forces exerted by pressure on the finger  10 ′ of the sensor  14 . 
     The role of the spacer  38  is important here and great attention must be taken in its manufacture. On passing on the forces exerted on the finger  10 ′, the spacer  38  must not transmit parasitic forces. Care should therefore be taken that the spacer is in contact neither with the core  12  supplying the finger  10 ′ with electrical energy nor with the intermediate tubular part  30 . 
     Care should also be taken that the spacer&#39;s own vibration mode does not perturb the pressure measurement of the sensor  14 . Care must therefore be taken that the own vibration frequency of that spacer  38  is far away from the bandwidth of the sensor  14 . In choosing a ceramic as material for producing that spacer  38 , these problems may be solved. 
     As shown in  FIGS. 3 and 4 , the head of the glow plug may comprise an overmolded part  44  of synthetic material. This part may incorporate the gripping zone  28  of the glow plug head. The overmolding with synthetic material makes it possible to ensure good fluid-tightness of the glow plug head and ensure good protection of the electrical connections of the electrical wires connected to the contact elements of the sensor  14 . 
     A glow plug  2  comprising a pressure sensor  14  such as the glow plug described above makes it possible to make an excellent pressure measurement within a combustion chamber. The measurement of this pressure is totally decoupled from the parasitic noise liable to perturb the pressure measurement. Thus even where there are injectors situated at each cylinder, the pressure sensor mounted in a glow plug according to the invention makes it possible to get free from the deformations of the cylinder head induced by the stresses exerted by those injectors. As for the deformation zone on the intermediate tubular part, this makes it possible to obtain good sensitivity for the measurement made by the sensor. 
     As is apparent in particular from  FIG. 4 , despite the introduction of an intermediate tubular part, the glow plug according to the invention remains easy to assemble. It is a matter of stacking parts which are sealed by an overmolding of plastics materials at the glow plug head. 
     The present invention is not limited to the preferred embodiment described above by way of non-limiting example. It also concerns all the variant embodiments accessible to the person skilled in the art. 
     Thus, for example, the present invention may perfectly well be envisaged with a conventional finger. The drawing of the glow plug must then be adapted to that finger of greater size but it is of course still possible to introduce an intermediate tubular part to achieve the decoupling between the body of the glow plug and the finger thereof. 
     The fixing of the finger in the intermediate tubular part and respectively of the intermediate tubular part in the body is provided above by interference fit. Another way of fixing (welding, for example) may be envisaged. 
     An alternative to the use of a spacer may be envisaged. There may for example be provided a nut fastened to the core supplying the finger with electrical energy and on which the lower face of the pressure sensor would come to rest. To withstand the high mechanical forces to transmit, rigidifying that core may then have to be envisaged. If the spacer is kept, it may take a different form to that described above. That form is advantageous since it makes it possible to have only one part between the finger and the sensor. However, it would be possible to have a spacer associated with a washer for bearing upon which would form the interface between the spacer and the sensor.