Abstract:
A glow plug ( 20 ) for an internal combustion engine ( 110 ) essentially includes:
       a body ( 27 ) intended to be fixed to the engine, and   a core ( 29 ) extending in a direction of elongation ( 11 ), connected to the body and intended to extend inside the combustion chamber ( 9 ) of the engine,   measurement elements ( 200, 55, 65 ) intended to determine the pressure internal to the combustion chamber ( 9 ) through the displacement of the core ( 29 ) under the effect of pressure,
 
characterized in that the glow plug is equipped with elements ( 50, 60 ) that make it possible to quantify the displacement of the core with respect to the cylinder head ( 5 ) of the engine on which the glow plug is installed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is the 35 USC 371 national stage of international application PCT/EP2003/014936, filed on 29 Dec. 2003, which designated the United States of America. 
   FIELD OF THE INVENTION 
   The present invention relates to the control of internal combustion engines and more specifically to the measuring of the pressure in the combustion chamber of these engines. 
   BACKGROUND OF THE INVENTION 
   Internal combustion engines have “standard” settings established on test bed for controlling the injection of fuel according to the operating circumstances encountered. These settings do not unfortunately take account of the conditions of ageing of the engine in real time, particularly within the actual combustion chamber (fouling of the injectors, loss of compression, deterioration of the injector nozzle opening pressures, etc.). 
   Admittedly, it has been proposed for engines to be equipped with knock sensors. These sensors, of the accelerometer type, therefore allow the presence of shockwaves that carry the risk of damaging the engine to be detected, and avoided, by modifying the injection parameters. However, these sensors provide only a very imperfect solution to the improvement of engines to make them less polluting and/or more efficient. 
   This being the case, the applicant company has set itself the task, in order to optimise the operation of the engine, of measuring the pressure within the combustion chamber. 
   Various devices have already been proposed for measuring this pressure. However, these are either not very reliable or too expensive because of their cost price or because of the costs that they incur (modifying the cylinder head or the engine block in order to introduce a sensor). 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to determine reliably and at a limited cost, the internal pressure in the combustion chamber. 
   To do this, the invention proposes a glow plug for an internal combustion engine essentially comprising a body intended to be fixed to the engine, and a core extending in a direction of elongation, the core being connected to the body and intended to extend inside the combustion chamber of the engine, and measurement means intended to determine the pressure internal to the combustion chamber through the displacement of the core under the effect of said pressure, this glow plug being equipped with means that make it possible to quantify the displacement of the core with respect to the cylinder head of the engine on which the glow plug is installed. 
   The pressure within the combustion chamber tends to drive the core out (push it back). In consequence, the invention makes it possible to deduce the pressure within the combustion chamber from the relative displacement of the core of the glow plug with respect to the cylinder head, without having to pierce the engine block, by incorporating the sensor into the glow plug. 
   According to an additional feature, the means intended to allow detection of the displacement of the core of the glow plug with respect to the cylinder head of the engine is a yoke secured to said cylinder head, either by screwing or by welding, or by any equivalent known means. 
   According to an advantageous feature of the invention, the measurement means are placed between said yoke and a part integral with the core, so as to incorporate the measurement system into the glow plug as best as possible. 
   According to an additional feature, the measurement means comprise a tubular piezoelectric element extending around the core in the direction of elongation between a first end and a second end, said tubular piezoelectric element being connected to the core at the first end and to the cylinder head via the yoke at the second end. 
   A piezoelectric element allows a displacement to be converted into a corresponding electrical signal with great precision for small-amplitude displacements, as in the case of the present invention (a few hundred nanometers). This solution proves to be all the more advantageous as piezoelectric elements work out to be less expensive than and are less sensitive to temperature than deformation sensors of the strain gauge type. 
   The invention also relates to a vehicle equipped with a propulsion unit comprising, in addition to the glow plug:
         an internal combustion engine having a combustion chamber and on which said glow plug is mounted,   fuel-injection means for injecting fuel into the combustion chamber of the engine,   processing means for processing the information acquired by the measurement means, said processing means controlling the injection means on the basis of the information acquired by the measurement means in the course of one combustion cycle.       

   This solution has the aforementioned advantages in relation to the glow plug. 
   Advantageously, the processing means comprise a high-frequency filter to eliminate information relating to the vibration of the core with respect to the cylinder head. 
   The displacement of the core with respect to the cylinder head depends in particular on the pressure in the combustion chamber, on the injection of fuel and on the combustion. By using a filter that cuts off frequencies appreciably higher than the rotational speed of the engine, for example, above 200 Hz, it is possible to reduce the effect of these disruptive influences on the measurement of pressure in the combustion chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will emerge more clearly from the description which will follow, given the reference to the attached drawings in which: 
       FIG. 1  illustrates, in part section, a glow plug of the prior art mounted on a cylinder head of an internal combustion engine, 
       FIGS. 2 and 3  show two variants, on a different scale, of the mounting of a glow plug on a cylinder head. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a propulsion unit  1  comprising a glow plug  20 , fuel-injection means  130 , an electronic computer  4  and an engine block  110  of an internal combustion engine comprising, in particular, a cylinder head  5  and a combustion chamber  9 . 
   The glow plug  20  here consists of a glow plug for a diesel engine extending in a direction of elongation  11  and housed in an orifice  3  made for that purpose in the cylinder head  5 . In accordance with conventional glow plugs, the glow plug  20  comprises a body  27 , a core  29  extending inside the body and an insulating sealing collar  12  arranged between the body  27  and the core  29 , generally consisting of an elastomer seal. 
   The core  29  comprises a resistive electric element  18  protected by a sheath  10  extending into the combustion chamber  9 , and a rod  14  secured to the sheath  10  and connecting the resistive element  18  to a threaded current lead-in  40  to which the power supply to the resistive element  18  is connected. 
   The glow plug  20  is also equipped with a displacement sensor  200  consisting of a sensitive piezoceramic element  45  placed between two contact rings  70   a  and  70   b  and electrically insulated from the core  29  and from the body  27  by two insulating rings  90   a ,  90   b.    
   The sensitive element  45 , the contact rings  70   a ,  70   b  and the insulating rings  90   a ,  90   b  are tubular, which means that the current lead-in  40  and the rod  14  pass through them and are locally surrounded by them. 
   The sensor  200  is connected, at a first end  200   a  in the direction of elongation  11 , to the core  29  by means of a fixing nut  7  and at the other end  200   b  in the direction of elongation  11  to the body  27  by a spacer piece  6  resting against this body  27 , said insulating rings  90   a ,  90   b  being interposed, in the case of one of them, between the contact ring  70   a  and the nut  7 , and, in case of the other, between the contact ring  70   b  and the spacer piece  6 . 
   The computer  4  is electrically connected to the contact rings  70   a ,  70   b  between which it measures the difference in voltage on the one hand, and to the injection means  130 , so as to control the amount of fuel injected into the combustion chamber  9  and the instant at which it is injected, on the other hand. 
   During internal combustion, the pressure in the combustion chamber  9  increases and the glow plug  20  is subjected to this. This pressure has a tendency to “drive” the glow plug outwards. The core  29  and in particular the sheath  10 , although held in position with respect to the body  27 , move slightly by a few microns with respect to said body  27 , which for its part is secured to the cylinder head  5 , according to the pressure inside the combustion chamber. 
   These miniscule displacements do not endanger the integrity of the glow plug  20  and make it possible to deduce the pressure in the combustion chamber  9 . Specifically, the variation in the relative position of the core  29  with respect to the body  27  in the direction of elongation  11  modifies the pressure exerted on the piezoelectric element  45 , and this induces a potential difference across the contact rings  70   a ,  70   b.    
   The information relating to the potential difference in the course of one combustion cycle across the contact rings  70   a ,  70   b  is processed by the computer  4  which, by reference to an operating model already saved in memory, determines the injection of fuel in real time, making it possible to take account of the state of the engine whatever its operating history. 
   The computer  4  comprises signal processing means for eliminating parasitic information. In particular, the computer comprises a high-frequency filter to eliminate the variations in voltage between the contact rings  70   a ,  70   b  that relate to vibrations of the core with respect to the body and the frequency of which is above 200 hertz for example. 
   A preferred embodiment is depicted in  FIG. 2 . Once again, we have the glow plug  20  of  FIG. 1 , the outer body  27  of which is threaded over its portion labeled  270  and engages directly in the tapping  30  of the orifice  3  in a direction of elongation and of mounting  11 . 
   The spacer piece  6  here is replaced by a stirrup-shaped yoke  50  projecting at the rear of the cylinder head  5  to serve, via its transverse wall  51 , as a bearing surface, the position of which is fixed with respect to the cylinder head for the pressure/displacement sensor  55 . This yoke  50 , added to the glow plug  20  of the prior art, is secured to the cylinder head  5  and makes it possible to make a mechanical connection between the latter and the glow plug  20 , on the one hand, and to exert pressure on the displacement sensor  55  via its transverse wall  51  once mounting has been achieved, on the other hand. 
   The rear end  29   a  of the central core  29  has, beyond the body  27 , at the opposite axial end to the combustion chamber  9  (and therefore in the direction of elongation  11 ), a bearing surface  57  for the sensor  55 , which sensor is therefore interposed between the surfaces  51  and  57 , so that the variations in pressure in the chamber  9  are transmitted to the core  29  and therefore to the sensor  55  via the surface  57 , the yoke  50  being fixed with respect to the cylinder head  5 . 
   It should be noted that, in this embodiment, the yoke  50  makes it possible to dispense with the nut  7  depicted in  FIG. 1 . 
   Furthermore, in this embodiment, the sensor  55  is pressurised by the increase in pressure in the chamber  9  whereas in the embodiment of  FIG. 1 , the increase in pressure in the chamber  9  will cause a relaxation of the measurement sequence. 
   Although the body  27  of the glow plug  20  is screwed, and therefore fixed, to the cylinder head  5 , the yoke  50  provides a direct mechanical connection between the cylinder head and the central core  29 , so as to get around the problem of any play there might be between the body  27  screwed to the cylinder head. By eliminating this play in this way, the sensor  55  is more capable of determining the pressure in the chamber  9 . 
   Furthermore, and contrary to  FIG. 1 , the yoke  50  makes it possible to eliminate from the measurement any vibration that the glow plug  20  may exhibit with respect to the cylinder head  5 , because the relative movement is, in this instance, evaluated directly between the cylinder head and the core  29  of the glow plug  20  rather than between the core  29  and the body  27 . 
   Although the yoke  50  has been depicted in the manner of a bell-shaped stirrup, here with an inverted U-shaped cross section, other shapes could have been anticipated. 
   Likewise, a connection other than by screwing (at  59 ) between the yoke and the cylinder head  5  could be anticipated (a bayonet system for example, or even welding if these fixing means can allow the “reference” pressure exerted on the sensor  55  in the direction of elongation  11  to be modified). 
   In theory, the yoke  50  could be welded, or even manufactured directly as an integral part of the cylinder head  5  (for example by casting). 
   This figure again shows the computer  4  which, on the basis of the potential difference measured across the terminals of the contact rings  21   a  and  21   b  allows the fuel-injection means  130  to be controlled. As in  FIG. 1 , the contact rings  21   a  and  21   b  are insulated from the remainder of the installation by the insulating rings  23   a  and  23   b.    
   In  FIG. 3 , the body  27  of the glow plug  20  is once again screwed into the threaded orifice  3  of the cylinder head  5  and the rear part  29   a  of the central core  29  is connected to the transverse bearing bar  57 . 
   However, in this version, the transverse bar  57  is placed above the yoke  60 , which is fixed (for example by welding at  61 ) with respect to the cylinder head). 
   The transverse wall  63  of the yoke extends at right angles to the direction of elongation  11  so as to exhibit a fixed bearing surface  63   a , for the pressure sensor  65 , which sensor is interposed between the yoke (transverse wall  63 ) and the bearing bar  57 , subjected to the pressure of the nut  67  which therefore pulls on the core  29  in the direction  11  in order to exert an appropriate reference pressure on the sensor. 
   Of course, the invention is not in any way restricted to the embodiment which has just been described by way of nonlimiting example. Thus, it could be anticipated for the invention to be applied to other types of internal combustion engine, such as a controlled-ignition engine, particularly a gasoline engine. As a controlled-ignition engine plug also comprises a body intended to be fixed to a cylinder head and a core extending inside the body (between which items a spark is generated), measurement means according to the invention may be placed between the body and the core to measure their relative displacement. 
   As a variant (not depicted), the spacer piece  6  may be omitted if the configuration of the plug  20  lends itself to direct mounting of the sensor  200  on the body  27  of the plug.