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EP1747406A1 - Glow plug provided with a pressure sensor - Google Patents

Glow plug provided with a pressure sensor

Info

Publication number
EP1747406A1
EP1747406A1 EP05758221A EP05758221A EP1747406A1 EP 1747406 A1 EP1747406 A1 EP 1747406A1 EP 05758221 A EP05758221 A EP 05758221A EP 05758221 A EP05758221 A EP 05758221A EP 1747406 A1 EP1747406 A1 EP 1747406A1
Authority
EP
European Patent Office
Prior art keywords
finger
glow plug
sensor
tubular
tubular part
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP05758221A
Other languages
German (de)
French (fr)
Inventor
Bernd Last
Cyrille Patri
Alain Ramond
John Burrows
Sandro Goretti
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Continental Automotive France SAS
Federal Mogul Italy SRL
Original Assignee
Siemens VDO Automotive SAS
Federal Mogul Ignition SRL
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Siemens VDO Automotive SAS, Federal Mogul Ignition SRL filed Critical Siemens VDO Automotive SAS
Publication of EP1747406A1 publication Critical patent/EP1747406A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23Q—IGNITION; EXTINGUISHING-DEVICES
    • F23Q7/00—Incandescent ignition; Igniters using electrically-produced heat, e.g. lighters for cigarettes; Electrically-heated glowing plugs
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23Q—IGNITION; EXTINGUISHING-DEVICES
    • F23Q7/00—Incandescent ignition; Igniters using electrically-produced heat, e.g. lighters for cigarettes; Electrically-heated glowing plugs
    • F23Q7/001—Glowing plugs for internal-combustion engines
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02P—IGNITION, OTHER THAN COMPRESSION IGNITION, FOR INTERNAL-COMBUSTION ENGINES; TESTING OF IGNITION TIMING IN COMPRESSION-IGNITION ENGINES
    • F02P19/00—Incandescent ignition, e.g. during starting of internal combustion engines; Combination of incandescent and spark ignition
    • F02P19/02—Incandescent ignition, e.g. during starting of internal combustion engines; Combination of incandescent and spark ignition electric, e.g. layout of circuits of apparatus having glowing plugs
    • F02P19/028—Incandescent ignition, e.g. during starting of internal combustion engines; Combination of incandescent and spark ignition electric, e.g. layout of circuits of apparatus having glowing plugs the glow plug being combined with or used as a sensor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23Q—IGNITION; EXTINGUISHING-DEVICES
    • F23Q7/00—Incandescent ignition; Igniters using electrically-produced heat, e.g. lighters for cigarettes; Electrically-heated glowing plugs
    • F23Q7/001—Glowing plugs for internal-combustion engines
    • F23Q2007/002—Glowing plugs for internal-combustion engines with sensing means

Definitions

  • the present invention relates to a glow plug comprising a pressure sensor.
  • a glow plug comprising a pressure sensor.
  • an internal combustion engine in particular a diesel engine
  • knowing the value of the pressure inside each cylinder makes it possible to better control the progress of combustion in this engine.
  • This information is then used to regulate the injection of fuel into each of the cylinders. Pollutant emissions from the engine can thus be reduced and consumption optimized.
  • each cylinder has a glow plug which makes it possible to heat the interior of the corresponding combustion chamber, in particular when the engine is started.
  • This glow plug is arranged in a threaded bore which passes through the cylinder head of the engine. It is already known to those skilled in the art to integrate a pressure sensor with a glow plug.
  • a glow plug generally comprises a tubular body having on its outer surface a threaded part allowing its fixing in a corresponding bore made in a cylinder head. Part of this body is then inside the combustion chamber and the other part outside of it. The part inside the combustion chamber has a finger inside which there is a heating electrode. This electrode is supplied by a core which crosses the tubular body of the spark plug. The part outside the combustion chamber is also called the spark plug head.
  • FIG. 1 shows an embodiment of a glow plug of the prior art provided with a pressure sensor.
  • the pressure sensor is disposed between, on the one hand the body of the glow plug on which it is supported and on the other hand a nut secured to the upper end of the core supplying electrical energy to the preheating electrode.
  • the core passes through the sensor.
  • Such a candle of the prior art makes it possible to measure the pressure exerted inside the combustion chamber on the finger of the glow plug.
  • other signals are superimposed on the signal corresponding to the pressure measured.
  • To make a good measurement of the pressure prevailing inside the combustion chamber of the cylinder in question it is necessary to be able to overcome the background noise disturbing the pressure measurement. This comes for example from deformations of the cylinder head in which is mounted the glow plug.
  • the object of the present invention is therefore to provide a glow plug fitted with a pressure sensor for which the signal corresponding to the measured pressure is not disturbed by spurious signals.
  • this glow plug offers a glow plug comprising: - a tubular body having means allowing it to be fixed in a bore, - a finger disposed inside the body of the plug and receiving a heating electrode, and - a sensor pressure.
  • this glow plug further comprises an intermediate tubular part; the finger is disposed inside the intermediate tubular part; the intermediate tubular part is mounted in the tubular body; the sensor is secured by its face opposite the finger to the intermediate tubular part, and the face of the sensor facing the finger is supported on the latter.
  • the pressure sensor can then measure the stresses resulting from the thrust exerted by the pressure prevailing in the corresponding combustion chamber without being hindered by stresses existing at the level of the bore in which the glow plug is fixed and which are retransmitted in part to the body of this candle.
  • the invention also proposes that the intermediate tubular part be secured to the tubular body, for example near the end of the tubular body intended to be in the combustion chamber of the corresponding engine. This area of the tubular body is generally not subjected to external stresses and the tubular intermediate piece is then not subjected to such stresses.
  • the intermediate piece is fixed to the tubular body by tight fitting. In this way, the attachment to the tubular body is excellent.
  • the finger can be fixed to the intermediate piece by tight adjustment or by welding.
  • the intermediate tubular part has a deformation zone between a first fixing zone in which the finger is secured to the intermediate tubular part and a second fixing zone in which the intermediate tubular part is secured to the tubular body.
  • This deformation zone gives freedom of movement to the finger and avoids creating tension between the tubular intermediate piece and the finger which would be detrimental to the pressure measurement between these two pieces.
  • a spacer is disposed between the finger and the corresponding face of the sensor inside the tubular intermediate piece. This spacer must be rigid in order to be able to transmit significant stresses.
  • the spacer is for example made of a ceramic material.
  • the spacer mentioned above can be in the form of an elongated tubular sleeve whose outside diameter is less than the inside diameter of the intermediate tubular part and ends on the side intended to come into contact with the pressure sensor by a zone frustoconical so as to have a bearing surface on the side of the sensor corresponding substantially to the surface of the sensor.
  • a single piece is disposed between the finger and the sensor. This is of course favorable for good transmission of forces but also allows easier mounting of the glow plug.
  • the senor is preferably placed in a housing provided for this purpose in the intermediate tubular part, near the end of this part opposite to that intended to receive the finger and the sensor is for example secured to the intermediate tubular part by a nut having a thread at its outer periphery engaged in a thread made at the inner end of the intermediate tubular part.
  • the nut can come to tighten the sensor against this spacer.
  • the present invention also relates to an internal combustion engine, in particular a diesel type engine, characterized in that it includes a glow plug as described above. Details and advantages of the present invention will emerge more clearly from the description which follows, given with reference to the appended diagrammatic drawing, in which: FIG.
  • FIG. 1 represents a glow plug of the prior art
  • FIG. 2 represents a glow plug according to the invention in side view
  • Figure 3 shows the glow plug of Figure 2 in longitudinal section
  • Figure 4 is an exploded perspective view of the candle shown in Figures 2 and 3.
  • Figure 1 shows in longitudinal section of a glow plug of the prior art comprising a pressure sensor so as to be able to measure the pressure in an engine cylinder.
  • This glow plug 2 is conventionally mounted in an engine 4, of the type Diesel, and more precisely in a cylinder head 6 of this 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 motor 4 by screwing.
  • the cylinder head 6 has a threaded bore passing through it and opening into a combustion chamber of said engine 4.
  • the body 8 has meanwhile on its outer surface a thread 16 corresponding to the thread made in the cylinder head 6.
  • a thread 16 cooperates with the threaded bore of the cylinder head 6, the spark plug 2 being in its position mounted in the engine 4, a part of the body 8 extends towards the inside of the engine 4, that is to say towards the combustion chamber, while another part extends towards the outside of the engine.
  • the body 8 is a tubular steel body inside of 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 tightened inside the body 8 and receives a preheating electrode (not shown).
  • the core 12 transmits electrical energy to the electrode located in the finger 10 and, therefore, is in contact with this electrode and is integral with the finger 10 which it extends inside the body 8.
  • a l opposite the finger 10, the free end of the core 12 allows its electrical connection to a supply conductor.
  • the pressure sensor is a piezoelectric sensor. It comprises a piezoelectric element 18 disposed between two contact elements 20 of electrically conductive material, the sensor itself being electrically isolated from the rest of the spark plug 2 by electrically insulating elements 22.
  • the pressure sensor 14 comes to take support by means of a support piece 24 on the body 8. The other face of the sensor bears against a nut 26 secured to the upper end of the core 12.
  • the part of the glow plug 2 at which is the pressure sensor 14 and located outside the engine is also called the spark plug head.
  • the body 8 has a gripping zone 28 used for mounting / dismounting the spark plug 2 relative to the cylinder head 6.
  • this gripping zone 28 is cylindrical with a hexagonal cross section. In this way, using a key, accessing from outside the cylinder head 6, the mounting and dismounting of the glow plug 2 are possible.
  • the diameter of this gripping zone 28 is, conventionally, greater than that of the rest of the body 8.
  • Figure 2 shows, in external view a glow plug according to the invention. Compared to the candle of Figure 1, there is a body 8 having on its outer surface a thread 16. We also recognize on the side of the head of the body 8 a gripping area 28.
  • the finger 10 ′ is very substantially smaller than the finger 10 in FIG. 1.
  • the glow plug in FIGS. 2 to 4 also includes a pressure sensor 14. This is also a piezoelectric sensor here. which can be of the same type as the glow plug sensor shown in Figure 1. While in Figure 1 the finger 10 is adjusted in the body 8, in the embodiment of Figures 2 to 4, in accordance with l invention, a tubular intermediate piece 30 is provided between the body 8 and the finger 10 '.
  • this tubular intermediate piece 30 is 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 arranged between the body 8 and the finger 10 but is disposed between the finger 10 'and the tubular intermediate piece 30. This appears in particular from the sectional view of FIG. 3.
  • the elements intended to be find on the side of the combustion chamber when the glow plug is fixed in the engine are at a lower level compared to elements located outside the combustion chamber, on the side of the head of the glow plug .
  • the finger 10 ' is tightly adjusted to the lower end of the tubular intermediate piece 30.
  • the intermediate tubular part 30 is itself fixed by adjustment with clamping at the lower end of the body 8 at a fixing zone 32.
  • the tip of the candle is also sometimes called the candle cone.
  • the tubular intermediate piece 30 extends freely inside the body 8. In its upper part, the tubular intermediate piece 30 widens and has a larger diameter area serving as a housing 34 for the sensor 14.
  • this sensor 14 is then located at the head of the spark plug, that is to say in the upper part of the latter.
  • the upper part of this housing 34 is tapped so as to be able to receive a nut 36 having an external thread.
  • This nut 36 is designed to exert a support on the upper face of the sensor 14.
  • the lower face of this sensor 14 rests on a spacer 38.
  • the latter connects 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 unnecessary to repeat the description of this sensor here.
  • the important difference 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 while in FIGS. 2 to 4 it works in compression.
  • the spacer 38 is a tubular piece to allow in particular at its center the passage of the core 12.
  • the outside diameter of the spacer 38 is adapted to the inside diameter of the tubular intermediate piece 30 so as to extend freely to the inside of it.
  • the diameter of the spacer 38 is adapted to the diameter of the finger 10 '.
  • the diameter of the spacer 38 is adapted to the diameter of the lower face of the sensor 14.
  • the spacer has in its upper part a frusto-conical area 40 allowing to pass gradually of the diameter corresponding to the finger 10 'to that of the sensor 14.
  • the pressure sensor 14 here measures only the stresses resulting from the pressure prevailing in the corresponding combustion chamber.
  • the various stresses exerted on the cylinder head 6 in which the glow plug is mounted in particular those which are not proportional to the pressure prevailing in the combustion chamber, have no influence on the pressure measured by the sensor 14.
  • the intermediate tubular part 30 is maintained in the body 8 at a region of the body which does not undergo the stresses linked to the deformation of the cylinder head 6 in which the glow plug 2 is screwed. Indeed, these deformations are found more at the level of the thread 16.
  • the sensor 14 is mounted on the one hand between the tubular intermediate piece 30 and on the other hand the finger 10 '.
  • care is taken in the design of the glow plug to provide a deformation zone 42 of the tubular intermediate piece 30.
  • This deformation zone 42 is located between a on the one hand the zone in which the finger 10 'is tightly adjusted in the tubular intermediate part 30 and on the other hand the zone in which the tubular intermediate part 30 is adjusted in the body 8.
  • This deformation zone 42 promotes the transmission of forces exerted by the pressure on the finger 10 'to the sensor 14.
  • the role of the spacer 38 is important here and great attention must be paid to the manufacture thereof.
  • the spacer 38 By retransmitting the forces exerted on the finger 10 ', the spacer 38 must not transmit parasitic forces. It should therefore be ensured that the spacer is not on the one hand in contact with the core 12 supplying the finger 10 'with electrical energy or on the other hand with the tubular intermediate piece 30. It should also be ensured that the own vibration mode of this spacer does not disturb the pressure measurement of the sensor 14. It is therefore necessary to ensure that the natural frequency of vibration of this spacer 38 is far from the bandwidth of the sensor 14. By choosing as material a ceramic to achieve this spacer 38, these problems can be solved. As shown in FIGS. 3 and 4, the head of the spark plug may include an overmolded piece 44 of synthetic material. This part can integrate the gripping zone 28 of the head of the spark plug.
  • a glow plug 2 comprising a pressure sensor 14 such as the spark plug described above allows an excellent measurement of the pressure inside a combustion chamber .
  • the measurement of this pressure is completely decoupled from parasitic noises which may disturb this pressure measurement.
  • the pressure sensor mounted in a glow plug according to the invention makes it possible to overcome the deformations of the head of the cylinder head induced by the stresses exerted by these injectors.
  • the deformation zone at the level of the intermediate tubular part makes it possible to obtain good sensitivity of the measurement carried out by the sensor. As is apparent in particular from FIG.
  • the glow plug according to the invention remains easy to assemble. It is a stack of parts that are sealed by an overmolding of synthetic material at the head of the candle.
  • the present invention is not limited to the preferred embodiment described above by way of nonlimiting example. It also relates to all the variant embodiments accessible to those skilled in the art. Thus for example the present invention is entirely possible with a conventional finger.
  • the design of the glow plug must then be adapted to this larger finger but it remains of course possible to introduce a tubular intermediate piece to effect decoupling between the body of the candle and the finger thereof.
  • the fastenings on the one hand of the finger in the intermediate tubular part and on the other hand of the tubular intermediate part in the body are provided above by tight fitting.
  • Another method of attachment can be envisaged.
  • An alternative to using a spacer can be considered.

Landscapes

  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Measuring Fluid Pressure (AREA)

Abstract

The inventive glow plug consists of a tubular body comprising means for fixing it into a bore, an intermediate tubular part (30) arranged inside said tubular body, a finger (10') disposed inside the intermediate tubular part (30) and comprising a heating electrode and a pressure sensor (14) connected by the face opposite with respect to the finger (10') to said intermediate tubular part (30), wherein the face of the sensor (14) disposed in front of the finger (10') is pressed thereto.

Description

Bougie de préchauffage comportant un capteur de pression Glow plug with pressure sensor
La présente invention concerne une bougie de préchauffage comportant un capteur de pression. Dans un moteur à combustion interne, notamment un moteur Diesel, on a remarqué que la connaissance de la valeur de la pression à l'intérieur de chaque cylindre permettait de mieux contrôler le déroulement de la combustion dans ce moteur. Cette information est alors utilisée pour réguler l'injection de carburant dans chacun des cylindres. Les émissions polluantes du moteur peuvent ainsi être réduites et la consommation optimisée. Dans un moteur de type Diesel, chaque cylindre comporte une bougie de préchauffage qui permet de réchauffer l'intérieur de la chambre de combustion correspondante, notamment au démarrage du moteur. Cette bougie de préchauffage est disposée dans un alésage taraudé qui traverse la tête de culasse du moteur. Il est déjà connu de l'homme du métier d'intégrer un capteur de pression à une bougie de préchauffage. De cette manière, l'alésage réalisé dans la tête de culasse pour recevoir la bougie de préchauffage sert également à accueillir un dispositif permettant la mesure de la pression à l'intérieur de la chambre de combustion. On évite ainsi de réaliser un alésage supplémentaire dans la tête de culasse. La réalisation d'un tel alésage créerait de nombreux problèmes dus notamment au manque de place ainsi que des problèmes de coût, d'étanchéité, d'accessibilité, etc.. Une bougie de préchauffage comporte généralement un corps tubulaire présentant sur sa surface extérieure une partie filetée permettant sa fixation dans un alésage correspondant réalisé dans une tête de culasse. Une partie de ce corps se trouve alors à l'intérieur de la chambre de combustion et l'autre partie à l'extérieur de celle-ci. La partie à l'intérieur de la chambre de combustion porte un doigt à l'intérieur duquel se trouve une électrode de chauffage. Cette électrode est alimentée par une âme qui traverse le corps tubulaire de la bougie. La partie à l'extérieur de la chambre de combustion est appelée aussi tête de bougie. Au niveau de cette tête, l'âme est connectée à une source d'énergie électrique. Il est connu également de placer un capteur de pression au niveau de cette tête. Ce capteur mesure alors la pression qui s'exerce sur le doigt de la bougie de préchauffage. Dans le principe, on mesure la force exercée par la pression régnant dans la chambre de combustion sur le doigt par une mesure des contraintes existant entre le corps de la bougie et le doigt. La figure 1 annexée représente une forme de réalisation d'une bougie de préchauffage de l'art antérieur munie d'un capteur de pression. Dans cette forme de réalisation de l'art antérieur, décrite plus dans le détail par la suite, le capteur de pression est disposé entre, d'une part le corps de la bougie de préchauffage sur lequel il prend appui et d'autre part un écrou solidaire de l'extrémité supérieure de l'âme alimentant en énergie électrique l'électrode de préchauffage. Dans cette forme de réalisation, l'âme traverse le capteur. Une telle bougie de l'art antérieur permet de mesurer la pression exercée à l'intérieur de la chambre de combustion sur le doigt de la bougie de préchauffage. Toutefois, d'autres signaux viennent se superposer au signal correspondant à la pression mesurée. Pour effectuer une bonne mesure de la pression régnant à l'intérieur de la chambre de combustion du cylindre considéré, il convient de pouvoir s'affranchir du bruit de fond perturbant la mesure de pression. Celui-ci provient par exemple des déformations de la tête de culasse dans laquelle est montée la bougie de préchauffage. La présente invention a alors pour but de fournir une bougie de préchauffage munie d'un capteur de pression pour laquelle le signal correspondant à la pression mesurée n'est pas perturbé par des signaux parasites. A cet effet, elle propose une bougie de préchauffage comportant : - un corps tubulaire présentant des moyens permettant sa fixation dans un alésage, - un doigt disposé à l'intérieur du corps de la bougie et recevant une électrode de chauffage, et - un capteur de pression. Selon l'invention, cette bougie de préchauffage comporte en outre une pièce tubulaire intermédiaire ; le doigt est disposé à l'intérieur de la pièce tubulaire intermédiaire ; la pièce tubulaire intermédiaire est montée dans le corps tubulaire ; le capteur est solidarisé par sa face opposée au doigt à la pièce tubulaire intermédiaire, et la face du capteur faisant face au doigt est en appui sur celui-ci. Le fait de munir la bougie de préchauffage d'une pièce tubulaire intermédiaire montée comme indiqué ci-dessus permet de réaliser un découplage entre le doigt et le corps de la bougie. Le capteur de pression peut alors mesurer les contraintes issues de la poussée exercée par la pression régnant dans la chambre de combustion correspondante sans être gêné par des contraintes existant au niveau de l'alésage dans lequel la bougie de préchauffage est fixée et qui sont retransmises en partie au corps de cette bougie. L'invention propose aussi que la pièce tubulaire intermédiaire soit solidarisée au corps tubulaire par exemple à proximité de l'extrémité du corps tubulaire destinée à se trouver dans la chambre de combustion du moteur correspondant. Cette zone du corps tubulaire n'est en général pas soumise à des contraintes extérieures et la pièce intermédiaire tubulaire ne subit alors pas de telles contraintes. Dans une forme de réalisation préférée, la pièce intermédiaire est fixée sur le corps tubulaire par ajustement serré. De la sorte, la fixation sur le corps tubulaire est excellente. On peut également envisager ici une fixation par soudure. De même le doigt peut être fixé à la pièce intermédiaire par ajustement serré ou par soudure. Pour une meilleure sensibilité de la mesure de pression au niveau du capteur de pression, la pièce tubulaire intermédiaire présente une zone de déformation entre une première zone de fixation dans laquelle le doigt est solidarisé à la pièce tubulaire intermédiaire et une seconde zone de fixation dans laquelle la pièce tubulaire intermédiaire est solidarisée au corps tubulaire. Cette zone de déformation donne de la liberté de mouvement au doigt et évite de créer une tension entre la pièce intermédiaire tubulaire et le doigt qui serait préjudiciable à la mesure de pression entre ces deux pièces. Pour transmettre les efforts exercés par la pression de la chambre de combustion sur le doigt de la bougie, une forme de réalisation prévoit qu'une entretoise est disposée entre le doigt et la face correspondante du capteur à l'intérieur de la pièce intermédiaire tubulaire. Cette entretoise doit être rigide pour pouvoir retransmettre des contraintes importantes. En outre, elle ne doit pas perturber la mesure de la pression par ses vibrations. Elle présente donc une fréquence propre de vibration très éloignée de la bande passante du capteur de pression. Pour satisfaire à ces exigences, l'entretoise est par exemple réalisée dans un matériau céramique. L'entretoise évoquée ci-dessus peut se présenter sous la forme d'une douille tubulaire allongée dont le diamètre extérieur est inférieur au diamètre intérieur de la pièce tubulaire intermédiaire et se termine du côté destiné à venir au contact du capteur de pression par une zone tronconique de manière à présenter une surface d'appui du côté du capteur correspondant sensiblement à la surface du capteur. Dans cette forme de réalisation, une pièce unique est disposée entre le doigt et le capteur. Ceci est bien entendu favorable pour une bonne transmission des efforts mais permet également un montage plus facile de la bougie de préchauffage. Dans la bougie de préchauffage selon l'invention, le capteur est de préférence placé dans un logement prévu à cet effet dans la pièce tubulaire intermédiaire, à proximité de l'extrémité de cette pièce opposée à celle destinée à recevoir le doigt et le capteur est par exemple solidarisé à la pièce tubulaire intermédiaire par un écrou présentant un filetage à sa périphérie extérieure engagé dans un taraudage réalisé à l'extrémité intérieure de la pièce tubulaire intermédiaire. Dans le cas où une entretoise est prévue entre le doigt et le capteur, l'écrou peut venir serrer le capteur contre cette entretoise. La présente invention concerne également un moteur à combustion interne, notamment moteur de type Diesel, caractérisé en ce qu'il comporte une bougie de préchauffage telle que décrite ci-dessus. Des détails et avantages de la présente invention ressortiront mieux de la description qui suit, faite en référence au dessin schématique annexé, sur lequel : La figure 1 représente une bougie de préchauffage de l'art antérieur, La figure 2 représente une bougie de préchauffage selon l'invention en vue de côté, La figure 3 représente la bougie de préchauffage de la figure 2 en coupe longitudinale, et La figure 4 est une vue en perspective éclatée de la bougie représentée sur les figures 2 et 3. La figure 1 représente en coupe longitudinale une bougie de préchauffage de l'art antérieur comportant un capteur de pression de façon à pouvoir mesurer la pression dans un cylindre de moteur. Cette bougie de préchauffage 2 est montée de manière classique dans un moteur 4, de type Diesel, et plus précisément dans une culasse 6 de ce moteur. Elle comporte un corps 8, un doigt 10, une âme 12 et un capteur de pression 14. Le corps 8 est adapté à être fixé au moteur 4 par vissage. A cet effet, la culasse 6 comporte un alésage taraudé la traversant et débouchant dans une chambre de combustion dudit moteur 4. Le corps 8 présente quant à lui sur sa surface extérieure un filetage 16 correspondant au taraudage réalisé dans la culasse 6. Lorsque le filetage 16 coopère avec l'alésage taraudé de la culasse 6, la bougie 2 étant dans sa position montée dans le moteur 4, une partie du corps 8 s'étend vers l'intérieur du moteur 4, c'est-à-dire vers la chambre de combustion, tandis qu'une autre partie s'étend vers l'extérieur du moteur. Le corps 8 est un corps tubulaire en acier à l'intérieur duquel sont logés notamment le doigt 10 et l'âme 12. Le doigt 10 fait saillie à partir du corps 8 tubulaire à l'intérieur de la chambre de combustion. Il est ajusté avec serrage à l'intérieur du corps 8 et reçoit une électrode de préchauffage (non représentée). L'âme 12 transmet l'énergie électrique à l'électrode située dans le doigt 10 et, de ce fait, est en contact avec cette électrode et est solidaire du doigt 10 qu'elle prolonge à l'intérieur du corps 8. A l'opposé du doigt 10, l'extrémité libre de l'âme 12 permet sa connexion électrique à un conducteur d'alimentation. Le capteur de pression est un capteur piézo-électrique. Il comprend un élément piézo-électrique 18 disposé entre deux éléments de contact 20 en matériau conducteur d'électricité, le capteur étant lui-même isolé électriquement du reste de la bougie 2 par des éléments électriquement isolants 22. Le capteur de pression 14 vient prendre appui par l'intermédiaire d'une pièce d'appui 24 sur le corps 8. L'autre face du capteur vient en appui contre un écrou 26 solidaire de l'extrémité supérieure de l'âme 12. La partie de la bougie de préchauffage 2 au niveau de laquelle se trouve le capteur de pression 14 et située à l'extérieur du moteur est aussi appelée tête de la bougie. Au niveau de cette tête, le corps 8 présente une zone de préhension 28 utilisée pour le montage/démontage de la bougie 2 par rapport à la culasse 6. Habituellement, cette zone de préhension 28 est cylindrique de section transversale hexagonale. De cette manière, à l'aide d'une clé, en accédant depuis l'extérieur de la culasse 6, le montage et le démontage de la bougie de préchauffage 2 sont possibles. Le diamètre de cette zone de préhension 28 est, de manière classique, supérieur à celui du reste du corps 8. La figure 2 représente, en vue extérieure une bougie de préchauffage selon l'invention. Par rapport à la bougie de la figure 1 , on retrouve un corps 8 présentant sur sa surface extérieure un filetage 16. On reconnaît également du côté de la tête du corps 8 une zone de préhension 28. A l'autre extrémité de la bougie de préchauffage, se trouve un doigt 10'. Il s'agit dans l'exemple de réalisation représenté au dessin d'un doigt 10' en céramique. Un tel doigt présente comme premier avantage d'avoir une durée de vie supérieure aux doigts que l'on rencontre généralement sur les bougies de l'art antérieur. Il est également de forme plus compacte. Ainsi, le doigt 10' est très sensiblement plus petit que le doigt 10 de la figure 1. La bougie de préchauffage des figures 2 à 4 comporte également un capteur de pression 14. Il s'agit ici aussi d'un capteur piézo-électrique qui peut être du même type que le capteur de la bougie de préchauffage représentée sur la figure 1. Alors que sur la figure 1 le doigt 10 est ajusté dans le corps 8, dans la forme de réalisation des figures 2 à 4, conformément à l'invention, une pièce intermédiaire tubulaire 30 est prévue entre le corps 8 et le doigt 10'. Cette pièce intermédiaire tubulaire 30 a pour but d'isoler le doigt 10' du corps 8. Une différence importante entre les capteurs de l'art antérieur et le capteur selon l'invention décrite ici est que le capteur de pression n'est plus disposé entre le corps 8 et le doigt 10 mais est disposé entre le doigt 10' et la pièce intermédiaire tubulaire 30. Ceci ressort notamment de la vue en coupe de la figure 3. Dans la suite de la description on considérera que les éléments destinés à se trouver du côté de la chambre de combustion lorsque la bougie de préchauffage est fixée dans le moteur sont à un niveau inférieur par rapport à des éléments se trouvant à l'extérieur de la chambre de combustion, du côté de la tête de la bougie de préchauffage. Le doigt 10' est ajusté avec serrage à l'extrémité inférieure de la pièce intermédiaire tubulaire 30. De manière classique, seule la partie supérieure du doigt 10' est à l'intérieur de la pièce tubulaire intermédiaire 30, la partie inférieure du doigt 10' faisant saillie en dehors de la pièce qui la porte pour se trouver à l'intérieur de la chambre de combustion et chauffer le mélange gazeux qui s'y trouve. L'âme 12 alimentant le doigt 10' en énergie électrique traverse toute la pièce tubulaire intermédiaire 30. La pièce tubulaire intermédiaire 30 est elle-même fixée par ajustement avec serrage à l'extrémité inférieure du corps 8 au niveau d'une zone de fixation 32. L'extrémité de la bougie est également appelée parfois cône de la bougie. Au dessus de la zone de fixation 32 de la pièce intermédiaire tubulaire 30 sur le corps 8, la pièce intermédiaire tubulaire 30 se prolonge librement à l'intérieur du corps 8. Dans sa partie supérieure, la pièce intermédiaire tubulaire 30 s'évase et présente une zone de plus grand diamètre servant de logement 34 pour le capteur 14. Dans la bougie, ce capteur 14 se trouve alors au niveau de la tête de la bougie, c'est-à- dire dans la partie supérieure de celle-ci. La partie supérieure de ce logement 34 est taraudée de manière à pouvoir recevoir un écrou 36 présentant un filetage extérieur. Cet écrou 36 est prévu pour exercer un appui sur la face supérieure du capteur 14. La face inférieure de ce capteur 14 repose quant à elle sur une entretoise 38. Cette dernière relie le doigt 10' au capteur 14. Le capteur 14 est par exemple un capteur du type de celui décrit en référence à la figure 1. Il est inutile de reprendre ici la description de ce capteur. La différence importante entre le capteur 14 de la figure 1 et celui des figures 2 à 4 est que sur la figure 1 le capteur 14 travaille à la traction alors que sur les figures 2 à 4 il travaille à la compression. Ceci ne modifie toutefois pas sa structure et on retrouve notamment un élément piézo-électrique disposé entre deux éléments de contact auxquels sont connectés des fils électriques. L'entretoise 38 est une pièce tubulaire pour permettre notamment en son centre le passage de l'âme 12. Le diamètre extérieur de l'entretoise 38 est adapté au diamètre intérieur de la pièce intermédiaire tubulaire 30 de manière à s'étendre librement à l'intérieur de celle-ci. Dans sa partie inférieure, le diamètre de l'entretoise 38 est adapté au diamètre du doigt 10'. Dans sa partie supérieure, le diamètre de l'entretoise 38 est adapté au diamètre de la face inférieure du capteur 14. Ainsi, comme on le remarque sur le dessin, l'entretoise présente dans sa partie supérieure une zone tronconique 40 permettant de passer progressivement du diamètre correspondant au doigt 10' à celui du capteur 14. Le capteur de pression 14 mesure ici uniquement les contraintes issues de la pression régnant dans la chambre de combustion correspondante. Les diverses contraintes s'exerçant sur la tête de culasse 6 dans laquelle la bougie de préchauffage est montée, notamment celles qui ne sont pas proportionnelles à la pression régnant dans la chambre de combustion, sont sans influence sur la pression mesurée par le capteur 14. En effet, la pièce tubulaire intermédiaire 30 est maintenue dans le corps 8 au niveau d'une zone du corps qui ne subit pas les contraintes liées à la déformation de la tête de culasse 6 dans laquelle la bougie de préchauffage 2 est vissée. En effet, ces déformations se retrouvent plutôt au niveau du filetage 16. Le capteur 14 est monté d'une part entre la pièce intermédiaire tubulaire 30 et d'autre part le doigt 10'. Pour obtenir une bonne mesure de la pression exercée sur le doigt 10', on veille dans la conception de la bougie de préchauffage à prévoir une zone de déformation 42 de la pièce intermédiaire tubulaire 30. Cette zone de déformation 42 se trouve entre d'une part la zone dans laquelle le doigt 10' est ajusté avec serrage dans la pièce intermédiaire tubulaire 30 et d'autre part la zone dans laquelle la pièce intermédiaire tubulaire 30 est ajustée dans le corps 8. Cette zone de déformation 42 favorise la transmission des efforts exercés par la pression sur le doigt 10' au capteur 14. Le rôle de l'entretoise 38 est ici important et une grande attention doit être portée à la fabrication de celle-ci. En retransmettant les efforts exercés sur le doigt 10', l'entretoise 38 ne doit pas transmettre de forces parasites. Il convient donc de veiller que l'entretoise ne soit pas d'une part en contact avec l'âme 12 alimentant le doigt 10' en énergie électrique ni d'autre part avec la pièce intermédiaire tubulaire 30. Il convient de veiller également que le mode de vibration propre de cette entretoise ne vienne pas perturber la mesure de pression du capteur 14. Il faut donc veiller à ce que la fréquence propre de vibration de cette entretoise 38 soit éloignée de la bande passante du capteur 14. En choisissant comme matériau une céramique pour réaliser cette entretoise 38, ces problèmes peuvent être résolus. Comme montré sur les figures 3 et 4, la tête de la bougie peut comporter une pièce surmoulée 44 en matière synthétique. Cette pièce peut intégrer la zone de préhension 28 de la tête de la bougie. Le surmoulage avec de la matière synthétique permet d'assurer une bonne étanchéité de la tête de la bougie de préchauffage et assure une bonne protection des connexions électriques des fils électriques reliés aux éléments de contact du capteur 14. Une bougie de préchauffage 2 comprenant un capteur de pression 14 telle la bougie décrite ci-dessus permet de réaliser une excellente mesure de la pression à l'intérieur d'une chambre de combustion. La mesure de cette pression est totalement découplée des bruits parasites pouvant venir perturber cette mesure de pression. Ainsi même dans le cas de la présence d'injecteurs placés au niveau de chaque cylindre, le capteur de pression monté dans une bougie de préchauffage selon l'invention permet de s'affranchir des déformations de la tête de la culasse induites par les contraintes exercées par ces injecteurs. La zone de déformation au niveau de la pièce tubulaire intermédiaire permet quant à elle d'obtenir une bonne sensibilité de la mesure réalisée par le capteur. Comme il ressort notamment de la figure 4, malgré l'introduction d'une pièce tubulaire intermédiaire, la bougie de préchauffage selon l'invention reste d'un montage aisé. Il s'agit d'un empilement de pièces que l'on vient sceller par un surmoulage de matière synthétique au niveau de la tête de la bougie. La présente invention ne se limite pas au mode de réalisation préférentiel décrit ci-dessus à titre d'exemple non limitatif. Elle concerne également toutes les variantes de réalisation accessibles à l'homme du métier. Ainsi par exemple la présente invention est tout à fait envisageable avec un doigt classique. Le dessin de la bougie de préchauffage doit alors être adapté à ce doigt de taille plus importante mais il reste bien entendu possible d'introduire une pièce intermédiaire tubulaire pour réaliser le découplage entre le corps de la bougie et le doigt de celle-ci. Les fixations d'une part du doigt dans la pièce tubulaire intermédiaire et d'autre part de la pièce intermédiaire tubulaire dans le corps sont prévues ci- dessus par ajustement serré. Un autre mode de fixation (soudure par exemple) peut être envisagé. Une alternative à l'utilisation d'une entretoise peut être envisagée. On peut par exemple prévoir un écrou solidaire de l'âme alimentant le doigt en énergie électrique et sur lequel viendrait reposer la face inférieure du capteur de pression. Pour encaisser les efforts mécaniques importants à transmettre, une rigidification de cette âme est alors peut être à envisager. Si cette entretoise est conservée, elle peut prendre une forme différente de celle décrite ci-dessus. Cette forme est avantageuse car elle permet de n'avoir qu'une seule pièce entre le doigt et le capteur. Toutefois on pourrait avoir une entretoise associée à une rondelle d'appui qui ferait l'interface entre l'entretoise et le capteur. The present invention relates to a glow plug comprising a pressure sensor. In an internal combustion engine, in particular a diesel engine, it has been noted that knowing the value of the pressure inside each cylinder makes it possible to better control the progress of combustion in this engine. This information is then used to regulate the injection of fuel into each of the cylinders. Pollutant emissions from the engine can thus be reduced and consumption optimized. In a diesel type engine, each cylinder has a glow plug which makes it possible to heat the interior of the corresponding combustion chamber, in particular when the engine is started. This glow plug is arranged in a threaded bore which passes through the cylinder head of the engine. It is already known to those skilled in the art to integrate a pressure sensor with a glow plug. In this way, the bore made in the cylinder head to receive the glow plug also serves to accommodate a device for measuring the pressure inside the combustion chamber. This avoids making an additional bore in the cylinder head. The realization of such a bore would create many problems due in particular to the lack of space as well as problems of cost, sealing, accessibility, etc. A glow plug generally comprises a tubular body having on its outer surface a threaded part allowing its fixing in a corresponding bore made in a cylinder head. Part of this body is then inside the combustion chamber and the other part outside of it. The part inside the combustion chamber has a finger inside which there is a heating electrode. This electrode is supplied by a core which crosses the tubular body of the spark plug. The part outside the combustion chamber is also called the spark plug head. At this head, the soul is connected to a source of electrical energy. It is also known to place a pressure sensor at this head. This sensor then measures the pressure exerted on the finger of the glow plug. In principle, we measure the force exerted by the pressure prevailing in the combustion chamber on the finger by a measurement of the stresses existing between the body of the candle and the finger. Figure 1 attached shows 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, on the one hand the body of the glow plug on which it is supported and on the other hand a nut secured to the upper end of the core supplying electrical energy to the preheating electrode. In this embodiment, the core passes through the sensor. Such a candle of the prior art makes it possible to measure the pressure exerted inside the combustion chamber on the finger of the glow plug. However, other signals are superimposed on the signal corresponding to the pressure measured. To make a good measurement of the pressure prevailing inside the combustion chamber of the cylinder in question, it is necessary to be able to overcome the background noise disturbing the pressure measurement. This comes for example from deformations of the cylinder head in which is mounted the glow plug. The object of the present invention is therefore to provide a glow plug fitted with a pressure sensor for which the signal corresponding to the measured pressure is not disturbed by spurious signals. To this end, it offers a glow plug comprising: - a tubular body having means allowing it to be fixed in a bore, - a finger disposed inside the body of the plug and receiving a heating electrode, and - a sensor pressure. According to the invention, this glow plug further comprises an intermediate tubular part; the finger is disposed inside the intermediate tubular part; the intermediate tubular part is mounted in the tubular body; the sensor is secured by its face opposite the finger to the intermediate tubular part, and the face of the sensor facing the finger is supported on the latter. The fact of providing the glow plug with a tubular part intermediate mounted as indicated above allows a decoupling between the finger and the body of the candle. The pressure sensor can then measure the stresses resulting from the thrust exerted by the pressure prevailing in the corresponding combustion chamber without being hindered by stresses existing at the level of the bore in which the glow plug is fixed and which are retransmitted in part to the body of this candle. The invention also proposes that the intermediate tubular part be secured to the tubular body, for example near the end of the tubular body intended to be in the combustion chamber of the corresponding engine. This area of the tubular body is generally not subjected to external stresses and the tubular intermediate piece is then not subjected to such stresses. In a preferred embodiment, the intermediate piece is fixed to the tubular body by tight fitting. In this way, the attachment to the tubular body is excellent. One can also consider here a fixing by welding. Similarly, the finger can be fixed to the intermediate piece by tight adjustment or by welding. For better sensitivity of the pressure measurement at the pressure sensor, the intermediate tubular part has a deformation zone between a first fixing zone in which the finger is secured to the intermediate tubular part and a second fixing zone in which the intermediate tubular part is secured to the tubular body. This deformation zone gives freedom of movement to the finger and avoids creating tension between the tubular intermediate piece and the finger which would be detrimental to the pressure measurement between these two pieces. To transmit the forces exerted by the pressure of the combustion chamber on the finger of the spark plug, an embodiment provides that a spacer is disposed between the finger and the corresponding face of the sensor inside the tubular intermediate piece. This spacer must be rigid in order to be able to transmit significant stresses. In addition, it must not disturb the measurement of the pressure by its vibrations. It therefore has a natural vibration frequency very far from the bandwidth of the pressure sensor. To meet these requirements, the spacer is for example made of a ceramic material. The spacer mentioned above can be in the form of an elongated tubular sleeve whose outside diameter is less than the inside diameter of the intermediate tubular part and ends on the side intended to come into contact with the pressure sensor by a zone frustoconical so as to have a bearing surface on the side of the sensor corresponding substantially to the surface of the sensor. In this embodiment, a single piece is disposed between the finger and the sensor. This is of course favorable for good transmission of forces but also allows easier mounting of the glow plug. In the glow plug according to the invention, the sensor is preferably placed in a housing provided for this purpose in the intermediate tubular part, near the end of this part opposite to that intended to receive the finger and the sensor is for example secured to the intermediate tubular part by a nut having a thread at its outer periphery engaged in a thread made at the inner end of the intermediate tubular part. In the case where a spacer is provided between the finger and the sensor, the nut can come to tighten the sensor against this spacer. The present invention also relates to an internal combustion engine, in particular a diesel type engine, characterized in that it includes a glow plug as described above. Details and advantages of the present invention will emerge more clearly from the description which follows, given with reference to the appended diagrammatic drawing, in which: FIG. 1 represents a glow plug of the prior art, FIG. 2 represents a glow plug according to the invention in side view, Figure 3 shows the glow plug of Figure 2 in longitudinal section, and Figure 4 is an exploded perspective view of the candle shown in Figures 2 and 3. Figure 1 shows in longitudinal section of a glow plug of the prior art comprising a pressure sensor so as to be able to measure the pressure in an engine cylinder. This glow plug 2 is conventionally mounted in an engine 4, of the type Diesel, and more precisely in a cylinder head 6 of this 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 motor 4 by screwing. For this purpose, the cylinder head 6 has a threaded bore passing through it and opening into a combustion chamber of said engine 4. The body 8 has meanwhile on its outer surface a thread 16 corresponding to the thread made in the cylinder head 6. When the thread 16 cooperates with the threaded bore of the cylinder head 6, the spark plug 2 being in its position mounted in the engine 4, a part of the body 8 extends towards the inside of the engine 4, that is to say towards the combustion chamber, while another part extends towards the outside of the engine. The body 8 is a tubular steel body inside of 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 tightened inside the body 8 and receives a preheating electrode (not shown). The core 12 transmits electrical energy to the electrode located in the finger 10 and, therefore, is in contact with this electrode and is integral with the finger 10 which it extends inside the body 8. A l opposite the finger 10, the free end of the core 12 allows its electrical connection to a supply conductor. The pressure sensor is a piezoelectric sensor. It comprises a piezoelectric element 18 disposed between two contact elements 20 of electrically conductive material, the sensor itself being electrically isolated from the rest of the spark plug 2 by electrically insulating elements 22. The pressure sensor 14 comes to take support by means of a support piece 24 on the body 8. The other face of the sensor bears against a nut 26 secured to the upper end of the core 12. The part of the glow plug 2 at which is the pressure sensor 14 and located outside the engine is also called the spark plug head. At this head, the body 8 has a gripping zone 28 used for mounting / dismounting the spark plug 2 relative to the cylinder head 6. Usually, this gripping zone 28 is cylindrical with a hexagonal cross section. In this way, using a key, accessing from outside the cylinder head 6, the mounting and dismounting of the glow plug 2 are possible. The diameter of this gripping zone 28 is, conventionally, greater than that of the rest of the body 8. Figure 2 shows, in external view a glow plug according to the invention. Compared to the candle of Figure 1, there is a body 8 having on its outer surface a thread 16. We also recognize on the side of the head of the body 8 a gripping area 28. At the other end of the candle preheating, there is a 10 'finger. In the example of embodiment shown in the drawing, there is a 10 ′ ceramic finger. Such a finger has the first advantage of having a longer lifespan than the fingers that are generally encountered on candles of the prior art. It is also more compact in shape. Thus, the finger 10 ′ is very substantially smaller than the finger 10 in FIG. 1. The glow plug in FIGS. 2 to 4 also includes a pressure sensor 14. This is also a piezoelectric sensor here. which can be of the same type as the glow plug sensor shown in Figure 1. While in Figure 1 the finger 10 is adjusted in the body 8, in the embodiment of Figures 2 to 4, in accordance with l invention, a tubular intermediate piece 30 is provided between the body 8 and the finger 10 '. The purpose of this tubular intermediate piece 30 is 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 arranged between the body 8 and the finger 10 but is disposed between the finger 10 'and the tubular intermediate piece 30. This appears in particular from the sectional view of FIG. 3. In the following description, it will be considered that the elements intended to be find on the side of the combustion chamber when the glow plug is fixed in the engine are at a lower level compared to elements located outside the combustion chamber, on the side of the head of the glow plug . The finger 10 'is tightly adjusted to the lower end of the tubular intermediate piece 30. Conventionally, only the upper part of the finger 10' is inside the intermediate tubular part 30, the lower part of the finger 10 'protruding outside the room that carries it to be inside the combustion chamber and heat the gas mixture 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 by adjustment with clamping at the lower end of the body 8 at a fixing zone 32. The tip of the candle is also sometimes called the candle cone. Above the attachment zone 32 of the tubular intermediate piece 30 on the body 8, the tubular intermediate piece 30 extends freely inside the body 8. In its upper part, the tubular intermediate piece 30 widens and has a larger diameter area serving as a housing 34 for the sensor 14. In the spark plug, this sensor 14 is then located at the head of the spark plug, that is to say in the upper part of the latter. The upper part of this housing 34 is tapped so as to be able to receive a nut 36 having an external thread. This nut 36 is designed to exert a support on the upper face of the sensor 14. The lower face of this sensor 14 rests on a spacer 38. The latter connects 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 unnecessary to repeat the description of this sensor here. The important difference 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 while in FIGS. 2 to 4 it works in compression. This does not, however, modify its structure and there is in particular a piezoelectric element disposed between two contact elements to which electrical wires are connected. The spacer 38 is a tubular piece to allow in particular at its center the passage of the core 12. The outside diameter of the spacer 38 is adapted to the inside diameter of the tubular intermediate piece 30 so as to extend freely to the inside of it. In its lower part, the diameter of the spacer 38 is adapted to the diameter of the finger 10 '. In its upper part, the diameter of the spacer 38 is adapted to the diameter of the lower face of the sensor 14. Thus, as can be seen in the drawing, the spacer has in its upper part a frusto-conical area 40 allowing to pass gradually of the diameter corresponding to the finger 10 'to that of the sensor 14. The pressure sensor 14 here measures only the stresses resulting from the pressure prevailing in the corresponding combustion chamber. The various stresses exerted on the cylinder head 6 in which the glow plug is mounted, in particular those which are not proportional to the pressure prevailing in the combustion chamber, have no influence on the pressure measured by the sensor 14. In Indeed, the intermediate tubular part 30 is maintained in the body 8 at a region of the body which does not undergo the stresses linked to the deformation of the cylinder head 6 in which the glow plug 2 is screwed. Indeed, these deformations are found more at the level of the thread 16. The sensor 14 is mounted on the one hand between the tubular intermediate piece 30 and on the other hand the finger 10 '. To obtain a good 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 tubular intermediate piece 30. This deformation zone 42 is located between a on the one hand the zone in which the finger 10 'is tightly adjusted in the tubular intermediate part 30 and on the other hand the zone in which the tubular intermediate part 30 is adjusted in the body 8. This deformation zone 42 promotes the transmission of forces exerted by the pressure on the finger 10 'to the sensor 14. The role of the spacer 38 is important here and great attention must be paid to the manufacture thereof. By retransmitting the forces exerted on the finger 10 ', the spacer 38 must not transmit parasitic forces. It should therefore be ensured that the spacer is not on the one hand in contact with the core 12 supplying the finger 10 'with electrical energy or on the other hand with the tubular intermediate piece 30. It should also be ensured that the own vibration mode of this spacer does not disturb the pressure measurement of the sensor 14. It is therefore necessary to ensure that the natural frequency of vibration of this spacer 38 is far from the bandwidth of the sensor 14. By choosing as material a ceramic to achieve this spacer 38, these problems can be solved. As shown in FIGS. 3 and 4, the head of the spark plug may include an overmolded piece 44 of synthetic material. This part can integrate the gripping zone 28 of the head of the spark plug. Overmolding with synthetic material ensures good sealing of the glow plug head and provides good protection of connections electrical wires connected to the contact elements of the sensor 14. A glow plug 2 comprising a pressure sensor 14 such as the spark plug described above allows an excellent measurement of the pressure inside a combustion chamber . The measurement of this pressure is completely decoupled from parasitic noises which may disturb this pressure measurement. Thus even in the case of the presence of injectors placed at the level of each cylinder, the pressure sensor mounted in a glow plug according to the invention makes it possible to overcome the deformations of the head of the cylinder head induced by the stresses exerted by these injectors. The deformation zone at the level of the intermediate tubular part makes it possible to obtain good sensitivity of the measurement carried out 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 stack of parts that are sealed by an overmolding of synthetic material at the head of the candle. The present invention is not limited to the preferred embodiment described above by way of nonlimiting example. It also relates to all the variant embodiments accessible to those skilled in the art. Thus for example the present invention is entirely possible with a conventional finger. The design of the glow plug must then be adapted to this larger finger but it remains of course possible to introduce a tubular intermediate piece to effect decoupling between the body of the candle and the finger thereof. The fastenings on the one hand of the finger in the intermediate tubular part and on the other hand of the tubular intermediate part in the body are provided above by tight fitting. Another method of attachment (welding for example) can be envisaged. An alternative to using a spacer can be considered. One can for example provide a nut integral with the core supplying the finger with electrical energy and on which the underside of the pressure sensor would come to rest. To absorb the significant mechanical forces to be transmitted, this stiffening of this core can then be considered. If this spacer is kept, it may take a different form from that described above. This shape is advantageous because it allows to have only one piece between the finger and the sensor. However, there could be a spacer associated with a support washer which would interface between the spacer and the sensor.

Claims

REVENDICATIONS
1. Bougie de préchauffage (2) comportant : - un corps (8) tubulaire présentant des moyens (16) permettant sa fixation dans un alésage, - un doigt (10') disposé à l'intérieur du corps (8) de la bougie et recevant une électrode de chauffage, et - un capteur de pression (14), caractérisée en ce qu'elle comporte une pièce tubulaire intermédiaire (30), en ce que le doigt (10') est disposé à l'intérieur de la pièce tubulaire intermédiaire (30), en ce que la pièce tubulaire intermédiaire (30) est montée dans le corps (8) tubulaire, en ce que le capteur (14) est solidarisé par sa face opposée au doigt (10') à la pièce tubulaire intermédiaire (30), et en ce que la face du capteur (14) faisant face au doigt (10') est en appui sur celui-ci. 1. Glow plug (2) comprising: - a tubular body (8) having means (16) allowing it to be fixed in a bore, - a finger (10 ') disposed inside the body (8) of the candle and receiving a heating electrode, and - a pressure sensor (14), characterized in that it comprises an intermediate tubular part (30), in that the finger (10 ') is disposed inside the part intermediate tubular (30), in that the intermediate tubular part (30) is mounted in the tubular body (8), in that the sensor (14) is secured by its face opposite the finger (10 ') to the tubular part intermediate (30), and in that the face of the sensor (14) facing the finger (10 ') is supported thereon.
2. Bougie de préchauffage (2) selon la revendication 1 , caractérisée en ce que la pièce tubulaire intermédiaire (30) est solidarisée au corps (8) tubulaire à proximité de l'extrémité du corps (8) tubulaire destinée à se trouver dans la chambre de combustion du moteur (4) correspondant. 2. Glow plug (2) according to claim 1, characterized in that the intermediate tubular part (30) is secured to the tubular body (8) near the end of the tubular body (8) intended to be in the combustion chamber of the corresponding engine (4).
3. Bougie de préchauffage (2) selon l'une des revendications 1 ou 2, caractérisée en ce que la pièce intermédiaire (30) est fixée sur le corps (8) tubulaire par ajustement serré. 3. Glow plug (2) according to one of claims 1 or 2, characterized in that the intermediate piece (30) is fixed to the tubular body (8) by tight adjustment.
4. Bougie de préchauffage (2) selon l'une des revendications 1 ou 2, caractérisée en ce que la pièce intermédiaire (30) est soudée sur le corps (8) tubulaire. 4. Glow plug (2) according to one of claims 1 or 2, characterized in that the intermediate piece (30) is welded to the tubular body (8).
5. Bougie de préchauffage (2) selon l'une des revendications 1 à 4, caractérisée en ce que le doigt (10') est fixé à la pièce intermédiaire (30) par ajustement serré. 5. Glow plug (2) according to one of claims 1 to 4, characterized in that the finger (10 ') is fixed to the intermediate piece (30) by tight adjustment.
6. Bougie de préchauffage (2) selon l'une des revendications 1 à 4, caractérisée en ce que le doigt (10') est soudé à la pièce intermédiaire (30). 6. Glow plug (2) according to one of claims 1 to 4, characterized in that the finger (10 ') is welded to the intermediate piece (30).
7. Bougie de préchauffage (2) selon l'une des revendications 1 à 6, caractérisé en ce que la pièce tubulaire intermédiaire (30) présente une zone de déformation (42) entre une première zone de fixation dans laquelle le doigt (10') est solidarisé à la pièce tubulaire intermédiaire (30) et une seconde zone de fixation (32) dans laquelle la pièce tubulaire intermédiaire (30) est solidarisée au corps tubulaire (8). 7. Glow plug (2) according to one of claims 1 to 6, characterized in that the intermediate tubular part (30) has a deformation zone (42) between a first fixing zone in which the finger (10 ' ) is secured to the intermediate tubular part (30) and a second fixing zone (32) in which the intermediate tubular part (30) is secured to the tubular body (8).
8. Bougie de préchauffage (2) selon l'une des revendications 1 à 7, caractérisée en ce qu'une entretoise (38) est disposée entre le doigt (10') et la face correspondante du capteur (14) à l'intérieur de la pièce intermédiaire tubulaire (30). 8. Glow plug (2) according to one of claims 1 to 7, characterized in that a spacer (38) is disposed between the finger (10 ') and the corresponding face of the sensor (14) inside of the tubular intermediate piece (30).
9. Bougie de préchauffage (2) selon la revendication 8, caractérisée en ce que l'entretoise (38) est réalisée dans un matériau céramique. 9. Glow plug (2) according to claim 8, characterized in that the spacer (38) is made of a ceramic material.
10. Bougie de préchauffage (2) selon l'une des revendications 8 ou 9, caractérisée en ce que l'entretoise (38) se présente sous la forme d'une douille tubulaire allongée dont le diamètre extérieur est inférieur au diamètre intérieur de la pièce tubulaire intermédiaire (30) et se termine du côté destiné à venir au contact du capteur de pression par une zone tronconique (40) de manière à présenter une surface d'appui du côté du capteur (14) correspondant sensiblement à la surface du capteur (14). 10. Glow plug (2) according to one of claims 8 or 9, characterized in that the spacer (38) is in the form of an elongated tubular sleeve whose outside diameter is less than the inside diameter of the intermediate tubular part (30) and ends on the side intended to come into contact with the pressure sensor by a frustoconical zone (40) so as to have a bearing surface on the sensor side (14) corresponding substantially to the surface of the sensor (14).
11. Bougie de préchauffage (2) selon l'une des revendications 1 à 10, caractérisée en ce que le capteur (14) est placé dans un logement (34) prévu à cet effet dans la pièce tubulaire intermédiaire (30), à proximité de l'extrémité de cette pièce opposée à celle destinée à recevoir le doigt (10') et en ce que le capteur (14) est solidarisé à la pièce tubulaire intermédiaire (30) par un écrou (36) présentant un filetage à sa périphérie extérieure engagé dans un taraudage réalisé à l'extrémité intérieure de la pièce tubulaire intermédiaire (30). 11. Glow plug (2) according to one of claims 1 to 10, characterized in that the sensor (14) is placed in a housing (34) provided for this purpose in the intermediate tubular part (30), near from the end of this piece opposite to that intended to receive the finger (10 ') and in that the sensor (14) is secured to the intermediate tubular piece (30) by a nut (36) having a thread at its periphery outer engaged in a thread made at the inner end of the intermediate tubular part (30).
12. Moteur à combustion interne (4), notamment moteur de type Diesel, caractérisé en ce qu'il comporte une bougie de préchauffage (2) selon l'une des revendications 1 à 11. 12. Internal combustion engine (4), in particular diesel type engine, characterized in that it includes a glow plug (2) according to one of claims 1 to 11.
EP05758221A 2004-04-27 2005-04-19 Glow plug provided with a pressure sensor Withdrawn EP1747406A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR0404463A FR2869391B1 (en) 2004-04-27 2004-04-27 PREHEATING CUP INCLUDING A PRESSURE SENSOR
PCT/FR2005/000932 WO2005111501A1 (en) 2004-04-27 2005-04-19 Glow plug provided with a pressure sensor

Publications (1)

Publication Number Publication Date
EP1747406A1 true EP1747406A1 (en) 2007-01-31

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP05758221A Withdrawn EP1747406A1 (en) 2004-04-27 2005-04-19 Glow plug provided with a pressure sensor

Country Status (7)

Country Link
US (1) US7825352B2 (en)
EP (1) EP1747406A1 (en)
JP (1) JP4662981B2 (en)
KR (1) KR20070043931A (en)
CN (1) CN100543370C (en)
FR (1) FR2869391B1 (en)
WO (1) WO2005111501A1 (en)

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DE102013200285B4 (en) * 2013-01-11 2021-11-04 Robert Bosch Gmbh Ceramic mass, its use and ceramic injection molding process
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Also Published As

Publication number Publication date
CN100543370C (en) 2009-09-23
KR20070043931A (en) 2007-04-26
JP4662981B2 (en) 2011-03-30
JP2007534918A (en) 2007-11-29
FR2869391B1 (en) 2006-07-14
US20070227487A1 (en) 2007-10-04
FR2869391A1 (en) 2005-10-28
US7825352B2 (en) 2010-11-02
WO2005111501A1 (en) 2005-11-24
CN1954179A (en) 2007-04-25

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