FR2845462A1 - Pre-heating plug for internal combustion engine includes vibration damping system to reduce noise on pressure output signal - Google Patents

Abstract

The pre-heating plug has an internal axis comprising a low part (204m) and a high part (204n) fixed to the envelope (201). The distance between the two fixings (204m, 204n) may be approximately 80 mm, nearly double the length of that in other designs, with a vibration damper (230) to reduce production of noise on the output signal. The pre-heating plug is designed to suppress the noise produced by the vibration of the internal axis (204) of the plug (100), which has a combustion pressure sensor (300), on the output signal from the pressure sensor. The low part (first part of fixing 204m) of the internal axis (204) is inserted and fixed in the tubular element (202). The high part (second part of fixing 204n) of the internal axis is fixed to the envelope (201). The distance between the two fixings (204m, 204n) may be approximately 80 mm, nearly double the length of that in other designs, having bearing on the engine casing thickness and the installation space in an engine with multiple valves. A vibration damper (230) is mounted around the internal axis, between the two fixing points.

Description

PREHEATING PLUG WITH COMBUSTION PRESSURE SENSOR

  The present invention relates to a glow plug with a combustion pressure sensor for detecting the combustion pressure in

  a combustion chamber of an engine.

  As shown in a cross-sectional view in FIG. 7, one of the prior art combustion pressure sensor glow plugs is shown in JP2001-124336A (pages 3-5, FIG.

attached to this document).

  A glow plug J100, shown in FIG. 7, has a cylindrical body or shell 201 for screwing into a motor yoke 1, thereby having a first end thereof in a combustion chamber of a engine. Thus, a thread 201b is formed on an outer surface of the body 201. In addition, inside the body 201 is fixed a tubular element 202

  having a first end protruding from said end of the body 201.

  The tubular element 202 is fixed by welding or press fitting into a fixing portion Ki at said end of the body 201.

  In addition, inside the tubular element 202 is disposed a heating element 203. In addition, insulating powder 205 is charged between the tubular element 202 and the heating element 203 thus constituting a

heating device 206.

  In addition, inside the body 201 is disposed an inner shaft 204 serving as a metal electrode. A first end of the inner axis 204 is electrically connected to the heating element 203, while its other end

  protrudes from the other end of the body 201.

  The first end of the inner shaft 204 is attached to the tubular member 202, while the other end of the inner shaft 204 is secured to the body 201 through a cylindrical ring 207 (the sealing).

  Furthermore, at the other end of the body 201 is disposed a combustion pressure sensor 300 which is fixed to the body 201 by a nut 211 disposed at

  the other end of the inner axis 204.

  The combustion pressure sensor 300 is constituted by a piezoelectric device for detecting the combustion pressure from a transfer of a force acting on the tubular element 202, when the pressure of

  combustion is generated in the engine.

  The glow plug J100 is caused to vibrate to a very small extent by an axial load due to the combustion pressure (pressure inside the combustion chamber 1a).

  The internal axis 204 attached to the tubular element 202 also vibrates under the effect of the displacement of the tubular element 202, which modifies a load exerted on the combustion pressure sensor 300 by the nut 211, the pressure of combustion being detected from a signal provided by the combustion pressure sensor 300.

  Recently, direct fuel injection has been the rule in diesel engines because of improved fuel consumption and exhaust control. Thus, the glow plug evolves in

  according to the above evolution of the motor system.

  Specifically, the thicker the cylinder head of the engine, the more the four-valve engine is used and the space for installation of the glow plug is limited, and it was necessary to design a longer glow plug, for example two times longer compared to the classic candle

  for the engine according to the prior art.

  Thus, the inner axis 204 becomes about twice as long.

  Therefore, a support distance "t" between the two attachment points for the inner axis 204 becomes, for example, about 80 mm.

  In this way, the resonance frequency of the inner axis 204 changes to a lower frequency range, for example less than 5 kHz, and the amplitude of the vibrations increases. Consequently, the mechanical vibrations of the internal axis 204 are transmitted to the combustion pressure sensor 300, thereby superimposing a noise of mechanical vibrations on the combustion pressure signal and on

  lowering the signal-to-signal ratio of the sensor signal.

  The present invention aims at eliminating a noise, caused by vibrations of an internal axis of a combustion pressure sensor glow plug, merging with the signal delivered by the sensor, even if the internal axis

becomes longer.

  An engine glow plug according to the present invention comprises five elements: (1) an envelope having a first end attached to a motor yoke; (2) a tubular member mounted within the casing and projecting from the first end of the casing; (3) a heating device disposed within the tubular member; (4) an inner shaft mounted within the casing, inserted through a first end into the tubular member and electrically connected to the heater; and (5) a combustion pressure sensor housed inside the casing at another end of the casing and detecting a

  combustion pressure transmitted via the inner axis.

  Their mutual relations with respect to the structure are such that: a first end of the inner axis is attached to the tubular member; another end of the inner shaft is attached to the casing; and at least one vibration damper for damping vibrations of the inner shaft is disposed about the axis

  interior between the first and the other ends of the inner axis.

  Because of the vibration damper, the vibrations of the inner shaft can be suppressed, even if the length of the inner shaft is increased to accommodate conditions imposed by the motor. Therefore, noise superimposed on the combustion pressure as a result of vibrations of the inner axis can be suppressed. Moreover, the results of experiments conducted by the inventors have shown that it is preferable that the distance between the vibration damper and the first end of the internal axis is less than or equal to 40 mm; the distance between the vibration dampers when there are at least two is less than or equal to 40 mm; and the distance between the vibration damper and the other end of

  the inner axis is less than or equal to 40 mm.

  Thus, if the support distance (length of the non-fixed portion) of the inner axis is less than or equal to 40 mm, the noise level of the combustion pressure signal can be suppressed to a degree necessary for a

practical use.

  Moreover, the results of experiments carried out by the inventors have shown that it is preferable for the hardness of the vibration damper to be greater than or equal to a durometer hardness A 10 and less than or equal to one.

hardness D90 according to durometer.

  If the vibration damper is too soft, the retaining force of the inner shaft weakens. On the other hand, if it is too hard, it retains too much the inner shaft, which prevents the combustion pressure from being transmitted to the pressure sensor via the inner shaft. As a result, it has been found that

  hardness range of the vibration damper indicated above is suitable.

  In addition, rubber or resin is preferably used for the vibration damper.

  The invention and many of the advantages attached to it will become readily apparent with reference to the following description, given in

  FIG. 1 is a cross-sectional elevational view of a combustion pressure sensor preheating plug according to the present invention. The glow plug is attached to a diesel engine cylinder head; Figure 2 is an enlarged cross-sectional view of the combustion pressure sensor; FIG. 3 is a partial cross-sectional view of the preheating plug of the present invention for explaining a simplified model of combustion pressure transmission paths; Figures 4a and 4b show the combustion pressure wave signal detected by a pressure sensor and a pressure gauge; FIG. 5 illustrates an experimental relationship between a noise level in% and the hardness of the vibration damper, the noise ratio being a ratio of a noise width to a maximum height "h" in the pressure signal the sensor, shown in Figure 4a; Fig. 6 shows a glow plug variant according to the present invention, wherein a plurality of vibration dampers are disposed between the first and second inner shaft attachment portions which are inserted into the glow plug casing. ; and Fig. 7 is a cross-sectional view of a prior art glow plug with a combustion pressure sensor disclosed in JP2001-124336A (pages 3 - 5, Fig. 1). 30

  With reference to the drawings, a preferred embodiment of the present invention will now be explained.

  The structure of the glow plug Figure 1 is a cross-sectional elevational view of a glow plug with a combustion pressure sensor according to the present invention.

  invention. The glow plug is attached to a cylinder head of a diesel engine.

  The glow plug 100 comprises: a candle body 200 which is provided with a heater and transmits the combustion pressure; and a combustion pressure sensor 300 for detecting a combustion pressure by transforming into a piezoelectric signal a force applied to the body 200 of the

  candle due to the combustion pressure.

  The candle body 200 comprises: a cylindrical metal casing 201 screwed into a motor yoke 1 so that a first end (the bottom) of the casing 201 is disposed in a combustion chamber 1a, while its other end (the top) is disposed outside the cylinder head 1 of the engine; a cylindrical tubular element (cladding tube) 202 fixed in the casing 201 so that a first end of the tubular member 202 projects from the casing 201 while its other end is fixed inside the casing 201; envelope 201; a heating element (heating coil) 203 fixed to the first end of the tubular element 202; and an inner shaft 204 constituting an electrode rod secured within the housing 201 such that a first end of the inner shaft is inserted into the tubular member 202 and is electrically connected to the heating element 203 while his other end

  protrudes from the other end of the envelope 201.

  In the cylinder head 1 of the engine is a female thread in which the male thread of the body 200 of the candle is screwed in the axial direction of the candle (longitudinal direction of the candle).

  The casing 201 is made of sulfur-free cutting tool steel or carbon steel. The radius of a first end (the lower end or the combustion chamber side) of the envelope 201 is smaller than that of its other stepped end. The first end of the casing 201 is provided with a thread 201b.

  Around the outer surface of the other end of the casing 201 is formed a hexagonal head 201a for screwing the body 200 of the spark plug into the cylinder head 1 of the engine. A portion of each corner of the hex head 201 may be partially removed to reduce the size of its outer shape.

  In addition, at the first end (lower end) of the casing 201 is formed a conical seat surface 201c closely in contact with another seat surface of the thread of the cylinder head 1 of the engine, which prevents the

gas leaks.

  The tubular element (cladding tube) 202 is made of refractory alloy and

  A first end of the tubular member 202 is closed and protrudes from the first end of the casing 201 and is provided with the heating element (FIG. heating coil 203) of NiCr and CoFe, while its other end disposed inside the casing 201 is open.

  The first end (lower end) of the inner shaft 204 is inserted into the other end (upper end) of the tubular member 202.

  The other end (upper end) of the heating element 203 is attached to the first end (lower end) of the tubular member 202, while the first end (lower end) of the heating element 203 is attached to the first end (lower end) of the inner axis 204. In addition, an insulating powder 205, for example magnesium oxide, is charged between

  the heating element 203 and the inner axis 204 and the tubular element 202.

  The inner surface of the tubular member 202 is hammered to increase the loading density of the insulating powder 205, which improves the thermal conductivity and strongly retains the inner axis 204 and the element.

heating 203.

  In the present embodiment, the tubular member 202, the heating element 203 and the insulating powder 205 constitute a heater 206 which is secured within the first end of the casing 201. The end of the device 206 (the first end of the tubular element 202) is arranged

in the combustion chamber la.

  The outer surface of the heater 206 (outer surface of the tubular member 202) is tightly fitted or welded to the inner surface of the casing 201, thereby forming a fastening portion K 1. Thus, the combustion gases of the fuel coming from the combustion chamber la can not

  not penetrate inside the envelope 201.

  The fixing portion KI covers the inner peripheral surface of the heater 206 and the casing 201 in the axial direction.

  Furthermore, between the other end of the tubular element 202 and the inner shaft 204 is disposed a sealing member 205a for enclosing the powder

insulation 205 during hammering.

  On the other hand, the cylindrical ring 207, for example of silicone rubber, of fluororubber, EPDM, NBR or H-NBR, is inserted from the other end (upper end) of the inner shaft 204 to the other. inside each other

  end (upper end) of the envelope 201.

  The cylindrical ring 207 serves for centering and suppressing vibrations of the inner shaft 204 and for sealing against water and airtightness of the casing 201. If the inner surface of the casing 201 envelope in contact with the cylindrical ring 207 is conical, it is in closer contact, which improves

  watertightness and airtightness.

  In addition, an insulating sleeve 210, for example made of resin (phenolic resin, PPS, laminated mica) or ceramic material such as alumina, is disposed at the other end of the internal axis 204. Furthermore, a hole 201d is formed in the hexagonal head 201a and a receiving portion 201e is formed between the

  hole 201d and the outer surface of the inner axis 204.

  Thus, the cylindrical sensor 300 of combustion pressure is received in the receiving part 201e. After the combustion pressure sensor 300 has been attached, the insulating sleeve 210 is put in place on the inner shaft 204, the nut 211 is tightened tight onto a thread 204a, thereby pushing the sensor downwardly. 300 from

  combustion pressure under the effect of an axial force exerted by the nut 211.

  On the other hand, an O-ring 208 is disposed between the inner surface of the hole 201d and the outer surface of the combustion pressure sensor 300, while the cylindrical ring 209 is inserted between the inner surface of the combustion pressure sensor 300 and the outer surface of the inner shaft 204. The O-ring 208 and the cylindrical ring 209 are for example in

  silicone rubber, fluorinated rubber, EPDM, NBR or H-NRB.

  In this case, the O-ring 208 serves to seal the water and the airtightness of the shell 201, while the cylindrical ring 209 serves

  both to provide watertightness and airtightness of the casing 201 and to suppress vibrations of the inner shaft 204. If the inner peripheral surface of the combustion pressure sensor 300 which In contact with the cylindrical ring 209 is tapered, this is in closer contact, which improves watertightness and airtightness.

  Furthermore, the combustion pressure sensor 300 is electrically isolated from the nut 211 by the insulating sleeve 210. Moreover, the combustion pressure sensor 300 is electrically isolated from the internal axis 204.

by the cylindrical ring 209.

  Moreover, a connecting bar 2 is fixed to the thread 204a by

  a nut 212. The connection bar 2 is electrically connected to a power source not shown and is electrically connected to the cylinder head 1 of the engine via the internal axis 204, the heating element 203, the tubular element 202 and the casing 201. Under these conditions, the cylinder head 1 of the motor is grounded.

  Thus, the glow plug 100 presented above heats the heating device 206 to contribute to the ignition of the diesel engine.

  For connections between the engine valves, it is preferable to use very flexible conductive cables (automotive wiring), which allows each tubular member 202 for each valve to vibrate independently. The vibration damper The (first) lower fastening portion 204m of the inner shaft 204 is inserted into and attached to the tubular member 202 by hammering the tubular member 202 and using the insulating powder 205 On the other hand, the (second) upper attachment portion 204n of the inner axis 204 is attached to the casing 201 by

  via the cylindrical ring 207.

  The length between the two attachment portions 204m and 204n becomes, for example, about 80mm, about twice the conventional length.

  Furthermore, the glow plug 100 according to the present invention is characterized in that a vibration damper 230 is disposed about the inner axis between the fastening portions 204m and 204n. The shock absorber

  vibration 230 serves to damp the vibrations of the inner axis 204.

  The vibration damper 230 is of rubber or resin of which.

  the hardness is preferably greater than or equal to Al 0 from durometer and less than or equal to D90 from durometer. Moreover, the vibration damper 230 can

  be a molded piece or a liquid that eventually solidifies or gels.

  The rubber molded part may be fluororubber, silicone rubber, H-NBR. In addition, the resin molded part may be epoxy, phenolic or acrylic resin. On the other hand, the liquid rubber may be an ordinary temperature polymerization, hot polymerization or multiple polymerization type silicone rubber. On the other hand, the liquid resin may be a heat-curing epoxy resin or a multi-polymerization type. The pressure sensor

  Figure 2 is an enlarged cross-sectional view of the combustion pressure sensor 300.

  The pressure sensor 300 is an assembly in which a piezoelectric ceramic material 302 of annular shape is held between an envelope

  metal ring 303 and an annular electrode 301.

  The piezoelectric ceramic material 302 is for example made of lead titanate or lead titanate and zirconate, for example of 0.4 mm thick. In addition, an insulator 304 is disposed between the receiving portion 201e and

  the electrode 301 of annular shape.

  The insulation 304 is for example made of natural mica, laminated mica, ceramic material such as alumina, a polyimide film, resin such as a phenolic resin, for example about 0.2 mm thick. The insulator 304 serves to electrically isolate the ring-shaped electrode 301 so that a signal delivered by the pressure sensor 300 is not short-circuited to

the envelope 201.

  It is preferable to grind or polish the surfaces of the metal shell 303 and the electrode 301 which are in contact with the piezoelectric ceramic material 302 to form a surface with a roughness of less than 6.3Z, by

  for example, 3.2Z or 1.6Z, which ensures uniformity of contact with the piezoelectric ceramic material 302 and prevents the piezoelectric ceramic material 302 from cracking. A magnetic material such as SUS430 can be used for the metal shell 303 and the electrode 301 which facilitates grinding or polishing.

  Furthermore, it is not easy to grind or polish the contact surface of the metal shell 303, because the metal shell comprises: a cylindrical flange 303a and a protruding pipe 303d. As a result, the cylindrical flange 303a can be separated from the protruding pipe 303d and these can be welded after grinding or polishing the cylindrical flange 303a.

  Furthermore, to the right of the flange 303a is formed a through hole in the axial direction of the spark plug to insert a protective tube 303b. The protection tube 303b is fixed by welding or brazing. Naturally, in the insulating sleeve 210 is formed a through hole 210a for the insertion of the protective tube 303b. The protective tube 303b serves for insertion and attachment of a shield wire 305 for providing a signal from the pressure sensor 300 so that a core 305a of the shield wire 305 is welded and electrically connected to the electrode

metal ring 301.

  A shield 305b (electrically insulated from the core 305a) of the shielded wire 305 is mated with the protection tube 303b, thereby electrically connecting the

  shield 303b to metal shell 303 which is grounded.

  In the pressure sensor 300 is disposed a piece of the piezoelectric ceramic material to simplify the pressure sensor 300 and give it a low center of gravity. Thus, a vibration noise generated by the pressure sensor 300 itself can be reduced, which improves the signal-to-noise ratio of the

signal issued.

  However, as described in the aforementioned JP2001-124336A (FIG. 2 appended thereto), it is possible to employ two pieces of the piezoelectric ceramic material. In this case, one of the two pieces of the piezoelectric ceramic material is further disposed, in place of the insulator 304 under the electrode 301, thereby connecting the two pieces parallel to each other and

  achieving double sensitivity and better signal-to-noise ratio.

Mounting of the pressure sensor

  First, there is fixed around the protruding tube 303d a heat-shrinkable insulating tube 306.

  Then, the piezoelectric ceramic material 302 is placed, then the electrode 301 around the projecting tube 303d. In the present case, the insulating tube 306 prevents short circuits between the piezoelectric ceramic material 302,

  the electrode 301 and the metal casing 303.

  Then, by resistance welding or by laser welding, the core 305a of the shielded wire 305 is connected to the electrode 301.

  Next, the shield 305b of the shield wire 305 is mated with the shielding tube 303b, thereby providing electrical connection between the shield 305b and the metal shield 303, securing the shield wire 305, and close contact therebetween.

  shielded wire 305 and protection tube 303b.

  Thus, the pressure sensor 300 which is a unitary body comprising the metal envelope 303, the electrode 301 and the shielded wire 305 is disposed through the insulator 304 in the receiving portion 201 e. The pressure sensor 300 is enclosed in the metal casing 303 and the body 200 of the spark plug, which ensures complete sealing and shielding

  300 electrical pressure sensor complete.

  Mounting the glow plug

  Referring to Figures 1 and 2, we will explain the mounting of the glow plug with combustion pressure sensor.

  To begin, the cladding 201 is made with the heating device 206 connected to the inner axis 204. The outside diameter of the tubular element 202 is set to a value greater than 60 μm by 140 μm to that of the

  inside diameter of the envelope 201.

  The tubular element 202 is force-fitted into the casing 201, which associates integrally the casing 201, the internal axis 204 and the heating device 206. To ensure a more perfect air tightness, it is possible to weld or Brazing the heater 206 using silver solder.

  Then, the vibration damper 230 is disposed between the casing 201 and the inner shaft 204, before the cylindrical ring 207 is inserted since

the top of the inner axis 204.

  If, for example, the vibration damper 230 is a 5 mm wide rubber resin molded part, it protrudes from the upper opening of the casing 201 and is disposed between the casing 201 and the casing 201. the inner axis 204. The radial compression of the vibration damper 230 is 10 to 30%, which secures it to the inner shaft 204 and the casing 201. In addition, the inner and outer peripheral surfaces outside the vibration damper 230

  are coated with a lubricating oil. Thus, the vibration damper 230 is force-fitted from the top of the inner axis 204 to a given position.

  In this case, if necessary, several vibration dampers can be used. The vibration damper 230 is effectively introduced into the casing 201 if it is a molded part.

  On the other hand, if the vibration damper is for example rubber or liquid resin, it is injected at the upper opening of the casing 201 from a nozzle of a dispenser for an assay. In this case, vacuum degassing can be employed depending on the viscosity of the rubber or liquid resin, thereby removing the bubbles and making a very dense filling. Then, the liquid is cured under given conditions.

  Thus, the rubber or liquid resin is easily introduced into a space between the inner shaft 204 and the casing 201. The rubber or liquid resin is particularly effective for a high displacement motor whose vibrations become stronger.

  After loading the vibration damper 230, the cylindrical ring 207 and the insulator 304 are lowered from the top of the inner shaft 204.

  Furthermore, after arranging the O-ring 208 on the step formed inside the hexagonal head 201a, the pressure sensor 300 is disposed in the receiving portion 201 e.

  Then, the cylindrical ring 209 is lowered from the top of the inner axis. Then, the O-ring 309 is lowered from the top of the shield wire 305. The insulating sleeve 210 is then lowered from the top of the inner axis 204, and at the same time the shielded wire 305 is pulled out through through hole 210a.

  The water-tight and airtight O-ring 309 made of a material such as silicone rubber, fluororubber, EPDM, NBR or H-NRB is compressed between the through-hole 210a of the insulating sleeve 210 and

  an upper end surface of the protection tube 303b.

  The insulating sleeve 210 is for example made of phenolic resin, PPS, laminated mica or ceramic material based on alumina, and preferably consists generally of a low density material with a high modulus of elasticity.

and low creep value.

  The lightweight insulating sleeve 210 provides a low center of gravity to the pressure sensor 300, which reduces the noise level of the vibrations. On the other hand, the low creep insulating sleeve 210 can retain its unchanged shape over time, which reduces the output variations due to a variation of the pressure.

  exerted on the pressure sensor 300 by the nut 211.

  Creep is further reduced if a phenolic resin mixed with glass fibers is heated for 3 to 20 hours at 175 to 205 ° C.

  Then, after combining the insulating sleeve 210 and the shielded wire 305, the pressure sensor 300 is firmly fixed in the receiving part 201e.

  screwing the nut 211 along the thread 204a.

  The nut 211 can be prevented from loosening by deforming and matting part of its hexagonal surfaces or by first applying a

  thread locking adhesive on the threaded surface of the nut 211.

  Furthermore, for the tight braking of the nut 211, the periphery of the insulating sleeve 210 may be provided with two flat surfaces (opposite to one another)

  retained in the nut 211 during screwing, for example using a wrench.

  Thus, the pressure sensor 300 is more securely fixed without torsion of the piezoelectric ceramic material 302 and without a welded portion between the electrode and the core 305a, which prevents the degradation and rupture of the material.

  piezoelectric ceramic 302 and core 305a.

  The casing 201 of the assembled glow plug 100 is threaded into the cylinder head 1 of the engine and the connecting bar 2 is arranged on the

  thread 204a and is fixed by the nut 211, as shown in FIG.

  Combustion Pressure Detection FIG. 3 is a partial cross-sectional view of the glow plug according to the present invention for explaining a simplified model.

  transmission paths of the combustion pressure.

  At the start of the motor, an electric current is supplied, via the connection bar 2, to the heating device 206 to facilitate ignition of the diesel engine. After starting the engine, the combustion pressure is transmitted via a first and a second path R1 and R2, as

  shown in FIG. 3, to the pressure sensor 300.

  The first path Ri goes to the pressure sensor 300 via the envelope 201 which is firmly fixed to the cylinder head 1 of the engine by the thread 201b.

  The second path R2 goes to the pressure sensor 300 via the insulating powder 205, the inner shaft 204, the nut 211 and the insulating sleeve 210 which

are likely to vibrate.

  The tubular element 202, as well as the internal axis 204, is also displaced in the axial direction of the spark plug by the combustion pressure transmitted by R2, since the tubular element 202 can be displaced in the axial direction of the spark plug. (going up and down in FIG. 3) by an elastic force of the envelope 201, even if the tubular element 202 is attached to the envelope 201 at the level

of the fixing part KM.

  Consequently, a displacement along R2 becomes longer than following Ri. According to the displacement mentioned above, the load exerted by the nut 211 on the pressure sensor 300 changes, which modifies the loads

  electrical generated by the piezoelectric ceramic material 302.

  The generated electrical charges are delivered in the form of an electrical signal via the electrode 301 as shown in FIG. 2, to the core 305a and are at the same time delivered in the form of an electrical signal. via metal enclosure 303 (to earth) and protection tube 303b

  as shown in Fig. 2, to shield 305b.

  The above-mentioned electrical charges (output signal) are applied to a charge amplifier, not shown, for converting the electrical charges into voltage and for amplifying the converted voltage. The amplified signal voltage is applied to a motor control unit (ECU), not shown, of the motor.

  thereby controlling combustion.

  Combustion pressure signal FIG. 4a is a curve of the combustion pressure (established by the pressure sensor and a pressure gauge) as a function of the crankshaft angle when the engine is running at 1200 rpm under a load of 40 N. The sensor pressure

  corresponds to a signal almost identical to that of the manometer pressure.

  Figure 4b is a graph showing a relationship between the pressure of the sensor and the pressure of the manometer. The pressure of the sensor is almost linear with respect to the gauge pressure. The variation of load affecting the pressure sensor 300 under the effect of the combustion pressure is accurately reflected by a high signal-to-noise ratio, an absence of hysteresis and an excellent reaction of the

sensor pressure signal.

  Experimental Results The noise of the combustion pressure sensor output signal 300 is suppressed and the output signal is stabilized even though the inner axis 204 is of greater length than in the prior art. Indeed, the vibration damper 230 is disposed between the attachment portions 204m and 204n, which shortens the distance (t) as shown in Figure 1, between the upper end of the vibration damper 230 and the fixing part 204n. Thus, even if the length of the inner axis 204 is increased, its resonant frequency does not decrease and its vibration level is suppressed. On the other hand, the distance "t", shown in FIG. 7, between the fixing points of the inner shaft 204 of the pre-heat glow plug J100 according to the prior art becomes larger. Therefore, in the prior art glow plug, the resonant frequency is reduced and the vibration level is increased by using a

inner axis longer.

  In contrast, in the present invention, even if the mechanical vibrations of the inner axis are superimposed on the combustion pressure, the

  noise component of the pressure signal is suppressed.

  The allowable resonant frequency of the inner axis 204 is greater than 5 kHz, since the frequency range of the combustion pressure is less than 5 kHz and, therefore, the vibration frequency greater than 5 kHz can be eliminated by a low pass filter.

  With regard to the vibration damper 230, its hardness is preferably greater than or equal to A10 from durometer and less than or equal to D90

  according to durometer, according to the results of experiments carried out by the inventors.

  If the hardness is less than A 10 according to the durometer, the vibration damper 230 becomes too soft to immobilize the inner shaft 204 and suppress its vibrations, whereas if the hardness is greater than D 90 according to durometer, the vibration damper 230 fixes too firmly the inner shaft 204 to transmit to the pressure sensor 300 the slight displacement d to the combustion pressure, which

  greatly reduces the output sensitivity and thus the signal-to-noise ratio.

  Thus, at first sight, it is expected that the small displacement of the inner shaft 204 can be transmitted to the sensor 300 if a small space, for example 50pim, is provided between the inner shaft 204 and the damper vibration 230 or between the envelope 201 and the vibration damper 230. However, it is in

  makes it difficult to control the dimensions of the small space mentioned above.

  FIG. 5 illustrates an experimental relationship between a noise level in% and the hardness of the vibration damper 230, the noise ratio being a ratio of a noise width to a maximum height "h" of the pressure signal of the sensor

shown in Figure 4A.

  As shown in FIG. 5, the noise level is about 4% for the prior art glow plug, whose distance "t",

  as shown in Figure 7, between the two attachment points is 40mm.

  On the other hand, when the distance between the two fastening portions 204m and 204n of the glow plug according to the present invention has been increased to mm, it has been confirmed that the vibration damping rubber silicone hardness A10, A50, A90 after durometer and epoxy resin with durometer D90 durometer is acceptable because these materials lower the noise level below the standard level of 4%. Hardness greater than D90 from durometer is not acceptable because the transmission of combustion pressure is prevented, as well as

that has already been explained.

  Although the vibration damper 230 may be disposed at any height between the fastening portions 204m and 204n of the inner shaft 204 as shown in FIG. 1, the distance "t" between the fastening portion 204m (attachment portion to the tubular member 202) and the vibration damper 230 or between the attachment portion 204n (attachment portion to the casing 201) and the damper

  230 vibration is preferably less than or equal to 40mm.

  The distance "t" shown in FIG. 1 between the attachment portion 204n and the vibration damper 230 is reduced to less than 40mm. Indeed, the distance "t" in conventional candles is of the order of 40 mm.

  Moreover, it is possible to use several vibration dampers 230 as shown in FIG. 6.

  As shown in FIG. 6, two vibration dampers 230 are disposed between the first attachment portion 204m and the second attachment portion 204n. For the same reason as in the preceding explanations, a distance "tl" between the first attachment portion 204m and a lower damper 230, a distance "t2" between the lower damper 230 and the upper damper 230 and a distance "t3 "between the upper damper and the second fastening portion 204n

  will preferably be less than or equal to 40mm.

  The glow plug according to the present invention can be

  used in the short glow plug according to the prior art.

Claims (4)

  An engine glow plug (100), comprising: a shroud (201) having a first end attached to a cylinder head (1) of the engine; a tubular member (202) installed within said casing (201) and projecting from said first end of said casing (201); a heater (206) disposed within said tubular member (202); an inner shaft (204) installed within said casing (201), inserted at a first end of itself into said tubular member (202) and electrically connected to said heater (206); and a combustion pressure sensor (300) housed within said shell (201) at another end of said shell (201) and detecting a combustion pressure transmitted through said inner shaft (204), characterized in that: said first end of said inner shaft (204) is attached to said tubular member (202); another end of said inner pin (204) is attached to said shell (201); and one or more vibration dampers (230) for damping vibrations of said inner shaft (204) are disposed about said inner shaft (204)   between the first and the other end of said inner axis (204).   2. Glow plug (100) according to claim 1, characterized in that: a distance between said vibration damper (230) and said first end of said inner shaft (204) is less than or equal to 40mm; a distance between said vibration dampers (230) is less than or equal to 40 mm; and   another distance between said vibration damper (230) and said other end of said inner shaft (204) is less than or equal to 40 mm.   3. Glow plug (100) according to claim 1, characterized in that the hardness of said damper (230) of vibration is greater   or equal to A 10 from durometer and less than or equal to D90 from durometer.   4. Glow plug (100) according to claim 1, characterized in that said damper (230) vibration is rubber or resin.