Combustion pressure sensor

A combustion pressure sensor for detecting a pressure in the combustion chamber of an internal combustion engine includes a heat releasing member disposed between a housing and a pressure transmitting member at a position forward of a load detecting section and slidable relative to the pressure transmitting member for releasing heat through heat transfer from combustion gas, which has entered an axial hole of the housing from the combustion chamber, to the housing.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority from Japanese Patent Applications 2006-116993, filed Apr. 20, 2006 and 2007-050256, filed Feb. 28, 2007, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combustion pressure sensor for detecting a combustion pressure in the combustion chamber of an internal combustion engine such as a diesel engine.

2. Description of the Related Art

Combustion pressure sensors adapted to be mounted to an internal combustion engine such as a diesel engine for detecting the combustion pressure in a combustion chamber of the internal combustion engine are known and used heretofore. In one example disclosed in Japanese Patent Application Publication JP 2005-90954 A (corresponding to US 2005/0061063 A1), the combustion pressure sensor is formed as an integral part of a glow plug. As shown inFIG. 8hereof, the disclosed combustion pressure sensor9includes a pressure transmitting member91(serving also as a heating rod) slidably displaceable in the axial direction relative to a housing92to transmit a pressure in the combustion chamber to a pressure sensor93disposed on a rear end of the combustion pressure sensor9for the detection of a combustion pressure in the combustion chamber.

In order to prevent entry of hot combustion gas from between the pressure transmitting member (heating rod)91and the housing92, the combustion pressure sensor9further has a seal member94and a membrane95. To this end, the seal member94and the membrane95are arranged to tightly block or seal a clearance between the pressure transmitting member (heating rod)91and the housing92. The membrane95is connected by welding to the housing92and the pressure transmitting member91so that the hot combustion gas can be trapped within a front end portion of the housing92.

With this arrangement, however, due to contraction in shape of the housing99that may occur in the axial direction of the housing92when the combustion pressure sensor9is mounted to the internal combustion engine, the pressure transmitting member91, which is connected to the housing92via the membrane95, is also displaced in the axial direction relative to the housing92. As a consequence of this displacement, a force or pressure is transmitted to the pressure sensor93, which will cause an unwanted change in the sensor output. This may lead to fluctuation in the initial value of the combustion pressure sensor9and deterioration of the sensor accuracy. To secure application to the engine control purposes, the combustion pressure sensor9requires collection using another sensor, which will incur additional cost. In some cases, engine control is rendered difficult to achieve.

Another conventional combustion pressure sensor is disclosed in Japanese Patent Application Publication JP 2006-084468 A2 (corresponding to US 2006/0053875 A1), which includes a seal in the form of a bellows-shaped component provided between the pressure transmitting member (heating rod) and the housing. However, since the seal is disposed in a front end portion of the housing, the seal is likely to cause thermal deterioration under the effect of high-temperature combustion gas. Accordingly, in a severe environment, sealing property between the pressure transmitting member and the housing tends to deteriorate, making it difficult to provide a sufficient level of durability.

SUMMARY OF THE INVENTION

With the foregoing difficulties in view, an object of the present invention is to provide a combustion pressure sensor, which has very good detection accuracy and durability.

According to the invention, there is provided a combustion pressure sensor for detecting a combustion pressure in a combustion chamber of an internal combustion engine, comprising: a hollow cylindrical housing adapted to be mounted to the internal combustion engine in such a manner that a front end portion of the hollow cylindrical housing is located in front of the combustion chamber; a pressure transmitting member slidably inserted through an axial hole of the hollow cylindrical housing such that a pressure receiving front end portion of the pressure transmitting member protrudes from the front end portion of the housing into the combustion chamber for transmitting a combustion pressure in the combustion chamber; a load detecting section disposed between the housing and the pressure transmitting member for detecting variations in load acting between the housing and the load transmitting member; and a heat releasing member disposed between the housing and the pressure transmitting member at a position forward of the load detecting section and slidable relative to the pressure transmitting member for releasing heat through heat transfer from combustion gas, which has entered the axial hole of the housing from the combustion chamber, to the housing.

In the combustion pressure sensor, since the heat releasing member is disposed between the housing and the pressure transmitting member at a position forward of the load detecting section, and since the heat releasing member is arranged to release heat through heat transfer from combustion gas, which has entered the axial hole of the housing from the combustion chamber, to the housing, the head of the combustion gas is released through the housing to the surrounding engine head portion. Thus, the heat of the combustion gas can never reach the rear end side of the combustion pressure sensor, so that the effect of thermal load on the load detecting section can be reduced. The combustion pressure sensor as a whole is highly durable in construction.

Furthermore, the heat releasing member is disposed in a slidable manner relative to the pressure transmitting member, and the pressure transmitting member is slidable relative to the housing. With this arrangement, even when the housing is deformed into axially contracted configuration due to a load acting in the axial direction of the housing when the combustion pressure sensor is mounted to the internal combustion engine, it is possible to prevent a load induced by the deformation of the housing from transmitting to the load detecting section. Especially because the heat releasing member is disposed to be slidable relative to the pressure transmitting member, and because the pressure transmitting member is slidable relative to the housing, a deformation of the housing can never be transferred to a deformation of the pressure transmitting member. As a result, the load detecting section is completely free from the effect of a stress that may occur due to deformation of the housing.

It will be appreciated that in the combustion pressure sensor of the invention, an unwanted change in the sensor output, which might otherwise occur at the time of mounting to the internal combustion engine, can be avoided. The combustion pressure sensor is therefore able to retain the desired detection accuracy. In performing engine control operation, the combustion pressure sensor1does not require correction using another sensor, which will incur additional cost.

As thus far described, it is possible according to the present invention to provide a combustion pressure sensor, which has very good detection accuracy and durability.

The term “front side” is used herein to refer to a side of the combustion pressure sensor from which the combustion pressure sensor is inserted in the combustion chamber of the internal combustion engine. Accordingly, the term “rear side” is used herein to refer to a rear side of the combustion pressure sensor, which is opposite to the front side.

Preferably, the pressure-receiving front end portion of the pressure transmitting member contains an internal glow plug having a heating element that generates heat when energized, and an electric conductor means through which electric power is supplied to the heating element. With this arrangement, since a combustion pressure detecting function and a blow plug function can be integrated into a single component, it is possible to achieve cost reduction, space saving, and easy assembly.

The glow plug preferably comprises a ceramic glow plug having a ceramic body and a heating element embedded in the ceramic body. Since the ceramic glow plug has an improved heat resistance property, it is possible to extend the service life of the combustion pressure sensor, which is used in a high temperature environment.

Preferably, the heat releasing member is formed from a material having a thermal conductivity greater than 15 W/mk. In this instance, by virtue of the heat releasing member, heat of the combustion gas, which has entered into the axial hole of the housing, can be efficiently released to the housing. This will achieve efficient reduction of thermal load that is exerted on the load detecting section.

The heat releasing member is preferably made of an elastic material and disposed in a prestressed state between the housing and the pressure transmitting member. The thus arranged heat releasing member insures sufficient heat transfer from the pressure transmitting member to the housing and smooth sliding movement of the pressure transmitting member relative to the housing.

Preferably, the heat releasing member is formed of a metal mesh member. The metal mesh member possesses the necessary degree of thermal conductivity and elasticity that are due for a heat releasing member to be disposed between the pressure transmitting member and the housing.

In one preferred form of the invention, the housing includes a housing body and a housing front end member fixed to a front end of the housing body, and the heat releasing member is disposed on the inside of at least one of the housing body and the housing front end member. This arrangement allows for easy arrangement the heat releasing member between the housing and the pressure transmitting member. Stated more specifically, the heat releasing member is mounted on the inside of either the housing body or the housing front end member while the housing body and the housing front end member are separated from each other. Subsequently, by joining together the housing body and the housing front end member, the heat releasing member can be readily placed between the housing and the pressure transmitting member. The combustion pressure sensor of this construction is easy to manufacture.

The heat releasing member may be disposed on the inside of the housing front end member or on the inside of the housing body. As a further alternative, the heat releasing member may be disposed on the inside of the housing body and the housing front end member across a joint between the housing body and the housing front end member.

The heat releasing member is preferably disposed on the inside of a front end portion of the housing front and member. With the heat releasing member thus arranged, heat of the combustion gas, which has entered between the housing and the pressure transmitting member, is released via the front end portion of the housing to the engine head. This provides further improvement in the heat releasing efficiency, leading to additional reduction of thermal load on the load detecting section.

Preferably, the combustion pressure sensor further includes a seal member that seals a clearance between the housing and the pressure transmitting member at a position rearward of the heat releasing member, the seal member being disposed on the inside of at lest one of the housing body and the housing front end member. By thus providing the seal member, the combustion gas is no longer possible to move into a rearward side of the sensor beyond the seal member. Additionally, since the seal member is disposed behind the heat releasing member, heat of the combustion gas is released by the heat releasing member to the surrounding engine head on the front side of the seal member. By thus releasing the combustion heat, it is possible to suppress deterioration by heat of the seal member. This leads to further improvement in the durability of the combustion pressure sensor.

The seal member may be disposed on the inside of the housing front end member or on the inside of the housing body. As a further alternative, the seal member may be disposed on the inside of the housing body and the housing front end member across a joint between the housing body and the housing front end member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings andFIG. 1in particular, there is shown a combustion pressure sensor1according to a first embodiment of the present invention. The combustion pressure sensor1is used for the detection of the combustion pressure in a combustion chamber61of an internal combustion engine such as a diesel engine, and for this purpose it generally comprises a housing2, a pressure transmitting member3, a load detecting section4, and a heat releasing member5.

The housing2is a hollow cylindrical member adapted to be mounted to the internal combustion engine in such a manner that a front end portion21of the hollow cylindrical housing2is disposed in front of, or directed toward, the combustion chamber61.

The pressure transmitting member3is an elongated rod-like member slidably inserted through an axial hole22of the hollow cylindrical housing2such that a pressure receiving front end portion31of the rod-like pressure transmitting member3protrudes from the front end portion21of the housing2into the combustion chamber61for transmitting a combustion pressure in the combustion chamber61to the load detecting section4.

The load detecting section4is disposed between the housing2and the pressure transmitting member3for detecting variations in load acting between the housing2and the pressure transmitting member3.

The heat releasing member5is disposed between the housing2and the pressure transmitting member3at a position forward of the load detecting section4and slidable relative to the pressure transmitting member3for releasing heat through heat transfer from combustion gas, which has entered the axial hole22of the housing2from the combustion chamber61, to the housing2.

The heat releasing member5is formed of a metal mesh member having a thermal conductivity greater than 15 W/mK and a certain degree of elasticity. The elastic heat releasing member5is disposed in a prestressed states between the housing2and the pressure transmitting member3. Stated more specifically, the heat releasing member5is elastically deformed or distorted in a radially compressed configuration by and between the housing2and the pressure transmitting member3so that an elastic force acts on both of an inner circumferential wall of the axial hole22of the housing2and an outer circumferential surface of the rod-like pressure transmitting member3. The axial hole22of the housing2has a retainer portion221disposed intermediately between a forward end (lower end inFIG. 1) of the housing2and a rear end (upper end inFIG. 1) of the housing2, the retainer portion211having a larger diameter than the rest of the axial hole22. The heat releasing member5is held or retained in the retainer portion221.

As shown inFIG. 2, the heat releasing member5is in the form of a circular split ring (C-shaped ring) having an axial slit51. The ring-like heat releasing member5has an outer circumferential surface52held in pressure contact with the circumferential wall of the retainer portion221of the axial hole22of the housing2, and an inner circumferential surface53held in pressure contact with the outer circumferential surface of the pressure transmitting member3. The degree of elasticity of the heat releasing member5is determined such that the heat releasing member5can retain sufficient ability to slide relative to the pressure transmitting member3while keeping itself in positive contact with the housing2and the pressure transmitting member3.

The heat releasing member5is formed from, for example, metal fibers knitted or woven into a mesh structure, which is shaped into a split circular ring (C-shaped ring). The shape of the heat releasing member5should by no means be limited to the C shape as in the illustrated embodiment but may include a continuous circular ring-like configuration.

Referring back toFIG. 1, the combustion pressure sensor1is inserted through an engine head portion62of the internal combustion engine (diesel engine, for example) and firmly secured to the engine head portion62. The engine head portion62has a mounting hole621for mounting the combustion pressure sensor1to the engine head portion62. The mounting hole621has a tapered portion622for abutment with the front end portion21of the housing2of the combustion pressure sensor1. The combustion pressure sensor1has a beveled portion211formed on an outer circumferential surface of the front end portion21of the housing2, and an externally threaded portion23formed on the outer peripheral surface of a rear end portion of the housing2. The engine head portion62has an internally threaded portion (not designated) formed on a read end portion (upper end portion inFIG. 1) of a circumferential wall of the mounting hole621. The externally threaded portion23of the housing2is threaded with the internally threaded portion of the engine head portion62with the beveled portion211of the housing front end portion21held in abutment with the tapered portion622of the mounting hole621, so that the combustion pressure chamber1is mounted to the engine head portion62.

The load detecting section4is formed by a detecting element41which produces an electric signal during detection of combustion within the combustion chamber61, and a pair of electrode plates42and43disposed in sandwiching relation to the detecting element41for taking out the electric signal produced by the detecting element41. A holder member44is disposed on a rear side of the load detecting section4(which is directed away from the combustion chamber61) for holding the load detecting section4in position on the rear end of the housing2. In order to improve the sensitivity of the load detecting section4(i.e., to make the load detecting section4sensitive to variations in load that may occur during detection of combustion pressure), the detecting element41and the electrode plates42,43are initially held under a preload condition with a compressive force or load applied thereto from the holder member41.

The combustion pressure sensor1of the foregoing construction operates as will be discussed below.

The pressure receiving front end portion31of the pressure transmitting member3is subjected to a combustion pressure generated during combustion of an air-fuel mixture within the combustion chamber61, whereupon the pressure transmitting member3is displaced in the axial direction toward the rear end (upper end inFIG. 1) of the combustion pressure sensor1, thereby transmitting the combustion pressure to the load detecting section4. With the combustion pressure thus transmitted, the initial compressive load applied in advance to the load detecting section4is caused to vary and the load detecting section4generates an output signal corresponding to the transmitted combustion pressure. The detecting element may use a piezoelectric element in which instance since the piezoelectric element produces a voltage or charge corresponding to a change in load applied thereto, the load detecting section4is able to produce an output nearly proportional to the combustion pressure.

Heat of high-temperature combustion gas generated during combustion is received by the heat releasing member5, which in turn transmits via the housing2to the engine head portion62. By thus releasing the head, undue temperature rise within the housing2can be avoided.

Various advantageous effects achieved by the combustion pressure sensor1of the first embodiment of the present invention will be enumerated as follows.

In the combustion pressure sensor1, since the heat releasing member5is disposed between the housing2and the pressure transmitting member3at a position forward of the load detecting section4, and since the heat releasing member5is arranged to release heat through heat transfer from combustion gas, which has entered the axial hole22of the housing2from the combustion chamber61, to the housing2, the head of the combustion gas is released through the housing2to the surrounding engine head portion62. Thus, the heat of the combustion gas can never reach the rear end side of the combustion pressure sensor1, so that the effect of thermal load on the load detecting section4can be reduced. The combustion pressure sensor1as a whole is highly durable in construction.

Furthermore, the heat releasing member5is disposed in a slidable manner relative to the pressure transmitting member3, and the pressure transmitting member3is slidable relative to the housing2. With this arrangement, even when the housing2is deformed into axially contracted configuration due to a load acting in the axial direction of the housing2when the combustion pressure sensor1is mounted to the internal combustion engine, it is possible to prevent a load induced by the deformation of the housing2from transmitting to the load detecting section4.

Stated more specifically, at the time of mounting the combustion pressure sensor1to the engine head portion62, the combustion pressure sensor1is inserted in the mounting hole621of the engine head portion62and the externally threaded portion23of the housing2is threaded into the internally threaded portion of the mounting hole621until the beveled portion211at the front end portion21of the housing2comes in abutment with the tapered portion622of the mounting hole621. To secure firm and reliable mounting of the combustion pressure sensor1to the engine head portion62, the externally threaded portion23is further turned in a tightening direction until the beveled portion211of the housing2exerts a certain force or pressure on the tapered portion622of the engine head portion62. With this tightening of the threaded portion23, it may occur that the housing2slightly deforms in an axially contracted configuration at its axial portion extending between the externally threaded portion23and the beveled portion211.

In this instance, however, since the heat releasing member5is disposed in a slidable manner relative to the pressure transmitting member3and since the pressure transmitting member3is slidable relative to the housing2, a deformation of the housing2can never be transferred to a deformation of the pressure transmitting member3. As a result, the load detecting section4is completely free from the effect of a stress that may occur due to deformation of the housing2.

It will be appreciated that in the combustion pressure sensor1of the invention, an unwanted change in the sensor output, which might otherwise occur at the time of mounting to the engine head portion62, can be avoided. The combustion pressure sensor1is therefore able to retain the desired detection accuracy. In performing engine control operation, the combustion pressure sensor1does not require correction using another sensor, which will incur additional cost.

Furthermore, by virtue of the thermal conductivity greater than 15 W/mk, the heat releasing member5can effectively release heat of the combustion gas, which has entered into the axial hole22of the housing2, to the housing2. By thus releasing the combustion heat, the thermal load on the load detecting section4can be effectively reduced.

Moreover, since the heat releasing member5is made of an elastic material and disposed in a prestressed state (or radially compressed condition) between the housing5and the pressure transmitting member5. The thus arranged heat releasing member5insures sufficient heat transfer from the pressure transmitting member3to the housing2and smooth sliding movement of the pressure transmitting member3relative to the housing2.

Yet, the heat releasing member is formed of a metal mesh member and hence is able to possess the necessary degrees of thermal conductivity and elasticity that are due for a heat releasing member5to be disposed between the pressure transmitting member3and the housing2.

It will be appreciated that the combustion pressure sensor1according to the first embodiment of the present invention excels in detection accuracy and durability.

FIG. 3shows in longitudinal cross section a combustion pressure sensor1A of the type including a build-in or internal glow plug7according to a second embodiment of the present invention.

The glow plug7includes a heating element71that generates heat when energized, and a lead wire72forming an electric conductor means through which electric power is supplied to the heating element71. The heating element71is disposed inside a pressure receiving front end portion31of a pressure transmitting member3, and the lead wire72connected at one end to a rear end (upper end inFIG. 3) of the heating element71. A front end (lower end inFIG. 3) of the heating element71is electrically connected to the pressure transmitting member3and eventually grounded through the heat releasing member5, housing2and engine head portion62.

The heating element71and lead wire71of the glow plug7are received in an axial hollow internal portion33of the pressure transmitting member3. The hollow internal portion33is filled with an insulating material34that provides an electric insulation between the heating element71and the pressure transmitting member3except for the respective front ends (lower ends inFIG. 3) where the heating element3and the pressure transmitting member3are electrically connected together.

With this arrangement, when the glow plug7is energized, the heating element71generates heat to thereby increase the ambient temperature within the combustion chamber61via the pressure transmitting member3.

Other structural parts of the combustion pressure sensor1A of the second embodiment are the same as those of the combustion pressure sensor1of the first embodiment described above with reference toFIGS. 1 and 2, and further description thereof can be omitted.

In the second embodiment shown inFIG. 3, since a combustion pressure detecting function and a glow plug function are integrated into a single component, it is possible to achieve substantive cost reduction, space saving, and easy assembly.

The combustion pressure sensor1A of the second embodiment is also able to achieve the same advantageous effects as those described with reference to the first embodiment shown inFIGS. 1 and 2.

FIG. 4shows in longitudinal cross section a combustion pressure sensor1B according to a third embodiment of the present invention. The combustion pressure sensor1B differs from the sensor1of the first embodiment shown inFIGS. 1 and 2in that the housing2is formed by a housing body24and a housing front end member25fixed to a front end (lower end inFIG. 4) of the housing body24, and the heat releasing member5is disposed inside the housing front end member25.

The housing front end member25has an outside diameter substantially equal to an outside diameter of the housing body24, and an inside diameter larger than the diameter of the axial hole22formed in the housing body24. The housing front end member25has a front end portion (lower end portion inFIG. 4) bent in a radial inward direction to form an annular retaining portion251for retaining thereon the heat releasing member5. The annular retaining portion251has a central hole252formed therein in axial alignment with the axial hole22of the housing body24and having the same diameter as the axial hole22. The outer circumferential surface of the annular retaining portion251has a beveled front end portion211.

The heat releasing member5is firmly retained on the retaining portion251within the housing front end member25while an inner circumferential surface53of the ring-shaped heat releasing member5is held in sliding contact with the outer circumferential surface of the rod-like pressure transmitting member3under the elasticity of the heat releasing member5. Thus, the heat releasing member5is disposed inside the front end portion (lower end portion inFIG. 4) of the housing front end member25. The housing front end member25is connected at its rear end portion (upper end inFIG. 4) to the housing body24by welding, for example.

Other structural parts of the combustion pressure sensor1B of the third embodiment are the same as those of the combustion pressure sensor1of the first embodiment described above with reference toFIGS. 1 and 2and further description thereof can be omitted.

In the third embodiment shown inFIG. 4, it is possible to provide the heat releasing member5between the housing2and the pressure transmitting member3without difficulty. Stated more specifically, at a first step of assembly, the heat releasing member5is disposed inside the housing front end member25while the housing front end member25is standing along as a separate part which is structurally independent from the housing body24. Then, the housing body24and the housing front end member25are brought together while the pressure transmitting member3is extending through the axial hole22of the housing body24, a central hole of the ring-shaped heat releasing member5, and the central hole252of the housing front end member25. While keeping this condition, the housing body24and the housing front end member25are joined together by welding. With this arrangement, it is readily possible to arrange the heat releasing member5between the housing2and the pressure transmitting member3. The combustion pressure sensor1B having a two-piece housing2is easy to manufacture.

Furthermore, since the heat releasing member5disposed inside the front end portion of the housing front end member25is able to release heat of the combustion gas from the front end portion of the housing2to the engine head portion62, the heat releasing efficiency of the combustion pressure sensor1is very high and the thermal load on the load detecting section4decreases further.

Obviously, the combustion pressure sensor1B of the third embodiment is also able to achieve the same advantageous effects as those described with reference to the first embodiment shown inFIGS. 1 and 2.

FIG. 5shows in longitudinal cross section a combustion pressure sensor1C according to a fourth embodiment of the present invention, which is different from the combustion pressure sensor1B of the third embodiment shown inFIG. 4in that a seal member11is arranged to seal a clearance between the housing2and the pressure transmitting member3at a position rearward (upward ofFIG. 5) of the heat releasing member5within the housing front end member25.

The seal member11has an annular fixed end portion (rear end portion)111firmly connected to a front end face (lower end face inFIG. 5) of the housing body24, and a generally hollow cylindrical body portion112extending from an inner peripheral edge of the annular fixed end portion111toward the heat releasing member5in confronting relation to the outer circumferential surface of the pressure transmitting member5. The body portion112is in the form of a bellows and has a tubular free end portion (front end portion)113opposite to the fixed end portion111of the seal member11. The free end portion113is in sealing contact with the outer circumferential surface of the pressure transmitting member3, so that the clearance between the housing2and the pressure transmitting member3is closed.

In practice, the free end portion113of the seal member11may be either in direct contact with, or slightly separated from, the outer circumferential surface of the pressure transmitting member3. In the latter case, however, a clearance formed between the free end portion113of the seal member11and the outer circumferential surface of the pressure transmitting member3must be small enough to prevent passage therethrough of the combustion gas so as not to allow the entry of the combustion gas beyond the seal member11. As a further alternative, the free end portion (front end portion)113of the seal member11may be connected to the pressure transmitting member5in which instance, since the bellows-shaped body portion112of the seal member11can be extended and contracted in the axial direction thereof, the seal member11does not obstruct movement of the pressure transmitting member5in the axial direction. The seal member11may be made of stainless steel.

Other structural parts of the combustion pressure sensor1C of the fourth embodiment are the same as those of the combustion pressure sensor1B of the third embodiment described above with reference toFIG. 4and further description thereof can be omitted.

In the fourth embodiment shown inFIG. 5, the seal member11effectively prevents the combustion gas from entering a rear side of the combustion pressure sensor1C beyond the seal member11. Additionally, since the seal member11is disposed behind the heat releasing member5as viewed from the combustion chamber61, it is possible to release the heat of the combustion gas to the surrounding engine head portion62on the front side of the seal member11. By thus releasing the combustion heat, it is possible to suppress deterioration by heat of the seal member11. This leads to further improvement in the durability of the combustion pressure sensor1C.

Obviously, the combustion pressure sensor1C of the fourth embodiment is also able to achieve the same advantageous effects as those described with reference to the first embodiment shown inFIGS. 1 and 2.

FIG. 6shows in longitudinal cross section a combustion pressure sensor1D according to a fifth embodiment of the present invention. The combustion pressure sensor1D of this embodiment differs from the combustion pressure sensor1B of the third embodiment shown inFIG. 4in that the pressure transmitting member3has a built-in or internal ceramic glow plug70.

The ceramic glow plug70has a ceramic body701and a heating element710embedded in the ceramic body701. The heating element710has a negative terminal connected to a hollow cylindrical glow holder73and eventually grounded via the pressure transmitting member3, heat releasing member5, housing2and engine head portion62. The positive terminal of the heating element710is connected to an end of a lead wire72disposed in an axial hollow interior portion33of the pressure transmitting member3. The lead wire72forms an electric conductor means through which electric power is supplied to the heating element710.

The load detecting section4of the combustion pressure sensor1D is held by a holder member44from the rear end side of the sensor1D, in the same manner as the first embodiment shown inFIGS. 1 and 2. The holder member44holds the load detecting section4in such a manner that the detecting element41and the electrode plates42,43are initially loaded with a compressive force or load so as to improve the sensitivity of the load detecting section4. The holder member44is integral with the pressure transmitting member3. More particularly, the holder member44is formed integrally with a rear end (upper end inFIG. 6) of the pressure transmitting member3. The holder member44has an outer peripheral edge portion firmly connected to the rear end of the housing2.

With this arrangement, when a pressure receiving front end portion31of the pressure transmitting member3is subjected to a combustion pressure in the combustion chamber61, the preloaded compressive load on the load detecting section4decreases. By detecting a change in the compressive load, the load detecting section4produces an output signal corresponding to the combustion pressure in the combustion chamber61.

Other structural parts of the combustion pressure sensor1D of the fifth embodiment are the same as those of the combustion pressure sensor1B of the third embodiment described above with reference toFIG. 4and further description thereof can be omitted.

In the fifth embodiment shown inFIG. 6, since a combustion pressure detecting function and a glow plug function are integrated into a single component, it is possible to achieve substantive cost reduction, space saving, and easy assembly, as in the same manner as the second embodiment shown inFIG. 3. Furthermore, since the ceramic glow plug70is highly resistant to heat, the combustion pressure sensor1D incorporating such highly heat-resistant ceramic glow plug70can exhibit a prolonged service life in a high-temperature ambient.

Obviously, the combustion pressure sensor1D of the fifth embodiment is also able to achieve the same advantageous effects as the third embodiment shown inFIG. 4.

FIG. 7shows in longitudinal cross section a combustion pressure sensor1E according to a sixth embodiment of the present invention. The combustion pressure sensor1E of this embodiment is structurally the same as the combustion pressure sensor1D of the fifth embodiment shown inFIG. 7with the exception that the load detecting section4employs strain gages410.

The load detecting section4is formed by an annular disc-like support portion440formed integrally with a rear end portion (upper end portion in FIG.7) of the pressure transmitting member3, and a strain gage410adhered or cemented in an appropriate pattern to a rear end face of the support portion440. The supporting portion40has an outer peripheral edge portion firmly secured to the rear end of the housing2. The strain gage410may be a resistance strain gage consisting of a material of strip that is cemented to a part of the support portion under measurement and that changes in resistance with elongation or compression (or a change in length of the strip) under strain to measure pressure applied thereto.

With this arrangement, when combustion pressure in the combustion chamber61is transmitted by the pressure transmitting member3to the load detecting section4, the support portion440strains or becomes distorted. Strain of the support portion440is detected by the strain gage410with the result that the combustion pressure in the combustion chamber61can be detected by the combustion pressure sensor1E.

Other structural parts of the combustion pressure sensor1E of the sixth embodiment are the same as those of the combustion pressure sensor1D of the fifth embodiment described above with reference toFIG. 6and further description thereof can be omitted.

The combustion pressure sensor1E of the sixth embodiment also has exhibits excellent detection accuracy and durability.

It will be appreciated that the combustion pressure sensor1E of the sixth embodiment is also able to achieve the same advantageous effects as the fifth embodiment shown inFIG. 6.

Obviously, various minor changes and modifications are possible in the light of the above teaching. It is to be understood that within the scope of the appended claims the present invention may be practiced otherwise than as specifically described.