The present invention relates to a heating rod and in particular but not exclusively to a heating rod of a glow plug used for preheating a diesel engine. It also relates to a glow plug and to a method for manufacturing such a heating rod and such a glow plug.

A heating rod of a glow plug is in the form of a rigid rod or bar, which extends longitudinally according to an axis, called the principal axis, from the body of the plug into the combustion chamber. Throughout the text, the term "distal" and its derivatives designate directions, elements or parts which are situated axially on the side of the free extremity of the heating rod, which is intended to extend into the combustion chamber, and the term "proximal" and its derivatives designate directions, elements or parts which are situated axially on the opposite side, i.e. towards the connection towards the outside of the cylinder head of the engine on which the glow plug is intended to be fitted.

A heating rod also extends from the proximal side into the plug body, and has a proximal extremity which is equipped with an electrical connection which forms a first electrical power supply terminal of the heating rod, in general with an electrode which extends axially beyond the proximal extremity of the heating rod.

Throughout this text, "cylinder" and its derivates refer to the mathematical and geometrical definition that is to say a surface or a volume generated by a straight line remaining parallel to a fixed direction and moving on a base line. It includes not only the right circular cylinder but also any cylinder with non-circular base. The expression "globally cylindrical" includes not only the perfectly cylindrical forms but also those which can provide irregularity or variation in regard with a perfect cylindrical form, without changing the piece function.

Throughout the text, "electrically insulator" and its derivates relate to a dielectric material which electrical resistance is sufficient to prevent any passing of an electric current in the predetermined operation conditions of the heating rod. A glow plug includes a cylindrical plug body having a threaded external portion for fitting on a cylinder head, and at the distal portion of the plug body a cylindrical internal housing for receiving a proximal portion of the heating rod extending from said proximal extremity. Said housing has an opening for the passage of the heating rod such that the heating rod extends axially, projecting (on the distal side) beyond said proximal portion and the opening. The heating rod has a heating distal portion extending from said opening as far as the distal extremity of the heating rod.

A known heating rod includes:

- an external sheath made of a metal alloy, which is an electrically conductive material, axially extending along said main axis, and comprising an axially distal closed end and an axially proximal open end,

- a heating wire located in the external sheath, made of an electrically conductive material, having an electrically conductive external surface and helically wound around a core,

- a core made of an electrically insulating material and located inside the heating wire and,

- a filling electrical insulating powder.

A heating rod of a glow plug must very quickly make a high temperature available to assist the ignition process, subsequently maintaining this temperature regardless of boundary conditions or even adapting it to suit them. The glow plug is located in the combustion chamber, and must sustain the operating conditions in the combustion chamber, in particular a temperature which can be up to 1400°C and a pressure which can be up to 20MPa. Also, during pre-heating, a high current flows via an electrode to the heating wire, therefore all the elements of a heating rod which are electrically conductive must sustain high current up to some amperes.

A known heating rod for a glow plug is generally manufactured as followed. First a core is inserted into a heating wire. The distal end of the external sheath is made in a tapered shape and closed. The proximal end of the heating wire encircling the core is then connected to a distal end of an electrical terminal supply, such as an electrode. The heating rod connected to the electrode and the core are then disposed in the external sheath and the heating wire distal free end is welded to the external sheath distal closed end. Then the external sheath is filled with the electrical insulating powder. A sealing is placed between the external sheath proximal side and the electrode so as to seal the heating rod. Thereafter the heating rod is subjected to a swaging step which reduces the heating rod diameter in order to make it compact. The thus-produced heating rod is inserted into the receiving housing of the glow plug body to complete the glow plug.

In a known heating rod, the external sheath has the function to assemble and to maintain all heating rod pieces together. The external sheath is made of an electrically conductive material. The electrically insulating powder located between the external sheath and the heating wire prevents any contact of both pieces with one another which could cause a short-circuit, and thus damage the glow plug and make its lifetime shorter. Moreover, the electrically insulating powder has also a thermally conductive role. The heat generated by the heating wire has to be transmitted from the heating wire to the external sheath to thereby directly preheat the cylinder interior of a diesel engine or the like. A major problem of glow plugs is to increase their thermal conductivity in order to enhance their efficiency.

EP 1 471 307 aims to increase the thermal conductivity of a heating rod. As mentioned is this document, heat generated by the heating wire cannot be effectively conducted to the external sheath. Indeed the heat is conducted not only to the external sheath, but also to the core located inside the heating wire and which function is to maintain the heating wire by preventing him from deforming itself. As a result, the glow plug may fail to function as an effective heat source. This document proposes two solutions to overcome this problem. Firstly a core formed of an insulating material having thermal conductivity lower than that of the electrical insulating powder; and secondly an average particle size of the electrical insulating powder smaller than that of the electrical insulating material of the core.

However using an electrically insulating powder as insulator and filler poses problems. Indeed the vibration filling process of the heating rod causes a segregation of the powder (so that large-size grains fill the heating rod distal end). This results in an unequal distribution of the powder density and of the temperature in the heating rod, which may cause a local overheating, and consequently a shorter life time of the glow plug.

The invention is thus aimed at overcoming these disadvantages of the prior art, by proposing a heating rod in which the segregation of the electrical insulating powder is avoided.

It is also an object of the invention to reduce the cumbersome of the glow plug, by proposing a more compact and thinner heating rod, and so a quicker responding heating rod.

The invention is also aimed at optimally filling the heating rod and thus at providing a better hold of the heating rod pieces during the reduction step.

The invention is also aimed at decreasing the energy consumed by the heating rod, by providing a heating rod in which the current level flowing through the heating wire can be decreased for the same thermal efficiency.

The invention is, in particular, aimed at increasing the thermal conductivity of the heating rod and the glow plug efficiency.

The invention is also aimed at increasing the life time of the glow plug.

Additionally, the invention is aimed at overcoming manufacturing problems of the heating rod and of the glow plug, by proposing a cheaper process for manufacturing the heating rod.

The invention is also aimed at facilitating the manufacturing process of the heating rod. The invention is in particular aimed at avoiding any positioning error.

The invention is also aimed at making the manufacturing process of the heating rod faster by allowing the filling of several heating rods at the same time.

The invention is also aimed at reducing the level of a swaging step of the manufacturing process. To this end, the invention concerns a heating rod, including:

(i) an external sheath:

- axially extending along a longitudinal axis,

- comprising an axially closed end and an axially open end with said both ends axially opposite,

- having an external sheath inner dimension,

- made of an electrically conductive material,

(ii) at least one heating wire:

- made of at least one electrically conductive material,

- having an electrically conductive external surface,

- located in the external sheath and circumscribed in a volume having an outer dimension, named wire outer dimension,

(iii) electrically insulating means at least radially located in the heating rod between the external sheath and the heating wire, comprising at least one internal sleeve having:

- at least one open end,

- an inner dimension corresponding to said wire outer dimension so that said at least one heating wire is housed in it,

- an outer dimension corresponding to said external sheath inner dimension so that it is housed in said external sheath,

- longitudinally extending in said external sheath at least along at least a part of said at least one heating wire,

characterized in that it comprises a rotation blocking device adapted for rotatably blocking of said internal sleeve with regard to said external sheath around said external sheath longitudinal axis, and in that said rotation blocking device comprises at least one external recess formed in an external face of said internal sleeve, and an electrically insulating filling powder located into at least one said at least one external recess of said internal sleeve and into a space between the internal sleeve and the external sheath, so as to rotatably block said internal sleeve relative to said external sleeve. Such an electrically insulating means, which comprises at least one internal sleeve rotatably blocked with regards to the external sheath by said at least one external recess has numerous advantages, and makes it possible to simultaneously solve the various problems mentioned above.

A heating rod according to the invention thus comprises a rotation blocking device adapted for rotatably blocking said internal sleeve with regard to said external sheath around said external sheath longitudinal axis. In order to effectively provide with this rotation blocking, and also to ease the filling of the heating rod, the internal sleeve has a particular blocking structure comprising at least one external recess formed in the external face of said internal sleeve. Here, "external" used as regards said at least one external recess and said external face of said internal sleeve, means "oriented towards the external sheath".

Advantageously, and according to the invention, said at least one external recess is longitudinally extending along said external face of said internal sleeve. Advantageously, and according to the invention, at least one of said at least one external recess is chosen in the group consisting of at least one external longitudinal groove and at least one longitudinal split of said internal sleeve. In some embodiments, said at least one external recess axially extends along a part only of the length of the internal sleeve. In some embodiments, said at least one external recess axially extends along the entire length of the internal sleeve. There is no impediment to implement another device or other particular structures in order to effectively provide with a rotation and/or translational blocking, as for instance to solder and/or to glue and/or to fasten the internal sleeve to the external sheath and/or to implement a mechanical barrier...

Advantageously, a heating rod according to the invention also comprises a translation blocking device adapted for translational blocking said internal sleeve with regard to said external sheath along said external sheath longitudinal axis. In order to effectively provide with such a blocking, and also to ease the filling of the heating rod, the internal sleeve preferably has a particular blocking structure comprising at least one longitudinal external recess, such as at least one external longitudinal groove, extending along a part only of the length of the external face of said internal sleeve. Alternatively or in combination, there is no impediment to implement another device or other particular structures in order to effectively provide with a translational blocking, as for instance to solder and/or to glue and/or to fasten the internal sleeve to the external sheath and/or to implement a mechanical barrier...

Advantageously and according to the invention, the internal sleeve is made of a thermally conductive and electrically insulating material.

Advantageously and according to the invention, the internal sleeve is a ceramic pre- sintered piece. The "ceramic" term refers to solid electrically insulating inorganic materials, which can be formed by sintering, casting or material removal. The ceramic materials also sustain high temperature, and have high mechanical and oxidation resistances and a good electrically insulating property. They include non-metallic inorganic and/or mineral materials made from compounds of at least one metal and at least one non-metal, such as electro-melted magnesium oxide MgO, aluminum oxide Al ₂ O ₃ , aluminum nitride A1N, a mixture of A1N and MgO, zirconium oxide ZrO ₂ , silicon nitride S1 ₃ N ₄ ... The internal sleeve is pre- sintered in the meaning of being sintered before its use for manufacturing a heating rod, i.e. before being inserted in the external sheath.

In some embodiments, advantageously, a heating rod according to the invention is also characterized in that said inner dimension of said internal sleeve is substantially the same than said wire outer dimension so that there is a contact between an outer surface of said at least one heating wire and an internal surface of said internal sleeve. A clearance may be provided, at least for the mounting, between said at least one heating wire and said internal sleeve.

In particular said internal sleeve and said at least one heating wire can deform or move relative to said external sheath during a swaging step. Such a swaging step made by rotation forging, may cause damages to said at least one heating wire, and may even break it. Advantageously, a heating road according to the invention is also characterized in that it also includes a powder that:

- fills said heating rod,

- blocks said internal sleeve relative to said external sheath, - is made of an electrically insulating powder material.

In some embodiments for a heating rod according to the invention said at least one heating wire is located in said external sheath and circumscribed in a volume having said wire outer dimension and an inner dimension, named wire inner dimension. Advantageously, a heating rod according to the invention also includes a inner core:

- located in said heating rod inside said at least one heating wire,

- having an outer dimension corresponding to said wire inner dimension so that it is housed in said at least one heating wire,

- made of an electrically insulating material.

Advantageously according to the invention, said least one heating wire comprises:

- a glow resistor with one end welded to said external sheath axially closed end,

- a regulation resistor connected at one end to an electric supply connection,

- both free ends of said glow resistor and of said regulation resistor being welded together.

Unlike EP 1 471 307 which teaches that the filling powder must have a higher density than the core in order to increase the thermal conductivity of the heating rod, the inventors have surprisingly found that choosing a higher initial density for the internal sleeve and for the core than for the filling powder, can enhance the efficiency of the heating rod. Thus advantageously and according to the invention, the internal sleeve and the core have an initial density which is higher - in particular by 10% to 30% higher - than the one of the filling powder. This reduces their porosity and improves the thermal conductivity of the heating rod.

The invention also concerns a glow plug including:

(i) a plug body comprising:

- an external fixation thread,

- a receiving housing for a heating rod, (ii) a heating rod including at least one heating wire,

(iii) an electric supply connection for said heating wire, characterized in that said heating rod is according to the invention.

Additionally, the invention provides a method for manufacturing a heating rod comprising:

(i) providing an external sheath:

- axially extending along a longitudinal axis,

- including an axially closed end and an axially open end with said both ends axially opposite,

- having an external sheath inner dimension,

- made of an electrically conductive material,

(ii) providing at least one heating wire:

- made of an electrically conductive material,

- having an electrically conductive external surface, - inserted into the external sheath and housed into a volume having an outer dimension, named wire outer dimension,

(iii) providing electrically insulating means at least radially placed into the heating rod between the external sheath and said at least one heating wire, and comprising at least one internal sleeve:

- having at least one open end,

- extending longitudinally into said external sheath at least along at least a part of said heating wire,

- housed into said external sheath thanks to an outer dimension corresponding to said external sheath inner dimension,

- and said at least one heating wire being housed into said internal sleeve thanks to an inner dimension corresponding to said wire outer dimension, characterized in that said internal sleeve is rotatably blocked with regard to said external sheath around said external sheath longitudinal axis by at least one external recess formed in the external face of said internal sleeve, and an electrically insulating powder located into said at least one external recess of said internal sleeve and into a space between the internal sleeve and the external sheath. Advantageously and according to the invention, at least one of said at least one external recess is chosen in the group consisting of at least one external longitudinal groove and at least one longitudinal split of said internal sleeve.

A manufacturing method according to the invention may be implemented according to many different embodiments. In some embodiments a method according to the invention, is further characterized in that it comprises:

- inserting said internal sleeve into said external sheath,

- then inserting said at least one heating wire, into said internal sleeve.

In other embodiments, a method according to the invention comprises:

- inserting said at least one heating wire into said internal sleeve,

- then inserting said internal sleeve, with said at least one heating wire inside, into said external sheath.

In other embodiments, a method according to the invention comprises:

- inserting said at least one heating wire into said external sheath,

- then inserting said internal sleeve into said heating rod between said external sheath and said at least one heating wire.

A manufacturing method according to an embodiment of the invention is further characterized in that it comprises the following steps:

- inserting said inner core into said at least one heating wire,

- inserting said internal sleeve into said external sheath,

- inserting said at least one heating wire (with said inner core inside) into said internal sleeve,

- connecting one end of said at least one heating wire to an electric supply connection,

- then joining a free end of at least one heating wire to said external sheath closed end, - then lifting said electric supply connection to a predetermined high,

- then filling said heating rod with an electrically insulating powder material,

- applying vibrations to said heating rod,

- then sealing said heating rod with a gasket,

- then closing said heating rod at its axially open end by rotation forging,

- then rotation forging of said heating rod.

An advantage of the invention is that the internal sleeve provides a guidance means for inserting the inner core encircled by said at least one heating wire, into the external sheath, which eases the manufacturing process. This also provides a heating rod that is symmetrical in relation to the main axis of the heating rod; i.e. the inner core is exactly at the middle of the heating rod, and thus said at least one heating wire and the external sheath are exactly at the same distance from each other, at any side of the heating rod.

The invention also concerns a heating rod, a glow plug and a method for manufacturing a heating rod and/or a glow plug characterized in combination by all or some of the features mentioned above or below.

Other features, objectives and advantages of the invention will be appreciated from a reading of the following description, which sets out by way of non-limiting examples, embodiments of the invention, and in which:

- figure 1 is a schematic axial view of a glow plug according to the invention,

- figure 2 is a schematic axial section view of a glow plug according to the invention, with partial cutaways,

- figure 3 is a schematic axial section view of a heating rod according to the invention,

- figure 4 is a schematic axial section view of an internal sleeve, according to a first embodiment of the invention, comprising longitudinal grooves, - figure 5 is a schematic radial section view of the heating rod, according to a second embodiment of the invention, comprising an internal sleeve with longitudinal grooves,

- figure 6 is a schematic radial section view of the heating rod, according to the first embodiment of the invention, comprising an internal sleeve with a longitudinal split.

A glow plug illustrated in figures 1 and 2 comprises a plug body 51 and a heating rod 52. A proximal connecting part of the plug body 51 includes an electrical connector 53. The connector 53 receives an electric supply voltage in order to supply electrical heating wires 59, 60 of the heating rod 52. The distal side of the connector 53 is connected to an electrode 56 which is a round steel rod. The electrode 56 axially extends into the plug body, is connected to the connector 53 at its proximal end and to the heating wires 59, 60 at its distal end. A current flows from the connector 53 to the electrode 56 and then flows through the heating wire 59, 60. The current is adapted to supply the heating wires 59, 60 with electric power, so that said heating wires 59, 60 generate heat at the heating rod distal end by Joule effect.

The plug body 51 is formed of a metallic shell used as a receiving housing 55 for the heating rod 52. The housing 55 comprises a proximal open end across which the electrode 56 proximal end connects to the connector 53, and a distal open end across which the heating rod 52 is inserted. The heating rod 52 includes an external sheath 58 axially extending along a main axis 50 of the glow plug and from the distal end of the electrode 56 to the distal end of the heating rod 52. Said external sheath 58 comprises an open proximal end and a closed distal end. The external sheath 58 and the housing 55 are both in contact and made of an electrically conductive material, preferably a metallic material. This contact forms a seal between the heating rod 52 and the plug body 51.

The heating wires 59, 60 comprise a glow resistor 59 and a regulation resistor 60. The glow resistor 59 extends between the distal closed end of the external sheath 58 and the distal end of the regulation resistor 60. The proximal end of the regulation resistor 60 is fastened by welding to the distal end of the electrode 56. The distal end of the regulation resistor 60 is fastened by welding to the proximal end of the glow resistor 59. The distal end of the glow resistor 59 is fastened by welding to the internal surface of the distal closed end of the external sheath 58.

In such a glow plug numerous elements are electrically conductive. Thus insulator pieces are needed to prevent contacts therebetween. An insulating washer 54 is inserted between the connector 53 and the housing 55 to electrically insulate them. Moreover an electrically insulating separating material 65 is located between the housing 55 and the electrode 56. Additionally, an insulating gasket 57 is inserted at the heating rod proximal end, between the external sheath 58 and the electrode 56 in order to electrically insulate them and to sealingly close the heating rod 52.

At least one electrically insulating internal sleeve 63 is inserted between the heating wires 59, 60 and the external sheath 58. The heating wires 59, 60 forming the glow resistor 59 and the regulation resistor 60, are relatively soft, so they may bend or become eccentric during a rotation forging step. Said internal sleeve 63 prevents contacts between the external sheath 58 and the heating wires 59, 60. Additionally the heating rod 52 comprises a inner core 61 inserted into the heating wires which extend according to a helically winding around said inner core 61. Thus the inner core 61 maintains the heating wires 59, 60 in position, in particular during a rotation forging process.

The plug body 51, its housing 55, the external sheath 58, the internal sleeve 63 and the inner core 61 are preferably globally cylindrical and symmetrical around said main axis 50, but other forms could be considered.

The external sheath 58 holds all the pieces of the heating rod, transmits the electric current from the glow resistor 59 to the housing 55, and also transmits the heat outside of the heating rod 52. Metallic alloys sustaining high temperatures and devoid of transformation phases are used as material for the external sheath 58.

The glow resistor 59 is a helically wound filament (heating wire), whose distal end is welded in a bore-hole to the distal end of the external sheath 58, and whose proximal end is welded to the regulation resistor 60 distal end. Thanks to the bore-hole and to its tight closure, good electrical conductivity is achieved from the glow resistor 59 to the external sheath 58. The wire forming the glow resistor 59 is advantageously made of ferrite steel that contains chromium and aluminum apart from iron. Optimal mechanical and thermal properties are achieved through the following composition: 22% of Cr, 5.3% of Al and 72.7% of Fe. Such alloy is for instance Kanthal AF® (trademark of Sandvik Heating Technology).

The regulation resistor 60 is advantageously made of a helically wound pure nickel wire since nickel has a positive temperature regulation coefficient. The resistor value of the regulation resistor 60 varies with the temperature unlike the glow resistance 59 which the resistor value does not depend on the temperature. Therefore the regulation resistor 60 prevents the temperature from rising over a predetermined level.

The inner core 61 located inside said heating wires 59, 60, has an outer dimension corresponding to the helically wound wire inner dimension, that is to say the internal diameter of the smaller heating wire spires, so that the inner core 61 is housed in the heating wires winding. Said outer dimension of the inner core 61 is substantially the same than said helically wound wire inner dimension, so that there is a contact between an inner surface of the wires and an outer surface of the inner core. A mounting clearance may exist between said helically wound wire inner dimension and said outer dimension of the inner core 61. This clearance is defined by the ratio of the outer dimension of the inner core 61 on the helically wound wire inner dimension. Said ratio is preferably comprised between 0.95 and 1.

The heating rod 52 preferably comprises one inner core 61; however, there is no impediment to insert several smaller inner cores, or to insert a smaller inner core and to complete the space inside the heating wire spires with some electrically insulating powder.

The inner core 61 can be made from various materials that must be electrically insulating and sustain high temperature of the operating conditions. It is preferably made of a ceramic material such as aluminum oxide Al ₂ O ₃ . The inner core 61 is also pre- sintered, for example at a temperature between 900°C and 1300°C, before being inserted into the heating rod 52.

The glow resistor 59 and the regulation resistor 60 are electrically insulated from the external sheath 58 by the internal sleeve 63. The internal sleeve 63 has not only good electrical insulation properties but also good thermal conductivity at high temperatures and thus provides an improved transfer of heat generated by an electrical current flowing through the heating wires 59, 60.The internal sleeve 63 is preferably made of a ceramic insulating material as for instance electro-melted magnesium oxide MgO.

The internal sleeve 63 axially extends in the external sheath

58 at least along at least a part of the heating wires 59, 60. However, there is no impediment to have several internal sleeves 63. In particular, a heating rod according to the invention may have several internal sleeves 63 axially placed in the extension of each other and/or laterally placed in the extension of each other around main axis 50 and/or concentrically placed around each other between a part of the external sheath 58 and the heating wires 59, 60. Thus several heating rod configurations according to the invention are possible, in particular the following ones:

- preferably, only one internal sleeve axially extending into the external sheath 58 along at least the whole axial length of the heating wires 59, 60,

- only one internal sleeve axially extending in the external sheath 58 along only an axial part of the heating wires 59, 60 and powder to axially complete the axial remaining part of the external sheath 58 and of the heating wires 59, 60 not covered by this internal sleeve,

- several internal sleeves axially extending on top of each other and/or laterally placed in the extension of each other around main axis 50 and/or concentrically placed around each other in the external sheath 58 all along the heating wires 59, 60,

- several internal sleeves axially extending on top of each other and/or laterally placed in the extension of each other around main axis 50 and/or concentrically placed around each other in the external sheath 58 along only a part of the heating wires 59, 60, and powder to axially complete the axial remaining part of the external sheath 58 and of the heating wires 59, 60 not covered by the internal sleeves.

Additionally in some embodiments at least one internal sleeve 63 may also extend along at least a part of the electrode 56.

Moreover, each internal sleeve 63 is a ceramic block, which may be pre-sintered for example at a temperature between 900°C and 1300°C.

The inner dimension of the internal sleeve 63 corresponds to the helically wound wire outer dimension, that is to say the external diameter of the larger heating wire spires, so that the heating wires 59, 60 are housed in said internal sleeve 63. The term "larger" covers as well "larger or equal", and "strictly larger", that is to say this term encompasses the case where the inner dimension of the internal sleeve 63 is strictly larger than the helically wound wire outer dimension, and also the case where the inner dimension of the internal sleeve 63 is substantially the same than the helically wound wire outer dimension. Preferably the inner dimension of the internal sleeve 63 is substantially the same than the helically wound wire outer dimension, advantageously with a mounting clearance between said inner dimension of the internal sleeve 63 and said helically wound wire outer dimension. This mounting clearance is defined by the ratio of the helically wound wire outer dimension on the inner dimension of the internal sleeve 63. Said ratio is preferably comprised between 0.95 and 1.

The outer dimension of the internal sleeve 63 corresponds to the external sheath inner dimension, that is to say both dimensions are substantially the same. Additionally said outer dimension of the internal sleeve 63 is smaller than said external sheath inner dimension so that the internal sleeve 63 is housed in said external sheath 58. The term "smaller" covers as well "smaller or equal", and "strictly smaller", that is to say this term encompasses both the case where the outer dimension of the internal sleeve 63 is strictly smaller than the external sheath inner dimension, and also the case where the outer dimension of the internal sleeve 63 is substantially the same than the external sheath inner dimension. Preferably the outer dimension of the internal sleeve 63 is substantially the same than the external sheath inner dimension, advantageously with a mounting clearance between said outer dimension of the internal sleeve 63 and said external sheath inner dimension. This mounting clearance is defined by the ratio of the outer dimension of the internal sleeve 63 on the external sheath inner dimension. Said ratio is preferably comprised between 0.95 and 1.

The internal sleeve 63 has both open axial ends: a proximal open end allowing the passing of the electrode 56 and/ or of the proximal end of the regulation resistor 60; and a distal open end allowing the passing of the distal end of the glow resistor 59 and/or of the inner core 61. At least one of those open axial ends may comprise a device to electrically insulate the different heating wires 59, 60 ends from each other at the passing through this open axial end.

One function of the internal sleeve 63 is to transfer the heat from the heating wires 59, 60 to the external sheath 58. Because the internal sleeve 63 is pre- sintered, a better level of strength is reached (compared to a ceramic powder) which is suited for handling and embedding the heating rod 52. In particular, the internal sleeve 63 allows manufacturing of a very slim heating rod 52 during a rotational forging step which reduces the heating rod radial size. Also having a thinner heating rod 52 increases the heat transfer efficiency by reducing the path between the heating wires 59, 60 and the external sheath 58, which decreases heat losses.

The manufacture of the internal sleeve 63 by a pre- sintering step contributes to an initial density of the internal sleeve 63 by 10% to 30% higher than the density of a ceramic powder, which leads to a better thermal conductivity of the heating rod 52.

A pre- sintered internal sleeve 63 with good electrical insulating and thermal conductivity properties, provides with an equal temperature distribution over the entire volume of the heating rod 52 and prevents any local overheating.

Furthermore a higher initial density provides with a more equal final density of the internal sleeve 63 after a rotational forging reduction step, which results in a better thermal conductivity. In this way, a better redirection of the heat from the heating wires 59, 60 to the external sheath 58, and hence to the surface of the heating rod 52, is achieved. Improving the heat transfer decreases the temperature difference between the glow resistor 59 and the external sheath 58. Thus the level of continuous electrical current flowing through the heating wires 59, 60, in order to maintain the glow plug at a predetermined temperature, can be decreased. This also decreases the danger of breaking for the heating wires 59, 60 and therefore increases the durability of the heating rod 52.

In addition the heating rod 52 also comprises a powder 62 adapted to optimally fill the heating rod 52 and hence to hold together all the heating rod pieces. The powder 62 is made of a ceramic powder, preferably an electro-melted magnesium oxide MgO powder, but it could be made of other ceramic powder which is thermally conductive, electrically insulating and which sustains the predetermined operating conditions of the combustion chamber. The choice of the ceramic powder and also the choice of the materials of the inner core 61 and of the internal sleeve 63, influence the thermal conductivity of the heating rod.

The powder 62 is located into at least one groove (several grooves 64 on figure 5) or into at least one split (one split 66 on figure 6) of the internal sleeve 63, into the space between the internal sleeve 63 and the external sheath 58, and also into the space between the inner core 61 and the internal sleeve 63 comprising the heating wires 59, 60. In other words, the powder 62 is located anywhere where there is air to expel. The size of each groove 64, or the size of each split 66, must largely exceed the average size of the grains of the powder 62. Also the groove(s) 64, or the split(s) 66, allow additional filling of the heating rod 52 by the powder 62.

The powder 62 is preferably filled into the heating rod 52 by a vibration process. By filling the external sheath 58, the powder 62 expels air, and thus prevents a subsequent burning of the heating wires during the glowing phase. Additionally, the powder 62 has also the function of holding the internal sleeve 63 and the inner core 61 in place in relation to the external sheath 58. In order to effectively provide with such a blocking, and also to ease the filling of the heating rod, the heating rod 52 also includes a rotation and/or translation blocking device adapted for rotatably and/or translational blocking said internal sleeve 63 with regard to the external sheath 58 around and/or along said glow plug main axis 50. Preferably, said blocking device is made of a particular structure of said internal sleeve 63 and/or of said external sheath 58. The above mentioned grooves 64, or the above mentioned split 66 of the internal sleeve 63 constitute such a blocking device.

In some embodiments, the internal sleeve 63 may thus have two or more grooves 64 axially and/or transversally extending along at least a part of the internal sleeve 63. Each groove 64 may have a depth and a width of at least 0.1mm, and preferably, of minimum 0.30 mm. Each groove 64 may have a cross- section with various shapes, such as a U-shape, a V-shape, a rectangular shape... Moreover grooves 64 are preferably disposed on both the external and the internal surfaces of the internal sleeve 63. In some other embodiments, grooves 64 are disposed only on one of said surfaces. The grooves 64 on the external surface and the grooves 64 on the internal surface are not radially in front of each others, in order to assure sufficient strength of the internal sleeve 63 during the assembly process.

Preferably, the internal sleeve 63 has four grooves 64, as represented in the figures 4 and 5, distributed at the same distance from each other on each surface of the internal sleeve 63, with each groove 64 on the internal surface located substantially at the middle between two grooves 64 of the external surface. They also preferably axially extend along the entire length of the internal sleeve 63. The shape of the cross-sections of the grooves are all the same, and may be an arc of circle or a U-shape. These selected preferential shape, size and distribution of the grooves 64 provide with an optimal strength of the internal sleeve 63 during the assembly process.

In some embodiments, the internal sleeve 63 may also have at least one split 66 axially extending along at least a part of the internal sleeve 63. This split 66 has a width ranging from 0.1mm to 2mm, and preferably from 0.30mm to 1mm along the entire length of the internal sleeve 63. The cross-section of the split 66 can have various shapes, like for instance a U-shape, a V-shape, a rectangular shape, a trapezium shape... In some embodiments (not represented), the internal sleeve 63 may have several splits 66 (as soon as they do not separate the internal sleeve in several parts). Preferably, the internal sleeve 63 has one split 66, as represented in figure 6, axially extending along the entire length of the internal sleeve 63, having a double trapezium shape being wider at the internal and external surfaces of the internal sleeve 63, thus forming a restricted width between said surfaces, substantially at the middle of its thickness. Also these selected preferential shape, size and distribution of the split 65 provide with an optimal strength of the internal sleeve 63 during the assembly process.

In order to prevent the diffusion of the powder 62 and the intrusion of any foreign body into the heating rod 52, a gasket 57 is inserted between the proximal end of the heating rod 52 and the distal part of the electrode 56. The gasket 57 is preferably an O-ring and is made of material sustaining the predetermined operating conditions of the combustion chamber as for example a fluoro elastomere such as Viton ^® (trademark of DuPont Performance Elastomers L.L.C.).

An embodiment of a method for manufacturing heating rod 52 and a glow plug according to the invention is described thereafter.

The inner core 61 is inserted into the regulation resistor 60 and the glow resistor 59. Both resistors are positioned in a way that the proximal end of the regulation resistor 59 and the distal end of the glow resistor 60 come in contact to be welded together by a method such as laser welded.

The proximal end of the regulation resistor 60 is fastened to the distal end of said electrode 56. This step can be done by various means such as mechanical and/or welding steps.

Said internal sleeve 63 is inserted into said external sheath 58.

The heating wires forming said glow resistor 59 and said regulation resistor 60 with the inner core 61 inserted inside, are inserted into the internal sleeve 63 and the external sheath 58.

The distal end of the glow resistor 59 is welded to the distal closed end of the external sheath 58 by a welding step such as an arc welding step. The fastening of the glow resistor 59 and of the regulation resistor 60 in their correct position in the external sheath 58 prevents any short-circuit contact between the heating wires forming said glow resistor 59 and said regulation resistor 60 and the external sheath 58.

All the above described steps are in a preferably chronological order, but this chronological order can be modified without changing the final heating rod functioning.

The electrode 56 is slightly lifted at its proximal end to a predetermined height from the heating rod 52, which is fixed. This lifting height may be of several millimeters. This lifting step expands the regulation resistor 60 and the glow resistor 59. This lifting of the electrode 56 changes the temperature distribution in the heating rod. The global temperature of the heating rod 52 remains the same, as the heating wires 59, 60 length does not change; but the temperature per unit of surface of the heating rod 52 decreases as the length of the heating wires forming said glow resistor 59 and said regulation resistor 60 per unit of surface is decreased. Therefore this lifting step makes the temperature distribution more uniform along the heating rod 52. Also, with the lifting of the electrode 56, the total length of the glow resistor 59 and the regulation resistor 60 becomes longer. This step also allows an optimal and easier filling of several heating rods 52 simultaneously.

After that, the heating rod 52 is filled with said powder 62 on a vibration table. Vibration movements allow compact and optimal filling of the heating rod 52 by the powder 62.The excess powder 62 is withdrawn by suction.

Then the gasket 57 is put inside the heating rod 52 between the distal part of the electrode 56 and the proximal part of the external sheath 58.

The external sheath 58 is then closed at its proximal end by rotation forging. In this way the gasket 57 and the powder 62 can not fall from the external sheath 58. This step allows an easier handling of the heating rod 52 in subsequent operations. The assembly of said heating rod is thus finished.

Thereafter follows a step of rotation forging, with which the required diameter and electric characteristics of the heating rod are achieved. The rotation forging step reduces the content of atmospheric air in pores of the heating rod 52, by firmly compressing the powder 62. This results in a low porosity and a reduced diameter, and increased the length of the heating rod 52. If necessary, the thinner distal end of the heating rod 52 is also shaped by additional rotation forging.

Once said heating rod 52 is manufactured, it is inserted into said housing 55 of a glow plug, force fit to a predetermined depth.

The internal sleeve 63 facilitates the manufacturing method of a heating rod 52 according to the invention. Indeed, the level of the rotation forging process can be decreased because the internal sleeve 63 and the inner core 61 are pre-sintered. Additionally, the heating wires 59, 60 with the inner core 61 inserted inside, are more easily inserted into the external sheath 58 thanks to that fact that the internal sleeve 63 constitutes a landmark; and there are less placement mistake risks for the heating wires 59, 60 with the inner core 61 inserted inside.

There are many other possible variations of heating rod, of a glow plug and of their manufacturing method, from the embodiments above described and illustrated in the figures. In particular, there is no impediment to design a glow plug comprising a heating rod with several internal sleeves 63 and/or several inner cores 61. It is also possible to design a glow plug comprising a heating rod with ceramic powder to at least partly replace the inner core 61. Also, it is possible to use a material different than a ceramic for the internal sleeve 63 as soon as this material is electrically insulating and sustains said predetermined operating conditions. In particular, the internal sleeve 63 and the inner core 61 could be formed with another method than by sintering, like casting or milling or other. Additionally, there is no impediment to use said glow plug and/or said heating rod in other applications than to preheating a diesel engine as for instance for preheating water.