Patent Publication Number: US-7223098-B2

Title: Ignition unit

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an ignition unit for igniting the burner of a burning appliance (e.g., a water heater, a heating apparatus et cetera) which makes utilization of the combustion of liquid fuel (e.g., kerosene) or gas fuel (e.g., gas). This invention relates more particularly to an ignition unit which is configured such that fuel, blown out from the burner, is ignited by a spark discharge from an ignition electrode and is burned. 
   2. Description of the Related Art 
   Such a type of ignition unit is called the “ignitor”. One such example of the ignition unit of this type has been known in the art, which is comprised of an ignition electrode capable of generation of spark discharges and a transformer mounted in the inside of a casing. The transformer receives a supply of electric power from a commercial power source and then voltage-transforms it into a high voltage for discharge at the ignition electrode, and the transformer and the ignition electrode are connected together so that an electric current of such a high voltage flows to the ignition electrode (see Japanese Utility Model Kokoku Pub. No. 1989-8859). 
   In addition, there is proposed another example of the above-described ignition unit which is characterized in that the connection between a high voltage output part on the casing side and an ignition electrode is established when the former is inserted into the latter (see Japanese Patent Kokai Pub. No. 2001-248838). In this prior art technique, the casing-side high voltage output part is formed by a pair of elastic plates which are in elastic contact with each other. And, a plate-like connection part of the ignition electrode on the base end side thereof is inserted between these elastic plates and, as a result of such insertion, the connection part is held firmly between the pair of elastic plates. 
   SUMMARY OF THE INVENTION 
   And now, generally the casing is formed of synthetic resin. An electronic component for generation of high voltages is disposed in the inside of the casing while on the other hand the ignition electrode is disposed near the burner. And, fuel blown out of the burner towards the combustion chamber is ignited by a spark discharge from the ignition electrode and combustion starts. Accordingly, in a burning appliance such as a water heater and the like, the ignition electrode constituting an ignition unit is disposed at a position where the ignition electrode is exposed to the burning chamber and therefore receives heat of combustion directly therefrom, in other words at a position inside the burner case, while on the other hand the casing is disposed at a position outside the burner case so that it will not receive heat of combustion directly from the burning chamber. Consequently, the ignition unit is assembled after an operation of connecting together the ignition electrode and the casing housing therein the electronic component for high-voltage generation and assembly operations such as individual fixing operations. 
   Conventional ignition units, however, have the following various drawbacks. In the first place, it is required that, in addition to the connection of the ignition electrode to the high voltage output part of the casing, the conductive connection to the casing side be established with respect to the ground side for spark discharges from the ignition electrode. In the second place, even when the connection between the ignition electrode and the high voltage output part of the casing is established not by a cord but by direct mechanical joint as proposed in the aforesaid patent document (2001-248838), it is required that, in order to prevent the occurrence of a relative displacement between the ignition electrode and the casing housing therein the high voltage output part, they must be united together strongly as a single piece. In the third place, in the case of employing a technique in which the ignition electrode and the casing are not united together but are just connected together and they are individually fixed to the burner case or the like by screws or other fixing means, such fixing operations make the assembly of an ignition unit extremely complex. In the fourth place, it must be arranged such that excessive thermal influence by heat of combustion from the side of the burner case is prevented from acting on the casing at the time of the unification or fixation. In the fifth place, when establishing the connection between the ignition electrode and the high voltage output part of the casing by an insertion operation, this requires a structure capable of guaranteeing that the ignition electrode and the high voltage output part are connected together definitely because of the flow of a high voltage electric current. Finally, in the case where the ignition electrode being located in the vicinity of the burner and the high voltage output part of the casing are brought into direct connection with each other, accordingly, the connection area is situated near the burner case, therefore producing the requirement that the occurrence of leakage of a high voltage (for example, spark leak) from the edge of the connection area be avoided. 
   Bearing in mind the above-described drawbacks, the present invention was made. Accordingly, an object of the present invention is to provide an ignition unit capable of eliminating the aforesaid various drawbacks. 
   In order to achieve the above-described object, the present invention provides an ignition unit comprising: a bracket which is fixed to a dividing wall constituting a part of an ignition target; a spark plug which is supported by the bracket so as to penetrate through the dividing wall and which ignites the ignition target by a spark discharge; and a main unit which has a housing containing therein a high voltage output part for the spark discharge and which is mounted onto the bracket, wherein: the bracket is made of an electrically conductive material so as to constitute, together with the dividing wall, a part of a ground-side circuit from the ignition target which has received a spark discharge from the spark plug; the spark plug projects to near the ignition target so that an ignition electrode part on the side of a leading end is able to issue a spark discharge to the ignition target at a position on one side through the bracket; the spark plug is supported by the bracket so that a high voltage input terminal on the side of a base end projects in a predetermined direction at a position on the other side through the bracket; and the main unit has a high voltage output terminal which is connected to the high voltage output part and which projects outwardly and a ground-side connection terminal which projects in parallel with the high voltage output terminal. And, the housing of the main unit and the bracket have sections respectively integrally formed therewith, these sections together constituting a mount mechanism for guiding an insertion operation and for holding a conductive connection state in order that both the conductive connection of the high voltage output terminal to the high voltage input terminal and the conductive connection of the ground-side connection terminal to the bracket can be established by insertion engagement of the main unit into the bracket in the projecting direction of the high voltage output terminal and the ground-side connection terminal. 
   In the case of this invention, with the bracket fastened to the dividing wall of the ignition target by for example screws or the like, the spark plug is supported by the bracket, and the ignition electrode part is located on the side of the ignition target interior to the dividing wall while on the other hand the high voltage input terminal is located outside the dividing wall. Both the conductive connection of the high voltage output terminal on the main unit side with respect to the high voltage input terminal of the spark plug on the bracket side and the conductive connection of the ground-side connection terminal on the main unit side with respect to the bracket are completed simultaneously by carrying out a single insertion engagement operation of inserting the main unit into the bracket from the aforesaid projection direction, i.e., from a certain direction. At the time of such an insertion engagement operation, the mount mechanism ensures that the direction of insertion and the progress of insertion are guided. Besides, it is further ensured that the aforesaid conductive connections are maintained once the mounting operation is completed. Stated another way, it is possible to unite together the main unit and the bracket supporting thereon the spark plug by assembling by direct connection without an intervention (e.g., a high voltage cord), such as by an assembly operation by the conductive connection between the high voltage output terminal and the high voltage input terminal and the conductive connection between the ground-side connection terminal and the bracket. And, since the mount mechanism ensures that the aforesaid insertion engagement is guided, this makes it possible to not only facilitate the foregoing assembly operation but also ensure the execution thereof. And, in a state in which the main unit is mounted onto the bracket by the aforesaid insertion engagement operation, a predetermined high voltage is applied, through the high voltage output terminal and the high voltage input terminal, to the ignition electrode part from the high voltage output part and, as a result, a spark discharge is shot to the ignition target such as the burner or the like. Upon receipt of the spark discharge, fuel from the ignition target is ignited and combustion starts. On the other hand, conduction to the ground-side connection terminal is established from the ignition target in receipt of the spark discharge through the ground-side connection circuit made up of the dividing wall (such as the burner case and so on) and the bracket fixed to the dividing wall. 
   In accordance with the present invention, more preferable working effects will be obtained by addition of the following specific particulars and by giving more detailed specifics. 
   In the first place, it is possible that the housing of the main unit and the bracket have sections respectively integrally formed therewith, wherein the sections together make up a lock mechanism for locking the main unit and the bracket in a mount state established by the mount mechanism. And, the lock mechanism has a convex portion which is formed in one of the housing of the main unit and the bracket and which projects in a direction orthogonal to the direction of the insertion engagement, and a concave portion which is formed in the other of the housing of the main unit and the bracket and into which the convex portion is engaged from the direction orthogonal to the insertion engagement direction, and the convex and concave portions are arranged so that the convex portion reaches a position of engagement into the concave portion by the insertion engagement, and that the convex portion is placed in a state in which the convex portion is engaged into the concave portion with the main unit mounted onto the bracket by the mount mechanism. By virtue of the addition of such a lock mechanism, a state in which the main unit is mounted onto the bracket is established, in other words the high voltage output terminal and the high voltage input terminal are brought into conductive connection with each other, and, in addition, the ground-side connection terminal and the bracket are brought into conductive connection with each other, is maintained more reliably. Stated another way, since the convex portion forming the lock mechanism is engaged, relative to the direction orthogonal to the insertion engagement direction, into the concave portion, this prevents the relative displacement of the main unit to the bracket with respect to all the plane directions orthogonal to the direction in which the convex portion is engaged into the concave portion. As the result of this, besides the positioning of a mount fixation position with respect to the insertion engagement direction and the slipping-off prevention of the main unit, it is possible to strongly hold the mount state of the main unit by preventing the occurrence of displacement of the main unit with respect to other directions, thereby making it possible to strongly unit together the main unit and the bracket. In other words, by virtue of the addition of the lock mechanism, it is ensured that the mount fixation position of the main unit is determined reliably in an operation of mounting the main unit onto the bracket. In addition, after the mounting of the main unit, the main unit is fixed in a slipping-off prevention manner. The mount state of the main unit is held more strongly and, as a result, the main unit and the bracket are united together strongly. 
   In the above-described lock mechanism, the concave portion is an engagement hole which is formed so as to penetrate through the bracket; the convex portion is formed at a leading end side portion of a projected piece which extends and projects from the housing of the main unit in the same direction as the high voltage output terminal; and the projected piece is configured so that an elastic pressing force acts in a direction in which the convex portion is engaged into the engagement hole. By the application of an elastic pressing force from the projected piece, it becomes possible to ensure that the lock state in which the convex portion is engaged into the engagement hole is maintained definitely and, in addition, there is established a state in which elastic pressing force acts against the bracket from the side of the main unit, thereby making it possible to reinforce the unification of the main unit and the bracket to a further extent. 
   In the second place, it is possible that the mount mechanism is provided with a guide rail which projects from an outer surface of the housing of the main unit and which extends in parallel with the insertion engagement direction, and a clamp piece which is formed in the bracket so as to slide in the insertion engagement direction along over the housing&#39;s outer surface with the guide rail clamped by the clamp piece. In this case, a more concrete structure serving as the mount mechanism is specified. And, an insertion engagement operation is carried out in which either the main unit or the bracket is slid along over the housing&#39;s outer surface in the direction in which the guide rail extends, i.e., in the insertion engagement direction, with the main unit-side guide rail clamped by the bracket-side clamp piece. Because of this, the direction of an insertion operation is determined automatically and, in addition, it is ensured that the insertion operation is guided by the sliding of the main unit or the bracket being slid along over the housing&#39;s outer surface, with the guide rail clamped by the clamp piece. Therefore, the process up to the mount operation is guided without fail. In this case, the number of guide rails may be one, two, three or more. Additionally, as the clamp piece for clamping guide rails, a single clamp piece having a specific cross-sectional shape may be used. Alternatively, the guide rails may be clamped between end edges of two or three flat plate-like clamp pieces. 
   In addition, it may be arranged such that the guide rail is provided with a concave groove which opens at a side surface of the guide rail and which extends in a straight line along the guide rail, and that an end edge of the clamp piece which extends in the insertion engagement direction is engaged into the concave groove. By performing an insertion engagement operation with the end edge of the clamp piece engaged into the concave groove of the guide rail, it becomes possible to mount the main unit onto the bracket in such a restricted state that there occurs no displacement with respect to all of the orthogonal triaxial direction. This makes it possible to more strongly and highly realize the unification of the main unit and the bracket. 
   Furthermore, it may be arranged such that the housing of the main unit is provided with two guide rails while on the other hand the bracket has a central clamp piece which is inserted between the two guide rails and two outer clamp pieces which are arranged outside the two guide rails respectively, and that the central clamp piece is arranged so that, when the main unit is mounted onto the bracket, the guide rails are clamped between the central clamp piece and each of the outer clamp pieces, with the central clamp piece spaced apart from the outer surface of the housing so as to define a space therebetween. In this case, it becomes possible for the central clamp piece to shield the housing&#39;s outer surface, with a layer of air intervened between the housing&#39;s outer surface and the central clamp piece. Accordingly, even when, with the combustion of the ignition target, radiation heat is received from the dividing wall of the ignition target to which the bracket is fixed, it becomes possible to suppress thermal influence on the main unit as much as possible by heat reflection by the central clamp piece and heat insulating action by the air layer. Because of this, the durability of the main unit whose housing contains therein the high voltage output part and so on is improved, thereby making it possible to improve the reliability of the ignition unit itself. 
   In the third place, it may be arranged such that the high voltage output terminal is accommodated in the inside of a tubular cover which projects from the housing of the main unit and which opens at a leading end thereof. As a result of such an arrangement, in the state in which the conductive connection between the high voltage output terminal and the high voltage input terminal is established, it becomes possible to avoid the possibility of occurrence of the leak of electricity caused by a spark leak from the sharp end of the high voltage output terminal or from the sharp end of the high voltage input terminal. Especially in the case where the dividing wall to which the bracket is attached fixedly is situated in the vicinity of the high voltage output terminal or the like placed in the aforesaid conductive connection state, the occurrence of such electricity leakage is avoided effectively. 
   Additionally, in the case where such a tubular cover is added, it may be arranged such that the high voltage input terminal is shaped like a rod while on the other hand the high voltage output terminal has a leading end opening which receives therein the rod-shaped high voltage input terminal by the insertion engagement of the main unit, and that an inner surface of the tubular cover is shaped like a funnel the diameter of which gradually decreases from a portion thereof which opens at a greater diameter than that of the leading end opening of the high voltage output terminal to the leading end opening of the high voltage output terminal. As a result of such arrangement, even when any one of the high voltage output and input terminals or both undergo a slight displacement in an insertion engagement operation of the main unit to the bracket, it is possible to provide guidance to a conductive connection position by guiding the high voltage input terminal along over the funnel-shaped internal surface of the tubular cover. This therefore makes it possible to ensure that the conductive connection of the high voltage output terminal and the high voltage input terminal is established without fail. Because of this, the process to the mutual conductive connection of the main unit-side high voltage output terminal and the spark plug-side high voltage input terminal is guided reliably in the insert engagement of the main unit into the bracket, thereby making it possible to ensure that the conductive connection is established without fail. 
   In the fourth place, it may be arranged such that the ground-side connection terminal is formed of two elastic band plates which are placed one upon the other and which are bent in such a curve that their projected ends open outwardly from each other, and that the bracket has integrally formed therewith a plate terminal which extends in the insertion engagement direction and which is sandwiched between the two elastic band plates by the insertion engagement. As a result of the insertion engagement operation of the main unit into the bracket, the plate terminal is driven between the projected ends of the two elastic band plates making up the ground-side connection terminal, thereby ensuring that the conductive connection therebetween is established without fail, with the plate terminal fastened between the two elastic band plates by the action of elastic resilience. Besides, it is arranged such that the projected ends are bent so as to open outwardly from each other, so that, even when the ground-side connection terminal and the plate terminal are slightly displaced from each other in the insertion engagement operation, guidance to the conductive connection position is provided without fail and, as a result, the conductive connection is established. Because of this, in mounting the main unit onto the bracket by an insertion engagement operation, the guidance to the conductive connection of the ground-side connection terminal and the maintenance of the conductive connection are ensured more definitely. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view illustrating in an exploded manner an example of an embodiment of the present invention when being attached to a burning appliance; 
       FIG. 2  is a perspective view illustrating in an exploded manner an ignition unit of the embodiment; 
       FIG. 3  is a perspective view illustrating the ignition unit of  FIG. 2  in a one-piece state as a result of the assembly of its individual parts; 
       FIG. 4  is a partial top plan view of the ignition unit when attached to the burning appliance of  FIG. 1 ; 
       FIG. 5  is a perspective view illustrating a rear surface of a main unit when viewed from above; 
       FIG. 6  is an enlarged cross-sectional explanatory view taken along line A—A of  FIG. 4 ; 
       FIG. 7  is cross-sectional explanatory view taken along line B—B of  FIG. 6 ; and 
       FIG. 8  is an enlarged cross-sectional explanatory view taken along line C—C of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinbelow, embodiments of the present invention will be described with reference to the drawings. 
   Referring to  FIG. 1 , there is shown an exploded perspective illustration of an ignition unit G of the present invention which is incorporated into a combustion system F for hot water supply equipment. This combustion system F is made up of: a combustion burner  11  formed by combustion tubes arrayed side by side in a plurality of rows; a burner case  13  which contains therein the combustion burner  11  to divisionally define a space for a combustion chamber  12  above the combustion burner  11 ; and an fan  14  which provides a supply of combustion air to the combustion burner  11 . The combustion system F serves to heat water to a predetermined level of temperature by use of the combustion heat of the combustion burner  11  and supplies the hot water to a hot water tap or the like. More specifically, a storage water heater body (not shown in the figure) which houses therein for example a heat exchanger of the fin and tube type is fixedly joined to an upper flange  131  of the burner case  13 , wherein water which is made to flow through the inside of the heat exchanger is heat-exchange heated by the heat of combustion of the combustion burner  11 . In the present invention, the combustion burner  11  is a “ignition target”, and the shell wall of the burner case  13  is a “dividing wall”. The burner case  13  is formed of a plate of electrically conductive steel and constitutes a part of a ground-side circuit by combination with the combustion burner  11 , as will be described later. 
   The ignition unit G is an integrated assembly of a bracket  2  which is fixed to the burner case  13  by screws  21 ,  21 , a spark plug  3  supported by the bracket  2 , and a main unit  4  whose housing (chassis)  41  contains therein a high voltage output part. In the figure, a heat-resisting gasket  5  is shown. The heat-resisting gasket  5  is put between the bracket  2  and the burner case  13 , thereby blocking the transfer of heat from the burner case  13  to the bracket  2  and preventing the leakage of exhaust combustion gas. Formed through the burner case  13  are a through hole  132  of a given shape by which a spark plug  3  is arranged so as to pass therethrough, and a through hole  133  serving as an supervising window. The bracket  2  is attached in positional association with the through holes  132 ,  133 . 
   Hereinafter, referring mainly to  FIGS. 2 and 3 , each of the component parts  2 ,  3 ,  4  of the ignition unit G will be described in detail.  FIG. 2  is a perspective view illustrating the ignition unit G in an exploded manner, while  FIG. 3  is a perspective view illustrating the ignition unit G in a one-piece state as a result of the assembly of its individual parts. In the following description, the directions in which orthogonal three axes (X, Y, Z) extend will be used as directional references in some cases (see  FIG. 1  or  2 ). In the figures, the X-axis direction is a vertical direction, the Y-axis direction is a front-to-rear direction, and the Z-axis direction is a horizontal direction. With respect to the axes X, Y, or Z, the same will be applied to the other figures. 
   As shown in detail in  FIGS. 2 and 3 , the spark plug  3  is composed of a body  31  made of insulating material such ceramic or the like, a rod-like electrode conductor  32  which passes completely through the body  31  in the Y-axis direction, and a flame rod  33  for ignition detection which passes completely through the body  31 . In the present embodiment, the flame rod  33  is just provided in the same body  31  through the use of the spark plug  3 . Therefore, the provision of the flame rod  33  is not essential for the spark plug  3  of the present invention. However, in accordance with the present embodiment, it is possible to install the flame rod  33  together with the spark plug  3  without making use of other special install means. The body  31  is comprised of: an internal portion  311  which is fitted into the through hole  132  of the burner case  13 ; an external portion  312  which is fitted into a retention hole  232  of the bracket  2  which will be described later; and a flange portion  313  which projects from the boundary position of the portions  311 ,  312  to the outer circumference, extending greatly beyond the through hole  132  and the retention hole  232 . The flange portion  313  is for positioning and engagement with respect to the bracket  2 . More specifically, by the flange portion  313  being engaged into a retention portion  23  of the bracket  2  which will be described later, the positioning of the spark plug  3  to the bracket  2  to a given retention position is determined. A leading end of the conductor  32 , which is a rear side end thereof is situated in the inside of the combustion chamber  12 , is bent in a curve towards the combustion burner  11 , and is extended to near the combustion burner  11  to form an ignition electrode part  321 , while on the other hand a base end of the conductor  32  which is a front side end thereof is situated outside the burner case  13  and is bent in a curve in a given direction (bent horizontally to be oriented to the right relative to  FIGS. 2 and 3 ) to form a high voltage input terminal  322  (see also  FIG. 4 ). 
   The bracket  2 , formed by bending a metal sheet material which is an electrically conductive material, is electrically connected to the burner case  13  and to the combustion burner  11  by the screws  21 ,  21 , whereby the bracket  2  itself constitutes a part of the ground-side circuit. The bracket  2  is provided with an supervising window  22 , a retention portion  23  for supporting the body  31  of the spark plug  3 , a mounting portion  24  on which to mount the main unit  4 , and an engagement hole  25  for restricting the main unit  4  in the mount state. The supervising window  22  is formed by fitting a sheet of heat-resistance glass or transparent quartz into a through hole. The state of flames produced by the combustion burner  11  can be inspected from the outside through the supervising window  22 . The retention portion  23  is composed of a concave part  231  into which the flange portion  313  of the body  31  of the spark plug  3  is engaged from the side of the burner case  13  and a retention hole  232  which has a hole shape corresponding to that of the outer circumference of the outer portion  312  and which opens at the bottom of the concave part  231 . The supervising window  22  and the retention portion  23  are formed in a flat plate portion  26  which is fixed to the burner case  13  while being brought into close contact with the gasket  5 . 
   The mounting portion  24  is formed as follows. That is, the mounting portion  24  is so bent in a curve from a lateral one side position (the right-hand side in  FIGS. 2 and 3 ) of the flat plate portion  26  towards the front side as to form a step with respect to the flat plate portion  26 . Because of this, the mounting portion  24  is situated at an offset position outwardly (frontwardly) spaced a given distance apart from the burner case  13 , with the flat plate portion  26  fixed to the burner case  13 . Such arrangement keeps the main unit  4  away from the burner case  13 , thereby achieving improvement in the heat resistance of the main unit  4 . In addition, a projected piece  27  which is bent in a curve from a lower edge position of the flat plate  26  and which projects for a predetermined distance towards the front side is formed in a continuous manner. Further, a flange piece  28  which is bent in a curve downwardly from a leading end position of the projected piece  27  is formed in a continuous manner. And, a plate terminal  271 , which is brought into conductive contact with a ground-side connection terminal  46  of the main unit  4  which will be described later, is formed by a one side end edge of the projected piece  27  (the right-hand side end edge in  FIGS. 2 and 3 ). In addition, an engagement hole  25  as a concave portion is formed at a predetermined position of the flange piece  28 , and penetrates therethrough. A guide portion  281 , which is so bent in a curve as to open diagonally outwardly, is formed, extending from the engagement hole  25  to an end portion of the flange piece  28 . Together with a projected piece  44  and a convex portion  43  of the main unit  4  which will be described later, the engagement hole  25  makes up a “lock mechanism” of the present invention. 
   The mounting portion  24  constitutes, together with guide rails  42 ,  42  of the main unit  4  which will be described later, a “mount mechanism” of the present invention. The mounting portion  24  is made up of a central clamp piece  241  and outer clamp pieces  242 ,  242  which are located outside the central clamp piece  241  facing each other thereacross (see also  FIG. 5 ). A slit  243  of a given width s is defined between the central clamp piece  241  and each of the outer clamp pieces  242  (see  FIG. 6 ). On the other hand, the central clamp piece  241  is bent in a curve at the root thereof, expanding in parallel with each of the outer clamp pieces  242  at a position nearer to the burner case  13  than each of the outer clamp pieces  242 . 
   On the other hand, in the main unit  4  the housing  41  which is made of synthetic resin for insulation and protection houses therein an electric circuit board (not shown) on which to mount electronic components such as a transformer constituting a high voltage output part. The main unit  4  has two rows of guide rails, i.e., the guide rails  42 ,  42 , constituting a part of the mount mechanism, the projected piece  44  provided with the convex portion  43  constituting a part of the lock mechanism, a high voltage output terminal  45  operable to apply to the spark plug  3  a high voltage generated by the electric circuit board, the ground-side connection terminal  46 , and an electric power supply terminal  47  for turning on the electric circuit board. 
   Both the guide rails  42 ,  42  are formed integrally with the housing  41  so that they extend in parallel with one another on a rear surface  411  of the housing  41  in the Z-axis direction, with a space of a given size defined therebetween. It is set such that the distance between the guide rails  42 ,  42  agrees with the width of the central clamp piece  241 , i.e., the width relative to the X-axis direction (see  FIG. 6 ). In addition, each guide rail  42  is thick over the full length thereof or has a partially thickened portion (in the present invention each guide rail  42  is thickened partially). A concave groove  421 , which opens outwardly (upwardly and downwardly) at a position flush with the rear surface  411  and which extends in the Z-axis direction, is formed in the thickened portion. And, the groove width of the concave groove  421  is set to a dimension corresponding to the thickness of the end edge of each of the outer clamp pieces  242 , while on the other hand the thickness of each of the guide rails  42  at the groove bottom of the concave groove  421  is set to the aforesaid width s. In addition, on the side of the bracket  2 , the concave groove  421  opens in such a manner that it gradually increases in groove width for facilitating insert and engagement operation guidance (see reference numeral  422  in  FIG. 7 ). 
   Therefore, as shown in  FIG. 5 , the rear surface  411  of the main unit  4  is brought into abutment with the outer clamp pieces  242 ,  242  of the mounting portion  24  of the bracket  2 . Then, when the main unit  4  is made to approach the bracket  2  in the Z-axis direction, in other words when the main unit  4  is shifted to the left, end edges of the outer clamp pieces  242 ,  242  are inserted and engaged into the concave grooves  421  of the guide rails  42 . The main unit  4  is now mounted on the mounting portion  24  of the bracket  2 , as shown in  FIGS. 6 and 7 . In this mount state, the one pair of the guide rails  42 ,  42  are placed in such a state that they are sandwiched tightly by the outer clamp pieces  242 ,  242  from the outside over a given length relative to the Z-axis direction, while at the same time the end edges of the outer clamp pieces  242 ,  242  are placed in such a state that they are closely fitted to the concave grooves  421 ,  421  respectively over a given length relative to the Z-axis direction. This therefore ensures that the displacement relative to the X-axis direction and the Y-axis direction is prevented from occurring, thereby enabling the housing  41  of the main unit  4  to be secured to the bracket  2 . In addition, in such a mount state, both the outer clamp pieces  242 ,  242 , although they are in an abutment relationship with the rear surface  411  of the main unit  4 , covers it and, in addition, the central clamp piece  241  is spaced from the rear surface  411  of the housing  41  and is therefore placed in such a state that it covers the rear surface  411  between the guide rails  42 ,  42  with a space P defined between itself and the rear surface  411 . Therefore, radiation heat from the burner case  13  is reflected by the central clamp piece  241  and, in addition, it is possible to insulate the radiation heat by a layer of air of the space P. Because of this, it becomes possible to achieve improvement in thermal durability by avoiding thermal influence on the main unit  4  as much as possible. 
   By the above-described insertion engagement operation of the main unit  4 , in addition to the mounting of the main unit  4  to the bracket  2 , the conductive connecting of the high voltage output terminal  45  to the high voltage input terminal  322 , the conductive connecting of the ground-side connection terminal  46  to the plate terminal  271 , and the engaging of the convex portion  43  of the projected piece  44  into the engagement hole  25  are all completed at the same time. More specifically, the projected piece  44 , the high voltage output terminal  45  housed in the tubular cover  48 , and the ground-side connection terminal  46  all project from their respective predetermined positions of a left-hand side surface  412  of the housing  41  of the main unit  4 . The predetermined positions used here are those at which, when the housing  41  is mounted onto the mounting portion  24 , (i) the onvex portion  43  is engaged into the engagement hole  25  while simultaneously the projected piece  44  is in abutment with a rear surface of the flange piece  28 , (ii) the high voltage output terminal  45  and the high voltage input terminal  322  lie coaxially and face to face with each other in the Z-axis direction; and (iii) the plate terminal  271  lies between a pair of elastic band plates  461 ,  461  (see  FIG. 5 ) which together constitute the ground-side connection terminal  46  with respect to the Z-axis direction. By virtue of such a positional arrangement, with the aforesaid insert engagement of the main unit  4  into the bracket  2 , the conductive connecting of the high voltage output terminal  45  and the high voltage input terminal  322 , the conductive connecting of the ground-side connection terminal  46  and the plate terminal  271 , and the engaging between the convex portion  43  and the engagement hole  25  are all completed at the same time that the main unit  4  is mounted integrally onto the bracket  2 . 
   The convex portion  43 , the high voltage output terminal  45 , and the ground-side connection terminal  46  will be described in detail. Preferably both the convex portion  43  and the projected piece  44  are made of material capable of exerting elastic resilience; for example, they may be made of synthetic resin and be formed integrally with the housing  41 . Alternatively, it may be arranged such that they are formed of synthetic resin or the like as a separate body and are adhered to the housing  41 . The left side of the convex portion  43  forms a slope while the right side thereof is provided with a stepped portion. As a result of such arrangement, when the convex portion  43  is engaged into the engagement hole  25 , the stepped portion of the convex portion  43  engages with the hole edge of the engagement hole  25 . In other words, the position of the main unit  4  to the bracket  2  with respect to the Z-axis direction is determined by the engagement. Once the engagement is established, the rightward difference movement of the main unit  4  is prevented, in other words the engagement functions to prevent the main unit  4  from slipping off from the engagement hole  25 . In addition, if the projected piece  44  exerts elastic resilience, this causes elastic resilience to act on the flange piece  28  by causing the projected piece  44  to get into the rear surface side of the flange piece  28 , thereby making it possible to further enhance the integration of the main unit  4  with the bracket  2 . 
   In addition, the high voltage output terminal  45  is formed by a tubular terminal having a leading end  451  which opens like a trumpet or funnel for receiving the rod-shaped high voltage input terminal  322 , as shown in  FIG. 8 . And, the tubular cover  48  houses therein the high voltage output terminal  45  and shields the whole or a part of the high voltage output terminal  45 . In the present embodiment, as the tubular cover  48 , there is shown one that is provided with a leading end  481  having a semicircular cross-sectional shape and a lateral given range portion  482  having a circular cross-sectional shape including the leading end  451  of the high voltage output terminal  45 . In other words, at least the leading end  451  of the high voltage output terminal  45  is shielded. Because of such arrangement, the occurrence of a leak of electricity from the leading end  451  to the burner case  13  caused by spark leakage or the like is prevented. In addition, an inner surface  483  of the tubular cover  48  extending to the leading end  451  is shaped like a funnel so that the diameter gradually decreases towards the leading end  451 . This therefore reliably guides the high voltage input terminal  322  to engage into the high voltage output terminal  45 , at the time of the insertion engagement of the main unit  4 . 
   Furthermore, the ground-side connection terminal  46  is formed by the pair of elastic band plates  461 ,  461  which are placed one upon the other and the elastic band plates  461 ,  461  have projected ends which are so bent in a curve as to spread outwardly from each other for providing guidance for the entrance of the plate terminal  271  between the elastic band plates  461 ,  461 . And, the plate terminal  271  is inserted between the elastic band plates  461 ,  461  by said insertion engagement operation, as a result of which the plate terminal  271  is sandwiched tightly between the elastic band plates  461 ,  461 , with elastic resilience acting thereon. 
   The above-described ignition unit G is attached to the combustion system F as follows. In the first place, with the spark plug  3  supported by the retention portion  23  of the bracket  2 , the bracket  2  is attached to the burner case  13  by the screws  21 ,  21 . At this time, the spark plug  3  is inserted, through the gasket  5 , into the through hole  132 . In the next place, the main unit  4  is mounted onto the mounting portion  24  of the bracket  2  by an insertion engagement operation, whereby the integration of the main unit  4  and the bracket  2 , the conductive connection between the high voltage output terminal  45  and the high voltage input terminal  322 , and the conductive connection between the ground-side connection terminal  46  and the plate terminal  271  are all completed by the mounting and lock mechanisms in a single work operation. In other words, the integration of the main unit  4  and the bracket  2  and both of the conductive connection between the high voltage output terminal  45  and the high voltage input terminal  322  and the conductive connection between the ground-side connection terminal  46  and the plate terminal  271  are completed by a single assembly operation. 
   It should be noted that the present invention is not limited to the above-described embodiments. The present invention includes other various embodiments. Stated another way, in the above-described embodiment the central clamp piece  241  is provided mainly for the purpose of heat shield and heat insulation, which is not essential in the mounting mechanism because in the mounting and assembling operations the outer clamp pieces  242 ,  242  mainly take charge of guidance and position retention. On the contrary, it may be arranged such that the provision of the outer clamp pieces  242 ,  242  is omitted and the mounting mechanism is made up of the central clamp piece  241  alone. In this case, it suffices if a concave groove is so formed as to open on the opposite side to each guide rail  42  and both side end edges of the central clamp piece are fitted into the concave grooves. 
   The conductive connection between the high voltage output terminal  45  and the high voltage input terminal  322  and the conductive connection between the ground-side connection terminal  46  and the plate terminal  271  should not be in any way limited to those depicted in the figures. Stated another way, any type of connection method may be employed as long as it provides conductive connections by the inserting of the main unit  4  from the Z-axis direction.