Glow plug for diesel engines and process for its manufacture

Abstract

 Glow plug for diesel engines, comprising:

• a body (12) of elongate shape having a penetrating cavity (26), and
• a heating element (14) including a metal sheath (16), an electrode (20) projecting from one end of the metal sheath (16) and a resistive element (22) located within the said sheath (16),

The heating element (14) is fixed to the body (12) by means of a radial compression from the exterior of a portion (32) of the body (12).

Description

[0001] The present invention relates to glow plugs for diesel engines and a process for their manufacture.

[0002] Glow plugs of the most common type comprise:

• a body of elongate shape having a penetrating cavity, and
• a heating element including a metal sheath, an electrode projecting from one end of the metal sheath and a resistive element located within the said sheath and insulated electrically from it.

[0003] At the present time, the assembly of the body and the heating element is carried out by interference-fitting the heating element into the cavity of the body. The insertion provides a forced fit and is carried out by applying an insertion force between the body and the heating element directed parallel to the longitudinal axis of the body and of the heating element. The insertion force must lie between two clearly defined values (typically 150 and 800 daN). An insertion force which is too low can cause insufficient interference which result in the detaching of the heating element from the body and/or an insufficient degree of gas-tightness. An insertion force which is too great can cause excessive interference and consequent seizing of one of the two elements and a loss of tightness due to micro-leaks. Moreover, in certain types of plug, the interference between the body and the heating element can produce excessive deformation of the threaded portion formed on the body (permitting the threaded connection of the plug to the cylinder head) and can affect in a more or less serious way the ease of fitting the plug to the engine.

[0004] The insertion force is closely related to the tolerances on the diameters between which the interference is established. Consequently, the tolerances must be very strict, especially as they are applied to relatively small dimensions (approximately 5 mm). The requirement to have very strict tolerances on the components considerably increases the production cost.

[0005] The object of the present invention is to provide a glow plug for diesel engines and a process for its manufacture which enable the aforesaid drawbacks to be overcome.

[0006] According to the present invention, this object is achieved by means of a glow plug and a process for its production having the characteristics forming the subject of the claims.

[0007] A preferred embodiment of the present invention will now be described in detail with reference to the attached drawings, provided purely by way of example and without restrictive intent, in which:

• Figure 1 is a side view in partial section of a plug according to the present invention,
• Figure 2 is a detail on a larger scale of the part indicated by the arrow II in Figure 1,
• Figure 3 is a side view of a heating element before fitting into the body, and
• Figure 4 is an axial section through the body before it is joined to the heating element.

[0008] With reference to Figure 1, the number 10 indicates a glow plug for diesel engines. The plug 10 comprises, in a known way, a metal body of elongate shape 12 and a heating element 14. The metal body 12 and the heating element 14 are two separate components which are fixed together permanently by the method which is described below.

[0009] With reference to Figure 3, the heating element 14 comprises a metal sheath 16 having a closed end of rounded shape 18. An electrode 20 consisting of a metal bar projects from a second end of the metal sheath 16. The electrode 20 is connected to a resistive element 22 (Figure 1) normally consisting of one or more spirals of resistive material connected electrically to the electrode 20 and to the metal sheath 16. The metal sheath 16 is also filled with a powdered insulating material.

[0010] The heating element 14 is produced in the following way. The sheath 16 is formed from a small metal tube, one of whose ends is rounded and partially closed. The coiled resistive element 22 is fixed to one end of the electrode 20 and is inserted into the metal sheath 16. One end of the resistive element 22 is then welded to the rounded end 18 of the sheath 16. This welding completes the closing of the end of the sheath element. The sheath is then filled with insulating powder and the open end of the sheath from which the electrode 20 projects is closed by the insertion of a ring of elastomeric material. A pre-closing operation is carried out at this point in the process, to prevent leaks of the magnesium oxide powder and any rotation of the rod. The heating element 14 is then subjected to a swaging process during which the sheath 16 is repeatedly compressed radially from the exterior to produce a reduction of the diameter and a increase in the length of the metal sheath 16. This operation is carried out with a variable-head swaging machine. In a preferred embodiment of the present invention, a pair of annular projections 24 are formed on the outer surface of the metal sheath 16. In a practical embodiment, the diameter of the sheath 16 at the end of the swaging can be of the order of 4.9 mm and the external diameter of the annular projections 24 can be of the order of 5.2 mm. The annular projections 24 are located in an area of the sheath 16 which is designed to establish the connection with the body 12. The number, shape and dimensions of the projections 24 are not essential for the purposes of the present invention and can be varied from those shown. For example, it is possible to provide a single projection having larger dimensions or more than two projections having smaller dimensions. These projections are formed during the stage of plastic deformation of the metal sheath 16. This is because it is possible to obtain a shaped profile of the outer surface of the sheath during the swaging, by deforming the swaged surface in different ways.

[0011] With reference to Figure 4, the body 12 is of elongate shape and has a penetrating cavity 26. The body 12 also has, in a known way, a threaded portion 28 and a hexagonal portion 30 designed to be engaged with a spanner to enable the plug to be screwed into or out of the cylinder head of an engine.

[0012] Preferably, the penetrating cavity 26 of the body 12 has two portions 26a and 26b having diameters slightly different from each other, and a shoulder 32 formed in the area of transition between the portions having different diameters 26a, 26b. The portion 26a of the cavity 26 has a diameter such that it receives the portion of the heating element 16 carrying the aforesaid annular protuberances 24. In a particular and non-restrictive example of embodiment, where the external diameter of the projections 24 is 5.2 mm, the internal diameter of the portion 26a of the cavity 26 can be, for example, 5.22 mm. An area 34 of increased thickness is preferably formed on the outer surface of the body 12 in the area in which the annular projections 24 will be positioned.

[0013] According to the present invention, the process for fixing the heating element 14 to the body 12 provides for the insertion of the heating element 14 without interference into the portion 26a of the cavity 26. The portion 34 of the body 12 is then subjected to a plastic deformation operation by radial compression from the exterior, preferably carried out by means of a variable-head swaging machine. Following a swaging operation, the inner surface of the cavity 26a adheres completely to the outer surface of the metal sheath 16. Figure 2 shows schematically the contact area between the sheath 16 and the body 12 at the end of the plastic deformation of the body. The protuberances 24 of the sheath 16 form a fastening which prevents the sheath from sliding out of the body 12, and at the same time forms a labyrinth structure which improves the gas-tightness of the junction area between the sheath 16 and the body 12.

[0014] During the stage of insertion of the heating element 14 into the body 12, the two annular protuberances can act as guides for pre-fitting into the body. In this stage it is not necessary to check the insertion force and consequently the tolerances of the body 12 and the heating element 14 can have values greater than those required for the process according to the known art. The swaging of the body 12 is carried out on a limited area of the body, and plastically deforms the material of the body on to the sheath 16 at the positions of the annular projections 24. At the end of the process, the inner surface of the body is matched perfectly to the profile imparted to it by the outer surface of the sheath which acts as a shaped swaging die. The deformation of the portion 34 of the body 12 does not change the dimensions of other parts of the body, except by a slight axial elongation of the body, since the swaging acts only on the area in which the body and sheath are fixed together, without affecting other areas of the body.

[0015] A particularly important advantage of the process according to the present invention consists in the fact that, with this assembly system, it is possible to considerably reduce the proportion of wastage of the product and to achieve a very low possibility of defects in the finished product. By using the correct equipment and machine parameter setting, it is possible to reduce the proportion of wastage in the assembly stage virtually to zero. Where tightness in respect of the leakage of gases from the combustion chamber is concerned, the process according to the invention is independent of the variation of the surface states of the components. Micro-seizing which is difficult to detect and which may occur with the process according to the known art is thus avoided. With the process according to the invention, the roughness of the contact surfaces provides an advantage in that it enables the gas-tightness to be increased. Where costs are concerned, the process according to the invention provides a significant advantage by comparison with the conventional insertion system, since it makes it possible to use lower-grade and therefore less costly machining of the components. With the process according to the invention, the body 12 could be produced by a plastic deformation machining process which is markedly less expensive than machining with removal of material, which is necessary with the process according to the known art to ensure sufficiently small tolerances.

Claims

1. Glow plug for diesel engines, comprising:

- a body (12) of elongate shape having a penetrating cavity (26), and

- a heating element (14) including a metal sheath (16), an electrode (20) projecting from one end of the metal sheath (16) and a resistive element (22) located within the said sheath (16),

   characterized in that the heating element (14) is fixed to the body (12) by means of a radial compression from the exterior of a portion (32) of the body (12).

2. Glow plug according to Claim 1, characterized in that the aforesaid sheath (16) has at least one annular projection (24) located in the area in which the sheath (16) is fixed to the body (12).

3. Process for producing a glow plug for diesel engines, comprising:

- a body (12) of elongate shape having a penetrating cavity (26), and

- a heating element (14) including a metal sheath (16), an electrode (20) projecting from one end of the metal sheath (16) and a resistive element (22) located within the said sheath (16),

   the process comprising a stage in which the heating element (14) and the body (12) are fixed together,
   characterized in that the said fixing is achieved by means of an operation of plastic deformation of a portion (32) of the body (12) by means of radial compression from the exterior.

4. Process according to Claim 1, characterized in that the aforesaid plastic deformation of the body (12) is achieved by means of swaging.

5. Process according to Claim 1, characterized in that it comprises the forming of at least one annular projection (24) on the outer surface of the sheath (16) before the heating element (14) is inserted into the aforesaid cavity (26).

Drawing