Monolithic glow plug probe/shell

Abstract

 An incandescent glow plug for preheating Diesel engines, comprising a main body (14) of elongated shape and having a threaded length (30) for connecting the plug into the cylinder head of the engine, and a sheathing element (12) projecting from one end of the main body and containing a spiral resistor (18) attached to a terminal (20) projecting from the second end of the main body (14).
The sheathing element (12) and the main body (14) are obtained integrally from a single monolithic workpiece.

Description

[0001] The present invention relates to an incandescent glow plug for preheating Diesel engines, as well as a method for the production of such plugs.

[0002] Incandescent glow plugs of known type are characterized by a main body of elongated shape with a threaded portion to permit it to be connected into the cylinder head. From one end of the main body there projects a sheathing element made of metallic material having outstanding resistance to high temperatures and to corrosion by the gases that develop in the combustion chamber. The sheathing element contains a heating resistor that receives its electricity supply via a terminal projecting from the other end of the main body. In the known solutions, moreover, the sheathing element is an element separate from the main body and is fixed to the latter by means of implanting or welding.

[0003] When conventional production techniques are used, increasing difficulty is being experienced in respecting the ever tighter fabrication tolerances imposed by engine makers as regards the concentricity of the sheathing element with respect to the mean diameter of the threaded portion and with respect to a conical seating seal provided on the main body. A second problem associated with incandescent glow plugs of the traditional type is that of assuring a perfect seal for the combustion gases in the sealing zone between the sheathing element and the main body.

[0004] With a view to solving the aforesaid problems, the present invention has as its object an incandescent glow plug and a method for its fabrication having the characteristics set out in Claims 1 and 3 hereinbelow.

[0005] The innovative aspect underlying the present invention consists essentially of the fact that both the sheathing element and the main body of the plug are obtained in integral form by starting from a single monolithic workpiece.

[0006] Thanks to this solution idea, it is possible to ensure a far greater manufacturing accuracy as regards the concentricity tolerances between the main body and the sheathing element than can be obtained with the known systems. Furthermore, the monolithic structure of the plug completely eliminates every kind of sealing problem in the zone of passage from the main body to the sheathing element.

[0007] Further characteristics and advantages of the present invention will become apparent in the course of the detailed description given below, which is provided solely by way of example and is not to be deemed limitative in any way, the said description making reference to the drawings attached hereto, where Figures 1 to 7 provide a schematic illustration of the production sequence for obtaining an incandescent glow plug in accordance with the present invention.

[0008] Referring to Figure 1, 10 indicates a tubular element of which the precise geometry (length, diameter, wall thickness) is determined as a function of the final dimensions of the plug it is proposed to produce. By way of example, the diameter d of the tubular element 10 could be of the order of 8-14 mm, with a wall thickness t of the order of 0.5-3 mm. The material from which the tubular element 10 is made is an alloy having a particularly high resistance to both high temperatures and the corrosion effects of the gases that develop in the combustion chamber of a Diesel engine. Materials having suitable characteristics include, for example, alloys of iron, nickel and chromium with a high chromium content (of the order of 20-24%, for example) and of a type similar to the alloys known by the commercial names of nicrofer 6023 or niccrofer 6025.

[0009] Turning now to Figure 2, the tubular element 10 is subjected to a hammering operation during the course of which there are obtained on the tubular element 10 two portions 12, 14 having diameters of, respectively, d and d' that are separated from each other by means of an intervening truncated conical surface 16. The hammering operation is performed by subjecting the end portion 12 to inward-acting radial forces that are applied by means of a series of rotating hammers and produce a plastic deformation of the portion 12 while yet maintaining the condition of coaxiality of the portions 12 and 14 with a high degree of accuracy. Although the hammering operation represents a preferred form of realization of the present invention, the protection range of the invention does not exclude other methods of obtaining the same result, including die forming operations and straightforward material removal by machining.

[0010] In the subsequent phase illustrated by Figure 3, the free end of the portion 12 is given an ogival shape by means of plastic deformation methods that are known per se. Thereafter, a spiral resistor 18, to which there has previously been attached a terminal 20, is inserted in the portion 12 and is welded to the end 22 shaped in the manner of an ogive. The terminal 20 projects beyond the far end of the portion 14, i.e. the free end away from the smaller-diameter portion 12. The spiral resistor 18 may consist either of a single material (monospiral) or of two or more spirals of different materials connected to each other in series. After the distal end of the resistor 18 has been attached to the end 22 by means of welding, powder of some ceramic material (magnesium oxide, for example) is introduced into the interior of the portion 12 and a gasket 24 is then slid onto the terminal 20 . Subsequently, as shown in Figure 5, the portion 12 is subjected to a second operation of plastic deformation in the course of which its diameter is further reduced to a value d'' and the ogive-shaped end 22 is completely closed. At this point, therefore, the portion 12 of the tubular element 10 assumes the conformation of a closed tubular element that acts as a sheath enclosing the heating resistor 18 .

[0011] The subsequent operations have the purpose of impressing upon the larger-diameter portion 14 the shape of the main body of a plug. This result is obtained by means of a plastic deformation operation that reduces the diameter of the portion 14, thus forming thereon two sections 26 and 28 having slightly different diameters. In particular, the section 28 will have a diameter d^III that, following a rolling operation, will make it possible to obtain a threaded length 30 having a diameter d^IV suitable for being connected into a threaded seating provided on the cylinder head of an internal combustion engine (not shown). Subsequently, the open end of the portion 28 is provided with a toroidal gasket and an insulating sleeve (neither of which is shown), after which the terminal part 32 of the portion 28 is shaped so as to confer upon it either a hexagonal or an octagonal profile. Lastly, as shown in Figure 7, the length of the terminal 20 that projects from the plug is provided with a connector 34 of such shape and size as may be desired, which is appropriately blocked in position. At this point the plug is completed and presents a main body 14 with a threaded portion 30 , a sheathing element 12 containing a spiral resistor and a conical sealing surface 16 that joins the sheathing element 12 to the main body 14 . This production method makes it possible to maintain the tolerances of the concentricity or coaxiality of the sheathing element 12 with respect to the conical sealing surface 16 and the mean diameter of the threaded length 30 within much smaller limits [than in the traditional methods]. Furthermore, by virtue of the fact that the main body 14 and the sheathing element 12 have been obtained from a monolithic body, the method ensures a seal that is enormously greater than in the case of the known solutions, where the tubular element 12 is a piece distinct and separate from the main body 14.

Claims

1. An incandescent glow plug for preheating Diesel engines, comprising:

- a main body (14) of elongated shape and having a threaded length (30) for connecting the plug into the cylinder head of the engine, and

- a sheathing element (12) projecting from one end of the main body (14) and containing a spiral resistor (18) attached to a terminal (20) projecting from a second end of the main body (14),
characterized in that the sheathing element (12) and the main body (14) are obtained in integral form from a single monolithic workpiece (10).

2. A plug in accordance with Claim 1, characterized in that it comprises a truncated conical sealing surface (16) situated between the sheathing element (12) and the main body (14).

3. A method for the production of an incandescent glow plug for preheating Diesel engines characterized in that it comprises phases of:

- predisposing a tubular element (10) made of a metallic alloy capable of resisting high temperatures and corrosion by the gases that develop in the combustion chamber, and

- obtaining integrally from the said tubular element (10) a main body (14) of elongated shape and having a threaded length (30) for connecting the plug into the cylinder head of the engine and a sheathing element (12) projecting from one end of the main body (14).

4. A method in accordance with Claim 3, characterized in that it also comprises phases of:

- obtaining on the said tubular element (10) at least two portions (12, 14) of different diameter (d', d) with an intervening truncated conical surface (16),

- conferring an ogival shape upon the free end (22) of the smaller-diameter portion (12),

- inserting a spiral resistor (18) into the smaller-diameter portion (12), and

- welding one end of the said spiral resistor (18) to the said ogive-shaped and (22).

5. A method in accordance with Claim 4, characterized in that the said smaller-diameter portion (12) is formed by means of plastic deformation resulting from the application of radial compression to the tubular element (10).

Drawing