Abstract:
A method of making a glow plug includes the steps of swaging a sheath of a heater having an internal heating coil therein and a compressible washer at the open end of the sheath to reduce the sheath&#39;s diameter and to force some of the washer out of the sheath to leave a washer portion of a diameter larger than the swaged sheath diameter. The heater is force fitted into a housing with the external washer being press fitted against an internal bore wall of the housing to form an air-tight seal with the bore wall. This eliminates the usual brazing step of brazing the sheath to the housing to seal against flow of combustion gases therebetween.

Description:
This is a division of application Ser. No. 483,365, filed Apr. 8, 1983, now U.S. Pat. No. 4,477,717. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of making glow plugs and to glow plugs which are used to ignite fuel in internal combustion engines with an internal electrical resistant element which is enclosed within a sheath and which is exposed to the fuel within the internal combustion chamber. 
     The present invention relates to a glow plug which is used in a diesel engine typically for powering an automotive engine for igniting the fuel quickly, for example, in under ten seconds, and which is produced at sufficiently low cost to be commercially competitive with existing glow plugs. It is to be appreciated that the glow plug is subjected to rather hostile, environmental conditions within the cylinder wherein engine vibrations are present, the temperature at the plug is at least 1100 degrees C, and the hot combustion gases are under high pressures and are corrosive in nature. 
     The heating element sheath projects outwardly into the combustion chamber from an encircling housing or casing which is usually threaded at one end and threaded into the cylinder block. The projecting portion of the tubular sheath is usually secured in a gas tight manner by brazing to the housing by a filler tight brazing at the end of the housing encircling the sheath. The brazing provides a gas tight seal between the sheath and the internal bore of the casing so that the high pressure gas for example at 400 psi at ignition time will not move along the interface between the sheath and the casing bore wall and eventually penetrate into the interior of the sheath at the open opposite end of the sheath. Such brazing is shown in U.S. Pat. No. 3,749,980. 
     The tubular heating element disposed within the housing has a central electrode projecting from its interior end which needs to be electrically isolated from the casing and which also needs to be sealed in a gas tight manner with respect to the sheath wall to prevent the intrusion of air bearing oxygen into the interior of the heating element. U.S. Pat. No. 4,252,091 discloses providing a grooved bushing having matching grooves to fit into the electrode and into the grooves formed in an end of the tubular sheath to provide a sealed, tortuous passage against the penetration of air into the interior of the heating element and into contact with the magnesium oxide and the heating element coil. Additionally, this patent discloses that a filler material having a high affinity to oxygen such as aluminum or magnesium may be placed over the top of the heating element and the bushing and captured below an &#34;O&#34; ring to assist in providing a gas tight seal against air intrusion into the interior of the tubular heater element. The present invention eliminates the necessity for the brazing operation such as disclosed in the U.S. Pat. No.  3,749,980 and provides a more simple and inexpensive interconnection between the glow plug housing and sheath. The present invention uses a compressible, insulative bushing which is compressed during assembly to provide an effective, air-tight seal not only between the surrounding housing and the tubular sheath but also between the externally projecting electrode and the tubular sheath to provide against penetration of oxygen into the interior of the heating element. 
     In accordance with the further aspect of the present invention there is provided a new and improved fast-start heating composite coil which uses a nickel element to allow the initial voltage to provide a fast start and a Kanthal coil portion to provide the heating. The increasing resistance of the nickel limits the maximum temperature. Such a fast heating of the plug followed by a self-regulating characteristic avoids the high operating temperatures which would be ultimately produced without the regulating feature of the nickel wire coil and yet, provides for more instantaneous starts of engines in cold climates wherein the delayed startup time has been a particular problem. 
     The present invention has solved the problem of how to provide a series resistive network for bringing the sheath up to ignition temperature and then to plateau so as not to exceed a predetermined temperature for example about 2100 degrees F. after ninety seconds of operation. The present invention provides a practical and effective fast start glow plug which will meet the necessary and commercially desired criteria for starting automotive engines in cold climates. 
     A general object of the invention is to provide a new and improved glow plug and method of making the same as contrasted with the prior art glow plugs as above described. Another object of the invention is to provide a new and improved mechanical seal in a glow plug to prevent the intrusion of oxygen into the interior of the heating element. 
     A still further object of the invention is to provide a new and improved fast start glow plug. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings in which: 
     FIG. 1 is cross-sectional view of a glow plug constructed in accordance with the preferred embodiment of the invention. 
     FIG. 2 is an enlarged partially sectional view of the heating element prior to extrusion. 
     FIG. 3 is a view of the heating element after extrusion. 
     FIG. 4 is a view of the bushing. 
     FIG. 5 is an enlarged view of the heating element having the composite coils and constructed in accordance with the preferred embodiment of the invention. 
     FIG. 6 is a view of the finished plug. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for purposes of illustration, the invention is embodied in a glow plug 10 which is formed within an internal heating element 11 which has one end 11a projecting outwardly from a housing or casing 12 which has a threaded portion 14 for threading into an engine block. The housing 12 has a central axial bore 13 in which is mounted a hollow-cylindrical sleeve or sheath 16 of the heating element. Typically, the sheath is formed of stainless steel or other suitable material and has an outer closed end 16a. Within the sleeve is a central electrical conductor 18 which passes through a central bushing or washer 19 and a nut 15 to an outer connector terminal 22. The inner end of the conductor is connected to a heating coil or element 20 disposed within the sheath. The internal end 20a of the heating element coil is electrically connected to the end 16a of the sheath 16. The conductor is spaced from the sleeve 16 and likewise the heating element coil 20 is spaced from the sleeve 16 and each is supported and rigidly held by granular, insulative material 23, such as magnesium oxide, or the like backed within the sheath 16 and about the conductor and the heating coil 20. 
     The sheath closed end 16a is inserted into the combustion chamber for ignition of the fuel and needs to be brought rapidly up to temperature by means of a first coil portion 38 (FIG. 2) of a material which has a relatively constant resistance with temperature as compared to a second coil portion 40 which has a large variation in resistance with temperature change. Within the combustion chamber the pressure may reach as high as 480 psi which pressure causes gases to try to flow along the interface between an internal wall in the casing defining the bore 13 for the casing 12 and the adjacent external surface 47 of the metallic sheath 16. An internal wall 42 defining the bore 13 is centered about the center line of the glow plug and extends axially within the casing 12. Heretofore, there was a braze or a weld formed at the end 12a of the bushing and the adjacent sheath to provide a gas tight seal. The present invention has an enlarged space, or gap in the form of a counterbore 46 at the end 12a of the casing to limit the amount of direct contact between the sheath surface 47 and the wall 42 of the casing bore 13. As will be seen, the gas pressure will flow up the counterbore and to the interface of the sheath wall 47 and the axial wall 42 of the bore 13 in the casing, or housing 12. If air under pressure reaches the outer end of the sheath, it must be sealed or air will tend to intrude through cracks and crevices into the interior of the sheath where it will attack the Nickel and Kanthal coils 40 and 38. 
     In accordance with the present invention the conventional brazing seal between the outer metal casing 12 and the sheath 16 of the tubular heating element is eliminated and a gas tight seal therebetween is achieved mechanically. This is achieved by using a compressible gasket or washer 50 which is compressed with sufficient pressure during assembly of the tubular heating element 11 and the housing 12 to provide a seal not only between the housing and the sheath 16 but also between the conductor 18 and the sheath 16 so that no gas will penetrate into the interior of the heating element. 
     In the preferred embodiment of the invention, the silicone washer 50 is trapped within the sheath 16 by a crimped end 52 on the sheath prior to swaging of the sheath in the known manner. In this known and conventional swaging operation, the diameter of the sheath is reduced considerably and its length is increased. During the swaging operation, the end 50a of the silicone washer expands to project outwardly of end 16b of the sheath as shown in FIG. 3 and retains a larger diameter than that of the external wall 47 of the sheath. During such a swaging operation, there is provided a tight internal first seal 55 (FIG. 2) between the internal portion 50b of the washer 50 and the adjacent internal sidewall 56 (FIG. 2) of the sheath. Likewise, during the swaging operation the compressed washer will be obtaining a very tight and second seal 57 between the internal bore wall 50c of the washer and the conductor 18. Thus, when the assembly has been swaged to provide the configuration of FIG. 3 the first seal 55 and the second seal 57 will have been formed. 
     In accordance with an important aspect of the invention, the mechanical seal between the casing 12 and the sheath 16 is achieved by compressing the silicone washer 50 within a tapered wall section 60 of the housing 12 adjacent the internal end of the counterbore 46. Herein the tapered wall has approximately a 10 degree taper and, is converging to a smaller diameter in the upward direction as viewed in FIG. 4 such that the projecting portion 50a of the silicone washer 50 is continually reduced in diameter as it is being compressed along the tapered wall section 60. The sheath end 16b is likewise being compressed by the tapered wall 60. It is this compression and compressing of the silicone washer under high force loading that provides an effective third seal which prevents the gases moving through the counterbore 46 and penetrating into the sheath and to the heating element as would allow oxygen to attack the heating coils 38 or 40. 
     Referring now in greater detail to the preferred embodiment of the invention, the silicone washer 50 is annular in shape and is placed within the internal bore of the sheath 16 and is placed against the magnesium oxide which surrounds the conductor and the internal coil prior to extrusion, as seen in FIG. 2. The preferred material is a silicone rubber capable of withstanding high temperatures and having a low compression set. 
     The sheath end 16b is crimped at 52. A very small recess is provided as shown at 65 between the end of the washer 50 and the end of the crimped sheath 16b. During the conventional swaging operation, the washer 50 is squeezed to project outwardly through and to fill the the space but also assumes a generally tapered or frusto-conical surface 66, as best seen in FIG. 3 with the portion 50a projecting outwardly beyond the end 16b of the sheath. This extruded external portion 50a of the silicone washer 50 has a substantially greater outer diameter than the outer diameter of the extruded sheath which has had its diameter reduced substantially from that shown in FIG. 2 to a smaller diameter after extrusion, and to have the overall appearance as shown in FIG. 3. In addition to the compressed washer seal, the preferred embodiment of the invention also uses a cement, or adhesive which is applied as a ring 70 onto the exterior wall 47 of the sheath 16 below the washer 50 for cementing engagement with the bore wall 42 of the housing 12. The preferred ring of cement is sold under the Trademark &#34;Lock Tite&#34; No. RC 620 by the Lock Tite Corporation. 
     In assembly, the sheath 16 with the washer 50 and the cement ring 70 thereon, as shown in FIG. 3, are pressed fitted into the housing to a predetermined dimension as measured from the external end 12a of the housing 12 to assure that there is the compression desired and that the cement is in engagement with the internal bore wall 42 of the housing at the desired location. 
     In accordance with another important aspect of the present invention the coils 38 and 40 are constructed in accordance with the preferred embodiment of the invention as shown in FIG. 5 with their respective ends 40a and 38a abutted end-to-end with a weld 75 therebetween to mechanically join the ends together and to electrically connect the ends together. This preferred weld is made by lazer welding or other percussion weld or a butt weld. This is in contrast to the type of side-by-side relationship of the coil end as shown in British patent publication No. 2,013,277A. With the present invention both of the coils 38 and 40 may be wound with the same hand whereas, in the British publication the coils are wound with opposite hands and the ends are laid parallel to each other for welding. The parallel ends of the British publication are more difficult to prevent from contacting the sheath and shorting out the coil. The preferred heating as shown in FIG. 5 has the Kanthal A-1 coil 38 with a larger diameter than the nickel coil 40 and it has a substantially reduced number of coils, for example, about seven coils with the coils reducing vary substantially in diameter from a maximum o.d. to the smallest diameter coil 38c (FIG. 5). In order to heat the hemispherical sheath end 16A as quickly as possible, the coil 38 is formed with the coils assuming a generally hemispherical shape to be close to the sheath end wall 16A. On the other hand, the nickel coil 40 has a substantial constant diameter throughout. In this preferred embodiment of the invention a small axially located aperture 72 is formed in the closed end 16a of the sheath and the straight end 20a of the coil is projected therethrough followed by an inert arc welding to seal the sheath to gas leakage and to ground the coil to the sheath end 16a. 
     It has been found by tests that when 11 volts are applied that the composite coil will heat the sheath to a temperature of 1562 degrees F. at a location of three millimeters up the sheath length from the tip 16a within six to ten seconds. After about 90 seconds the temperature will have plateaued off and will be in the range of 2040-2140 degrees F. Also, these plugs will withstand 12.5 volts for 120 seconds. 
     Because of the high temperature co-efficient of resistance of the nickel resistor coil 40 compared to the heating coil 38 from cold start to operating temperature, the resistance of the nickel coil 40 increased by 500 to 600 percent while the increase of resistance of the heating coil is less than 10 percent. This stabilized resistance of a coil 38 minimizes excessive starting current surge. 
     From the foregoing it will be seen that the present invention provides an improved mechanical seal for use in glow plugs of various constructions. The improved seal is of particular utility in the fast start glow plug herein described. 
     While a preferred embodiment of the invention has been shown and described it will be understood that there is no intent to limit the invention by such disclosure but rather it is intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention as defined in the appended claims.