A spark plug

Abstract: A spark plug structure comprising: a cylindrical metallic shell; a joint type insulator having a center bore, and including a front half piece and a rear half piece, each made of a tubular aluminium nitride (AlN), and the front and rear half pieces being joined at their respective end by means of a glass sealant, and encased into the metallic shell; a center electrode placed into the center bore of the insulator; an elongated terminal placed into the rear half piece of the insulator; an electrically conductive glass provided to seal respective spaces appeared between the center electrode, insulator and the terminal; the front half piece having an elongated projection, the length of which is more than 2.0 mm, and the rear half piece having a recess the depth of which is more than 2.0 mm, the front and rear half pieces being joined at the projection and the recess by means of an annular glass sealant which has thickness of less than 2.0 mm and length of more than 2.0 mm.

Description: The invention relates to a spark plug for use in an internal combustion engine in which an insulator includes two pieces joined end to end. In most spark plugs the insulator is mainly made of alumina (Al₂O₃). Due to the low thermal conductivity of alumina, the insulator is unable to loose sufficient heat in a combustion chamber of modern high efficiency engines. The heat laden insulator causes unfavorable preignition. AT-B-306,978 discloses a spark plug with a two part insulator to allow thermal expansion of the lower part. The join between upper and lower parts is stepped but no sealant provided. GB.A.-996,823 discloses a spark plug with a two part insulator, the lower part being of alumina and the upper of thermosetting plastic. The lower part has an annular protrusion into the upper but no cement is provided. According to Japanese Patent Publication JP-3. 55-46634, it is suggested that the insulator is made from alumina nitride (AlN) which is of good thermal conductivity so as to conduct heat from the combustion chamber. In order to save cost, it is proposed that the insulator is divided into two pieces, rear and front or perhaps better described as top and bottom. The front (bottom) piece is made from aluminium nitride (AlN) of good thermal conductivity, and the rear (upper) piece is made from alumina (Al₂O₃). The two pieces are joined at their adjacent ends by means of glass sealant. Due to the relatively poor strength at the joined portions, there is a risk of cracks occurring on the glass sealant, which loosen the joint portions at the time of providing the glass sealant. Further, cracks may occur on the insulator at the time of caulking the metallic shell which encases the insulator. Therefore, it is an object of this invention to provide a spark plug structure which is less liable to crack. According to the present invention there is provided a spark plug comprising: a cylindrical metallic shell, a ground electrode, and a tubular insulator having upper and lower pieces, within the metallic shell, the pieces being joined at adjacent ends by means of a glass sealant, a center electrode concentrically placed into a bore through the insulator with a lower end of the electrode protruding beyond the lower piece of the insulator to form a spark gap with the ground electrode; and a terminal protruding from the upper end of the insulator, wherein the lower and upper pieces of the insulator have at their adjacent ends respective ones of an elongate projection, the length of which is more than 2.0 mm, and an annular recess, the depth of which is more than 2.0 mm, which receives the projection and which is surrounded by an annular peripheral member having a thickness of more than 1.5 mm, the pieces being joined by means of an annular glass sealant which has thickness of less than 2.0 mm and a length of more than 2.0 mm and is between the projection and the interior of the recess, the upper piece of the insulator being made of alumina, and the lower piece of the insulator of sintered aluminum nitride having thermal conductivity of more than 60 W/mK, and outer surface of the lower piece being coated with a non-crystalline alumina layer of 1 - 30 micrometers in thickness. Preferably the projection is on the lower piece. The invention can provide a spark plug which has improved insulation to reduce preignition, and thermal shock even when used in a high efficiency engine in which the insulator is exposed to rapid cooling and heating cycles with huge differences of temperature and pressure. With the invention, enough strength is imparted to the glass sealant sufficiently to resist a load of 200 Kg which is applied when the glass sealant is provided. The recess is surrounded by an annular peripheral part, the thickness of which is more than 1.5 mm. This reduces cracks occurring on the insulator at the time of caulking the metallic shell. The invention will be more clearly understood from the following description which is given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a longitudinal cross sectional view of a spark plug of the invention; Fig. 2 is partly sectioned view of an insulator, with the upper part somewhat broken away; Fig. 3 is a graph showing the relationship between tensile load (Kg) and thickness (t) of glass sealant; Fig. 4 is a graph showing the relationship between tensile load (kg) and length (1) of glass sealant; Fig. 5 is a graph showing the relationship between caulking load (tons) and thickness (w); Fig. 6 is a graph showing the relationship between bonding area and bonding strength in a modified embodiment of the invention; Fig. 7 is a view similar to Fig. 2 according to another modified form of the invention; Fig. 8 is an enlarged view of a specified section of Fig. 7; Fig. 9 is a view similar to Fig. 1 according to another embodiment of the invention; Fig. 10 is a view similar to Fig. 2 according to another embodiment of the invention; and Fig. 11 is a view similar to Fig. 8 according to modified form of the invention. Referring to Figs. 1 and 2, a spark plug 100 according to the present invention, has a metallic shell 90 having a ground electrode 50a integral therewith. In the metallic shell 90, a tubular insulator 30 is concentrically placed. The insulator 30 is of joint type comprising rear (or upper) and front (or lower) half pieces 20 and 10. The front half piece 10 is made from aluminum nitride (AlN) of high thermal conductivity, while the rear half piece 20 is made of alumina (Al₂O₃) for the purpose of cost-saving. The front half piece 10 has an elongated projection 11, and the rear half piece 20 has a recess 21 at the ends where they are joined. The rear and front half pieces 20 and 10 are joined at the recess 21 and the projection 11 by means of an annular glass sealant 40. The recess 21 is surrounded by an annular peripheral part the thickness dimension (W) of which is determined to be more than 1.5 mm as described in detail hereinafter. The common length in which the rear and front half pieces 20 and 10 are joined, corresponds to the length (1) of the glass sealant 40. The glass sealant 40 is made from CaO, BaO, Al₂O₃ or SiO₂-based vitreous material, and determined by its length (l) and thickness (t) to be 4.0 mm and 1.0 mm respectively. It is noted that minimum limit of the length (l) is 2.0 mm, while the maximum limit of the thickness (t) is 2.0 mm to sufficiently resist the maximum load of 200 Kg applied to the glass sealant 40 when providing it. The recess 21 is, as mentioned before, surrounded by an annular peripheral part, the thickness dimension (W) of which is determined to be 3.0 mm by way of illustration. The thickness dimension (W) must be at least 1.5 mm to resist a maximum load of around 5 tons applied when the metallic shell 90 is squashed at an annular end 91 by means of caulking. On the other hand, the front half piece 10 of the insulator 30 has axial bores 13 and 14 of different diameter. The rear half piece 20 of the insulator 30 has an axial bore 22 communicated with the bores 13 and 14 so as to constitute a central bore as a whole. Into the axial bores 13 and 14, a center electrode 50 is placed with the front end somewhat extended beyond the front half piece 10 to form a spark gap (Sp) with the ground electrode 50a. The center electrode 50 has a flanged head 51 at its rear end, and is made from a copper-based core clad by a nickel based alloy. At the time of assembly, the center electrode 50 is inserted through the rear end of the axial bores 13, 14 and 22, and received at its flanged head 51 by a shoulder 14a of the diameter-increased bore 14. In this instance, the center electrode 50 may be adhered to an inner surface of the bore 13 by means of a heat-resistant adhesive 52. At the space in which the two bores 14 and 22 meet, a resistor 61 is placed with its upper head and bottom sandwiched by electrically conductive layers 60 and 60a for the purpose of noise reduction. Into the axial bore 22, an elongate terminal 80 is air-tightly inserted in a manner to sandwich the conductive layer 60 with the resistor 61. Now, Figs. 3 and 4 show the result of strength test carried out by changing the thickness (t) and length (l) of the glass sealant 40 which joins the rear and front half pieces 20 and 10. Fig. 3 shows the result of tensile test which the joint type insulator 30 has undergone under the ambient temperature of around 1000 degrees Celsius depending on the thickness dimension (t) of the glass sealant 40 with the length (l) as constant 4.0 mm. Fig. 4 shows the result of tensile test which the joint type insulator 30 has undergone under the ambient temperature of around 1000 degrees Celsius depending on the length dimension (l) of the glass sealant 40 with the thickness dimension (t) as constant 1.0 mm. As a result, it has found that the requirements of l ≧ 2.0 mm, t ≦ 2.0 mm are apparently obtained to resist the maximum load of 200 Kg. Fig. 5 shows the result of the strength test carried out by changing the thickness dimension (W) of the annular periphery 21a in the recess 21. In this strength test, various loads are measured when the cracks occurred on the annular periphery 21a at the time of caulking the metallic shell 90 as designated by (x). As the result of this test, it has found that it is necessary to arrange W ≧ 1.5 mm to cope with the maximum load of around 5 tons. As understood from the foregoing description, it is necessary to arrange dimensions (t), (l) and (W) as follows: t ≦ 2.0 mm, l ≧ 2.0 mm and W ≧ 1.5 mm. These dimensional arrangements enable to substantially prevent cracks from occurring on the joint type insulator 30. In a modified form of this invention, the front half piece 10 is made of sintered aluminum nitride (AlN) of more than 60 W/mK in thermal conductivity. On an outer surface of the front half piece 10, a non-crystalline aluminium layer of 1 - 30 micrometers coated by means of CVD or the like. The rear and front half pieces 20 and 10 are bonded by vitreous adhesive of high melting point. The front half piece 10 is coated with fine-structured alumina, so that the alumina layer is prevented from transforming into Trigonal corundum by oxidation, at the same time, prevented from being separated, thus contributing to long service life. Example 1 The alumina (Al₂O₃) layer is made by previously oxidizing the aluminum nitride piece 10 of 20 mm in length. The experiment is carried out at 5500 rpm X 4/4 in a six-cylinder engine with displacement of 2000 cc for 100 hours. After the experiment, oxidation degree is measured by EPMA, it is found from Table 1 that the thicker the alumina layer is, the lesser the formation of Al₂O₃ is as seen from sample A to sample E. The alumina layer of 1 micron is sufficient to protect the aluminum nitride from being oxidized into Al₂O₃ more than necessary. However, the upper limit of the thickness of the alumina layer is around 30 microns, because too much alumina causes separation. previous oxidation thickness of Al₂O₃ thickness of Al₂O₃ after 100 hours sample A no oxidation 0 µm 40 µm sample B oxidation 0.8 µm 35 µm sample C oxidation 1 µm 25 µm sample D oxidation 3 µm 20 µm sample E oxidation 10 µm 18 µm Example 2 The samples A to E as used in the experiment 1, are used in an anti-preignition test in a four-cylinder engine with displacement of 1600 cc. As seen in Table 2, the thickness of Al₂O₃ substantially has no effect on the anti-preignition. The samples C, D and A have figures similar to those of sample F which has no layer of Al₂O₃, and representing high heat-resistant characteristics compared to the prior art BPR6EY plug. Now, various kinds of Vitreous materials are listed in Table 3 to be applied to the annular glass sealant 40. These vitreous materials are of high melting point of more than 500 degrees Celsius, and of 32 - 80 X 10⁻⁷ in thermal expansion which falls between that of AlN and that of Al₂O₃. vitreous material thermal expansion (X10⁻⁹/°C) melting point (°C) sintered temp. (°C) volume resistance Logρ(Ω·m) at 150°C Na₂O₃·B₂O₃·SiO₂ -based glass I 75.5 697 990 11.2 ditto II 57.0 705 1050 11.4 ditto III 45.5 698 1050 11.5 Example 3 Fig. 6 of (a), (b) shows that Al₂O₃-coated (layer of 10 µm) front half piece 10 is stronger than non Al₂O₃-coated front half piece when bonding strength between the rear and front half pieces 20 and 10 is compared. As seen in Fig. 6 of (a), the bonding strength rapidly increases with the increase of the bonding area compared to that of (b). A further modified form of the present invention, the annular glass sealant 40 is made of vitreous material which has a melting point of more than 500 degrees Celsius, and has a temperature of 800 - 1400 degrees Celsius required when the sealant 40 is provided. The thermal expansion of the vitreous material falls within the range from 32 X 10⁻⁷ to 80 X 10⁻⁷. Maximum temperature which arises from the combustion chamber of the engine, corresponds to the temperature in which preignition occurs. At this time, the glass sealant rises its temperature as high as around 500 degrees Celsius. Accordingly, it is required for the glass sealant 40 to have a melting point of more than 500 degrees Celsius so as to properly function. A glass used for resistor has a melting temperature of 800 - 1000 degrees Celsius, so that the glass sealant 40 is desired to have a temperature of more than 800 - 1000 degrees Celsius which is required at the time of providing it. But, the temperature is preferably below 1400 degrees Celsius so as not to encourage oxidation of the aluminum nitride (AlN). The thermal expansion of the aluminum nitride is 32 - 48 X 10⁻⁷/°C, while that of alumina (Al₂O₃) is 69 - 80 X 10⁻⁷/°C. Therefore, it is necessary that the thermal expansion of the glass sealant 40 falls on the range between 32 - 48 X 10⁻⁷/°C and 69 - 80 X 10⁻⁷/°C to prevent cracks from occurring on the glass sealant 40. A power supply is normally 40 KV, so that it is necessary for the glass sealant 40 to have enough length (l) to withstand 40 KV at the temperature of 500 degrees Celsius. Vitreous examples which meet those requirements are shown at Table 4. vitreous material yield point (°C) thermal expansion (10⁻⁷/°C) withstand voltage at 500°C (KV/mm) B₂O₃ SiO₂ -based glass A 550 45 18.0 ditto B 715 67 22.5 BaO -based glass A 670 67 22.0 ditto B 710 68.5 23.5 The temperature of specified portion (A) Of Fig. 7 in the glass sealant 40 is measured with the use of spark plugs each corresponding to BPR4EY and BPR7EY. The engine used in this experiment is four series-cylinder, DOHC four-valve with the displacement of 16oo cc under the condition of 6000 rpm X 4/4. The ignition timing is represented by advance angles which is needed to cause preignition. The result is shown at Table 5 which teaches that the temperature of the glass sealant 40 reaches up to 500 degrees Calsius. From this result, it is apparently necessary to use vitreous material having a melting point of more than 500 degrees Celsius so as to ensure strength and electrical conditions of the glass sealant 40. spark plug ignition timing BTDC temperature at (A) BPR4EY 30° 485°C BPR7EY 57.5° 460°C An insulator is made by using materials as listed at Table 6. The insulator is applied to a spark plug corresponding to BPR4ES with the thermal expansion of the glass sealant varying as Table 7. The engine used in this experiment is water-cooling type of six series- cylinder, OHC with the displacement of 2000 cc under operating condition of 6000 rpm X 4/4 (one minute) and idling (one minute) for 200 hours. In this experiment, six test pieces are used at each case. The result of Table 7 shows that the thermal expansion of the glass sealant 40 is needed to fall between that of the aluminium nitride and that of alumina. material thermal expansion (10⁻⁷/°C) AlN 34 Al₂O₃ 80 vitreous material (yield point) thermal expansion (10⁻⁷/°C) result C (600°C) 24 two out of six...cracks at (a) D (550°C) 45 all (6/6)...no cracks E (595°C) 95 five out of six...cracks at(b) Then, the relationship between withstand voltage (KV/mm) and the length (l) of the glass sealant is checked in regard to the vitreous materials listed at Table 4. The experiment is carried out with the use of a spark plug corresponding to BPR4ES. In this experiment, voltage of 40 KV is applied to the section designated at (Y) of Fig. 7 under the ambient temperature of 500 degrees Celsius to check whether the glass sealant 40 is perforated or not. The result is shown at Table 8 in which it is represented by a cross when the glass sealant 40 is perforated, while it is represented by a circle when the glass sealant is not perforated. It is noted that the withstand voltage is simply expressed by the product of insulation withstand voltage and the length (l). vitreous material withstand voltage (KV/mm) l: length (mm) 0.5 1.0 1.5 2.0 2.5 3.0 B₂O₃, SiO₂ -based glass A 18.0 X X X ○ ○ ○ ditto B 22.5 X X X ○ ○ ○ BaO -based glass A 22.0 X X X ○ ○ ○ ditto B 23.5 X X X ○ ○ ○ Figs. 9 through 11 shows another embodiment of the invention. A spark plug 101 comprises a center electrode 104, a tubular insulator 102, a metallic shell 103 and a spiral thread 105 cut at an outer surface of the metallic shell 103. The insulator 102 is joint type including rear and front half pieces 108 and 106. The front half piece 106 is made from ceramic material of good thermal conductivity such as beryllium oxide (BeO) and aluminum nitride (AlN) each of which is transparent. The rear half piece 108 is made of alumina (Al₂O₃). Such is the structure of the front half piece 106 that the front half piece 106 permits release of the heat so as to prevent preignition even when the piece 106 is exposed to high temperature gas in the combustion chamber. Expensive material e.g. aluminum nitride (AlN) is only used for the front half piece 106, thus contributing cost-saving as a whole. The rear and front half pieces 108 and 106 are bonded at 107 by means of oxidation soldering, alumina cement or glass sealant. At the portion 107, the length of projection 109 falls within the range from 0.5 mm to 8.0 mm to ensure high voltage insulation, and ready manufacturing as seen Figs. 10 and 11. When the thermal expansion of the front half piece 106 is greater than that of the rear half piece 108, the two pieces 106 and 108 are joined as shown in Fig. 10. When the thermal expansion of the front half piece 106 is smaller than that of the rear half piece 108, the two pieces 106 and 108 are joined as shown in Fig. 11. It is noted that a resistor 112 is placed at a center bore 112a of the rear half piece 108 with the resistor 112 sandwiched between a terminal 113 and a center electrode 104 by way of an electrically conductive glass 111 and 111a.

Claims: A spark plug (100) comprising: a cylindrical metallic shell (90), a ground electrode (50a) and a tubular insulator (30) having upper and lower pieces (20, 10) within the metallic shell, the pieces being joined at adjacent ends by means of a glass sealant (40), a center electrode (50) concentrically placed into a bore (13, 22) through the insulator with a lower end of the electrode protruding beyond the lower piece (10) of the insulator to form a spark gap (Sp) with the ground electrode; and a terminal (80) protruding from the upper end of the insulator, wherein the lower and upper pieces of the insulator have at their adjacent ends respective ones of an elongate projection (11), the length of which is more than 2.0 mm, and an annular recess (21), the depth of which is more than 2.0 mm, which receives the projection and which is surrounded by an annular peripheral member (X) having a thickness (W) of more than 1.5 mm, the pieces being joined by means of an annular glass sealant (40) which has thickness of less than 2.0 mm and a length of more than 2.0 mm and is between the projection (11) and the interior of the recess, the upper piece (20) of the insulator being made of alumina, and the lower piece of the insulator of sintered aluminum nitride having thermal conductivity of more than 60 W/mK, and outer surface of the lower piece being coated with a non-crystalline alumina layer of 1 - 30 micrometers in thickness. A spark plug according to claim 1 wherein the projection (11) is on the lower piece (10). A spark plug according to claim 1 or 2 wherein the lower piece (10) of the insulator is made of transparent ceramic material such as transparent alumina, aluminum nitride, beryllium oxide. A spark plug according to any preceding claim wherein the glass sealant (40) has a melting point of more than 500 degrees Celsius, and a thermal expansion of between 32 X 10⁻⁷/°C and 80 X 10⁻⁷/°C and a sealing point between 800°C and 1400°C. A spark plug according to any preceding claim wherein the center electrode (50) is made of a copper-based core clad by a nickel-based alloy. A spark plug according to any preceding claim wherein the lower piece (10) of the insulator (30) extends about 20 mm from the lower end of the metallic shell (90).

Inventors: KOZUKA KAZUHIKI, OSHIMA TAKAFUMI, YAMADA SHIGEYASU, KOZUKA, KAZUHIKI, OSHIMA, TAKAFUMI, YAMADA, SHIGEYASU
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1989
Main classification: H01T13
IPC codes: H01T13