An igniter plug particularly for use with very low temperature liquid fuel

Abstract: An igniter plug for use with very low temperature liquid fuel including a metallic shell (1), an insulator (2) located within the metallic shell, and a center electrode (3) placed to pass through an inner bore of the insulator (211). A front end of the electrode (3) extends beyond a front end surface (21) of the insulator to be exposed to liquid fuel mixture. The insulator is made from a sintered ceramic which includes a silicon nitride component having a concentration ranging from 85% to 99% by weight. The front end surface of the insulator is retracted within the metallic shell by 1.5 mm to 2.5 mm.

Description: The present invention relates to an igniter plug which is exposed to extremely great temperature differences, and is particularly concerned with an igniter plug for use in a rocket propelled engine in which, for example, liquified hydrogen and liquified oxygen are employed as components of the liquid fuel. In a typical igniter plug for use in a rocket propelled engine in which liquid fuel mixture is ignited in the combustion chamber to generate propulsion, the igniter includes a metallic shell into which a tubular insulator is placed through which a center electrode passes. In this instance, the front end of the insulator somewhat extends beyond that of the metallic shell. Meanwhile, the plug is exposed to very low temperatures such as approximately 200 degrees Celsius below the freezing point as the engine is started. Once the engine has started, the plug is exposed to extremely high temperatures such as some thousands of degrees Celsius. The acqueous component, which emerges at the time of the fuel combustion, adheres to the extended front end, and is retained there. The acqueous component is then frozen by the liquified fuel by the time the engine is restarted. This freezing hinders the uniform rise of temperature of the insulator. The huge temperature difference causes a thermal shock to run through the insulator, and at the same time the freezing causes thermal stress in the insulator which eventually results in the formation of cracks. This is because the prior insulators have been made from an alumina ceramic material having a poor thermal conductivity at high temperatures, and a relatively great thermal coefficient of expansion of 8.5 to 10.0 X 10⁻⁶/K. Therefore, it is an object of this invention to provide an igniter plug which is capable of avoiding thermal shock from being induced, and at the same time, substantially preventing the expelled acqueous component from adhering and being retained on the insulator, thereby avoiding the freezing of the acqueous component and preventing the occurrence of such stresses. The British patent application GB-A-2097469 discloses a spark plug with an insulator made of substantially pure silicon nitride or of a silicon nitride based material with a content of at least 65% silicon nitride. In this spark plug, the end of the insulator is positioned within the metallic shell. According to an aspect of the present invention there is provided, an igniter plug for use with very low temperature liquid fuels comprising; a cylindrical metallic shell; a tubular insulator concentrically located within said metallic shell; a center electrode extending through an inner bore of said insulator, said electrode having an end extending beyond said insulator, said insulator being made from a sintered ceramic which includes a silicon nitride component, characterized in that the concentration of the silicon nitride component in said sintered ceramic is from 85% to 99% by weight; and in that the end of said insulator is positioned within said metallic shell and spaced 1.5 mm to 2.5 mm from the open end of the shell. The insulator is made of sintered ceramics which include a silicon nitrate component ranging from 85% to 99% by weight. The sintered nitride preferably has a small thermal coefficient of 3.0 X 10⁻⁶ to 3.5 X 10⁻⁶/K with a relatively good thermal conductivity, thus avoiding thermal shock from being induced on the insulator to prevent cracks even though the igniter is exposed to huge temperature differences. The reason the weight percent of the silicon nitride component is determined as above, is that a ceramic having a silicon nitride concentration of around 85% begins to have a low bending strength, while one exceeding around 99%, becomes poor in sintering and begins to have bad rigidity. On the other hand, the front end of the insulator is retracted inside the metallic shell by 1.5 mm to 2.5 mm, thus avoiding the acqueous component from adhering and being retained at the front end of the insulator. This holds the insulator free from freezing to prevent thermal stress from occurring due to non-uniform temperature rises, avoiding cracks from occurring on the insulator. These and other objects and advantages of the invention will be apparent upon reference to the following description, which is given merely by way of example, with reference to the drawings; in which: Fig. 1 is a part-sectional view of an igniter plug according to an embodiment of this invention; and Fig. 2 is a longitudinal cross sectional view of the igniter plug of Fig. 1 incorporated into a pre-burner spark device of a rocket propelled engine. Referring to Figs. 1 and 2 of the drawings, an igniter plug (R) is intended for use with very low temperature liquid fuels. The igniter plug (R) comprises a cylindrical metallic shell 1, a tubular insulator 2 and a center electrode 3. It is intended to be mounted in a pre-burner spark device 90 incorporated into a rocket propelled engine as described hereinafter. The metallic shell 1 is preferably made of Inconel (registered Trade Mark), and has a male thread 111 at an outer surface of its front portion for the sake of convenience when mounting the igniter plug (R) on the pre-burner spark device 90. At a middle portion of the metallic shell 1, an increased-diameter flange 13 is provided which interfits through a gasket 97 into a recess 91 formed at the pre-burner spark device 90. The shell 1 has a rear portion 12 extending axially which terminates at open rear end 121. The open rear end 121 is turned inward to act as a caulking against the insulator 2 which is concentrically located within the shell 1. An inner wall portion of the shell 1, is contoured to provide consecutively two annular shoulders 17 and 18 of different tapered degree at a portion of the flange 13. An increased-diameter portion 24 is provided at the middle of the insulator 2. At the boundary areas in which upper and lower surfaces of the portion 24 each meet the outer surface of the insulator 2 tapered surfaces 241 and 242 are provided. The increased-diameter portion 24 of the insulator 2 is received by the shoulder 17 through the tapered surface 241 and packing 171. At the tapered surface 241 and the caulked end 121 of the shell 1, annular packings 14 and 15 surround the insulator 2 respectively. The annular space between the inner surface of the shell 1 and the outer surface of the insulator 2 is filled with a talc 16 extending from the packing 14 to the packing 15. The insulator 2 is made from sintered ceramics including a silicon nitride component within the range from 85% to 99% by weight. A front end surface 22 of the insulator 2 has a bevelled portion 21 at its periphery, and is retracted inside the end surface 112 of the shell 1 by around 2.0 mm as designated at (Lg), but exaggerated in the drawing for the purpose of elucidation. The frontal part of the insulator 2 is located to have a slight clearance (Gp) with an inner wall 113 of a front part 11 of the metallic shell 1, and at the same time, the outer surface 212 of the frontal part is tapered, so that the clearance (Gp) progressively increases in the axial direction approaching the front end surface 22. The center electrode 3 is preferably made of platinum and rhodium-based alloy and allocated to pass through an axial bore 211 of the insulator 2. A rear end of the electrode 3 extends beyond an end portion 23 of the insulator 2 to have a terminal 31 to which a high voltage power source 93 is connectable by way of a tension cord (not shown). A front end 32 of the electrode 3 extends beyond both the front end surfaces 22 and 112 of the insulator 2 and the shell 1 to form a bulge portion having a circumference 322 and a front surface end 321. The front end 32 is located, so that the circumference 322 is surrounded by an annular wall 92 which is provided with a passage 92a in the pre-burner spark device 90 when the igniter plug (R) is mounted in the device 90 as shown in Fig. 2. The annular wall 92 is located to surround the bulge portion of the center electrode 3 to act as a ground electrode so as to cause a spark discharge from the center electrode 3 to the annular wall 92. The pre-burner spark device 90 includes a liquified hydrogen port 94 and liquified oxygen port 95 and the liquid fuel mixture of the liquified hydrogen and oxygen is to be fed into the device 90. During the process in which the mixture fuel passes through the passage 92a, the spark occurs between the circumference 322 and the annular wall 92 to ignite the fuel. The following effects are obtained. The insulator 2 is made of sintered ceramics including a silicon nitride component ranging from 85% to 99% by weight. The sintered ceramics has small thermal coefficient of 3.0 X 10⁻⁶ to 3.5 X 10⁻⁶/K with relatively good thermal conductivity, thus substantially preventing thermal shock from being induced in the insulator 2 which can cause cracks even though the igniter plug (R) is exposed to the huge temperature difference between the time when the engine just starts and the time at which the engine is in full operation. On the other hand, the front end surface 22 of the insulator 2 is retracted inside the metallic shell 1 by a range of 1.5 mm to 2.5 mm. This generally avoids the aqueous component from adhering to the outer surface 212 of the insulator 2. In particular, such is the structure of the igniter plug that the acqueous component is generally not retained at the clearance (Gp) under the influence of surface tension as is common with plugs where the insulator extends beyond the front end of the metallic shell. This stops the insulator 2 from freezing to prevent thermal stress from occurring due to non-uniform temperature rises, avoiding cracks from forming on the insulator 2 and thus contributes to a long servicing life. It will be appreciated that auxiliary agents such as magnesia (MgO), alumina (Al₂O₃) an yttrium oxide (Y₂O₃) may be added to the sintered ceramics. Further, it is noted that an igniter plug according to the invention may be applied to a rocket engine in which liquified hydrogen-kerosine based fuel, or sulfur-hydrazine based fuel is employed which results in no acqueous component being formed during combustion. In addition, the front end surface 22 of the insulator 2 may be retracted into the metallic shell by exactly 1.5 mm or 2.5 mm as alternative embodiments of this invention.

Claims: An igniter plug for use with very low temperature liquid fuels comprising; a cylindrical metallic shell (1); a tubular insulator (2) concentrically located within said metallic shell (1); a center electrode (3) extending through an inner bore (113) of said insulator (2), said electrode having an end (322) extending beyond said insulator (2), said insulator (2) being made from a sintered ceramic which includes a silicon nitride component, characterized in that the concentration of the silicon nitride component in said sintered ceramic is from 85% to 99% by weight, and in that the end (22) of said insulator (2) is positioned within said metallic shell (1) and spaced 1.5 mm to 2.5 mm from the open end (112) of the shell. An igniter plug according to claim 1, including an annular space (16) between the inner surface of a rear portion (12) of said metallic shell (1) and the outer surface of said insulator (2), said annular space (16) being filled with talc. An igniter plug according to claim 1 or 2, wherein the insulator (2) includes magnesia, alumina and yttrium oxide. An igniter plug according to any preceding claim, wherein said center electrode (3) is made of an alloy of platinum and rhodium-based metals. An igniter plug according to any preceding claim, wherein the end (32) of said center electrode (3) is bulged. An igniter plug according to any preceding claim, wherein a slight annular clearance (Gp) is provided between the front part of said insulator (2) and said metallic shell (1), said clearance progressively increasing in the axial direction approaching the front end (112) of said metallic shell (1).

Inventors: AOKI NOBORU C O NGK SPARK PLUG, SUZUKI TAKAHIRO O NGK SPARK PL, AOKI, NOBORU C/O NGK SPARK PLUG CO LTD, SUZUKI, TAKAHIRO/O NGK SPARK PLUG CO LTD
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1989
Main classification: H01T13
IPC codes: F23Q3, F02K9, H01T13