Glow plug

Abstract: A heater tube (3) comprises a smaller-diameter portion (31) and a larger-diameter portion (32). A first resistor (4) and a second resistor (5) are provided in the heater tube (3). The second resistor (5) has a positive resistance-temperature coefficient larger than that of the first resistor (4). The first resistor (4) and the second resistor (5) are electrically connected in series. The entire body of the first resistor (4) and 3/4 of the second resistor (5) are accommodated in the smaller-diameter portion (31).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a glow plug, used for improving the start-up ability of a diesel engine or the like. 2. Related Art: From DE-A-40 01 296 a glow plug is known, which comprises a cylindrical housing, a cup-shaped heater tube having a smaller diameter portion at a closed end and a larger diameter portion at an open end, wherein the larger diameter portion is fixed to an end of the housing. A first resistor is provided in the heater tube adjacent to the closed end and a second resistor is provided adjacent to the first resistor within the smaller diameter portion of the heater tube. This known glow plug also comprises a third resistor, which is electrically connected in series with the second and first resistor, wherein the resistance-temperature coefficient of the third resistor is larger than a resistance-temperature coefficient of the first and second resistor, wherein the first resistor and the second resistor are completely accommodated in the smaller diameter portion of the heater tube, whereas the third resistor is completely accommodated within the larger diameter portion of the heater tube. Fig. 8 shows a further conventional glow plug 1 disclosed in Unexamined Japanese Patent Application No. 3-99122, published in 1991. The conventional glow plug 1 comprises a first resistor 4 and a second resistor 5 both serving as a heat generating element. The second resistor 5 has a positive resistance-temperature coefficient larger than that of the first resistor 4. Both the first resistor 4 and the second resistor 5 are provided in a heater tube 3. With this arrangement, immediately after the glow plug 1 receives electric power, a large amount of electric current flows across the first resistor 4. The first resistor 4 generates heat. After an elapse of a predetermined time, the second resistor 5 increases the temperature. The resistance value of the second resistor 5 increases correspondingly. The electric power supplied to the first resistor 4 is decreased due to the increase of the resistance of the second resistor 5. This prevents the first resistor 4 from being burned out by excessive heat generation. The diameter of the heater tube 3 is small at a portion accommodating the first resistor 4 compared with at a portion accommodating the second resistor 5. With this arrangement, the heat capacity of the heater tube 3 becomes small in the vicinity of the first resistor 4 compared with in the vicinity of the second resistor 5. The temperature of the first resistor 4 can be quickly increased immediately after the glow plug 1 is activated. More specifically, an ordinary engine requires a preheating. Within a given preheating time (for example, 5 seconds), the front end of the glow plug 1 reaches a predetermined temperature (e.g., 800 °C). However, according to the above-described conventional glow plug, the temperature of the glow plug may increase excessively after the front end of the glow plug reaches the above-described predetermined temperature. The maximum temperature possibly increases up to 1,050 °C. The power supply voltage is set at approximately 12 V during the above-described preheating. After the preheating is finished, the engine is started. A generator voltage of a charging generator is controlled to a predetermined voltage by a regulator. The setting voltage of the regulator is increased to approximately 14 V, once the engine starts rotating. The increased voltage is supplied to the sufficiently heated glow plug 1. Thus, the temperatures of the resistors 4 and 5 of the glow plug 1 further increase. Such an excessive temperature increase may cause oxidation and burnout of the resistors 4 and 5. This possibly gives adverse, influence to the durability of the glow plug 1. SUMMARY OF THE INVENTION In view of the foregoing problems, it is an object of the present invention to provide a glow plug capable of maintaining quick heat generation ability as well as suppressing the excessive temperature increase in the glow plug after finishing the preheating. According to the present invention, this object is solved by the features of claim 1. An improved embodiment of the inventive glow plug results from the subclaim 2. The present invention provides a glow plug having various aspects, which will be described hereinafter. Reference numerals in parentheses, added in the following description, show the correspondence to the components described in preferred embodiments of the present invention. The reference numerals are thus merely used for the purpose of expediting the understanding to the present invention and not used for narrowly interpreting the scope of the present invention. According to one embodiment of the present invention, a glow plug comprises a cylindrical housing (2). An elongated cup-shaped heater tube (3) has a larger-diameter portion (32) which is fixed to an end (2a) of the housing (2). A first resistor (4) and a second resistor (5) are provided in an inside space of the heater tube (3). The second resistor (5) has a positive resistance-temperature coefficient larger than that of the first resistor (4). The second resistor (5) is electrically connected in series with the first resistor (4). Furthermore, the entire body of the first resistor (4) and at least part of the second resistor (5) are accommodated in a smaller-diameter portion (31) of the heater tube (3). With this arrangement, the heat capacitors of both the first resistor (4) and the second resistor (5) can be reduced. Not only the warm-up ability of the second resistor (5) is improved, but also the resistance increasing ability of the second resistor (5) is improved. Accordingly, the electric power supply to the first resistor (4) can be quickly suppressed. In addition, the temperature increase of the first resistor (4) can be quickly suppressed. Thus, it becomes possible to prevent any excessive temperature increase of the glow plug (1) after the temperature of the front end of the glow plug (1) reached the above-described setting temperature (e.g., 800 °C). Accordingly, even if the glow plug (1) receives the electric power approximately 14 V, an excessive temperature increase of the glow plug (1) is effectively prevented. The oxidation and burnout of the first and second resistors (4, 5) can be suppressed. The durability of the glow plug (1) can be adequately maintained. As described above, the first resistor (4) is also accommodated in the smaller-diameter portion (31). The first resistor (4) has a smaller heat capacity comparable with that of the conventional one. Thus, the temperature of the first resistor (4) can be quickly increased immediately after the glow plug (1) is activated. When installed in the combustion chamber of an internal combustion chamber, the glow plug (1) is subjected to an explosive pressure caused in the combustion chamber. The heater tube (3) may cause a displacement about a cylindrical peripheral portion (34) facing to and brought into contact with an inner cylindrical surface of one end (2a) of the housing (2). More specifically, the closed end (3a) of the heater tube (3) may shift in a radial direction with respect to the housing (2). In this respect, according to the present invention, the larger-diameter portion (32) of the heater tube (3) is brought into contact with the inner cylindrical surface of the housing (2). The connecting strength in the vicinity of the cylindrical peripheral portion (34) can be maintained sufficiently. It becomes possible to eliminate the possibility that the heater tube (3) may be bent or broken at the above-described cylindrical peripheral portion (34) during the repetitive operations of the glow plug (1) in the combustion chamber. According to the present invention, more than half of the second resistor (5) is accommodated in the smaller-diameter portion (31) of the heater tube (3). With this arrangement, an excessive temperature increase succeeding the preheating can be effectively suppressed compared with the previously described prior art. This effect was confirmed based on a later-described experiment. Furthermore, in one embodiment of the invention the larger-diameter portion (32) has an outer diameter in a range of 4.5 mm to 6.0 mm. Too much reducing the outer diameter of the large-diameter portion (32) is not preferable in that it becomes difficult to maintain a satisfactory strength in the vicinity of the cylindrical peripheral portion (34) where the heater tube (3) is brought into contact with the inner cylindrical surface of the housing (2). Too much increasing the outer diameter of the large-diameter portion (32) is not preferable in that it becomes difficult to suppress an overall size of the glow plug (1) and assure a smooth installation of the glow plug (1) to the engine. According to the invention the outer diameter of the smaller-diameter portion (31) is 0.6 to 0.9 times as large as the outer diameter of the larger-diameter portion (32). Too much reducing the outer diameter of the smaller-diameter portion (31) is not desirable in that the smaller-diameter portion (32) becomes too thin to manufacture. Too much increasing the outer diameter of the smaller-diameter portion (31) is not desirable in that the smaller-diameter portion becomes too thick to sufficiently reduce the heat capacity of the first and second resistors (4,5). BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanied drawings, in which: Fig. 1A is a cross-sectional view showing an overall arrangement of a glow plug in accordance with a first embodiment of the present invention; Fig. 1B is an enlarged view showing a first resistor and a second resistor connected each other in the glow plug in accordance with the first embodiment of the present invention; Fig. 2 is a diagram showing a current supply circuit of the glow plug in accordance with the first embodiment of the present invention; Fig. 3 is a graph comparatively showing heat generation characteristics at the front end of the glow plug in accordance with the present invention and a prior art; Fig. 4 is a graph showing an overshoot temperature in relation to a variation of an overlap ratio L1/L0 between a smaller-diameter portion of a heater tube and the second resistor, obtained in an experiment conducted in the resent invention; Fig. 5 is a graph showing a warm-up ability at the front end of the glow plug in relation to a variation of the overlap ratio L1/L0 between the smaller-diameter portion of the heater tube and the second resistor, obtained in an experiment conducted in the resent invention; Fig. 6 is a cross-sectional view showing an overall arrangement of a glow plug in accordance with a second embodiment of the present invention; Figs. 7A through 7C are enlarged views showing connecting structures in accordance with third through fifth embodiments of the present invention; and Fig. 8 is a cross-sectional view showing an overall arrangement of a conventional glow plug. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be explained hereinafter with reference to accompanied drawings. Identical parts are denoted by the same reference numerals throughout the drawings. First embodiment A glow plug 1 of the present invention shown in Fig. 1 is provided in each of a plurality of (e.g., four) cylinders (not shown) of a diesel engine. The glow plug 1 has a function of promoting firing and combustion of fuel during an engine start-up operation. The glow plug 1 comprises a cylindrical hollow housing 2 made of iron-group material. The housing 2 has a screw portion 21 which is detachably engageable with the cylinder of the engine. An elongated cup-shaped heater tube 3 is inserted into one end 2a of the housing 2. The heater tube 3 is fixed to the housing 2 by soldering. The heater tube 3 is made of a conductive member (such as stainless material) which is excellent in the heat durability and the resistivity to oxidation. The heater tube 3 has a smaller-diameter portion 31 at a closed end 3a. A larger-diameter portion 32 is integral with the smaller-diameter portion 31 and provided at an opened end 3b of the heater tube 3. A diameter of the larger-diameter portion 32 is larger than a diameter of the smaller-diameter portion 31. The closed end 3a of the heater tube 3 extends outward from the one end 2a of the housing 2 and exposed to the outside of the housing 2. The larger-diameter portion 32 of the heater tube 3 fits the inner cylindrical wall of the one end 2a of the housing 2. The larger-diameter portion 32 is fixed to the one end 2a of the housing 2. The heater tube 3 comprises a tapered portion 33 connecting the smaller-diameter portion 31 and the larger-diameter portion 32. The tapered portion 33 has a diameter increasing gradually from the smaller-diameter portion 31 to the larger-diameter portion 32. From the requirements in the manufacturing, the tapered portion 33 is inclined at approximately 15° with respect to the larger-diameter portion 32. The heater tube 3 has an inner hollow space which accommodates first and second coil-like resistors 4 and 5. These resistors 4 and 5 extend in an axial direction of the heater tube 3. The first resistor 4 is adjacent to the closed end 3a of the heater tube 3. The second resistor 5 is closer to the opened end 3b of the heater tube 3 than the first resistor 4. The entire body of the first resistor 4 and at least part (e.g., 3/4 the length of the second resistor 5) of the second resistor 5 are accommodated in the smaller-diameter portion 31. The first resistor 4 has one end 41 electrically connected to the closed end 3a of the heater tube 3. The other end 42 of the first resistor 4 is electrically connected to one end 51 of the second resistor 5. The other end 52 of the second resistor 5 is electrically connected to one end 61 of an intermediate shaft 6. The intermediate shaft 6 is inserted in the housing 2 and fixed to the housing 2. Insulating powder 30, comprising heat-resistive material (e.g., magnesia), is provided in the heater tube 3. The above-described one end 61 of the intermediate shaft 6, the first resistor 4 and the second resistor 5 are buried in this insulating powder 30. With this arrangement, the one end 61 of the intermediate shaft 6, the first resistor 4 and the second resistor 5 are electrically isolated from the heater tube 3. The first resistor 4 is made of a first conductive member (e.g., iron-chrome alloy or nickel-chrome alloy) which has a resistance change rate of approximately 1. The resistance change rate is defined as a ratio of a resistance change to a temperature change. For example, the resistance change is a difference between a resistance value at 1,000 °C and a resistance value at 20 °C. The temperature change is a difference between 1,000 °C and 20 °C, wherein 20 °C represents a room temperature and 1,000 °C represents a temperature of the first resistor 4 during the preheating of the glow plug 1. The second resistor 5 is made of a second conductive member (e.g., nickel, low carbon steel, or cobalt-iron alloy) having a resistance change rate in a range of 5 to 14. The present invention refers the above-described resistance change as a resistance-temperature coefficient which is represented by a gradient or inclination of a plotted line in a graph with an abscissa representing a temperature and an ordinate representing a resistance value. Accordingly, the second conductive member has a larger resistance-temperature coefficient than the first conductive member. The first resistor 4 and the second resistor 5 are connected by plasma arc welding. A fused portion 45 is formed at a connecting portion of the first resistor 4 and the second resistor 5. The other end 42 of the first resistor 4 is overlapped with the one end 51 of the second resistor 5. The plasma arc is applied to the overlapped portion between the first resistor 4 and the second resistor 5 to form the fused portion 45. The one end 61 of the intermediate shaft 6 extends into the heater tube 3 from the opened end 3b of the heater tube 3. The other end 62 of the intermediate shaft 6 is fixed to the other end 2b of the housing 2 by a nut 9. An O ring 7 and a resin bush 8 are interposed between the nut 9 and the housing 2. The O ring 7 is made of an insulating elastic member such as fluoro rubber. Thus, the intermediate shaft 6 is electrically isolated from the housing 2. Hereinafter, an arrangement of a current supply circuit of the above-described glow plug 1 will be explained. As shown in Fig. 2, there are a plurality of (e.g., four) glow plugs 1 connected in parallel with each other. A battery power source (i.e., power supply source) 20 produces a power voltage of 12 V. Each glow plug 1 receives the power voltage of 12 V through a relay 201. The glow plug 1 causes heat and starts preheating. A start-up ability of the engine can be improved. The glow plug 1 has a body earth. In the drawing, reference numeral 202 denotes an engine key switch. Reference numeral 203 denotes a control unit having a timer function. Reference numeral 204 denotes an engine cooling water temperature sensor. Reference numeral 205 denotes a start-up timing indicator. The control unit 203 receives a signal of the engine cooling water temperature sensor 204 in response to ON (turned-on) of the engine key switch 202. The start-up timing indicator 205 is activated in response to a signal of the control unit 203. The relay 201 is activated. A predetermined voltage (e.g., 12V) is applied from the power source 20 to the glow plug 1. The glow plug 1 starts preheating. There is a predetermined waiting time (e.g., 5 seconds) provided immediately after the turning-on (ON) operation of the engine key switch 202. After the waiting time has elapsed, the start-up timing indicator 205 is turned off in response to a signal sent from the control unit 203. The turning-off of the start-up timing indicator 205 allows a driver to visually recognize the termination of the preheating. Thus, the driver can start the engine. The cooling water temperature sensor 204 detects the temperature of the engine cooling water. When the engine cooling water temperature is lower than a predetermined value (e.g., 60 °C), electric power is continuously supplied to the glow plug 1 to increase the temperature of the engine. This operation is generally referred to as afterglow. When the engine cooling water temperature is substantially equal to the above-described predetermined value, the glow plug 1 is deactivated in response to a signal sent from the control unit 203. Performing the above-described afterglow is effective to promote the combustion of the fuel mixture in the cylinder. Vibrations of the engine can be suppressed. And, the white smoke of the exhaust gas can be eliminated. Once the engine starts rotating, a regulator increases the voltage level of the electric power supplied to the glow plug 1. For example, the glow plug 1 receives 14 V which is higher than the above-described predetermined voltage (12 V). According to the above-described arrangement, the entire body of the first resistor 4 and 3/4 of the second resistor 5 are completely accommodated in the smaller-diameter portion 31 of the heater tube 3. This arrangement is effective to reduce the heat capacity of the second resistor 5 as well as the first resistor 4. Not only the warm-up ability of the second resistor 5 but also the resistance increasing ability of the second resistor 5 can be improved. As a result, the electric power supply to the first resistor 4 can be quickly suppressed. The temperature of the first resistor 4 can be sensitively controlled. In other word, it becomes possible to effectively suppress the temperature overshoot (i.e., excessive temperature increase) of the glow plug 1 after the front end of the glow plug 1 reached the above-described setting temperature (e.g., 800 °C). The above-described increased power voltage (14 V) is applied to the glow plug 1 during the afterglow operation. However, the arrangement of the present embodiment can prevent the temperature of the glow plug 1 from increasing extraordinarily. The oxidation and burnout of the resistors 4 and 5 can be suppressed. The durability of the glow plug 1 is adequately maintained. As the first resistor 4 is also accommodated in the smaller-diameter portion 31, the first resistor 4 has a smaller heat capacity comparable with that of a conventional one. Thus, the temperature of the first resistor 4 can be quickly increased immediately after the glow plug 1 is activated. Hereinafter, an experimental result will be explained. An experiment was conducted to evaluate the effect of an overlap length L1 between the smaller-diameter portion 31 and the second heater 5. Experimental data relating the temperature overshoot and the quick warm-up ability were obtained by changing the value of the overlap length L1. Detailed content and result will be explained hereinafter. The tapered portion 33 is not included in the smaller-diameter portion 31. In this experiment, the first resistor 4 is made of a 80wt%Ni-20wt%Cr alloy. The second resistor 5 is made of a 92wt%Co-8wt%Fe alloy. In an assembled condition, the first resistor 4 and the second resistor 5 have a wire diameter of 0.35 mm. The first resistor 4 is 6 mm in length, 2.5 mm in diameter, and 0.60 mm in coil pitch. The second resistor 5 is 22 mm in length (L0), 2.5 mm in diameter, and 0.50 mm in coil pitch. The smaller-diameter portion 31 has an outer diameter of 4.3 mm. The larger-diameter portion 32 has an outer diameter of 5.0 mm. A saturated temperature of the glow plug 1 is set to 900 °C as a result of a combination of dimensions and materials of the first and second resistors 4 and 5 and the heater tube 3. Based on the above-described glow plug 1, the overlap length L1 between the second resistor 5 and the smaller-diameter portion 31 was variously changed. A total of five samples of the glow plug 1 were fabricated to set an overlap ratio (= L1/L0) to each of 0, 1/4, 1/2, 3/4 and 1. Each sample of the glow plug 1 is subjected to a power voltage of 11 V. A temperature change on the surface of the heater tube 3 was measured in relation to an elapsed time. Fig. 3 shows the measured results of two samples, one sample having the overlap ratio (L1/L0) of 0 corresponding to the prior art and the other sample having the overlap ratio of 1/2 corresponding to the present invention. From the measurement result, an overshoot temperature T (°C) was obtained. And, a time t (sec) was obtained as a warm-up period required for the front end of the glow plug 1 to reach 800 °C from the application of the power voltage. In each of Figs. 4 and 5, a curve plotted by white round points shows the measured values of the overshoot temperature T and the warm-up period t with respect to all of the above-described samples. The overshoot temperature T is defined as a difference between the maximum temperature and the saturated temperature of the glow plug 1. Furthermore, a similar experiment was conducted on the glow plug 1 having different dimensions. The outer diameter of the smaller-diameter portion 31 is set to 3.5 mm. In an assembled condition, the first and second resistors 4 and 5 have a wire diameter of 0.30 mm. A total of five samples of the glow plug 1 were fabricated to set an overlap ratio (= L1/L0) to each of 0, 1/4, 1/2, 3/4 and 1. The experiment was conducted in the same manner. Experimental result is shown by a curve plotted by black round points in Figs. 4 and 5. As understood from Fig. 4, the overshoot temperature T decreases with increasing overlap ratio L1/L0. Especially, the overshoot temperature T greatly decreases when the overlap ratio L1/L0 exceeds 1/2, compared with the value obtained when the overlap ratio L1/L0 is 0. Accordingly, having the overlap ratio L1/L0 larger than 0 is effective to prevent the temperature of the glow plug 1 from excessively increasing. This effect is realized when the smaller-diameter portion 31 accommodates at least part of the second resistor 5. Oxidation and burnout of the first and second resistors 4 and 5 can be suppressed. Accordingly, the glow plug 1 can maintain appropriate durability. As understood from Fig. 5, all samples of the glow plug 1 reached 800 °C within 5 seconds which corresponds to an ordinary preheating time. Thus, the first resistor 4 can be quickly warmed up. In other words, the warm-up ability of the glow plug 1 can be maintained adequately. Considering the experimental results, it is preferable that more than half of the second resistor 5 is accommodated in the smaller-diameter portion 31 of the heater tube 3. With this arrangement, a temperature overshoot (i.e., an excessive temperature increase) succeeding the preheating can be effectively suppressed. Furthermore, it is preferable that the larger-diameter portion 32 has an outer diameter in a range of 4.5 mm to 6.0 mm. Too much reducing the outer diameter of the large-diameter portion 32 is not preferable in that it becomes difficult to maintain a satisfactory strength in the vicinity of the cylindrical peripheral portion 34 where the heater tube 3 is brought into contact with the inner cylindrical surface of the housing 2. Too much increasing the outer diameter of the large-diameter portion 32 is not preferable in that it becomes difficult to suppress an overall size of the glow plug 1 and assure a smooth installation of the glow plug 1 to the engine. Furthermore, it is preferable that an outer diameter of the smaller-diameter portion 31 is 0.6 to 0.9 times as large as the outer diameter of the larger-diameter portion 32. Too much reducing the outer diameter of the smaller-diameter portion 31 is not desirable because the smaller-diameter portion 32 becomes too thin to manufacture. Too much increasing the outer diameter of the smaller-diameter portion 31 is not desirable because the smaller-diameter portion becomes too thick to sufficiently reduce the heat capacity of the first and second resistors 4 and 5. Second Embodiment As shown in Fig. 6, the diameter of the inner cylindrical wall of the housing 2 is differentiated at both ends. More specifically, the inner diameter of the housing 2 is larger at one end 2a of the housing 2 than at the other end 2b. A thin cylindrical portion 22 is provided at the one end 2a of the housing 2. The thin cylindrical portion 22 spatially surrounds the heater tube 3 so as to define a so-called pocket space 10 between them. Thus, the heat conduction between the heater tube 3 and the thin cylindrical portion 22 is worsened due to air intervening in the pocket space 10. With this arrangement, it becomes possible to prevent the heat of the heater tube 3 from leaking to the housing 2 at the region corresponding to the pocket space 10. Thus, the larger-diameter portion 32 keeps a significant amount of heat. Not only the warm-up ability of the second resistor 5 but also the resistance increasing ability of the second resistor 5 can be improved. As a result, the temperature of the first resistor 4 can be sensitively controlled. In other word, it becomes possible to effectively suppress the excessive temperature increase of the glow plug 1 after the front end of the glow plug 1 reached the above-described setting temperature (e.g., 800 °C). The heater tube 3 is brought into contact with and fixed to the inner cylindrical wall of the housing 2 at a portion 34 adjacent to the one end 2a of the housing 2. The portion 34 serves as a bottom of the pocket space 10. Third through Fifth Embodiments Third through fifth embodiments relate to the connecting structure between the first resistor 4 and the second resistor 5. Fig. 7A shows the connecting structure in accordance with the third embodiment. The other end 42 of the first resistor 4 is overlapped with the one end 51 of the second resistor 5 at their distal ends. Using the plasma arc welding, the first resistor 4 and the second resistor 5 are fixed each other at the overlapped portion. Fig.7B shows the connecting structure in accordance with the fourth embodiment. Both the other end 42 of the first resistor 4 and the one end 51 of the second resistor 5 extend in the axial direction. The other end 42 of the first resistor 4 is overlapped with the one end 51 of the second resistor 5 at their distal ends. Using the plasma arc welding, the first resistor 4 and the second resistor 5 are fixed each other at the overlapped portion. Each length of the first and second resistors 4 and 5 is defined by the length of their coil portions. The portion A extending in the axial direction is not included in the length of the first and second resistors 4 and 5. Fig.7C shows the connecting structure in accordance with the fifth embodiment. The other end 42 of the first resistor 4 extends parallel to the one end 51 of the second resistor 5 in the axial direction. The other end 42 of the first resistor 4 is overlapped with the one end 51 of the second resistor 5 entirely along the axially extending portions thereof. The axially extending overlapped portions are welded at a plurality of points (e.g., three points) by laser welding. In Figs. 7A through 7C, reference 45 denotes a fused portion between the first resistor 4 and the second resistor 5. Other Modifications According to the above-described embodiments, the first resistor 4 is made of the conductive member having a small positive resistance change rate. It is possible to fabricate the first resistor 4 by a conductive member having a negative resistance-temperature coefficient.

Claims: A glow plug comprising: a cylindrical housing (2); a cup-shaped heater tube (3) having a smaller-diameter portion (31) at a closed end (3a) and a larger-diameter portion (32) at an opened end (3b), said larger-diameter portion (32) being fixed to an end (2a) of said housing (2); and a heater resistive element accommodated in an inner hollow space of said heater tube (3) extending from the smaller-diameter portion (31) to the larger-diameter portion (32), said heater resistive element consisting of only two serially connected first and second resistors (4, 5), wherein a resistance-temperature coefficient of said second resistor (5) is larger than a resistance-temperature coefficient of said first resistor (4); and said first resistor, (4) is entirely located in said smaller-diameter portion, wherein an outer diameter of said smaller-diameter portion (31) is 0.6 to 0.9 times as large as an outer diameter of said larger-diameter portion (32); and said second resistor (5) spans from said smaller-diameter portion (31) to said larger-diameter portion (32) so that part of said second resistor (5) overlaps with said smaller-diameter portion (31), with an overlap ratio L 1 /L 0 not smaller than 1/2, where L 0 represents the length of said second resistor (5) and L 1 represents an overlap length. The glow plug in accordance with claim 1, wherein said larger-diameter portion (32) has an outer diameter in the range of 4.5 mm to 6.0 mm.

Inventors: ISHII HIROHISA, KOYAMA TAIJI, ISHII, HIROHISA, KOYAMA, TAIJI, Ishii, Hirohisa c/o Denso Corporation
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1997
Main classification: F23Q7
IPC codes: F23Q7