GLOW PLUG, ITS PRODUCTION PROCESS AND ION CURRENT DETECTOR

Abstract: The present invention is to provide a glow plug (1) with an ion current detecting function, which is mounted in a combustion chamber of a Diesel engine. The glow plug includes an insulator (8); a heating element (6) embedded in the insulator and energized through a pair of lead wires (11a, 11b) to generate heat; and an ion current detecting electrode (14) embedded in the insulator with a portion exposed into a flame in the combustion chamber so that an ionization state in the flame can be detected. The heating element of the glow plug promotes ignition and combustion of fuel in the combustion chamber (7) due to its heating action caused when running the heating element hot. The ion current detecting electrode detects an ionization state in the combustion flame. When detecting ion current, the ion current detecting electrode and the adjacent inner wall of the combustion chamber form two electrodes for capturing positive and negative ions existing between both when burning the fuel. In such structure, the ion current can be detected precisely in spite of very simple structure. It is therefore possible to effectively use the information for combustion control. Further, since the ion current detecting function is added to the glow plug, an inexpensive ion current detecting sensor can be provided.

Description: FIELD OF THE INVENTION The present invention relates to a glow plug for promoting ignition and combustion of fuel and an ion current detector using the glow plug. BACKGROUND ART From the standpoint of the environmental protection, further reduction of the discharged quantity of exhaust gas or soot has been recently required in not only gasoline engines but also Diesel engines. To meet the requirements, consideration has been given to improvements on various points such as improvements of engine, a reduction in emission gas by the post processing (using a catalyst or the like), improvements in characteristics of fuel or lubricating oil and improvements of engine combustion control systems. In connection with the above countermeasures, it is required to detect engine combustion conditions during engine operation. Such engine combustion conditions are to be detected by measuring cylinder internal pressure, combustion light, ion current and such. Of all the measuring methods for detecting the engine combustion conditions, the ion current measurement has been considered to be highly useful because it can be used for directly observing a chemical reaction resulting from the engine combustion, and therefore various types of ion current detecting methods have been proposed. Japanese Patent Laid-Open Application (JP-A) No. 7-259597 discloses a method for detecting ion current (ionization degree of fuel gases) due to combustion of fuel by a sleeve-like electrode attached to a mounting seat for a fuel injection nozzle, the sleeve-like electrode electrically insulated from the injection nozzle and a cylinder head of the engine, and connected to an external detection circuit. US Patent No. 4,739,731 discloses a sensor provided with a ceramic glow plug for detecting ion current (conductivity of ionized fuel gases). In this technique, an electric conductive layer made of platinum is formed on a surface of a heater (heating element) of the ceramic glow plug, and electrically insulated from a combustion chamber and a glow plug clamping fixture. An external power source (for 250-volt DC voltage) is provided for applying the voltage to the electric conductive layer to detect ion current resulting from the fuel combustion. In typical ion current detectors with a glow plug having such an ion current detecting function, ignition and combustion of fuel are generally promoted by a heating action of the heating element when the engine starts at low temperature. In this case, such a heating state of the heating element usually continues after warm-up of the engine has been completed until the combustion is stabilized (generally, referred to as "afterglow"). After completion of the afterglow, the heating action of the glow plug is stopped and the processing step of detecting ion current is started. However, the following drawbacks are present in the above conventional techniques. With the former technique (JP-A No. 7-259597), there is a need for ion current detection to provide a sleeve-like electrode insulated from the other portions, and this forces complicated work in preparing electrode materials and machining the electrode. Such an electrode for ion current detection is thus very expensive, and besides, becomes unusable earlier because of short-circuit between the electrode and the fuel injection nozzle or the cylinder by carbon generated in the combustion chamber. With the latter technique (USP No. 4,739,731), since the electrode for detecting ion current is provided on the heating element, and the electrode and the heating element are connected to different power sources through individual electric circuits, respectively, the circuit structure is complicated. In addition, since a large amount of expensive noble metal such as platinum is needed for ensuring heat and wear resistance of the electrode, the glow plug itself becomes very expensive. Further in this sensor, the electrode is almost completely exposed into the combustion chamber and the space between the housing and the electrode is narrow. For this reason, there is a danger that the electrode is shorted to the ground and the housing is made conductive due to adhesion of carbon to the electrode surface, resulting in an error in detecting ion current. Existing ion current detectors display only a heating action and cannot detect ion current during the afterglow period. Since in this period any result of ion current detection can not be used for performing combustion control, the combustion cannot be controlled optimally. Stated more specifically, it is difficult to control the combustion optimally during the afterglow period because such a result of ion current detection cannot be used in individual combustion operations, e.g., for performing feedback control of ignition stage and flame failure detection. When using the above conventional glow plug, carbon adheres to the circumference of the ceramic heating portion to reduce insulation resistance between the exposed electrode for ion current detection and the grounded portion (plug housing and cylinder head) insulated from the electrode. In this case, a flow of leakage current may be created through the adhered carbon even if no ion is derived from the combustion gases. When this happens, the ion current detected shows a waveform different from a desired one due to occurrence of the leakage current, and such an incorrect detection result causes deterioration in the accuracy of ignition stage and flame failure detections. The electric insulation between the exposed electrode and the ground portion is dependent on pressure in the combustion chamber. Especially, in the engine compressing process the insulation resistance drops and the leakage current becomes easy to flow. Also when using the glow plug, a sharp temperature change runs the danger that the ion current detecting electrode is broken by thermal vibration. Since a large amount of expensive noble metal such as platinum is needed for ensuring heat and wear resistance of the electrode, the glow plug itself becomes very expensive. Further, since the ion current detecting electrode supported at the tip of the glow plug directly touches a flame having a high temperature, stresses tend to be concentrated in the neighborhood of the ion current detecting electrode and could damage the ceramic glow plug such as to crack it. Document JP-A-59-160046 discloses an ignition timing detector arranged in a cylinder head in the manner that it is projected into a combustion chamber having an injection nozzle. The ignition timing detector is obtained by covering the outside of an ionic-current detecting core with an insulator and winding a nichrome wire serving as a heating member for a glow plug in the form of a coil around the insulator. One end of the nichrome wire is connected to a center electrode while the other end is connected to a cover. The timing when fuel injected into the combustion chamber is fired is detected from the ionic current passed between the core and the outer wall, and the actual combustion time is calculated in a control circuit from the ionic current and the output of a crank angle position detector. According to document JP-A-60-013985 a method wherein the ignition timing sensor is incorporated with an electric resistance heating member at the tip end of the flame sensor thereof to use it both for a preheating plug in combination is disclosed. The flame sensor section of the ignition timing sensor is provided with the electric resistance heating member, in which a high resistance wire member of tungsten or the like is wound around the tip end of a guide member in the shape of a coil and is incorporated into a casing. Both ends of the guide member are connected to an electric source through a connector and a switch while the flame sensor section functions as the preheating plug. Document JP-A-2-176322 discloses a conductive heater with a means which serves as an ignition source for fuel and a flame detecting means which determines the change of the current level relative to the applied voltage and the presence of flame. Prior to the igniter ignition command, the flame detecting device monitoring command is sent out so as to diagnose the insulation deterioration condition of a conductive heater in advance, and the diagnosis made is indicated on a first indicator with a lamp. After diagnosing the insulation deterioration, the current and voltage are applied to the conductive heater to bring same to the red-hot condition at 1,000 to 1,200 deg.C, which serves as the ignition source. After the conductive heater became red-hot, the fuel valve opening command is sent out. When the flame is formed upon opening the fuel valve, a flame ion current flows between the conductive heater and the grounding of the burner nozzle because of the conductivity of the flame. A determination is then made as to the presence of ignition and the burning condition, and the determination results are indicated on a second indicator with a lamp. Document DE 37 06555 A1 discloses a structure wherein an ion current detecting electrode is arranged in a glow plug of filament type and of sheath type. The filament type structure is just formed in such a manner that a hearing element is wound in the form of a coil around the periphery of a ceramic cylinder. An insulated ion electrode is arranged at the ceramic cylinder. All of the ion electrodes are made of a metal. Document EP 0 456 245 A2 discloses a structure wherein a heating element made of tungsten exhibiting positive-temperature-coefficient is provided such that a combustion state is monitored by reading the variation in resistance value of the heating element caused heat by the combustion flame. When monitoring combustion, a predetermined amount of current is applied to the heating element, thereby detecting a state of voltage drop due to increase in resistance caused by the increase in temperature. Therefore, it is an object of the present invention to provide a glow plug capable of detecting ion current precisely with a simple structure and an ion current detector using the glow plug. Another object of the present invention is to provide an ion current detector capable of detecting ion current precisely even for a period of glow of the glow plug and hence maintaining proper combustion of fuel based on the detection results. Still another object of present invention is to provide an ion current detector capable of detecting ion current precisely and hence performing precise control of individual processings such as ignition stage detection and flame failure detection based on the detection results. Yet another object of the present invention is to provide a relatively inexpensive glow plug having excellent durability, which can detect ion current precisely without any trouble from carbon adhesion and any damage to the ion current detector. Yet another object of the present invention is to provide a glow plug having excellent durability without suffering any damage such as a crack and showing ease of manufacture. The term "ion current" used here means current passing through ionized fuel gases in a combustion chamber. The ion current detecting electrode may be referred to as the ion current detecting electrode. To achieve the above objects, a glow plug according to the invention as claimed in claim 1 comprises an insulator made of an insulating ceramic, a heating element made of a conductive ceramic and embedded in the insulator and energized through a pair of lead wires to generate heat, a combination of said insulator and said heating element being sintered as a single unit; and a ion current detecting electrode embedded in said insulator with a portion of said ion current detecting electrode exposed to a flame produced in a combustion chamber so that a ionization state in the flame can be detected. In this case, the heating element of the glow plug acts to promote ignition and combustion of fuel in the combustion chamber by a heating action of the heating element. The ion current detecting electrode embedded in the heat resisting insulator detects the ionization state in the combustion flame. When detecting ion current, the ion current detecting electrode and the inner wall of the combustion chamber adjacent to the ion current detecting electrode form two electrodes for capturing positive and negative ions existing therebetween during fuel combustion. According to the glow plug, the ion current can be detected precisely with very simple structure, and information detected can be effectively used for combustion control. Further, since the glow plug is given an ion current detecting function, an inexpensive ion sensor can be provided. Since the glow plug of the invention is constructed such that the majority of the ion current detecting electrode is embedded in the heat resisting insulator except only a portion exposed to the outside, an amount of carbon adhered to the outer surface of the glow plug cannot establish an electric connection between the electrode and a housing (grounded side) to cause error detection of the ion current that may occur in the prior art (USP 4,739,731). The exposed portion of the ion current detecting electrode is preferably provided at the tip of the glow plug so that the exposed portion and the housing (inner wall side of the combustion chamber) will be separated as far from each other as possible. Although it is considered that some carbon adheres to the outer surface of the glow plug during operation, the carbon adhered is burnt off by a heating action of the heating element 7 (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can maintain its performance in detecting ion current for long periods. Further, since the heating element itself is embedded inside the heat resisting insulator, it never change its heating characteristics due to lowering of resistance or the like to maintain high heating performance for long periods. In other words, since such a construction could resist oxidation to wear the heating element, the sectional area is kept constant and the resistance does not vary. The construction also avoids damaging the heating element under thermal action such as thermal shock in the combustion chamber. In the glow plug of the invention, the heating element and the ion current detecting element is constructed as follows. The invention as claimed in claim 2 recites that the heating element and the ion current detecting electrode is electrically connected to each other. Stated more specifically, in the invention as claimed in claim 3 the heating element and the ion current detecting electrode are integrally formed, while in the invention as claimed in claim 4 lead wires reside between the heating element and the ion current detecting electrode. In all of claims 2 to 4, both the heating performance of the heating element and the performance in detecting ion current can be maintained for long periods as mentioned above. From the standpoint of the manufacturing process, it is considered that the invention as claimed in claim 3 shows the simplest way to manufacture the glow plug. The glow plug as claimed in claim 5 is such that the heating element and the ion current detecting electrode are insulated from each other. Since the heating element and the ion current detecting electrode are energized through individual paths, the ion current detecting electrode can detect ion current synchronously with the heating action of the heating element (i.e., the combustion condition can be grasped constantly). The invention as claimed in claim 6 recites that at least the portion of the ion current detecting electrode exposed to the flame is made of a conductive ceramic material. It is therefore possible to minimize oxidation wearing of the ion current detecting electrode even when it is exposed to hot combustion gases, and hence to further improve the durability of the performance in detecting ion current by the glow plug. The invention as claimed in claim 7 recites that the heating element and the ion current detecting electrode are produced dividedly from each other by using mixtures having different components or different particle sizes. Such divided production can change the resistance between the heating element and the ion current detecting electrode to provide a glow plug (ion current sensor) according to the application. In the case where the result of ion current detection is used for flame failure detection, only the present or absence of ion current is required for the determination. In such a case, it is possible to increase the resistance of the ion current detecting electrode to a relatively large value, e.g., about 5 MΩ or less (1Ω or so with the heating element). In the case where the result of ion current detection is used for ignition stage detection, it is desirable to reduce the resistance of the ion current detecting electrode as small as possible (500 kΩ or less) since the leading edge of ion current must be detected for an instant. Although the above description was made to the glow plug featured in that the glow plug itself can prevent the ion current detecting electrode and the housing (inner wall side of the combustion chamber) from conducting even when some carbon adheres thereto, the carbon may become adhered and accumulated during a long period of operation. To eliminate such a problem, an ion current detector as claimed in claim 8 features that the adhered carbon is removed without stopping the ion current detection by using the glow plug of claim 5 that can carry out the ion current detection by the ion current detecting electrode simultaneously with the heating action of the heating element. Specifically, the ion current detector comprises switching means for turning on or off the power supply to the heating element, leakage current detection means for detecting a leakage current flowing from the ion current detecting electrode in a predetermined stage before fuel combustion, and operation means for operating the switching means to temporarily energize the heating element when the leakage current detected is larger than a predetermined threshold. When the carbon adheres to the outer portion of the glow plug in the combustion chamber, the exposed portion of the ion current detecting electrode and the housing side are electrically conducted to reduce the insulation resistance. Consequently, a leakage current flows and a desired ion current waveform cannot be obtained. As shown in Fig. 24B, the leakage current flows before its ion current waveform is obtained (before point A in Figs. 24A and 24B). On the contrary, in the invention the carbon adhered state of the outer surface of the glow plug is estimated based on the leakage current, and if it is such a carbon adhered state, the adhered carbon will be burnt off by running the heating element hot. As a result, a desired waveform of ion current (e.g., the waveform shown in Fig. 24A) can be obtained at all times, and the detection result can be used for precise processings such as ignition stage detection and flame failure detection. When the carbon adheres to the outer surface of the glow plug, the insulation resistance between the ion current detecting electrode and the housing side depends on the pressure in the combustion chamber. In the invention as claimed in claim 9, since the detection of leakage current is carried out when the pressure in the combustion chamber rises, the presence or absence of the leakage current can be detected securely. The timing period of the pressure rise corresponds to the compression stage in a Diesel engine, for example. The leakage current may also be detected in correspondence to the timing period of fuel injection into the combustion chamber. The timing period of fuel injection corresponds to a period that elapses between the moment the pressure in the combustion chamber of the Diesel engine rises and the moment just before the fuel burns. It is therefore possible to detect the leakage current more securely under such a condition that the carbon adhered. In a manufacturing method, the heating element and the ion current detecting electrode are first produced, surrounded with the heat resisting insulator, and hot-pressed at a predetermined temperature. A portion of the heat resisting insulator is then cut to expose the ion current detecting electrode to the outside. According to the above manufacturing technique, the glow plug having such special structure can be made up without requiring any complicated process, thereby providing the glow plug having such an excellent ion current detecting function through the simple manufacturing method. In a further example, the heating element and the ion current detecting electrode are provided on a thin-plate like heat resisting insulation sheet to be wrapped around a rod-shaped heat resisting insulation solid-shaft. The heat resisting insulation sheet and the heat resisting solid shaft are heat-treated, and a portion of the heat-treated body of the heat resisting insulation sheet and the heat resisting solid shaft is cut so that the ion current detecting electrode will be exposed to the outside. According to another example the heating element and the ion current detecting electrode are provided on certain one of plural layer members. Then the plural layer members are so superposed that the layer member having the heating element and the ion current detecting electrode thereon will reside in a central portion. After that, the plural layer members put on top of each other are heat-treated, and a portion of superposed layer members is cut so that the ion current detecting electrode will be exposed to the outside. A glow plug having a heating element energized through a pair of conductive wires to generate heat is used, and the following ion current detector is constructed by using an ion current detecting function of the glow plug. In the glow plug, the conductive wire pair (lead wire pair) and the heating element are insulated from the grounded side such as a cylinder head. An ion current detector according to a further example includes switching means for switching over between a first state and a second state, in which the first state is for applying a supply voltage from a power source to the conductive wire pair, and the second state is for shutting the electric path between the conductive wire pair and the power source and applying the supply voltage between the heating element and a wall portion of a combustion chamber. Further, ion current detection means is provided for detecting ion current resulting from fuel combustion by using the voltage supplied from the power source in the second state. In the first state, the supply voltage is applied from the power source to the conductive wire pair to run the heating element hot. This state corresponds to the state in which ignition and combustion of fuel is being promoted when the engine starts at low temperature. In the second state, the electric path between the conductive wire pair and the power source is shut and the supply voltage is applied between the heating element and the wall portion of the combustion chamber. This state corresponds to the state in which ion current is detected. The ion current is detected by the ion current detection means. In such structure, the voltage application to the heating element is performed through the common conductive wire pair in both states, and the switching between both states is selectively performed by the switching means. It is therefore possible to simplify the structure of the ion current detector that uses the glow plug having the ion current detecting function, such as wiring of the conductive wires connected to the heating element, and other circuit arrangements associated with ion current detection, and hence to provide an inexpensive ion current detector. In this case, the ion current detection accuracy is never reduced in spite of such simple structure. For more concrete structure of the ion current detector, the power source is connected through the switching means to an electric path between the heating element and the wall portion of the combustion chamber, while according to another example the power source is connected directly to the electric path between the heating element and the wall portion of the combustion chamber. Both sufficiently meet requirements for realizing simplification of the structure. However, since the voltage is applied between the heating element and the wall portion of the combustion chamber directly without the switching means, it can show the following special effect. Although the ion current due to fuel combustion is originally weak, since the power supply circuit is constructed without passing through the switching means, the ion current can be detected more precisely. The switching means can be materialized by a switching circuit with plural switch contacts, or a semiconductor switching element (transistor, thyristor or the like), with some resistance thereon. The power source for applying voltage to the conductive wire pair in the first state and the power source for applying voltage between the heating element and the wall portion of the combustion chamber in the second state may be provided separately, or a common power source may be used therebetween. In either case, the ion current can be detected precisely. In particular, according to the latter example a power source exclusively used for the ion current detection is not required, e.g., a power source other than the vehicle battery, thus simplifying the structure. According to another example one end of the power source is connected to one conductive wire coupled to the heating element while the other is connected to a cylinder head of the Diesel engine for holding the glow plug. In this case, the structure for applying voltage between the heating element and the wall portion of the combustion chamber can be simplified when it is used in the Diesel engine. In another example a constant voltage circuit is provided between the power source and one wire of the conductive wire pair for regulating the supply voltage of the power source to a constant value. Since the ion current is originally weak, the ion current value detected is susceptible to variation of the applied voltage to cause a detection error when the applied voltage largely varies. The detection error also causes various problems. For example, when the output information of the ion current (wave height, area, etc.) is used for flame failure detection, the accuracy of the flame failure detection must be lowered. In contrast, the above structure permits the improvement in accuracy of the ion current detection and hence the improvement in accuracy of individual processings such as flame failure detection. According to a further example a plurality of glow plugs are connected in parallel and power-supply paths to individual glow plugs are switched at the same time by the switching means. In such structure, the switching circuit as the switching means and the detecting resistor as the ion current detection means can be shared by the glow plugs, thereby further simplifying the structure. For example, when the glow plugs are provided in the combustion chambers of a multiple cylinder engine, ion current can be detected for each cylinder in time series. The ion current detector can also be simplified with structure other than the above. For example an voltmeter for ion current detection is provided between one conductive wire of the glow plug and the ground contact. In this case, the voltmeter can be constructed by an amplifier measuring a potential difference from the ground with relatively simple structure, rather than by a differential amplifier having relatively complicated internal structure. In addition, preferably a capacitor is provided between one conductive wire of the glow plug and the voltmeter. In this case, the DC component of the supply voltage is cut by the capacitor. Therefore, even when a power source of a relatively high voltage (e.g., 50 volts) is used exclusively for the ion current detection, the voltage applied to the voltage detector (amplifier) never exceeds the withstand voltage since the high voltage of the power source is not directly applied to the voltage detector. As a result, inconvenient things such as damage to the voltage detector can be prevented. It should be noted that this structure becomes more effective in the case the supply voltage for the ion current detection is 30 volts or higher. In another example an ion current detecting resistor is provided on the grounded side of the power source for detecting ion current from a potential difference between both terminals of the ion current detecting resistor. In this case, the voltage waveform corresponding to the ion current waveform detected is plotted on a reference level of O volt. It is therefore unnecessary to use an expensive, complicated voltage detector even when using a supply voltage exceeding the withstand voltage of the voltage detector. Such structure is preferably materialized such that the heating-element power source and the ion current detecting power source are provided separately with the ion current detecting resistor provided on the grounded side of the latter power source. This is because the heating performance of the heating element may be lowered at heating time when the heating element and the ion current detecting resistor are connected in series. On the other hand, a glow plug can be used for the glow plug in the above ion current detector, in which a heating element portion having a heating element is so provided that it projects into the combustion chamber for burning fuel. An ion current detecting electrode to the inner wall of the combustion chamber is formed in the heating element. In this case, the heating element of the glow plug acts to promote ignition and combustion of fuel in the combustion chamber due to the heating action when the heating element is running hot. When detecting ion current rather than running the heating element hot, the heating element acts as the ion current detecting electrode for detecting the ion current resulting from the fuel combustion. In other words, when detecting ion current, the heating element and the inner wall of the combustion chamber adjacent to the heating element form two electrodes for capturing positive and negative ions existing therebetween when burning the fuel. It is therefore possible to detect the ion current precisely in spite of such simple structure, and hence to effectively use the information on the detected ion current for various combustion control. Further, since the ion current detecting function is given to the glow plug, an inexpensive ion current sensor can be provided. A further example of a glow plug comprises a heating element portion provided with a heat resisting insulator and a heating element embedded in the heat resisting insulator, in which a portion of the heating element is exposed from the heat resisting insulator and the exposed portion is used as an ion current detecting electrode to the inner wall of the combustion chamber. In such a case, since the exposed portion of the heating element effectively acts as the ion current detecting electrode, the following effect can be newly obtained. Although it is considered that some carbon adheres to the exposed portion of the heating element during operation of the glow plug, the adhered carbon is burnt off by the heating action of the heating element (e.g., due to glowing when the engine starts at low temperature). As a result, life of the glow plug is never reduced even in such structure in which an exposed portion is provided in the heating element for use as an ion current detecting electrode, so that the glow plug adds excellent durability that are useful for long periods. The heating element is preferably made of a ceramic material. In this case, if the heating element made of a ceramic material is so arranged that a portion of the heating element will be exposed into the combustion chamber, oxidation wearing of the heating element can be minimized even when it is exposed to hot combustion gases, thereby further improving the durability of the glow plug. According to a further example, the heating-element running state of the glow plug and the ion current detecting state of the glow plug are switched (switching means). In the ion current detecting state of the glow plug, combustion ions are captured between the glow plug electrode portion and the inner wall of the combustion chamber, and ion current is detected by current detection means such as an ion current detecting resistor. The switching means can be operated to temporarily switch over to the ion current detecting state at least immediately after the fuel ignition stage (operation means). Since the function of the glow plug for promoting ignition and combustion of fuel is given top priority in all the functions of the glow plug, for example, during the afterglow period when the engine starts at low temperature, the ion current detection has not been performed during the afterglow period in the prior art. In contrast, according to this example, the ion current detection period is temporarily provided within a range in which the heating function of the glow plug is never damaged even under the heating-element running state such as in the afterglow period. It is therefore possible to detect ion current precisely even in the glow period of the glow plug, and hence to maintain the fuel combustion properly using the result of the ion current detection. In another example, the operation means operates the switching means to switch over to the ion current detecting state for a predetermined period of time after each event of the fuel injection into the combustion chamber. In this case, since the ion current detection period is set based on the fuel injection timing, the ion current can be detected securely by setting the ion current detection period as short as possible, thereby minimizing lowering of the glow function of the glow plug. In a further example, the operation means operates at a predetermined frequency to switch over between the heating-element running state and the ion current detecting state. Even in such a case, the ion current detecting function and the heating-element running function can be united in the afterglow period. According to another example, the glow plug comprises a heating element energized through a pair of lead wires to generate heat, a heat resisting insulator embedding the heating element therein, and an ion current detecting electrode integrally formed with the heating element. Such a glow plug is used to detect ion current produced when burning fuel. In this case, the ion current can be detected precisely in spite of such very simple structure, and the information on the ion current detected can be effectively used for combustion control. In a further example, the heating-element running state of the glow plug and the ion current detecting state of the glow plug are switched (switching means). Brief description of the normal switching action is as follows. For example, when the glow plug is held in the heating-element running state at the time of low-temperature start of the engine, and warm-up of the engine is completed by the heating action of the heating element, the glow plug is switched from the heating-element running state to the ion current detecting state. In other words, combustion ions are captured between the exposed electrode portion of the glow plug and the inner wall of the combustion chamber, and the ion current is detected by the current detection means such as an ion current detecting resistor. A leakage current flowing from the exposed electrode portion is detected in a predetermined stage before fuel ignition under the ion current detecting state of the glow plug (leakage current detection means). When the leakage current detected by the leakage current detection means is larger than a given threshold, the switching means is operated to temporarily switch over from the ion current detecting state to the heating-element running state (operation means). The carbon adhered to the outer portion of the glow plug in the combustion chamber can cause a reduction in insulation resistance between the exposed electrode and the grounded portion, and hence a flow of leakage current. In this case, a desired waveform of the ion current may not be obtained. As shown in Fig. 6(b), the leakage current flows before a real waveform of the ion current is plotted (before point A in Fig. 6). In contrast, according to this example, the leakage current is detected in a predetermined stage (in the timing period of fuel injection in Fig. 6), so that the carbon adhesion to the outer glow plug can be estimated based on the leakage current. If such a carbon adhesion occurs, the glow plug is changed to the heating-element running state and the adhered carbon is burnt off. It is therefore possible to constantly detect a desired ion current waveform (e.g., the waveform shown in Fig. 24A), and hence to perform ignition detection or flame failure detection precisely using the detection result. When the carbon adheres to the outer glow plug, the insulation resistance between the exposed electrode and the grounded side depends on the pressure in the combustion chamber. For this reason, as the pressure rises, the insulation resistance is reduced and the leakage current tends to flow. To avoid such an inconvenient thing, the leakage current is detected when the pressure in the combustion chamber rises. In this case, the presence or absence of the leakage current can be detected securely. The pressure rise in the combustion chamber corresponds to the compression process in the Diesel engine. According to another example the leakage current is detected in response to the timing of fuel injection into the combustion chamber. For example, the timing period of fuel injection corresponds to a period that elapses between the moment the pressure in the combustion chamber of the Diesel engine rises and the moment just before the fuel burns. It is therefore possible to detect the leakage current more securely under such a condition that the carbon adhered. On the other hand, according to yet another example the operation means holds the switching means in the heating-element running state for a period of time according to the leakage current value detected by the leakage current detection means. In other words, the more the carbon adheres to the outer glow plug, the larger the leakage current value will be. It is therefore possible to burn off the adhered carbon securely by setting the hold time of the heating-element running state according to the leakage current value. According to a further example a high-pass filter is provided to the signal output portion of the ion current detector for detecting ion current, and the detection signal is input to the signal processor. Since the high-pass filter is incorporated in the system circuitry, the ion current due to combustion can be separated from the leakage current due to a failure of insulation even when the carbon adheres to the ion current detecting electrode of the glow plug, thereby detecting the ion current securely. If the combustion condition information such as ignition stage is judged from the output waveform of the high-pass filter, the judgment processing becomes easy to perform. A study confirms that the cut-off frequency of the high-pass filter is preferably set from 50 Hz to 5 kHz, more preferably 100 to 500 Hz. In another example, the threshold for use in judging leakage current by the operation means is set to a value near the acceptable maximum value. When the switching means is operated mainly aiming at removal of the adhered carbon, the threshold for use in judging leakage current should be set small. However, the use of the above structure permits separation between the leakage current and the ion current even when some leakage current flows. If the threshold for use in judging leakage current is set large within the acceptable range, the number of times the adhered carbon is burnt off will be reduced. It is therefore possible to detect ion current frequently, and hence to detect the combustion conditions frequently. Another example comprises comparison means for inputting an output signal of the high-pass filter and comparing the input signal with the threshold for use in detecting combustion conditions. Since the output of the high-pass filter is compared with the threshold for use in detecting combustion conditions, the processing for detecting combustion conditions can be easily carried out. According to a further example the conductive heating-element and the ion current detecting electrode are provided inside the insulator. At least the exposed portion contacting the flame is constructed of the above conductive mixed-sinter with adding a sintering auxiliary made of more than one kind of oxide of rare-earth element. The structure of the mixed sinter is composed of a first crystal phase and a grain boundary phase between the first crystal phases. Portion of the grain boundary phase or the entire grain boundary phase is crystallized into a second crystal phase containing the sintering auxiliary. The ion current detecting electrode is thus composed of the first crystal phase as a crystal phase of either the conductive ceramic material or the nonconductive ceramic material or both, and the grain boundary phase between the crystal phases. The portion of the grain boundary phase or the entire grain boundary phase is crystallized into the second crystal phase while the grain boundary phase in the conventional or typical conductive mixed-sinter are made into glass phase in whole. The conductive heating-element and the ion current detecting electrode are provided in the insulator in either way that molded parts of them are produced separately, embedded in the ceramic powder material for the insulator and molded integrally, or that the molded parts of the conductive heating-element and the ion current detecting electrode are inserted between two molded parts of insulator produced in advance. The molded parts of the insulator, the conductive heating-element and the ion current detecting electrode are made up such that main composition of ceramic powders for the molded parts is mixed with paraffin wax and other resin and the mixture is injection-molded. The ion current detecting electrode is produced of a mixed material that contains a sintering auxiliary made of an oxide or oxides of rare-earth element in addition to the conductive ceramic powder and the nonconductive ceramic powder. As discussed above, the mixed sinter made of such a material is composed of the first crystal phase and the grain boundary phase between the first crystal phases, with portion of the grain boundary phase or the entire grain boundary phase crystallized into the second crystal phase. As is similar to the ion current detecting electrode, for the conductive heating-element and the insulator, it is also preferable to use such a material that contains a sintering auxiliary made of an oxide or oxides of rare-earth element in addition to the conductive ceramic powder and the nonconductive ceramic powder. The conductive heating-element and the insulator can thus be made in excellent structure with portion of the grain boundary phase or the entire grain boundary phase crystallized into the second crystal phase. As is similar to the above examples, the glow plug is run hot by the current passing therethrough to promote ignition and combustion of fuel in the combustion chamber, while the ion current detecting electrode form two electrodes with the inner wall of the combustion chamber adjacent to the ion current detecting electrode to detect ion current in the combustion flame. In such structure, precise detection of the ion current and hence the effective use of the information for combustion control is permitted. Further, since the glow plug has the ion current detecting function in addition to the original heating function, it can be manufactured with compact structure and at low cost. Although some carbon may adhere onto the surface of the ion current detecting electrode during burning fuel, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods. At least the exposed portion contacting the flame is constructed of the mixed sinter having the above structure. In other words, the structure of the mixed sinter is composed of the first crystal phase and the grain boundary phase between the first crystal phases, with portion of the grain boundary phase or the entire grain boundary phase crystallized into a second crystal phase containing the sintering auxiliary. For this reason, the melting point of the grain boundary phase and the corrosion resistance can be improved more largely than the conventional structure composed of amorphous glass phases without the second crystal in each grain boundary phase. It is therefore possible to improve the performance of the ion current detecting electrode that could resist thermal shock, oxidation and corrosion, and hence to prevent any damage to the ion current detecting electrode, thereby improving reliability of the accuracy in detecting ion current and reliability of the glow plug. Further, since the glow plug is such that the conductive heating-element, the lead wires and ion current detecting electrode are integrally provided inside the insulator, the structure of the glow plug is simplified. It is therefore possible to detect ion current precisely without carbon adhesion and hence to provide a glow plug exhibiting excellent durability without any damage to the ion current detecting electrode. The content of the sintering auxiliary to the total weight of the conductive ceramic material and the nonconductive ceramic material in the ion current detecting electrode is preferably set in a range from 3 to 25 wt%. If less than 3 wt%, the mixed sinter can not improve its compactness and is difficult to form the second crystal phase in each grain boundary phase. If it exceeds 25 wt%, the grain boundary phase is made into a glass phase without being crystallized. In this case, the melting point of the grain boundary phase is reduced to lower the resistance to thermal shock and corrosion. The second crystal phase of the ion current detecting electrode preferably exists in each grain boundary phase with a degree of crystallization of more than 5%. If less than 5%, the resistance to oxidation and corrosion can not be so improved since the melting point increases due to existence of the second crystal phase. The conductive ceramic material is preferably made of more than one kind of the materials such as metallic carbide, nitride and boride. In this case, the second crystal phase can be formed easily. The ion current detecting electrode provided in the glow plug according to a further example has an exposed portion exposed from the insulator into the flame. The exposed portion also has a ground portion with a surface roughness Rz of 0.1 to 30 µm (an average roughness of 10 points). The surface roughness Rz of the ground portion is an average roughness of 10 points determined under the provision of JIS B 0601, and the value is in a range of between 0.1 µm and 30 µm. If less than 0.1 µm, the ion current can not be detected sufficiently. If more than 30 µm, a crack or cracks may be developed due to thermal shock or the like. The ground portion is controlled within the above range by grinding it with a grindstone or the like. In this case, a desired surface roughness Rz is obtained by regulating the grain size of the abrasive of the grindstone and other grinding conditions. In arranging the conductive heating-element and the ion current detecting electrode in the insulator, molded parts for the conductive heating-element and the ion current detecting electrode are previously produced, embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by an injection molding. The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that two products (green sheets) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of one product with a conductive material in a desired form by a printing technique such as screen printing, pad printing or hot stamp. The other product is so stacked that it will cover the printed portion and then firing is performed. The conductive heating-element, the lead wires and the ion current detecting electrode may be printed on two or more products. The conductive heating-element and the ion current detecting electrode may also be printed on different products and laminated together. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode printed and built therein is thus obtained. The insulator is cut as required and the ground portion is ground in the exposed portion of the ion current detecting electrode in a manner described above. Such a glow plug as the ground portion of a specific surface roughness Rz is provided in the exposed portion of the ion current detecting electrode can thus be obtained. As is similar to the above examples, the glow plug having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. The ground portion is provided in the exposed portion of the ion current detecting electrode. The ground portion has a surface roughness Rz ranging from 0.1 to 30 µm. Since the ground portion has lots of micron size irregularities, electric flux in the electric field between the ion current detecting electrode and the adjacent cylinder head is concentrated to the convexities in the irregularities, and potential gradients become sharp in the neighborhood of the convexities to which the electric flux is concentrated. Such sharp potential gradients attract charged particles of combustion gases into the neighborhood of the convexities. Consequently, the ion current detecting electrode having the ground portion of the specific surface roughness Rz attracts the charged particles in the combustion chamber due to considerable force, thereby further improving the accuracy in detecting ion current. Further, since in the glow plug the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. The area of the exposed portion provided at the tip of the ion current detecting electrode is preferably set in a range from 1x10 -6 to 0.5 cm 2 . Although the ion current detecting electrode can detect ion output as long as the area (S) of the exposed portion of the ion current detecting electrode is larger than 0, if the area of the exposed portion is less than 1x10 -6 cm 2 , the dimensions of the exposed portion will be very small such as 10 µm x 10 µm or smaller when it is formed by a printing technique, resulting in decreased productivity. On the other hand, if larger than 0.5 cm 2 , the area occupied by the ion current detecting electrode will become too large and hence the conductive heating-element will be made small, resulting in decreased productivity. The ion current detecting electrode can be electrically connected to the conductive heating-element. In this case, since the ion current detecting electrode and the conductive heating-element can be integrally molded, the manufacturing process becomes simple. Another example is made to the glow plug in which the conductive heating-element, the lead wires and the ion current detecting electrode are provided inside the insulator. According to this example, at least top portion of the ion current detecting electrode is covered with a nonconductive porous layer. The nonconductive porous layer has a communication hole opening from the surface of ion current detecting electrode into the flame. The porous layer has electrical nonconductivity. The nonconductive porous layer is made up by sintering nonconductive ceramic powder containing a main component such as Si 3 N 4 , Al 2 O 3 or SiO 2 . As the example shows, the conductive heating-element and the ion current detecting electrode can be provided in the insulator in the following manner: The conductive heating-element and the ion current detecting electrode are previously produced while the insulator having grooves for accommodating them are prepared. The conductive heating-element and the ion current detecting electrode are then embedded in the grooves and integrally baked. The conductive heating-element, the ion current detecting electrode and the insulator may be made of ceramic powder. As is similar to the above examples, the glow plug having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. Since the top portion of the ion current detecting electrode is covered with the nonconductive porous layer, the ion current detecting electrode is never exposed to the direct fire of the flame. For this reason, the ion current detecting electrode is not subjected to stress concentration due to thermal shock by the hot flame, and hence any damage such as crack development. Further, since the nonconductive porous layer has the communication hole, ions flow into a space between the ion current detecting electrode and the cylinder head through the communication hole, thereby detecting the ions accurately. It is therefore possible for the structure to detection current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function), it can be manufactured with compact structure and at low cost. Furthermore, since the conductive heating-element is embedded in the insulator, it is never corroded by the combustion flame, so that excellent durability and good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. Although some carbon may adhere onto the surface of the insulator during burning fuel, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods. In the glow plug, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability and easy to manufacture. The thickness of the nonconductive porous layer is preferably between 0.2 mm and 1.5 mm. If less than 0.2 mm, damage such as crack development may be caused due to thermal shock by the flame. If more than 1.5 mm, the thickness will become too large and a crack or cracks may be developed due to stress concentration by the hot flame. The nonconductive porous layer and the insulator are preferably made of the same material. In this case, the junction between both is improved, and the resistance to thermal shock is also improved since both has the same coefficient of linear expansion. The ion current detecting electrode,and the conductive heating-element can be combined. In this case, the conductive heating-element is covered with the nonconductive porous layer at the tip of the glow plug main body. The ion current detecting electrode can be made of a conductive ceramic material containing MoSi 2 , WC, TiN or the like, or refractory metal such as W, Mo or Ti. The top portion of the insulator is preferably formed into a semi-spherical shape. In this case, since the acute angle portion is removed from the tip of the insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance. Further, since thermal stress concentration is prevented, the resistance to thermal shock can also be improved. The communication hole formed in the nonconductive porous layer can have any hole diameter as long as it is penetrated from the surface of the nonconductive porous layer to the surface of the ion current detecting electrode. For example, the communication hole has only to pass current when the tip of the glow plug is immersed in an alcoholic solution containing water at a ratio of 50 to 50 and a voltage of 12 volt or so is applied between the tip and the solution. In a further example, the glow plug is so constructed that the ion current detecting electrode will be electrically connected to the midway of the heating element, and that, when R1 denotes electric resistance of a first heating section of the heating element from a first end of the heating element, corresponding to a positive side in passing a DC current through the heating element, to a center of a first connecting portion, at which the ion current detecting electrode is first connected to the heating element; R2 denotes electric resistance of a second heating section of the heating element from the center of the first connecting portion, where a connection between the heating element and the ion current detecting electrode is first established, to a second end of the heating element corresponding to a negative side in passing a DC current through the heating element; and r denotes electric resistance between the first connecting portion and the opening end of the ion current detecting electrode, it will satisfy the relationship of R2 > r. The first connecting portion is a portion at which the ion current detecting electrode is first connected to the conductive heating-element in a path from the positive end to the negative end of the conductive heating-element. Such definition is made by taking into account both a single ion current detecting electrode and a plurality of ion current detecting electrodes provided for the conductive heating-element. When a plurality of ion current detecting electrodes are provided, the first heating section corresponds a path from the positive end to the closest ion current detecting electrode and the second heating section is a path from the negative end to the adjacent ion current detecting electrode. In other words, the second heating section may be connected to one or plural ion current detecting electrodes. To set the relationship between the electric resistance R2 of the second heating section and the electric resistance of the ion current detecting electrode to R2 > r, both materials, or width, thickness or length of the conduction path can be changed. As an example of material change, the second heating section and the ion current detecting electrode can be constructed at different mixing rates between the conductive ceramic powder and the nonconductive ceramic powder. As a material for the conductive heating-element and the ion current detecting electrode, at least one kind of metallic silicide, carbide, nitride or boride such as MoSi 2 , Mo 5 Si 3 , Mo x Si 3 c y (x=4-5; y=0-1), MoB, WC or TiN is used. As a nonconductive ceramic material, Si 3 N 4 , Al 2 O 3 , BN or the like is used. As a sintering auxiliary, more than one kind of oxide of rare-earth element is added. Hereinbelow, the use of MoSi 2 as a conductive ceramic material, Si 3 N 4 as a nonconductive ceramic material and a mixture of Y 2 O 3 and Al 2 O 3 as a sintering auxiliary is shown. To achieve conductivity, the conductive heating-element and the ion current detecting electrode is constructed by making the grain size of Si 3 N 4 larger than that of MoSi 2 so that conductive particles of MoSi 2 will be linked together around a nonconductive particle of Si 3 N 4 . Specifically, MoSi 2 having a mean diameter of 1 µm and Si 3 N 4 having a mean diameter of 15 µm are used. With the sintering auxiliary, the mean diameter is 1 µm as well. The mixing ratio of MoSi 2 to Si 3 N 4 is properly selected within a range of 10-60 to 90-40 (wt%). If the mixing ratio is set as MoSi 2 : Si 3 N 4 = 20 : 80 in the second heating section of the conductive heating-element and as MoSi 2 : Si 3 N 4 = 40 : 60 in the ion current detecting electrode, the relationship of R2 > r will be achieved. The sintering auxiliary of Y 2 O 3 and Al 2 O 3 is added at a total rate of 10 % per weight. As the sintering auxiliary, more than one kind of oxide of rare-earth element other than Y 2 O 3 , such as Yb 2 O 3 , La 2 O 3 or Nd 2 O, may be used. Although the mixture of the conductive ceramic material and the nonconductive ceramic material is used for the conductor, there may be used only the conductive ceramic material or a mixture of the nonconductive ceramic material and metal powder instead of the conductive ceramic material, or only metal powder or a metal wire is also possible. The insulator is made of a ceramic sinter that is constructed by adding a sintering auxiliary of Y 2 O 3 and Al 2 O 3 to the main composition of conductive ceramic MoSi 2 and nonconductive ceramic Si 3 N 4 . To achieve nonconductivity, the insulator is constructed by making the grain size of Si 3 N 4 equal to or slightly smaller than that of MoSi 2 so that the conductive particles of MoSi 2 are surrounded with the nonconductive particles of Si 3 N 4 and divided into parts. Specifically, MoSi 2 having a mean diameter of 0.9 µm and Si 3 N 4 having a mean diameter of 0.6 µm can be used. It is preferable to select an identical or similar mixing ratio among the conductive heating-element, the ion current detecting electrode and the insulator because such a case makes differences small such as in thermal expansion coefficient. As the sintering auxiliary, more than one kind oxide of rare-earth element other than Y 2 O 3 , such as one combined with, yttrium, lanthanum and neodymium, may be used. From the standpoint of heater characteristics of the glow plug, it is also preferable to set the electric resistance R2 of the second heating section in a range of between 0.1 Ω and 5 Ω and the electric resistance r in a range of between 0.05 Ω and 2.5 Ω. In arranging the conductive heating-element and the ion current detecting electrode in the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced, embedded in the insulator and molded integrally. The lead wires are connected simultaneously with this molding process. Refractory metal or its alloy such as tungsten and molybdenum can be used for the lead wires. Alternatively, the molded body of the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up such that main materials of ceramic powders are premixed with a binder of resin and the like, and the mixture is injection-molded. The molded parts are then baked. The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that a product (green sheet) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of the product with a conductive material by a printing technique such as screen printing, pad printing or hot stamp. The product is then rolled and baked. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode printed and built therein is thus obtained. The firing of the injection-molded body or printed body is performed by a hot press method. For example, the body is pressurized at 400 kg/cm 2 under one atmosphere of Ar gas and baked at a temperature of 1800 °C for 60 min. Next, operation and effects of the above example will be described. The glow plug is energized to generate heat by passing current therethrough so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function), it can be manufactured with compact structure and at low cost. The electric resistance R2 of the second heating section is set larger than the electric resistance r of the ion current detecting electrode. For this reason, when the carbon adhered to the surface of the insulator of the glow plug and caused an electrical short between the ion current detecting electrode and the cylinder head, the carbon between the ion current detecting electrode and the cylinder head can be burnt off securely by applying a DC current across the conductive heating-element. When burning off the carbon, the DC current flows from the positive end to the cylinder head through the first heating section, the ion current detecting electrode and the adhered carbon since the relationship between the electric resistance R2 of the second heating section of the conductive heating-element and the electric resistance r of the ion current detecting electrode exhibits R2 > r. For this reason, the carbon on the surface of the insulator is heated and burnt due to the heat by combination with the air in the combustion chamber. Since the carbon is thus burnt off, the electrical short due to carbon adhesion can be easily eliminated. It is therefore possible to detect ion current accurately for long periods. Further, since the conductive heating-element is embedded in the insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. In the glow plug, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided inside the insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. The electric resistance R2 of the second heating section is preferably set to more than twice the electric resistance r of the ion current detecting electrode as recited in claim 50. In this case, the carbon can be burnt off more securely. The ion current detecting electrode can be constructed of the main composition of a conductive ceramic material made of more than one kind of metallic silicide, carbide, nitride or boride, or a mixture of a conductive ceramic material and a nonconductive ceramic material. In this case, the heat resistance can be improved and the expansion coefficient can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved. The ion current detecting electrode can also be constructed of the main composition of a material made of one kind of refractory metal having a melting point of 1200 °C or higher, or a mixture of a refractory metal material and a nonconductive ceramic material. In the former case, since the metal material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced. In the latter case, the high-temperature resistance and the resistance to oxidation can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that excellent durability can be obtained. Since the conductive heating-element of the glow plug is energized to generate heat up to a temperature from 1000 to 1100 °C, the melting point must be set to 1200 °C by taking into account the heat resistance of the ion current detecting electrode. The exposed portion of the ion current detecting electrode exposed from the insulator preferably has a portion made of more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd. In this case, the resistance to wear and oxidation of the ion current detecting electrode can be improved. According to another example, the ion current detecting electrode of the glow plug is electrically connected to the midway of the conductive heating-element. The tip of the ion current detecting electrode is exposed from the insulator into the flame, with positioning it more than 2 mm away from the tip of the housing supporting the main body for the insulator and the ion current detecting electrode. In arranging the conductive heating-element and the ion current detecting electrode inside the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced, embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by an injection molding. The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that two products (green sheets) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of one product with a conductive material in a desired form by a printing technique such as screen printing, pad printing or hot stamp. The other product is so stacked that it will cover the printed portion and then firing is performed. The conductive heating-element, the lead wires and the ion current detecting electrode may be printed on two or more products and laminated together. The conductive heating-element and the ion current detecting electrode may also be printed on different products and electrically conducted in the laminating process or after baked. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode printed and built therein is thus obtained. As is similar to the above examples, the glow plug having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. The tip of the ion current detecting electrode is located more than 2 mm away from the top portion of the housing. For this reason, even when some carbon is accumulated on the surface of the glow plug main body, the ion current detection can be performed securely. If the distance between the tip position of the ion current detecting electrode and the top portion of the housing is less than 2 mm, the detection ratio of ion output will be reduced gradually as the distance becomes short. In contrast, the present example is to set the distance to 2 mm or longer, so that the ion output can be detected securely. Such a reduction in the detection ratio with less than 2 mm distance seems to be caused as follows. If the distance (L) between the tip position of the ion current detecting electrode and the top portion of the housing is less than 2 mm, the insulation resistance between the ion current detecting electrode and the housing will be reduced largely to be a simulated short state when the carbon has been accumulated on the glow plug main body. It is therefore difficult to detect ion current. In contrast, since the present example is to set the distance (L) to 2 mm or longer, the insulation resistance is not so much reduced that a simulated short will occur even when the carbon has been accumulated on the glow plug main body. Even if the insulation resistance is reduced due to long-period operation, the carbon can be burnt off by a hating action caused when the conductive heating-element is electrically conducted as will be described later. It is therefore possible for the glow plug to detection current securely. In the glow plug, since the conductive heating-element, the lead wires and the ion current detecting electrode 9 are provided integrally, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. When R (Ω) denotes the total electric resistance of the conductive heating-element and B (Ω) denotes the electric resistance from the positive end of the conductive heating-element to the tip of the ion current detecting electrode, it is preferable to satisfy the relationship of B (Ω) ≥ R (Ω)/3. In this case, an optimum current can be passed through a circuit among the conductive heating-element, the ion current detecting electrode and the adhered carbon even when the carbon has been so accumulated that a simulated short will occur. For this reason, the carbon can be burnt off by the conductive heating-action of this circuit. After such a simulated short is relieved, the current flows through the conductive heating-element to further promote the carbon burn-off. If the electric resistance B (Ω) is very large, the resistance of the circuit among the conductive heating-element, the ion current detecting electrode and the adhered carbon becomes large. In this case, almost normal current flows through the entire conductive heating-element and the adhered carbon can be burnt off by the heating action of the conductive heating-element even if the adhered carbon exists. It is therefore possible to easily burn and destroy the carbon accumulated on the glow plug main body with maintaining the original heating function of the glow plug constantly. To set the relationship between R (Ω) and B (Ω) to B (Ω) ≥ R (Ω)/3, materials for the conductive heating-element and the ion current detecting electrode, or width, thickness or length of the conduction path can be changed. As an example of material change, the mixing ratio between raw materials of the conductive ceramic powder and the nonconductive ceramic powder is controllable. The length of the conduction path may also be changed by changing the connect position of the ion current detecting electrode to the conductive heating-element. The invention as claimed in claim 1 is applied to the glow plug constituted of the housing and the main body retained in the housing. The main body includes the insulator; the conductive heating-element provided inside the insulator; the pair of lead wires electrically connected to both ends of the conductive heating-element, drawn out to the outside of the insulator; and the ion current detecting electrode provided inside the insulator for detecting an ionization state in the flame. The tip of the ion current detecting electrode is exposed from the insulator so that it will contact the flame. The glow plug of the present invention features that when K denotes the coefficient of linear expansion of the ion current detecting electrode, H denotes the coefficient of linear expansion of the conductive heating-element and S denotes the coefficient of linear expansion of the insulator, the relationship among them is defined as H ≥ S and H ≥ K. If the coefficient of linear expansion H is smaller than S or K, tensile stress will be set up on the surface of the glow plug main body because such compressive stress as will be described later can not be applied thereon. For this reason, there is high possibility of crack development in the glow plug main body to make it difficult to improve the durability of the glow plug. In arranging the conductive heating-element and the ion current detecting electrode inside the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced. The molded body is then embedded in the powder material for the insulator and molded integrally. Alternatively, the molded body of the conductive heating-element and the ion current detecting electrode may be inserted between two insulator molded parts separately produced. The insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by mixing resin containing main components of the molded body such as ceramic powder and paraffin wax and injection molding the mixture. After that, pressure firing is performed including degreasing, and the baked body is cut to be a ceramic heater with an ion current detecting function. As is similar to the above examples, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. In the present invention, the relationship among the coefficients of linear expansion H, K and S of the conductive heating-element, the ion current detecting electrode and the insulator is defined as H ≥ S and H ≥ K. In other words, the conductive heating-element has a coefficient of linear expansion larger than those of the ion current detecting electrode and the insulator. For this reason, compressive stress is maintained on the surface of the glow plug main body when in use. As mentioned above, the glow plug main body is manufactured by molding the powder material and sintering it at a high temperature of about 1800 °C. Such a sinter is considered not to have any internal stress in a high-temperature state immediately after sintering. However, since the glow plug is usually used in a range of between room temperature and 1000 °C lower than the sintering temperature, the glow plug main body shrinks compared to the state immediately after sintering. At this time, the relationship among the coefficients of linear expansion H, K and S of the conductive heating-element, the ion current detecting electrode and the insulator is as H ≥ S and H ≥ K, i.e. the coefficient of linear expansion H of the conductive heating-element embedded inside is larger than the coefficients K and S of the insulator and the ion current detecting electrode exposed on the surface of the main body, so that compressive stress acts on the surface of the main body constantly. In the present invention, the compressive stress acts on the surface of the glow plug main body constantly when in use. As is well known, such compressive stress can be resistant to damage such as crack development much more than tensile stress. It is therefore possible for the glow plug of the present invention to prevent damage to the main body surface. Further, since the conductive heating-element is embedded in the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. The coefficients of linear expansion K, H and S preferably satisfy the following relationship: O s H-S ≤ 2.0x10 -6 (/°C) and 0 ≤ H-K ≤ 2.0x10 -6 (/°C). The case H-S is less than O is as described above. On the other hand, if H-S exceeds 2.0x10 -6 , the tensile stress of the conductive heating-element will become large to cause a sharp rise of the resistance of the conductive heating-element in long-period operation. The case H-K is less than O is as described above. On the other hand, if H-K exceeds 2.0x10 -6 , such a sharp rise of the resistance of the conductive heating-element will also be caused in long-period operation. The ion current detecting electrode can be constructed of a conductive ceramic material containing the main composition of more than one kind of metallic silicide, carbide, nitride or boride, or a mixture of a conductive ceramic material and a nonconductive ceramic material. In this case, the heat resistance can be improved and the expansion coefficient can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved. The ion current detecting electrode can also be constructed of a refractory metal material containing the main composition of more than one kind of metal having a melting point of 1200 °C or higher, or a mixture of the refractory metal material and a nonconductive ceramic material as recited in claim 59. In the former case, since the metal material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced. In the latter case, the high-temperature resistance and the resistance to oxidation can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that excellent durability can be obtained. Since the conductive heating-element of the glow plug is energized to generate heat up to a temperature from 1000 to 1100 °C, the melting point must be set to 1200 °C by taking into account the heat resistance of the ion current detecting electrode. Another example is applied to the glow plug including the insulator, the conductive heating-element provided inside the insulator, and the ion current detecting electrode provided inside the insulator for detecting an ionization state in the flame. In the glow plug, a conductive layer is provided on the surface of the insulator so as to cover the exposed portion of the ion current detecting electrode exposed from the insulator, with establishing an electrical connection to the ion current detecting electrode. The conductive layer is provided on an area wider than the area of the exposed portion so that the exposed portion of the ion current detecting electrode exposed from the insulator can be covered with the conductive layer. While the conductive layer is electrically connected to the ion current detecting electrode, the conductive layer has conductivity itself. For this reason, the conductive layer can act to effectively extend the area of the exposed portion of the ion current detecting electrode. In arranging the conductive heating-element and the ion current detecting electrode in the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced, such as one shown in Fig. 100, while joining the lead wires thereto. The molded body is then embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up such that the powder materials are mixed with resin containing paraffin wax for its main ingredients and the mixture is injection-molded. Pressure firing is then performed including degreasing, and after that, the baked body is cut to be a cylindrical shape with round tip, thus manufacturing a ceramic heater with an ion current detecting function. The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that a product (green sheet) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of the product with a conductive material in a desired form by a printing technique such as screen printing, pat printing or hot stamp. The product is then rolled and baked. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode built therein is thus obtained. In either technique, the ion current detecting electrode is manufactured to be exposed on the surface of the insulator. To form the conductive layer on the surface of the insulator, for example, the conditions of the insulator such as shape and roughness are first tailored according to the need. The conductive layer is then printed out on the surface of the insulator in a desired form by a printing technique such as pat printing or a cylinder-screen printing. Other techniques such as plasma coating and evaporation coating can also be used for formation of the conductive layer. As is similar to the above examples, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. The conductive layer electrically connected to the ion current detecting electrode is provided on the surface of the insulator. For this reason, the conductive layer acts as the exposed portion of the ion current detecting electrode to extend the area of the exposed portion. It is therefore possible to detect ion current more securely and precisely compared to an ion current detecting electrode with no conductive layer, and hence to further improve the fuel control. In the glow plug, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion while effectively extending the area of the ion current detecting electrode exposed to the flame. It is preferable to make the insulator partially exposed from the conductive layer so that an edged portion or portions will be formed on the conductive layer. In this case, the edged portion displays a tendency to absorb ions (edge effect) compared to the flat portion. For this reason, the ion current detection can be performed in quick response to make it possible, as will be described later, to sharpen the angle of leading edge in the ion current detection stage and to increase the peak value. As will also be described later, the edged portion formed by partially exposing the insulator from the conductive layer includes not only a case where the conductive layer has a pattern such as a net so that the insulator will be exposed from meshes of the net, but also a case where the conductive layer is made into a solid layer so that the edged portion is formed on the boundary between the solid layer and the exposed portion of the insulator. The edged portion is preferably rectangle in cross section. Such a rectangular-edged portion can be formed in steps without smoothing the boundary with the insulator. In this case, the edge effect can be further amplified. The conductive layer can be made in net structure so that the insulator will be exposed from meshes of the net. In this case, many rectangular-edged portions can be formed in the meshes and this make it possible to display the edge effect more securely. The conductive layer can be constructed of metal or conductive ceramic material. As such metal, a mixed material of refractory metal and active metal is preferably used. In this case, the active metal improves adhesion of the conductive layer to the insulator while the refractory metal improves the durability. For such refractory metal, platinum, noble metal such as gold, nickel, steel and chrome are cited, which can be used alone or by mixing them. For such active metal, titanium, zirconium, hafnium and vanadium are cited, which can also be used alone or by mixing them. A preferable combination is of gold and nickel of more than 90 wt% with active vanadium for the remainder. In such a combination, gold and nickel maintain the durability while vanadium improves adhesion to the insulator. As the conductive ceramic material, various kinds of metallic silicide, carbide, nitride or boride can be used. Silicide is preferably selected in view of the oxidation resistance. It is also preferable to mix an oxide-type ceramic material such as aluminum oxide or silicon dioxide so as to improve adhesion to the insulator. The thickness of the conductive layer is preferably between 1 µm and 20 µm. If less than 1 µm, waves or waste matters due to combustion will collide with or crash the conductive layer to wear it heavily, resulting in loss of durability. The thickness is preferably 5 µm or thicker. If more than 20 µm, the thermal expansion coefficient is made largely different from that of the insulator, so that crack development can occur due to thermal changes to cause the conductive layer to come off from the insulator. The thickness is preferably 15 µm or thinner. According to a modification of the glow plug as claimed in claim 1, the insulator includes a first insulating substrate, a covering insulating substrate provided on the front face of the first insulating substrate, and a second insulating substrate stacked on the back face of the first insulating substrate, the heating element is formed by printing between the front face of the first insulating substrate and the covering insulating substrate, the pair of lead wires are formed by printing between the front face of the first insulating substrate and the covering insulating substrate so as to be connected to both ends of the heating element, and the ion current detecting electrode is provided between the first and second insulating substrates. Since the conductive heating-element and the lead wires are formed by printing between the front face of the first insulating substrate and the covering insulating substrate while the ion current detecting electrode is provided between the first and second insulating substrates, the glow plug is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. Since the conductive heating-element is printed and embedded between the first insulating substrate and the covering insulating substrate, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic, thereby improving the durability. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. Although the ion current detecting electrode may be subjected to carbon adhesion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods. The conductive heating-element is formed by printing, e.g., on the front face of the first insulating substrate. Such printing formation is performed according to the following exemplary procedure. The conductive heating-element and the lead wires are formed on the front face of a product (green sheet) with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product is composed of ceramic material for the first insulating substrate as will be described later. The conductive heating-element and the lead wires can also be formed by printing on the covering insulating substrate. The second insulating substrate, the first insulating substrate and the covering insulating substrate are stacked in this order. Junction among them is made as will also be discussed in the following 51st embodiment. In other words, the substrates are all made into products of ceramic material, stacked one on another and joined by firing. Alternatively, the substrates may be joined with adhesive. As discussed above, the conductive heating-element and the lead wires are formed by printing between the first insulating substrate and the covering insulating substrate. For this reason, the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact. Since the conductive heating-element and the lead wires are never exposed into the combustion flame, the durability of the glow plug can also be improved. Further, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally together with the covering insulating substrate, the first insulating substrate and the second insulating substrate, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. Each outer surface of the first insulating substrate and the covering insulating substrate can have a curved surface portion. In this case, the curved surface portion is used to easily cut the laminated body of the first insulating substrate, the second insulating substrate and the covering insulating substrate to be a cylindrical shape in cross section (see Fig. 4). Another example features that the glow plug is formed by stacking the first insulating and the second insulating substrate together with the conductive heating-element, the lead wires connected to both ends of the conductive heating-element and the ion current detecting electrode for detecting an ionization state in the flame being provided therebeteen. In this case, the conductive heating-element, the lead wires and the ion current detecting electrode can be provided in parallel between the first insulating and the second insulating substrate. For this reason, the glow plug can be manufactured easily. The ion current detecting electrode is preferably formed by printing on the front face of the second insulating substrate. In this case, since the ion current detecting electrode is previously formed by printing on the second insulating substrate, the first insulating substrate can be stacked thereon to make the manufacturing process simple. The ion current detecting electrode is preferably made of a conductive wire and provided between the front face of the second insulating substrate and the back face of the first insulating substrate. In this case, since the ion current detecting electrode is previously made into a wire, it can be provided merely by inserting the wire between the first insulating substrate and the second insulating substrate. For this reason, the glow plug can be manufactured easily. For the conductive wire, a metal wire and a sinter of ceramic material may be cited. The tip of the ion current detecting electrode is preferably exposed at the top portion of the second insulating substrate so as to be exposed into the flame. In this case, the responsiveness of ion current detection and the detection accuracy (S/N ratio) can be improved. The ion current detecting electrode can be made of more than one kind of ceramic material such as MoSi 2 , WC and TiN. In this case, the heat resistance can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved. The ion current detecting electrode can be made of refractory metal containing more than one kind of metals W, Mo and Ti. In this case, since the material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced. The exposed portion of the ion current detecting electrode exposed from the second insulating substrate is preferably provided with more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd. In this case, the ion current detecting electrode can have improved resistance to wear and oxidation. The top portion of the rod-like insulator is preferably made into a semi-spherical shape. In this case, since the acute angle portion is removed from the tip of the rod-like insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance. In a modification of the glow plug of claim 1, the insulator is a rod-like insulator, the heating element is formed by printing inside the rod-like insulator, the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator, and the ion current detecting electrode is provided inside the rod-like insulator with electrical insulation from the heating element established. The ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. Furthermore, since the conductive heating-element is printed and embedded inside the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. Although the surface of the ion current detecting electrode may be subjected to carbon adhesion during fuel combustion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current accurately for long periods. The conductive heating-element is formed by printing inside the rod-like insulator. Such printing formation is performed according to the following exemplary procedure. The conductive heating-element and the lead wires are formed on the front face of a product (green sheet) with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product may be composed of ceramic material for the first insulating substrate. The product is then rolled and baked. The rod-like insulator with the conductive heating-element and the lead wires printed and built therein can thus be obtained. On the other hand, the ion current detecting electrode is inserted in and fixed to a hollow portion of the rod-like insulator, formed at the axial center of the product in the rolling process or other stage, through an electrically nonconductive material before firing or after firing. The conductive heating-element and the lead wires are thus formed by printing inside the rod-like insulator. For this reason, the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact. Since the conductive heating-element and the lead wires are never exposed into the combustion flame, the durability of the glow plug can also be improved. Further, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally inside the rod-like insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. In a further modification of the glow plug of claim 1, the insulator is a rod-like insulator constituted of an electrically insulating core shaft with a hollow portion therein and an insulating substrate covering the outer core shaft, the heating element is formed by printing between the core shaft and the insulating substrate inside the rod-like insulator, the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator, and the ion current detecting electrode is inserted in and fixed to the hollow portion of the core shaft with electrical insulation from the heating element established. In this case, since the rod-like insulator is constituted of the core shaft and the insulating substrate, the glow plug can be made easily. The conductive heating-element is preferably formed by printing on the inside surface of the insulating substrate. In this case, since the conductive heating-element and the lead wires can be previously formed by printing on the insulating substrate in the form of sheet, the glow plug can be manufactured easily because the sheet-like insulator has only to be wound around the core shaft. A method of manufacturing a glow plug, comprises the steps of preparing the product of the core shaft having the hollow portion and composed of electrically nonconductive ceramic material, and inserting the ion current detecting electrode into the hollow portion, forming the conductive heating-element and the lead wires on the surface of the product of the insulating substrate composed of electrically nonconductive ceramic material by using a printing technique, placing the product of the core shaft on the printed surface of the insulating substrate and winding the insulating substrate around the outer core shaft, and heating and baking the core shaft and the insulating substrate. In this case, the glow plug can be easily manufactured. In another modification of the glow plug of claim 1, the insulator is a rod-like insulator; the heating element is provided inside the rod-like insulator; the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator; and the ion current detecting electrode is put in a groove with electrical insulation from the heating element established, the groove provided axially on the outer surface of the rod-like insulator. As will be described later, the conductive heating-element and the lead wires may be provided inside the rod-like insulator by printing them on the surface of a product (green sheet) of conductive material for the rod-like insulator with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product is then wound around the core shaft separately produced and the rolled body is baked. Alternatively, a laminating method in which an upper sheet with a groove thereon is stacked on the product with the printed portions such as conductive heating-element formed thereon can be used. The rod-like insulator with the conductive heating-element and the lead wires printed and built therein is thus obtained. On the other hand, the ion current is inserted in and fixed to a hollow portion of the rod-like insulator before firing or after firing, the hollow portion formed axially on the outer surface of the rod-like insulator. The glow plug is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost. Since the conductive heating-element is embedded inside the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic, thereby improving the durability. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. Furthermore, since the ion current detecting electrode has only to be put in the groove on the rod-like insulator, the glow plug can be easily manufactured. Although the surface of the ion current detecting electrode may be subjected to carbon adhesion during fuel combustion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods. Since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally inside the rod-like insulator, the structure is simplified. It is therefore possible to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability. The groove with the ion current detecting electrode therein is preferably filled with a nonconductive coating material so that the ion current detecting electrode can be covered therewith. In this case, the ion current detecting electrode can be easily fixed to the rod-like insulator. As such a nonconductive coating material, an electrically nonconductive ceramic material may be used. The conductive heating-element and the lead wires are preferably formed by printing on the inside surface of the insulator. In this case, since the conductive heating-element and the lead wires can be previously formed by printing on the insulating substrate in the form of sheet, the glow plug can be manufactured easily because the sheet-like insulator has only to be wound around the core shaft. Further, since the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact. The tip of the ion current detecting electrode is preferably exposed at the top portion of the rod-like insulator so as to be exposed into the flame. In this case, the responsiveness of ion current detection and the detection accuracy (S/N ratio) can be improved. The ion current detecting electrode can be made of more than one kind of ceramic material such as MoSi 2 , WC and TiN. In this case, the heat resistance can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved. The ion current detecting electrode can be made of refractory metal containing more than one kind of metals W, Mo and Ti. In this case, since the material can be used in the form of wire or plate, the cost associated with material preparation, machining and assembly can be reduced. The exposed portion of the ion current detecting electrode exposed from the second insulating substrate is preferably provided with more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd. In this case, the ion current detecting electrode can have improved resistance to wear and oxidation. The top portion of the rod-like insulator is preferably made into a semi-spherical shape. In this case, since the acute angle portion is removed from the tip of the rod-like insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance. Further, since thermal stress concentration is prevented, the resistance to thermal shock can also be improved. A further method of manufacturing a glow plug, comprises the steps of forming the conductive heating-element and the lead wires on the surface of the product of the insulating substrate composed of electrically nonconductive ceramic material by using a printing technique, placing the product of the core shaft of electrically nonconductive ceramic material on the printed surface of the insulating substrate and winding the insulating substrate around the outer core shaft while forming a groove axially among both of rolled-directional end surfaces of the insulating substrate and the core shaft, arranging the ion current detecting electrode inside the outer groove, and heating and baking the core shaft and the insulating substrate. In this case, the ion current detecting electrode can be joined more tightly to the substrate since the width of the substrate narrows due to shrinking of the substrate by firing. In this method, the glow plug can be easily manufactured. BRIEF DESCRIPTION OF DRAWINGS The present invention will be more apparent by reference to the following detailed description when considered in connection with the accompanying drawings. Fig. 1 is a diagram showing general structure of a glow plug according to a first embodiment of the present invention; Fig. 2 is an enlarged cross-sectional view of the main parts of the glow plug according to the first embodiment; Fig. 3 is a diagram showing the procedure for manufacturing the glow plug; Fig. 4 is a diagram showing the procedure for manufacturing the glow plug; Fig. 5 is a diagram showing the procedure for manufacturing the glow plug; Fig. 6 is a diagram showing the procedure for manufacturing the glow plug; Fig. 7 is a diagram of general structure of an ion current detecting system, which shows a heating-element running state; Fig. 8 is a diagram of general structure of the ion current detecting system, which shows an ion current detecting state; Fig. 9 is a flowchart of switching processing of a switching circuit; Fig. 10 is a chart showing an example of ion current waveforms; Fig. 11 is a graph showing heating characteristics of the glow plug; Fig. 12 is an enlarged sectional view of a glow plug according to a second embodiment; Fig. 13 is an enlarged sectional view of a glow plug according to a third embodiment; Fig. 14 is an enlarged sectional view of a glow plug according to a fourth embodiment; Fig. 15 is a diagram explaining the procedure for manufacturing the glow plug according to the fourth embodiment; Fig. 16 is a diagram explaining the procedure for manufacturing the glow plug according to the fourth embodiment; Fig. 17 is an enlarged sectional view of a glow plug according to a fifth embodiment; Figs. 18A, 18B, 19A and 19B are enlarged sectional views of glow plugs according to a sixth embodiment; Fig. 20 is an enlarged sectional view of a glow plug according to a seventh embodiment; Fig. 21 is a diagram explaining the procedure for manufacturing the glow plug according to the seventh embodiment; Fig. 22 is a diagram showing general structure of an ion current detecting system according to the seventh embodiment; Fig. 23 is a flowchart of switching processing of a glow relay; Figs. 24A and 24B are graphs showing examples of ion current waveforms; Fig. 25 is a diagram showing general structure of another ion current detecting system according to the seventh embodiment; Fig. 26 is a chart explaining a relationship between resistance and ion current waveform of an ion current detecting electrode; Fig. 27 is a graph showing a relationship between the content of impurities Ca, K and Na and the flexural strength under a high-temperature condition of 1200 °C; Fig. 28 is a graph showing a relationship between the content of a mixture of Na+Ca+K impurities and the flexural strength under the high-temperature condition of 1200 °C; Fig. 29 is a graph showing a relationship of flexural strength of alloys with respect to temperature, each alloy containing less than 0.1 % of impurities, 1 % Ca, 1% K or 1% Na; Figs. 30A, 30B, 30C and 30D are perspective views explaining the procedure for manufacturing the glow plug in other way; Fig. 31 is a perspective view explaining the procedure for manufacturing the glow plug in other way; Figs. 32A, 32B and 32C are perspective views explaining the procedure for manufacturing the glow plug in other way; and Fig. 33 is a perspective view explaining the procedure for manufacturing the glow plug in other way. BEST MODES FOR CARRYING OUT THE INVENTION Hereinbelow, the best modes for carrying out the invention will be described by referring to preferred embodiments. <<First Embodiment>> Referring to the accompanied drawings, description will be made below with respect to the first embodiment which embodies the present invention in a ceramic glow plug (hereinafter, simply called a glow plug) used as part of starting aids of a Diesel engine. The glow plug of the embodiment is provided in a combustion chamber (turbulence chamber) formed in a cylinder head of the Diesel engine, with portion of the glow plug being exposed into the combustion chamber. When the engine starts at low temperature, the glow plug acts to promote ignition and combustion of fuel sprayed from a fuel injection nozzle. In the embodiment, the glow plug also acts to detect active ions existing in the flame front during fuel combustion. Fig. 1 shows general structure of a glow plug 1 according to the embodiment. In the drawing, the glow plug 1 has a cylinder-like metal housing 4. A male screw portion 43 and a hex-head portion 44 are formed on the outer surface of the housing 4 so that the glow plug can be mounted in a cylinder head, as described later. An annular protection tube 46 is deposited in the upper portion of the housing 4. A ceramic heating unit 6 is retained in the housing 4, which is constituted of a U-type conductive heating element 7; a heat resisting insulator 8 having electrical nonconductivity; an ion current detecting electrode 14 formed integrally with the heating element 7; and two tungsten lead wires 9a, 9b embedded in the insulator 8 and connected to both ends of the heating element 7, respectively. Stated more in detail, the majority of the heating element 7 is embedded in the heat resisting insulator 8 and tightly retained. As shown in an enlarged view of Fig. 2, the end surface of the ion current detecting electrode 14 formed at the tip of the heating element 7 is placed on the same plane as the outer surface of the heat resisting insulator 8. In this case, since the heating element 7 and the ion current detecting electrode 14 are integrally formed, both are electrically connected at all times. In such structure, the exposed portion of the heating element 7 and an inner wall of a turbulence chamber 17 (dashed-line portion) of the Diesel engine, described later, form opposed electrodes for detecting ion current. Returning to Fig. 1, conductive chips 10a, 10b are embedded in the heat resisting insulator 8 and connected to upper ends of the tungsten lead wires 9a, 9b, respectively. Each of the conductive chips 10a, 10b is connected to a corresponding one of two lead wires 11a, 11b serving as external signal input lines of the glow plug 1. In addition, the housing 4 and the protection tube 46 are electrically insulated from the lead wires 11a and 11b by an insulating tube 12D and a rubber bush 12E. The lead wires 11a, 11b are fixed by a caulking force of the protection tube 46 as well as the rubber bush 12E. The structure of the ceramic heating unit 6 will be described in detail below. In the heating unit 6, the heating element 7, the ion current detecting electrode 14 and the heat resisting insulator 8 are sintered products made of a mixture of conductive ceramic powder (in the embodiment, molybdenum silicide (MoSi 2 ) powder) and nonconductive ceramic powder (in the embodiment, silicon nitride (Si 3 N 4 ) powder). A different point between them is that the heating element 7 and the ion current detecting electrode 14 have a mean diameter of MoSi 2 powder smaller than that of Si 3 N 4 powder, while the heat resisting insulator 8 has a mean diameter of MoSi 2 powder equal to or larger than that of Si 3 N 4 powder. By changing the grain size of each powder, the heating element 7 and the ion current detecting electrode 14 are produced dividedly from the heat resisting insulator 8. The mixing ratio of MoSi 2 powder to Si 3 N 4 powder in the heating element 7 and the ion current detecting electrode 14 is also made different from that in the heat resisting insulator 8. The actual mean diameter of each powder is as follows: with the heating element 7 and the ion current detecting electrode 14, MoSi 2 powder is between 1 µm and 3 µm and Si 3 N 4 powder is between 10 µm and 20 µm; and with the heat resisting insulator 8, MoSi 2 powder is about 1.1 µm and Si 3 N 4 powder is about 0.7 µm. With the mixing ratio of MoSi 2 powder to Si 3 N 4 powder, MoSi 2 powder is from 60 to 70 wt% and Si 3 N 4 powder is from 40 to 30 wt% in the former (the heating element 7 and the ion current detecting electrode 14); and MoSi 2 powder is from 20 to 30 wt% and Si 3 N 4 powder is from 80 to 70 wt% in the latter (heat resisting insulator 8). In both cases, Y 2 O 3 and Al 2 O 3 are added as auxiliaries at a rate of about 10 wt%. In the ceramic heating unit 6 having the above construction, since the heating element 7 and the ion current detecting electrode 14 are made up such that small particles of MoSi 2 powder (conductive ceramic powder) are linked together around a large particle of Si 3 N 4 powder (nonconductive ceramic powder), current flows through the heating element 7 and the ion current detecting electrode 14, and thereby the heating element 7 runs hot. On the other hand, since the heat resisting insulator 8 is made up such that small particles of Si 3 N 4 powder (nonconductive ceramic powder) are put between large particles of MoSi 2 powder (conductive ceramic powder), both powders are lined up to form an insulation layer having resistance larger than the heating element 7. Referring next to Figs. 3 to 6, a method of manufacturing the ceramic heating unit 6 will be described. At first, a binder is kneaded with the respective mixtures of MoSi 2 powder and Si 3 N 4 powder to form pastes. The pastes are then injection-molded into desired shapes of the heating element 7, the ion current detecting electrode 14 and the heat resisting insulator 8, respectively. In this molding process, plural sets of the heating elements 7 and the ion current detecting electrodes 14 are coupled together through a connecting bar 28 as shown in Fig. 3, with the tungsten lead wires 9a and 9b connected to each set. The ion current detecting electrode 14 is cut part-way off (along the dot-dash line of Fig. 3) to separate each set of the heating element 7 and the ion current detecting electrode 14 from the connecting bar 28. On the other hand, as shown in Fig. 4, two insulation parts 8a, 8b constituting the heat resisting insulator 8 are formed into a pair of semicylinders. Each mating surface of the insulation parts 8a, 8b has a groove portion 29 for accommodating the heating element 7, the lead wires 9a, 9b, and the ion current detecting electrode 14. As shown in Fig. 5, the integrated body of the heating element 7 and the ion current detecting electrode 14 is placed in the groove portion 29 so that they will be surrounded with the insulation parts 8a, 8b, and then hot-pressed at a temperature of between 1700 °C and 1800 °C. After that, the outer portion of the ceramic heating unit 6 is cut along the broken line of Fig. 6 to be a cylinder-like shape with a round tip. At this time, the heating element 7 is completely embedded in the heat resisting insulator 8, while the end surface of the ion current detecting electrode 14 is exposed at the tip of the ceramic heating unit 6. Referring next to Figs. 7 and 8, an ion current detecting system using the above glow plug 1 will be described. Figs. 7 and 8 are diagrams showing general structure of an ion current detecting system according to the embodiment. Fig. 7 shows a state in which the glow plug 1 (heating element 7) is running hot, i.e., where the glow plug 1 is promoting ignition and combustion of fuel at starting of the engine. Fig. 8 shows a state in which the glow plug 1 detects ion current resulting from fuel combustion. In both figures, a screw hole 16 is formed in a cylinder head 45 of the Diesel engine, and the glow plug 1 is screwed tightly in the screw hole 16. In fastening the glow plug 1 to the cylinder head 45, the hex-head portion 44 is held with a given tool and the male screw portion 43 of the plug 1 is screwed into the screw hole 16. The tip of the ceramic heating unit 6 of the glow plug 1 projects into the turbulence chamber 17 formed in the cylinder head 45. The turbulence chamber 17 communicates with a main combustion chamber 19 provided above a piston 18, and forms part of the combustion chamber. In the turbulence chamber 17, a tip of a fuel injection nozzle 20 is so arranged that the fuel will be sprayed from the fuel injection nozzle 20 into the turbulence chamber 17. A switching circuit 25 is provided between a 12-volt DC battery 34 and the glow plug 1, which switches the electric path between the battery 34 and the glow plug 1 according to the operating state of two 2-position switches 25. The switching circuit 25 maintains a heating-element running state (the state shown in Fig. 7) in normal operation in which no command signal is input from an electronic control unit (hereinafter, referred to as ECU) 30. When a command signal is input from the ECU 30, the heating-element running state goes to an ion current detecting state (the state shown in Fig. 8). At this time, two movable pieces of the switches 25 are interlocked. Terminals 23a and 24a of the switches 25 are connected to the lead wires 11a and 11b of the glow plug 1, respectively. Each of the switches 25 has a pair of contacts 23b, 23c or 24b, 24c to be selectively connected to the terminal 23a or 24a. As shown in Fig. 7, the heating-element running state is such that the terminal 23a and the contact 23b are closed and the terminal 24a and the contact 24b are closed. At this time, one lead wire 11a of the glow plug 1 is connected to the positive side of the battery 34 through the terminal 23a and the contact 23b, while the other wire 11b is connected to the negative side of the battery 34 through the terminal 24a and the contact 24b. The heating element 7 is thus maintained in the running state (i.e., current flows along the path indicated by the double-dot-and-dash arrows in Fig. 7). The contact 24b is also connected to a portion of the cylinder head 45. As shown in Fig. 8, the ion current detecting state is such that the terminal 23a and the contact 23c are closed and the terminal 24a and the contact 24c are closed, i.e., the switches 25 are both in the open state. In this case, the battery voltage is applied to the lead wire 11a through an ion current detecting resistor 26 existing in the electric path (the path indicated by the double-dot-and-dash arrows in Fig. 8) provided in parallel with one switch 23. In other words, the battery voltage is applied between the cylinder head 45 and the ion current detecting electrode 14 formed at the tip of the ceramic heating unit 6. As active ions are created in the flame front during combustion, application of the battery voltage accompanies an ion current flowing in the path as indicated by the double-dot-and-dash arrows in Fig. 8. The resistance of the ion current detecting resistor 26 is about 500 kΩ. The ion current flowing through the ion current detecting resistor 26 is detected by a potentiometer 27 as a potential difference between both ends of the resistor 26. The principle of ion current detection will be described in brief. When the fuel is fed to the turbulence chamber 17 through the fuel injection nozzle 20 and a combustion event occurs, a large number of positive and negative ions of ionized gases are created in the flame front due to combustion. Application of the battery voltage between the ion current detecting electrode 14 and the cylinder head 45 facing the electrode 14 causes the ion current detecting electrode 14 to capture the negative ions and the cylinder head 45 to capture the positive ions. As a result, the current path shown in Fig. 8 is formed, and an ion current flowing in the current path is detected as a potential difference between both ends of the ion current detecting resistor 26. The ECU 30 mainly includes a well-known microcomputer and an A/D converter, not shown, the microcomputer including a CPU, a ROM, a RAM, an I/O circuit and such. The ECU 30 receives a detection signal from the potentiometer 27. The ECU 30 also receives a detection signal from a water temperature sensor 36 for sensing temperature of engine cooling water, and a detection signal from an engine speed sensor 32 for sensing engine speed according to the crank angle of the engine. The ECU 30 thus detects a water temperature Tw and an engine speed Ne based on the detection signals from the sensors 36 and 32, respectively. When the Diesel engine starts at low temperature, the ECU 30 instructs the glow plug 1 to run the heating element 7 hot and to promote ignition and combustion of fuel. When warm-up of the Diesel engine is completed, the ECU 30 sends a switching command signal to the switching circuit 25 to change the system circuitry to the ion current detecting state so that the ion current due to combustion can be detected. At the beginning of engine start, the switching circuit 25 keeps the heating element in the running state. Hereinbelow, the switching process of the switching circuit 25 will be described with reference to a flowchart of Fig. 9. Such a switching action as shown in Fig. 9 is taken by an interrupt occurring at given timing. When starting the processing of Fig. 9, the ECU 30 determines at step 110 whether or not warm-up of the engine has been completed and the switching circuit 25 is in the ion current detecting state. Since a negative determination is made at step 110 at the beginning of engine start, the ECU 30 reads the water temperature Tw and the engine speed Ne at the subsequent step 120. The ECU 30 then determines at step 130, whether or not the water temperature Tw is higher than a predetermined warm-up temperature (60 °C in the embodiment), and at step 140, whether or not the engine speed Ne reaches a predetermined engine speed (2000 rpm in the embodiment) or more. If negative determinations are made at both steps, the ECU 30 regards the engine as not warmed up completely so further heating by the glow plug 1 (heating element 7) is necessary, and advances the processing to step 150. If a positive determination is made at either step 130 or step 140, the ECU 30 regards the engine as having been warmed up completely, or the heating by the glow plug 1 (heating element 7) as being unnecessary, and advances the processing to step 160. In the case the processing goes to step 150, the ECU 30 keeps the switching circuit 25 in the heating-element running state (the state of Fig. 7), and it ends the processing. In this state, ignition and combustion of fuel is promoted by the heating action of the glow plug 1. In the case the processing goes to step 160, the ECU 30 changes the switching circuit 25 from the heating-element running state to the ion current detecting state (the state of Fig. 8), and it ends this processing routine. In this state, an ion current produced when burning the fuel is detected by the ion current detecting resistor 26. The case where a positive determination is made at step 140 and the processing goes to step 160 includes a case, for example, where the engine is in a racing state and the engine speed Ne temporarily rises. In such a case, since the engine has not been warmed up completely, even if the switching circuit 25 has been changed once to the ion current detecting state, the ECU 30 will make a negative determination at step 110 in the next processing cycle, and goes to steps 130 and 140 again to execute the determinations. Once the engine speed Ne stops its temporal rise and starts to decrease (i.e., when the engine speed Ne becomes Ne < 2000 rpm), the ECU 30 returns the switching circuit 25 to the heating-element running state (step 150). The ECU 30 makes a positive determination at step 110 when the water temperature Tw becomes equal to or higher than 60 °C and warm-up of the engine is completed. The ECU 30 gives the positive determination every time at step 110 after the engine has been warmed up completely and the switching circuit 25 is changed to the ion current detecting state. The switching circuit 25 is thus maintained in the ion current detecting state (the state of Fig. 8). Fig. 10 is a chart of a current waveform resulting from the observation of ion current with an oscilloscope, the ion current produced when burning the fuel. In the drawing, a portion of the waveform with the voltage suddenly rising immediately after compression TDC (immediately after fuel injection) is of an ion current due to fuel combustion. In this waveform, point A shows a combustion start position, which corresponds to an ignition stage. The waveform has two peaks: the one, first peak B1, created by active ions in the diffused flame front, is observed early in the combustion event; and the other, second peak B2, created by a re-ionization effect due to an increase in internal pressure of the cylinder, is observed in the middle and late stages of the combustion event. The ECU 30 detects an actual ignition stage from the first peak B1 of the ion current waveform to perform feedback control of the ignition stage in such a manner that the actual ignition stage detected is made correspondent to a target ignition stage. The ECU 30 also detects a combustion condition, such as abnormal combustion or flame failure, from the second peak B2 of the ion current waveform to reflect the detection result on the fuel injection control. The result of the ion current detection is thus reflected on the fuel injection control so that the engine operation can be controlled precisely. Next, the effects of the embodiment will be described. (a) The glow plug 1 of the embodiment includes the heat resisting insulator 8; the heating element 7 embedded in the heat resisting insulator 8 and energized through the lead wire pair 9a, 9b (11a, 11b) to generate heat; and the ion current detecting electrode 14, formed integrally with the heating element 7, for detecting the ionization state in the combustion flame. The ion current detecting electrode 14 is constructed such that a portion (end surface) is exposed to the flame in the turbulence chamber 17. When detecting ion current, the ion current detecting electrode 14 and the adjacent inner wall of the turbulence chamber 17 form two electrodes (an electrode couple) for capturing combustion ions (positive and negative ions). It is therefore possible to detect ion current precisely with such very simple structure, and hence to provide the glow plug 1 for use as an inexpensive ion sensor. (b) Since the glow plug 1 is constructed such that the majority of the ion current detecting electrode 14 is embedded in the heat resisting insulator 8 except only a portion exposed to the outside, an amount of carbon adhered to the outer surface of the glow plug 1 cannot establish an electrical connection between the electrode and the housing (grounded side) to cause error detection of the ion current that may occur in the prior art (USP 4,739,731). Especially in the embodiment, since the exposed portion of the ion current detecting electrode 14 is provided at the tip of the glow plug 1, the exposed portion and the turbulence chamber 17 are spaced enough to solve the above problem more properly. The carbon adhered to the outer surface of the glow plug 1 is burnt off by a heating action of the heating element 7 (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug 1 can maintain its performance in detecting ion current for long periods. (c) Since the heating element 7 itself is embedded inside the heat resisting insulator 8, it could resist oxidation to wear the heating element 7. As a result, the heating element 7 cannot change its heating characteristics to maintain high heating performance for long periods. This construction also avoids damaging the heating element 7 under thermal action such as thermal shock in the turbulence chamber 17. Oxidation wearing of the heating element 7 causes a change in resistance to lower its heating performance as shown in Fig. 11 (see the broken line). In contrast, such lowering can be avoided in the embodiment (i.e., a characteristic indicated by the solid line is maintained). (d) In the glow plug 1 of the embodiment, the ion current detecting electrode 14 (and the heating element 7) is produced by molding a conductive ceramic material. It is therefore possible to minimize oxidation wearing of the ion current detecting electrode 14 even when it is exposed to hot combustion gases, and hence to further improve the durability of the performance in detecting ion current by the glow plug 1. (e) The ceramic heating unit 6 of the glow plug 1 (consisting of the heating element 7, the ion current detecting electrode 14 and the heat resisting insulator 8) is formed of a mixture of conductive ceramic powder (MoSi 2 powder) and nonconductive ceramic powder (Si 3 N 4 powder). It is therefore possible to provide a ceramic heating unit 6 having excellent resistance to heat and wear. Such a ceramic heating unit 6 can maintain a proper starting aid function when the engine starts at low temperature. (f) In the manufacturing process of the glow plug 1, the ceramic heating unit 6 (the heating element 7, the ion current detecting electrode 14 and the heat resisting insulator 8) is produced by molding a mixture of conductive ceramic powder and nonconductive ceramic powder, and then the ion current detecting electrode 14 is exposed to the outside by cutting the outer surface of the heat resisting insulator 8. According to this manufacturing process, the glow plug 1 having the ion current detecting function can be manufactured with such an easy way that any complicated step is not required. (g) In the ion current detector of the embodiment, the switching circuit is provided for switching over between the heating-element running state and the ion current detecting state. Application of voltage in the two states is carried out through the common lead wire pair 11a, 11b, and switching of both states is selectively performed by the switching circuit 25. It is therefore possible to simplify the structure of the ion current detector such as wiring of the lead wires 11a, 11b connected to the heating element 7, and other circuit arrangements associated with ion current detection, thereby providing an inexpensive ion current detector. (h) In the embodiment, the ion current detector is applied to a Diesel engine, in which one end of the battery 34 is connected to one lead wire 11a coupled to the heating element 7 with the other end connected to the cylinder head 45. It is therefore possible to simplify the structure of the opposed electrodes (the ion current detecting electrode 14 and the wall portion of the turbulence chamber 17) needed for detecting ion current. (i) Since the ion current detector of the embodiment is mainly designed to detect active ions in the flame front burning in the combustion chamber of the Diesel engine, the ion current can be detected with maintaining high combustion-ion density to increase the detection accuracy. It is therefore possible to detect the combustion condition precisely, and hence to reflect the detection result on fuel injection control. (j) In the embodiment, the battery 34 is provided for applying the supply voltage between the ion current detecting electrode 14 and the cylinder head 45 directly without passing through the switching circuit 25 (contact 23c). It is therefore possible to eliminate adverse effects such as noise caused by the switching action of the switching circuit 25. Resistance at each contact of the switching circuit 25 increases under oxidation, and in such a case, an increase in contact resistance involves a difficulty in detecting ion current that is originally weak. Such a difficulty, however, can be avoided according to the embodiment. (k) In the embodiment, since a normal vehicle battery 34 is used to detect the ion current, another power source does not need to be provided for ion current detection, thereby implementing the ion current detector without complicated arrangements. Referring next to Figs. 12 to 25, the structure of glow plugs according to second to seventh embodiments. In each embodiment, portions common to those in the first embodiment are given identical numbers and detailed description thereof is omitted. The following embodiments will be described mainly with respect to points different from the first embodiment. <<Second Embodiment>> Fig. 12 is sectional view showing the main parts of a glow plug according to the second embodiment. The glow plug 1 of the first embodiment is provided with the ion current detecting electrode 14 at the tip (round portion) of the heat resisting insulator 8, whereas the glow plug of the second embodiment is provided with an ion current detecting electrode 14A around the side of the heat resisting insulator 8. The end surface of the ion current detecting electrode 14A is exposed on the same plane as the side of the heat resisting insulator 8. Even in this case, the objects of the present invention can be achieved. The ion current detecting electrode 14A is molded as a body with the heating element 7, and both members 14A and 7 are electrically connected at all times. Since the heating element 7 itself is protected by the heat resisting insulator 8, there is no danger of impairing its heating characteristics. <<Third Embodiment>> Fig. 13 is a sectional view showing the main parts of a glow plug according to the third embodiment. In the glow plug of Fig. 13, an ion current detecting electrode 14B is electrically connected at the tip of the heat resisting insulator 8 to the heating element 7 through a lead wire 9c. In this case, the composition of the ion current detecting electrode 14B is the same as that of the heating element 7. Even such structure can achieve the objects of the present invention. <<Fourth Embodiment>> Fig. 14 is a sectional view showing the main parts of a glow plug according to the fourth embodiment. As shown in Fig. 14, the glow plug of the embodiment features that the end surface of an ion current detecting electrode 14C has a relatively large area in the top portion of the heat resisting insulator 8. The ion current detecting electrode 14C is formed laterally in a straight line (not shown) as seen from the lower side. Even such structure can achieve the objects of the present invention. In particular, since the area of the ion current detecting electrode 14C exposed to the combustion flame is large, the glow plug of this embodiment can detect ion current more precisely. Description will be made here with respect to featured points of a method of manufacturing the glow plug of Fig. 14 with reference to Figs. 15 and 16. At the beginning of the manufacturing process, the heating element 7 and the ion current detecting electrode 14C is made in the form such as shown in Fig. 15, by injection-molding a mixture of MoSi 2 powder and Si 3 N 4 powder. As shown in Fig. 15, the molding process is such that plural sets of the heating elements 7 and the ion current detecting electrodes 14C are coupled together through a connecting bar 28. The connecting bar 28 is then cut part-way off (along the dot-dash line of Fig. 15) to separate each set of the heating element 7 and the ion current detecting electrode 14C from the connecting bar 28. The set of the heating element 7 and the ion current detecting electrode 14C is surrounded with the heat resisting insulator 8 and hot-pressed at a temperature of between 1700 °C and 1800 °C. The outer portion of the ceramic heating unit 6 is then cut off along the broken line of Fig. 16 to be a cylindrical shape with a round tip. At this time, the heating element 7 is completely embedded in the heat resisting insulator 8, while the end surface of the ion current detecting electrode 14C is exposed laterally in a straight line in the top portion of the ceramic heating unit 6. <<Fifth Embodiment>> Fig. 17 is a sectional view showing the main parts of a glow plug according to the fifth embodiment. Although in the above embodiments the exposed end surface of the ion current detecting electrode is formed on the same plane as the outer surface of the heat resisting insulator 8, an ion current detecting electrode 14D of this embodiment projects from the outer surface of the heat resisting insulator 8. Even such a case can achieve the objects of the present invention as is similar to the above embodiments. Besides, the ion current detection in such structure can be further improved since the exposed area of the ion current detecting electrode 14D is large. The projection of the ion current detecting electrode 14D can be formed into any shape, such as a conic, pyramid-like, cylindrical, J-type or inverted T-type shape. Plural projecting electrode may also be provided. <<Sixth Embodiment>> Figs. 18A, 18B, 19A and 19B are sectional views showing the main parts of a glow plug according to the sixth embodiment. In the above embodiments, the heating element and the ion current detecting electrode are electrically connected either by molding them as a body (except in the third embodiment) or through the common lead wire pair (in the third embodiment). In contrast, this embodiment is such that the heating element and the ion current detecting electrode are separately formed and electrically connected by drawing out individual lead wires from both members (the heating element and the ion current detecting electrode), respectively. Figs. 18A and 18B show a case in which an ion current detecting electrode 14E is provided at the tip (round portion) of the heat resisting insulator 8. In Fig. 18A, lead wires 9a, 9b are drawn out of both ends of the U-type heating element 7, with one lead wire 9b connected to a lead wire 9d from the ion current detecting electrode 14E. The connection is established inside the heat resisting insulator 8. Fig. 18B shows slightly different structure in which wiring of the lead wires 9b, 9d and 9e to the heating element 7 and the ion current detecting electrode 14E are almost the same as those in Fig. 18A, but an arrangement of an external signal input portion is different. Specifically, the lead wire 9e is exposed on the side of the heat resisting insulator 8, and the exposed portion is connected to an external lead wire 9f through an annular conductor 55. The lead wires 9b and 9d are electrically connected through a conductive layer 57 provided on the end surface of the heat resisting insulator 8. On the other hand, Figs. 19A and 19B show a case in which an ring-shaped ion current detecting electrode 14F is provided around the side of the heat resisting insulator 8. In Fig. 19A, lead wires 62, 63 are drawn out of both ends of the U-type heating element 7, with one lead wire 63 connected to a lead wire 64 from the ion current detecting electrode 14F. The connection is established inside the heat resisting insulator 8. In Fig. 19B, wiring of the lead wires 62-64 to the heating element 7 and the ion current detecting electrode 14F are almost the same as those in Fig. 19A, but an arrangement of an external signal input portion is different. Specifically, the lead wire 62 is exposed on the side of the heat resisting insulator 8, and the exposed portion is connected to an external lead wire 66 through an annular conductor 65. The lead wires 63 and 64 are electrically connected through a conductor 67 provided on the end surface of the heat resisting insulator 8. In either of such cases as shown in Figs. 18A, 18B, 19A and 19B, the glow plug can detect ion current precisely with simple structure and maintain the heating performance of the heating element 7 for long periods, as is similar to those of the above embodiments, thus obtaining desired effects of the present invention. <<Seventh Embodiment>> The seventh embodiment as claimed in claims 5, 8 and 9 will be next described. Although in the first to sixth embodiments the heating element and the ion current detecting electrode are electrically connected, they are electrically insulated in this embodiment. This embodiment is to embody the present invention in an ion current detector using such a glow plug. Fig. 20 is a sectional view showing the main parts of a glow plug 1 according to the embodiment. In the drawing, the heating element 7 and an ion current detecting electrode 14G are embedded separately in the heat resisting insulator 8 of the ceramic heating unit 6, with a portion (top end surface) of the ion current detecting electrode 14G exposed at the tip of the heating unit 6. A pair of lead wires 72, 73 are connected to both ends of the heating element 7. One lead wire 72 is drawn out from the side of the heat resisting insulator 8 and electrically connected to the housing 4, and the other lead wire 73 is electrically insulated from the housing 4 and led to the outside of the heat resisting insulator 8. A lead wire 74 connected to the ion current detecting electrode 14G is electrically insulated from the housing 4 and the lead wire 73 of the heating element 7 and led to the outside of the heat resisting insulator 8. Description will be made here in brief with respect to the manufacturing process of the ceramic heating unit 6 with reference to Fig. 21. As is similar to the above embodiments, a binder is kneaded with respective mixtures of MoSi 2 powder and Si 3 N 4 powder to form pastes. The pastes are then injection-molded into desired shapes of the heating element 7, the ion current detecting electrode 14G and the heat resisting insulator 8, respectively. In this molding process, the heat resisting insulator 8 is formed into divided semicylinders between which the heating element 7, the lead wires 72, 73 connected to the heating element 7, the ion current detecting electrode 14G and the lead wire 74 connected thereto are put in place. In other words, they are positioned and accommodated in groove portions 75 formed in the heat resisting insulator 8. After all the members, such as the heating element 7 and the ion current detecting electrode 14G, are assembled and surrounded with the heat resisting insulator 8, the assembly is hot-pressed at a temperature of between 1700 °C and 1800 °C. After that, the outer portion of the ceramic heating unit 6 is cut to obtain a cylinder-like shape with a round tip. At this time, as shown in Fig. 20, the heating element 7 is completely embedded in the heat resisting insulator 8, while the end surface of the ion current detecting electrode 14G is exposed in the top portion of the ceramic heating unit 6. Referring next to Fig. 22, an ion current detecting system using the above glow plug 1 will be described. In the drawing, the housing 4 of the glow plug 1 is screwed in the cylinder head 45, and the tip of the ceramic heating unit 6 of the glow plug 1 is so arranged that it projects into the turbulence chamber 17 of the cylinder head 45. The tip of the fuel injection nozzle 20 is also provided in the turbulence chamber 17 for spraying the fuel into the turbulence chamber 17. One lead wire 72 of the heating element 7 is grounded through the housing 4, while the other lead wire 73 is connected through a glow relay 76 to the positive side of the battery 34 having a rating of 12 volts. Although the glow relay 76 is turned ON or OFF in response to a command signal from the ECU 30, it is maintained in the OFF state in normal operation. When the glow relay 76 is turned ON in response to the command from the ECU 30, the battery 34 supplies the heating element 7 with power to make the heating element 7 run hot. When the relay 76 is turned OFF, the running state of the heating element 7 is stopped. The lead wire 74 coupled to the ion current detecting electrode 14G is always connected to the positive side of the battery 34 through the ion current detecting resistor 26. Therefore, the ion current detection is performed each time the fuel injection nozzle 20 splays the fuel and a combustion event occurs. The resistance of the ion current detecting resistor 26 is about 500 kΩ. The ion current flowing through the ion current detecting resistor 26 is detected by the potentiometer 27 as a potential difference between both ends of the resistor 26. Hereinbelow, the switching process of the glow relay 76 will be described with reference to a flowchart of Fig. 23. Such processing as shown in Fig. 23 is executed by the ECU 30 when the ignition key is turned on to provide power for the engine. When starting the processing of Fig. 23, the ECU 30 determines at step 201 whether or not warm-up of the engine has been completed. Since a negative determination is made at step 201 at the beginning of engine start, the ECU 30 reads the water temperature Tw and the engine speed Ne at the subsequent step 202. The ECU 30 then determines at step 203, whether or not the water temperature Tw is higher than a predetermined warm-up temperature (60 °C in the embodiment), and at step 204, whether or not the engine speed Ne reaches a predetermined engine speed (2000 rpm in the embodiment) or more. If negative determinations are made at both steps, the ECU 30 regards the engine as not warmed up completely so further heating by the glow plug 1 (heating element 7) is necessary, and advances the processing to step 205. If a positive determination is made at either step 203 or step 204, the ECU 30 regards the engine as having been warmed up completely, or heating by the glow plug 1 (heating element 7) as being unnecessary, and advances the processing to step 206. In the case the processing goes to step 205, the ECU 30 keeps the glow relay 76 in the On state, and returns the processing to step 201. In this state, ignition and combustion of fuel is promoted by the heating action of the glow plug 1. In the case the processing goes to step 206, the ECU 30 changes the glow relay 76 from the ON state to the OFF state, and returns the processing to step 201. The case where a positive determination is made at step 204 and the processing goes to step 206 includes a case, for example, where the engine is in a racing state and the engine speed Ne temporarily rises. In such a case, the glow relay 76 is returned to the ON state (heating-element running state) again (step 205) once the engine speed Ne stops its temporal rise. When the water temperature Tw becomes equal to or higher than 60 °C and warm-up of the engine is completed, the ECU 30 turns the glow relay 76 to be OFF state (step 206), makes a positive determination at step 201, and advances the processing to step 207. Then, at step 207, the ECU 30 reads a current value Ip, detected by the ion current detecting resistor 26, at the timing of fuel injection at which the fuel injection nozzle 20 sprays the fuel, and at the subsequent step 208, it determines whether or not the current value Ip is a given threshold Ith or more. The current value Ip corresponds to a leakage current due to the carbon adhered to the outer surface of the ceramic heating unit 6. In the case a negative determination is made at step 208 (Ip < Ith), the ECU 30 returns the processing to step 201. In this case, the ECU 30 regards the carbon as not adhering to the outer surface of the ceramic heating unit 6, or as being allowable, so that the glow relay 76 is maintained in the OFF state. In the case a positive determination is made at step 208 (Ip ≥ Ith), the ECU 30 advances the processing to step 209 and changes the glow relay 76 from the OFF state to the ON state (heating-element running state). In this case, since it is considered that the carbon adhered to the outer surface of the ceramic heating unit 6 exceeds tolerable quantity, the insulation resistance between the ion current detecting electrode 14G and the grounded side (the housing 4 and the cylinder head 45) is reduced by the adhered carbon and a leakage current flows (where Ip ≥ Ith). Therefore, the glow relay 76 is turned ON and the heating element is made hot to burn off the adhered carbon. After that, the ECU 30 maintains the glow relay 76 in the ON state for a predetermined period of time (two seconds in the embodiment) at step 210, and returns it to the OFF state at the subsequent step 211. After returning to step 201 again, the ECU 30 performs optimum ON/OFF control of the glow relay 76 while monitoring the leakage current at step 207. In the embodiment, the glow relay 76 constitutes switching means as claimed. The processing step 207 corresponds to leakage current detecting means as claimed and the processing steps 208-211 correspond to operation means as claimed. Figs. 24A and 24B are charts of current waveforms resulting from the observation of ion current with an oscilloscope, the ion current produced when burning the fuel. Fig. 24A shows a case where no carbon adheres to the outer surface of the ceramic heating unit 6, and Fig. 24B shows a case where an amount of carbon adheres to the outer surface of the ceramic heating unit 6. In Fig. 24A, a portion of the waveform with the voltage suddenly rising immediately after the timing period of fuel injection is of an ion current due to fuel combustion. In this waveform, point A shows a combustion start position, which corresponds to an ignition stage. In the timing period of fuel injection, the current value is kept at about "0". The waveform has two peaks: the one, first peak B11, created by active ions in the diffused flame front, is observed early in the combustion event; and the other, second peak B12, created by a re-ionization effect due to an increase in internal pressure of the cylinder, is observed in the middle and late stages of the combustion event. The ECU 30 detects an actual ignition stage from the first peak B11 of the ion current waveform to perform feedback control of the ignition stage in such a manner that the actual ignition stage detected is made correspondent to a target ignition stage. The ECU 30 also detects a combustion condition, such as abnormal combustion or flame failure, from the second peak B12 of the ion current waveform to reflect the detection result on the fuel injection control. The result of the ion current detection is thus reflected on the fuel injection control so that the engine operation can be controlled precisely. In Fig. 24B, a leakage current exceeding the accepted level (the threshold value Ith) is observed in the timing period of the fuel injection. In this state, the adhered carbon is removed by the heating action of the heating element 7 (steps 209 to 211 of Fig. 23). If the adhered carbon is left as it is, the leakage current value gradually increases and the increased value of the leakage current may involve a difficulty in discriminating the second peak B12 from the first peak B11. Such a difficulty, however, can be avoided according to the embodiment. The ion current detecting system of the embodiment may be constructed as shown in Fig. 25. In Fig. 25, there are provided two DC power sources: one a heating-element power source 77 for energizing the heating element 7 to generate heat, and the other an ion current detecting power source 78 for detecting ion current. In this case, the glow relay 76 turning the glow plug 1 ON or OFF is provided between one lead wire 73 of the heating element 7 and the heating-element power source 77, while the ion current detecting resistor 26 is provided between the lead wire 74 of the ion current detecting electrode 14G and the ion current detecting power source 78. Even such structure can control the heating action of the heating element 7 while detecting ion current at all times. As an example a 12-volt DC power source (typical vehicle battery) is used as the heating-element power source 77 and a 50-volt DC power source is used as the ion current detecting power source 78. As discussed above, according to the seventh embodiment, the ion current can be detected precisely with very simple structure as is similar to the above embodiments, and besides, the following effects can also be obtained. (I) The glow plug 1 of the embodiment is constructed by insulating the heating element 7 from the ion current detecting electrode 14G. Since the heating element 7 and the ion current detecting electrode 14G are energized through individual paths, respectively, the ion current detecting electrode 14G can detect ion current synchronously with the heating action of the heating element 7 (i.e., the combustion condition can be grasped). (II) In the embodiment, a current value Ip indicative of a leakage current is detected in the timing period of fuel injection, and if the current value Ip detected is more than a predetermined threshold Ith, the glow relay 76 will be operated to make the heating element 7 generate heat temporarily (steps 209 to 211 of Fig. 23). In other words, the carbon adhered state of the outer surface of the ceramic heating unit 6 is estimated based on the current value Ip indicative of the leakage current, and if the amount of adhered carbon is regarded as exceeding an accepted value, the adhered carbon will be burnt off by the heating action of the glow plug 1. As a result, a desired waveform of ion current can be obtained at all times, and the detection result can be used for precise processings such as ignition stage detection and flame failure detection. Even in this case, the adhered carbon can be removed effectively without stopping the ion current detection. (III) In the embodiment, the leakage current (current value Ip) is detected in the timing period of fuel injection. The timing period of fuel injection corresponds to a period that elapses between the moment pressure in the combustion chamber of the Diesel engine rises and the moment just before the fuel burns. It is therefore possible to detect a leakage current securely under such a condition that the carbon adhered. The present invention can also be realized in the following embodiments other than the above embodiments. (1) Although in the above embodiments the heating element and the ion current detecting electrode are constructed of a mixture (a mixture of small MoSi 2 powder and large Si 3 N 4 powder) having the same composition (same grain size), they may be constructed of individual mixtures having different composition. By changing the composition between both members, the heating element and the ion current detecting electrode can vary in resistance from each other. For example, the grain size of MoSi 2 powder used as a conductive ceramic powder in the ion current detecting electrode can be made larger than that in the heating element (or grain size of Si 3 N 4 powder in the ion current detecting electrode can be made smaller than that in the heating element) to increase the resistance. Such divided production is carried out depending on the application. In the case where the result of ion current detection is used for flame failure detection, only the present or absence of ion current is required for the determination. In such a case, it is possible to increase the resistance of the ion current detecting electrode to a relatively large value, e.g., 5 MΩ or less (1Ω with the heating element). In the case where the result of ion current detection is used for ignition stage detection, it is desirable to reduce the resistance.of the ion current detecting electrode as small as possible (500 kΩ or less) since the leading edge of ion current must be detected for an instant. As shown in Fig. 26, the leading edge of ion current becomes gentle as the resistance of the ion current detecting electrode increases. (2) In the above embodiments, the heating element and the ion current detecting electrode are produced dividedly from the heat resisting insulator by changing the grain size and the mixing ratio between MoSi 2 powder as a conductive ceramic powder and Si 3 N 4 powder as a nonconductive ceramic powder. Such production may be changed, e.g., only either the grain size or the mixing ratio may be changed between MoSi 2 powder and Si 3 N 4 powder to produce both members dividedly. If the condition MoSi 2 powder < Si 3 N 4 powder is set in the grain size, the resistance becomes small enough to obtain a material for molding the heating element and the ion current detecting electrode as a conductive member. On the contrary, if the condition MoSi 2 powder > Si 3 N 4 powder is set in the grain size, the resistance becomes large enough to obtain a material for molding the heat resisting insulator as an insulation member. With the change of mixing ratio between powders, the resistance becomes small as the mixing ratio of MoSi 2 powder increases, whereas it becomes large as the mixing ratio of Si 3 N 4 powder increases. (3) In the above embodiments, the glow plug 1 may be a two-wire glow plug with two terminals provided at one end. In this case, the conductive lead wires 11a, 11b are electrically connected to the two terminals, respectively. (4) In the first embodiment, the switching circuit 25 having two 2-position switches 25 is provided in the ion current detecting system for switching over between the heating-element running state and the ion current detecting state (see Figs. 7 and 8). Such an arrangement in the ion current detecting system may be changed. In other words, other means can be used instead such as a semiconductor switch (transistor, thyristor or the like) capable of handling high DC current, as long as the means has the ability to switch over between the two states. With the ion current detecting system of the seventh embodiment (Figs. 22 and 25), the glow relay 76 as switching means may also be replaced by other means such as a semiconductor switch. (5) Although in the first embodiment a common DC power source (vehicle battery 34) is used in both the heating-element running state and the ion current detecting state, two DC power source may be used. Stated more specifically, a heating-element power source for energizing the heating element 7 to generate heat and an ion current detecting power source for detecting ion current are prepared. For example, a 12-volt DC power source (vehicle battery) is used as a heating-element power source and a 50-volt DC power source is used as an ion current detecting power source. (6) In the first embodiment, the switching circuit 25 is operated to switch over between the heating-element running state and the ion current detecting state according to the control program (routine in Fig. 9) executed by the ECU 30. Such a programmed operation of the switching circuit 25 may be changed. For example, the system may be maintained in the heating-element running state only for a predetermined period of time (one or two minutes) after starting the engine, and automatically changed from the heating-element running state to the ion current detecting state when the predetermined time has elapsed. Otherwise, the two states may be switched mechanically, i.e., a bimetal and a switch operated upon deformation of the bimetal may be adopted to switch over between the two states. (7) In the case the heating element and the ion current detecting electrode are constructed separately as in the sixth and seventh embodiments, the ion current detecting electrode may be produced from the following materials. (7-1) The ion current detecting electrode is constructed of a refractory metal. Since the heating element generates heat at a temperature of between 1000 °C and 1200 °C, the metal preferably has a melting temperature of 1300 °C or higher. As such a refractory metal, noble metals, such as Ir, Rh, Ru and Os, or its alloy are cited. Since such noble metals never form nitride or silicide from silicon nitride, the ion current detecting electrode can exhibit a high degree of sintering and excellent durability. Use of an alloy of noble metal and base metal. In this case, the thermal expansion coefficient of the ion current detecting electrode can be easily controlled. Use of a metal or its alloy having a melting temperature of 1300 °C or higher (e.g., Ni, Co, W, Mo or Ti). In this case, the ion current detecting electrode can be manufactured at low cost, and besides, it has the ability to easily control the thermal expansion coefficient as is similar to that in the above case. Use of a mixture of a powder consisting of a metal or its alloy having a melting temperature of 1300 °C or higher (e.g., Ni, Co, W, Mo or Ti), and a conductive ceramic material. In this case, it is possible to obtain an ion current detecting electrode exhibiting good junction with an insulator. (7-2) The ion current detecting electrode is constructed of a conductive ceramic material. For such a conductive ceramic material, metallic silicide, boride, carbide and nitride, or a mixture of such compounds are cited. In this case, the ion current detecting electrode can be baked simultaneously with a nonconductive ceramic material to improve the workability. The conductive ceramic material can also be mixed with a nonconductive ceramic material such as Al 2 O 3 , sialon (Si-Al-O-N compounds, e.g., Si 3 N 4 and Al 2 O 3 ) and BN. (7-3) The ion current detecting electrode is constructed of conductive glass. (7-4) The ion current detecting electrode is constructed of a semiconductor material (e.g., SiC+Si 3 N 4 ). Such a material acts as an insulator at room temperature and as an ion current detecting electrode at a high temperature. (7-5) The ion current detecting electrode is constructed of an alloy that contains a small percentage of impurities such as Na, Ca, K and Mg, the percentage being less than a given value (e.g., 0.5 % or less). In this case, the high temperature resistance of the ion current detecting electrode increases to improve resistance to thermal shock. Figs. 27 to 29 are graphs of experimental results, each of which shows the relationship between impurity content and flexural strength. Fig. 27 is a graph showing a relationship between the content (%) of impurities Ca, K and Na and the flexural strength (MPa) under a high-temperature condition of 1200 °C. According to the graph, a sufficient flexural strength (about 700 MPa) can be obtained in the case each impurity content is 0.5 % or less. Fig. 28 is a graph showing a relationship between the content (%) of a mixed impurity of Na+Ca+K and the flexural strength (MPa) under the high-temperature condition of 1200 °C. According to the graph, a sufficient flexural strength (about 700 MPa) can be obtained in the case the mixture content is 0.5 % or less. Fig. 29 is a graph showing the flexural strength (MPa) of alloys with respect to temperature (°C), each alloy containing less than 0.1 % of impurities, 1 % Ca, 1% K or 1% Na. According to the graph, the highest flexural strength (high temperature strength) is found in the alloy with impurity content of 0.1 % or less, and this experimental result indicates that the high temperature strength increases as the impurity content decreases. (8) Although in the above embodiments the ion current detecting electrode is injection-molded, it may be formed by a printing technique (such a printing technique may also be used for formation of the heating element 7). Further, the electrode may be molded as a sinter to be incorporated into the heat resisting insulator. In manufacturing the glow plug 1, the following methods (8-1), (8-2) and (8-3) may be applied as well. (8-1) Figs. 30A to 30D show a manufacturing process in which a heat resisting insulation sheet is rolled into a cylindrical shape to form a ceramic heating unit 6. In this process, raw materials such as a ceramic material and a resin binder are mixed and a thin-plate like sheet 91 is formed (Fig. 30A). Then, a heating element portion 92 and an ion current detecting electrode portion 93 are printed on the sheet 91, as shown in Fig. 30B, by a screen printing technique. The ion current detecting electrode 93 is formed into a shape projecting from the tip of the U-type portion of the heating element 92. Lead wire portions 94a and 94b are also printed thereon. Under such a condition as shown in Fig. 30B, a coating material, composed of a ceramic material and a resin binder, is printed on the face of the sheet 91. Such coating is carried out for the purpose of elimination of difference in step height between the printed portion and the sheet face and improvement in adhesion of the sheet 91 to a solid shaft in the process of rolling the sheet as described later. Terminal portions 95a and 95b are also printed with a conductive paste on the back face of the sheet 91 so that the lead wire portions 94a and 94b will be electrically connected thereto. On the other hand, a column-like solid shaft 96 is formed of the same materials as the sheet 91 (i.e., a mixture of ceramic material and a resin binder). The solid shaft 96 is then wrapped with the sheet 91 as shown in Fig. 30C. The sheet 91 is so wrapped around the solid shaft 96 that the sheet face on which the heating element portion 92 and the ion current detecting electrode portion 93 are printed will be rolled inwardly. In Fig. 30C, a groove portion 99 extending in the axial direction is formed between both of rolled-directional end surfaces of the sheet 91. The groove portion 99 is formed by setting the width of the sheet 91 to be smaller than the outside diameter of the solid shaft 96. Alternatively, the groove portion 99 may be formed such that, after both end surfaces of the sheet 91 have been joined together, one end surface, overlapped with the other end surface, is cut axially to form the groove portion 99 between both end surfaces.After that, the groove portion 99 is filled with an insulating coating 100, made of a ceramic material, as shown in Fig. 30D. The sheet 91 and the solid shaft 96 are degreased by preheating, then, baked by heating to form one unit. The sheet 91 and the solid shaft 96 are adhered and joined together due to firing shrinking, and hence the groove portion 99 is made narrow. The terminals 95a and 95b connected to the lead wire portions 94a and 94b are plated with Cu and Ni. Finally the top portion of such a column body as shown in Fig. 30D is cut into a spherical shape, thus obtaining a ceramic heating unit 6 like that shown in Fig. 2. In this case, the heating element 7 (heating element portion 92) is completely embedded in the heat resisting insulator 8 (sheet 91; solid shaft 96), while the end surface of the ion current detecting electrode 14 (ion current detecting electrode portion 93) is exposed in the top portion of the ceramic heating unit 6. (8-2) Figs. 31 and 32A-32C shows a manufacturing process in which a plurality of heat resisting insulation members are laminated to form a ceramic heating unit 6. In this process, a thin-plate like first layer member 101, and semicylinder-like second and third layer members 102, 103 are first prepared. The first to third layer members 101-103 are products (green sheets) of nonconductive ceramic material which are produced by press-molding a mixture of raw materials such as a ceramic material and a resin binder. With the first layer member 101, a heating element portion 104 and an ion current detection electrode 105 are printed on the face side by a screen printing technique using a conductive paste. The ion current detecting electrode 105 is formed into a shape projecting from the tip of the U-type portion of the heating element 104. Lead wire portions 106a and 106b are also printed thereon with a conductive paste. Then, as shown in Fig. 32A, the second and third layer members 102 and 103 are overlaid on both faces of the first layer member 101, respectively. The overlaid members are degreased by preheating, then, baked by heating to form one unit. After that, the integrated body (Fig. 32A) is cut into a column-like shape, as shown in Fig. 32B, and the ends of the lead wire portions 106a and 106b are plated with Cu and Ni. Finally, the top portion of the column body is cut into a spherical shape, thus obtaining a ceramic heating unit 6 like that shown in Fig. 2. In this case, the heating element 7 (heating element portion 104) is completely embedded in the heat resisting insulator 8 (first to third layer members 101-103), while the end surface of the ion current detecting electrode 14 (ion current detecting electrode portion 105) is exposed in the top portion of the ceramic heating unit 6. (8-3) In Fig. 33, a plurality of thin-plate like first to fifth layer members 111-115, made of a heat resisting insulation material (a mixture of a ceramic material, a resin binder and such), are first prepared. With the third layer member 113 to be placed in the center, a heating element portion 116 and an ion current detecting electrode portion 117 are printed on the surface by a screen printing technique using a conductive paste. The ion current detecting electrode 117 is formed into a shape slightly projecting from the tip of the U-type portion of the heating element 116. Lead wire portions 118a and 118b are also printed thereon with a conductive paste. The first to fifth layer members 111-115 are put on top of each other, degreased by preheating, and then baked by heating to form one unit. After that, the integrated body of the layer members is cut into a column-like shape, and the top portion of the column body is cut into a spherical shape, thus obtaining a ceramic heating unit 6 like that shown in Fig. 2. In this case, the heating element 7 (heating element portion 116) is completely embedded in the heat resisting insulator 8 (first to fifth layer members 111-115), while the H-type end surface of the ion current detecting electrode 14 (ion current detecting electrode portion 117) is exposed in the top portion of the ceramic heating unit 6. According to the embodiment (8-3), the plurality of heat resisting insulation members to be first prepared may be the same type of sheets. It is therefore possible to improve application flexibility of such a heat resisting insulation material to be prepared in advance, compared to the embodiment (8-2).The glow plug 1 having unique structure as aforementioned and exhibiting an excellent ion current detecting action can be manufactured even by the methods (8-1), (8-2) and (8-3). In such methods, complicated work can be avoided.In the process of manufacturing the ceramic heating unit 6, a column-like body is formed and the top portion thereof is cut into a spherical shape. Alternatively, a hexahedron or other cubic shape may be used for formation of a ceramic heating unit 6 to be cut into a column-like shape with a round tip. In the manufacturing methods (8-2) and (8-3), the number of layer members, made of a heat resisting insulation material, can be determined arbitrarily as long as the heating element and the ion current detecting electrode are selectively provided on a certain layer member to be placed substantially in the center. Further, the heating element portion and the ion current detecting electrode portion may be provided separately as long as the ion current detecting electrode has an exposed portion after final cutting (in the case the heating element portion and the ion current detecting electrode are provided separately, individual layer members may be used for formation of them). (9) Although the above embodiments teaches an all ceramic type glow plug, the glow plug may be other types. For example, a coil-like metal wire (e.g., tungsten wire) may be used as a heating element, which is embedded in a heat resisting insulator made of a ceramic material, with a portion of the metal wire electrically connected to an ion current detecting electrode exposed to the combustion flame. Even this case can provide an inexpensive glow plug having an ion current detecting function. The heating performance of the heating element can also be maintained for long periods. (10) Although in the seventh embodiment a leakage current (current value Ip) is detected at the fuel injection timing (step 207 of Fig. 23), such a detection of leakage current may be carried out in other stage. For example, the leakage current may be detected at a predetermined crank angle before TDC. The predetermined crank angle is given as timing at which a pulse of a predetermined number is output. The pulse of a predetermined number is determined by a detection signal from the engine speed sensor 32. When some carbon adheres to the outer surface of the glow plug, the insulation resistance between the exposed electrode and the grounded side depends on the pressure in the combustion chamber. For this reason, the detection of leakage current may be carried out at any time as long as the pressure in the cylinder is kept high prior to fuel ignition, i.e., at any time in the compression stage. Although the detection of leakage current are preferably carried out in the compression stage, it is not limited by such a timing period since the leakage current can be observed in any timing periods when a large amount of carbon adheres to the outer surface of the glow plug. (11) In the seventh embodiment, the glow relay 76 is held in the ON state (heating-element running state) for a predetermined period of time (two seconds) at step 210 of Fig. 23, but the hold time may be variable. For example, the hold time of the ON state may be set according to the current value Ip read at step 207 of Fig. 23, i.e., the hold time may be set longer as the current value Ip (leakage current) becomes large. In this case, it is possible to remove adhered carbon more securely. (12) In the ion current detecting system according to the seventh embodiment (Fig. 22), a constant current/voltage circuit 80 may be provided at a location shown by the broken-line frame. In this case, it is possible to avoid a drop of the voltage applied to the ion current detecting electrode 14G under the condition that the heating element is running hot (under the condition that the glow relay 76 is in the ON state), and hence to stabilize the detection accuracy. Since such an improvement is made by merely adding the constant current/voltage circuit 80, it does not require any complicated circuit arrangement and not involve an increase in cost. (13) In the above embodiments, the glow plug of the present invention is used in the ion current detector for detecting combustion ions in the combustion chamber of the Diesel engine having the turbulence chamber. However, it may be used in a so-called direct-injection engine which has a mechanism for directly injecting the fuel into the combustion chamber. The glow plug of the present invention may also be used in other types of apparatuses. For example, the glow plug of the present invention can be used in an apparatus for burning unburnt fuel in an exhaust pipe of a gasoline engine. In such an apparatus, the glow plug detects combustion ions produced by burning the unburnt fuel, and the ion current detected is used for judgment on the combustion conditions of the unburnt fuel. INDUSTRIAL APPLICABILITY As described above, the glow plug according to the present invention is effective for internal combustion engines, particularly in promoting ignition and combustion of fuel in Diesel engine. Since the glow plug is able to monitor combustion conditions by detecting ion current in the combustion chamber, it is also effective for engine control. Further, according to the ion current detector of the present invention, precise ion current detection can be realized. Furthermore, according to the glow plug manufacturing method of the present invention, a precise glow plug with simple structure can be manufactured.

Claims: A glow plug (1) with a portion exposed, in use, into a combustion chamber (17) for burning fuel comprising: an insulator (8) made of an insulating ceramic; a heating element (7) made of a conductive ceramic and embedded in the insulator (8) and energized through a pair of lead wires (9a, 9b; 9b, 9e; 11a, 11b; 62, 63; 72, 73) to generate heat, a combination of said insulator (8) and said heating element (7) being sintered as a single unit; and a ion current detecting electrode (14, 14A - 14G) embedded in said insulator (8) with a portion of said ion current detecting electrode (14, 14A - 14G) exposed to a flame in the combustion chamber (17) so that a ionization state in the flame can be detected. A glow plug (1) as set forth in claim 1, wherein said heating element (7) and said ion current detecting electrode (14, 14A - 14F) are electrically connected to each other. A glow plug (1) as set forth in claim 2, wherein said heating element (7) and said ion current detecting electrode (14, 14A, 14C, 14D) are integrally formed. A glow plug (1) as set forth in claim 2, wherein lead wires reside between said heating element (7) and said ion current detecting electrode (14B, 14E, 14F). A glow plug (1) as set forth in claim 1, wherein said heating element (7) and said ion current detecting electrode (14G) are insulated from each other. A glow plug (1) as set forth in any one of claims 1 to 5, wherein at least the portion of said ion current detecting electrode (14A - 14G) exposed to the flame is made of a conductive ceramic material. A glow plug (1) as set forth in any one of claims 1 to 6, wherein said heating element (7) and said ion current detecting electrode (14G) are produced dividedly from each other by using mixtures having different components or different particle sizes. A ion current detector using the glow plug (1) of claim 5 for detecting ion current resulting from fuel combustion, comprising: switching means (76) for turning on or off the power supply of said heating element (7); leakage current detecting means (207) for detecting a leakage current from said ion current detecting electrode (14G) in a predetermined stage before fuel combustion; and operating means (208 - 211) for operating said switching means (76) to temporarily energize said heating element (7) when the leakage current (Ip) detected by said leakage current detection means (207) is larger than a predetermined threshold (Ith). A ion current detector as set forth in claim 8, wherein said leakage current detection means (207) is operated to detect the leakage current (Ip)when pressure in the combustion chamber (17) rises.

Inventors: KURANO ATSUSHI, MURAI HIROYUKI, SATO YASUYUKI, SHIBATA MASAMICHI, KURANO, ATSUSHI, MURAI, HIROYUKI, SATO, YASUYUKI, SHIBATA, MASAMICHI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1997
Main classification: F02P17
IPC codes: F02D41, F23N5, F23Q7, F02P19, F02P17