Patent Publication Number: US-7711498-B2

Title: Glow plug degradation determination device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-201030 filed on Aug. 1, 2007. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a glow plug degradation determination device that determines degradation of a glow plug of an internal combustion engine. 
   2. Description of Related Art 
   As a device for detecting a breakage abnormality in a glow plug of an internal combustion engine, there is a known device that senses voltage upstream of a glow plug and voltage downstream of the glow plug and determines a breakage in the glow plug based on a difference between the sensed voltages, e.g., as described in Patent document 1: JP-A-H11-182400. There is also a known device that has a dedicated circuit for detecting a breakage of a glow plug and that detects the breakage of the glow plug by comparing an electric potential in a series circuit including the glow plug with a reference electric potential corresponding to voltage of a power supply, e.g., as described in Patent document 2: JP-A-2002-276524. 
   For example, as shown in  FIG. 15 , a glow plug having a ceramic heater has a tip end, in which a U-shaped ceramic resistive element  100  is placed in a ceramic base  102 . The ceramic resistive element  100  is formed of a conductive ceramic, which is made by bonding tungsten carbide (WC) grains with a grain boundary material (Si—Yb—O—N) as a grain boundary glass layer, for example. The ceramic base  102  is formed of an insulating ceramic, which is made by bonding grains including silicon nitride (Si 3 N 4 ) as a main ingredient with a grain boundary material (Si—Yb—O—N) as a grain boundary glass layer, for example. 
   As the glow plug is used for a longer period, the grain boundary material Yb +  on a positive electrode side is diffused to a negative electrode side by a migration effect due to a heat load in the ceramic resistive element  100 . As a result, fixing strength of the tungsten carbide (WC) is lowered and the positive electrode side of the ceramic resistive element  100  becomes porous, thereby increasing resistance of the ceramic resistive element  100 . 
   As shown in  FIG. 16 , the heater temperature THG of the glow plug decreases if the resistance value R of the glow plug increases. In such the case, preheating of an internal combustion engine cannot be performed sufficiently, causing increase of hydrocarbon (HC) and carbon monoxide (CO) in exhaust gas. In a glow plug having a metal heater, the resistance is lowered by degradation due to a heat load. 
   Conventional devices can detect a breakage of a glow plug but cannot accurately detect degradation of the glow plug, which causes change in resistance of the glow plug. That is, since the detection is performed during an operation of the internal combustion engine, the glow plug is cooled by an intake air, an exhaust gas or a fuel spray. Therefore, it has been difficult to accurately detect the degradation even if the resistance is sensed during the operation of the internal combustion engine. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a glow plug degradation determination device capable of accurately determining degradation of a glow plug. 
   According to an aspect of the present invention, a glow plug degradation determination device has a sensing device and a determining device. The sensing device energizes a glow plug provided for preheating an internal combustion engine and thereby sensing a sensing value corresponding to resistance of the glow plug. The determining device energizes the glow plug immediately after an operation of the internal combustion engine is stopped and determines degradation of the glow plug based on the sensing value sensed with the sensing device. 
   According to another aspect of the present invention, the determining device energizes the glow plug immediately after the operation of the internal combustion engine is stopped and determines the degradation of the glow plug when a preset energization time of the energization elapses. 
   According to another aspect of the present invention, the determining device energizes the glow plug with an energization voltage equal to or lower than a rated voltage of the glow plug. 
   According to another aspect of the present invention, the glow plug degradation determination device further includes a coolant temperature sensing device for sensing coolant temperature of a coolant of the internal combustion engine. The determining device varies the energization voltage for energizing the glow plug in accordance with the coolant temperature sensed with the coolant temperature sensing device. 
   According to another aspect of the present invention, when the determining device determines that the glow plug is degraded, the determining device stores a result of the determination and reports the degradation of the glow plug before a next start of the internal combustion engine. 
   According to another aspect of the present invention, the determining device is prohibited from performing the determination until a preset time elapses after performing the determination even if the internal combustion engine is started and the operation thereof is stopped again. 
   According to another aspect of the present invention, the glow plug degradation determination device further includes a provisionally determining device for provisionally determining the degradation of the glow plug based on the sensing value sensed with the sensing device during the operation of the internal combustion engine. After the provisionally determining device provisionally determines the degradation to be present, the determining device determines the degradation of the glow plug immediately after the operation of the internal combustion engine is stopped. 
   According to another aspect of the present invention, the glow plug degradation determination device further includes a rotation speed sensing device for sensing rotation speed of the internal combustion engine and a provisionally determining device for provisionally determining degradation of the glow plug based on fluctuation in the rotation speed sensed with the rotation speed sensing device during the operation of the internal combustion engine. After the provisionally determining device provisionally determines the degradation to be present, the determining device determines the degradation of the glow plug immediately after the operation of the internal combustion engine is stopped. 
   According to another aspect of the present invention, the provisionally determining device performs the provisional determination when the operation of the internal combustion engine is stable. 
   According to another aspect of the present inventions the provisionally determining device performs the provisional determination within an afterglow time. 
   According to another aspect of the present invention, the determining device determines the degradation of the glow plug immediately after the operation of the internal combustion engine is stopped even when the provisionally determining device cannot perform the provisional determination within the afterglow time. 
   According to another aspect of the present invention, the determining device energizes the glow plug when the internal combustion engine is stopped for idle stop and determines the degradation of the glow plug based on the sensing value sensed with the sensing device. 
   According to yet another aspect of the present invention, after the determining device performs the determination immediately after the internal combustion engine is stopped for the idle stop, the determining device is prohibited from performing the determination even if the operation of the internal combustion engine is stopped again. 
   The glow plug degradation determination device according to one of the above aspects of the present invention determines the degradation of the glow plug immediately after the operation of the internal combustion engine is stopped. Therefore, the degradation can be accurately determined without being affected by cooling due to intake air, exhaust gas or a fuel spray or by lowering of temperature of a coolant. Further, since the determination is started when the internal combustion engine is still warm, the determination can be performed with low power. 
   In the case where the device performs the determination of the degradation when a preset energization time elapses immediately after the operation of the internal combustion engine is stopped as in one of the above aspects of the present invention, the device can perform the determination accurately. Moreover, in the case where the device energizes the glow plug with the energization voltage equal to or lower than the rated voltage of the glow plug as in one of the above aspects of the present invention, the device can perform the determination with low power and inhibit shortening of a lifetime of the glow plug due to the determination. In the case where the device varies the energization voltage according to the coolant temperature of a coolant as in one of the above aspects of the present invention, the device can perform the determination with low power and inhibit the shortening of the lifetime of the glow plug due to the determination. 
   In the case where the device reports the degradation of the glow plug before the next start when the device determines that the glow plug is degraded as in one of the above aspects of the present invention, a driver can be informed of the degradation before the start. In the case where the device prohibits the determination until a preset time elapses after performing the determination even if the operation is stopped next time as in one of the above aspects of the present invention, the device can inhibit consumption of a power supply battery in a vehicle such as a delivery vehicle, which repeats stoppage and operation, due to repetition of energization for the determination. 
   Further, in the case where the device has a provisionally determining device and determines the degradation with the determining device after provisionally determining the degradation to be present as in one of the above aspects of the present invention, it is made unnecessary to perform the energization and the determination with the determining device every time. As a result, the shortening of the lifetime of the glow plug can be inhibited. In the case where the device performs the provisional determination when the internal combustion engine is in a stable operation as in one of the above aspects of the present invention, the device can improve the accuracy of the provisional determination. Further, in the case where the device performs the provisional determination within an afterglow time as in one of the above aspects of the present invention, shortening of the lifetime of the glow plug can be inhibited. In the case where the device performs the determination of the degradation with the determining device without lengthening the afterglow time when the device cannot perform the provisional determination within the afterglow time as in one of the above aspects of the present invention, shortening of the lifetime of the glow plug can be inhibited and deterioration of fuel consumption can be inhibited. 
   In the case where the device performs the determination of the degradation with the determining device when the internal combustion engine is stopped for idle stop as in one of the above aspects of the present invention, the device can perform the determination of the degradation and improve restart performance through the energization to the glow plug. In the case where the device is prohibited from performing the determination of the degradation even if the operation is stopped when the idle stop is repeated as in one of the above aspects of the present invention, shortening of the lifetime of the glow plug can be inhibited. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of an embodiment will be appreciated, as well as methods of operation and the function of the related parts, from a study of the following detailed description, the appended claims, and the drawings, all of which form a part of this application. In the drawings: 
       FIG. 1  is a schematic diagram showing an internal combustion engine for a vehicle applied with a glow plug degradation determination device according to an embodiment of the present invention; 
       FIG. 2  is a block diagram showing an ECU and a controller according to the embodiment; 
       FIG. 3  is a flowchart showing an example of determination processing performed by the ECU according to the embodiment; 
       FIG. 4  is a flowchart showing an example of determination energization processing performed by the ECU according to the embodiment; 
       FIG. 5  is a flowchart showing an example of provisional determination processing performed by the ECU according to the embodiment; 
       FIG. 6  is a flowchart showing an example of provisional determination energization processing performed by the ECU according to the embodiment; 
       FIG. 7  is a flowchart showing another example of determination processing performed by the ECU according to the embodiment; 
       FIG. 8  is a flowchart showing yet another example of determination processing performed by the ECU according to the embodiment; 
       FIG. 9  is a flowchart showing a further example of determination processing performed by the ECU according to the embodiment; 
       FIG. 10  is a time chart showing transitions of glow plug temperature and coolant temperature according to the embodiment; 
       FIG. 11  is a graph showing a relationship between temperature and resistance of a glow plug according to the embodiment; 
       FIG. 12  is a graph showing a relationship between the coolant temperature and an afterglow time, a preheating guard time or a preheating time according to the embodiment; 
       FIG. 13  is a graph showing a relationship between the coolant temperature and the afterglow time or a provisional determination start time according to the embodiment; 
       FIG. 14A  is a graph showing a relationship between the coolant temperature and energization voltage according to the embodiment; 
       FIG. 14B  is a graph showing a relationship between the energization voltage and an energization time according to the embodiment; 
       FIG. 14C  is a graph showing a relationship between the energization time and the glow plug temperature according to the embodiment; 
       FIG. 15  is a cross-sectional view showing a tip of a glow plug of a related art; and 
       FIG. 16  is a graph showing temporal changes in temperature and resistance of the glow plug of the related art. 
   

   DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT 
   Hereafter, an embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic structure diagram showing an internal combustion engine for a vehicle applied with a glow plug degradation determination device according to the embodiment. As shown in  FIG. 1 , the internal combustion engine  1  is a multi-cylinder diesel engine (in the example of  FIG. 1 , a four-cylinder diesel engine). In the engine  1 , a combustion chamber  8  is constructed of a cylinder  2 , a piston  4 , and a cylinder head  6 . 
   An air intake system of the internal combustion engine  1  is provided with an intake airflow passage  16  communicating with the combustion chamber  8  through an intake valve  12 . An exhaust system of the internal combustion engine  1  is provided with an exhaust flow passage  20  communicating with the combustion chamber  8  through an exhaust valve  18 . 
   An injector  21  for injecting fuel into the combustion chamber  8  is provided in the cylinder head  6  so that the injector  21  protrudes into the combustion chamber  8 . The injector  21  is supplied with high-pressure fuel. Further, each combustion chamber  8  is provided with a glow plug  22  for warming an interior of the combustion chamber  8 . 
   When the fuel is injected from the injector  21  into the combustion chamber  8 , the fuel causes self-ignition due to compression of the combustion chamber  8 , and an energy is produced by the combustion of the fuel. The energy is taken out as a rotational energy of a crankshaft  24  of the internal combustion engine  1  through the piston  4 . In order to suppress rise in temperature of the internal combustion engine  1  due to the combustion of the fuel, a coolant is supplied to each cylinder  2  and the like to cool the cylinder  2  and the like. 
   A sensing system of the internal combustion engine  1  includes a rotation speed sensor  26  for sensing rotation speed of the internal combustion engine  1  according to rotation of a camshaft (not shown), which rotates once while the crankshaft  24  rotates twice, and a coolant temperature sensor  28  for sensing temperature THW of the coolant. 
   An electronic control unit  50  (hereafter, referred to as an ECU  50 ) controls output characteristics (output torque, exhaust characteristics) of the internal combustion engine  1  by operating various actuators such as the injectors  21  based on sensing values of various sensors that sense operating states of the internal combustion engine  1  and requests of a driver. Specifically, the ECU  50  controls a temperature state of the glow plug  22  through a controller  60 . Further, the ECU  50  determines degradation of the glow plug  22  through the controller  60 . When the degradation of the glow plug  22  is determined, the ECU  50  indicates the degradation in an indicator  53 . 
     FIG. 2  is a block diagram showing a configuration of the ECU  50  and the controller  60 . The internal combustion engine  1  according to the embodiment is a four-cylinder engine. Therefore, as shown in  FIG. 2 , four glow plugs  22  are provided for the respective cylinders. In order to provide and break continuity between a battery  30  and the glow plugs  22 , the controller  60  is provided with switching elements SW 1 -SW 4  each constructed of a MOS transistor. 
   A control circuit  61  operates conduction control terminals (gates) of the switching elements SW 1 -SW 4  based on a command signal from the ECU  50  to control conduction states of the switching elements SW 1 -SW 4 . 
   The glow plugs  22  of the respective cylinders are connected with resistive elements R 1 -R 4  in parallel. Voltage drop amounts due to the resistive elements R 1 -R 4  are taken into a current detection circuit  62 . The current detection circuit  62  senses amounts of currents passing through the glow plugs  22  of the respective cylinders as sensing values based on the voltage drop amounts. The sensing values are outputted to the ECU  50  and a breakage detection circuit  66 . 
   The ECU  50  indirectly senses the temperature of the glow plug  22  of each cylinder based on the current flowing through the glow plug  22  of the cylinder. Since a resistance value R of the glow plug  22  has temperature dependence, the temperature THG of the glow plug  22  can be indirectly sensed based on the relationship therebetween. That is, the resistance value R of the glow plug  22  increases as the temperature THG of the glow plug  22  increases. More specifically, the resistance value R of the glow plug  22  is calculated from a value of voltage applied to the glow plug  22  and a value of the current flowing through the glow plug  22 , and then the temperature THG of the glow plug  22  is indirectly sensed based on the resistance value R. 
   If the value of the current flowing through the glow plug  22  is sensed as a sensing value when the voltage applied to the glow plug  22  is constant, the sensing value corresponds to the resistance value R. If the value of the voltage applied to the glow plug  22  is sensed as a sensing value when the value of the current flowing through the glow plug  22  is controlled to be constant, the sensing value corresponds to the resistance value R. When both of the voltage value and the current value fluctuate, the voltage value and the current value of the glow plug  22  may be sensed as the sensing values to calculate the resistance value R. 
   The breakage detection circuit  66  detects existence/nonexistence of a breakage abnormality in a closed-loop circuit including the glow plug  22  of each cylinder based on whether the current flows through the glow plug  22 . The detection result of the breakage is outputted as diagnostic information to the ECU  50  through the control circuit  61 . 
   The ECU  50  takes in a signal related to the temperature THG of the glow plug  22 , an output signal of the coolant temperature sensor  28  and a voltage value of the battery  30 . The ECU  50  converts the signals and the value into digital data with an A/D converter  51 . Digital data outputted by the A/D converter  51  is taken into a microcomputer  52 . The microcomputer  52  generates the command signal based on the input data and outputs the command signal to the controller  60  through an output circuit  54 . The diagnostic information outputted by the controller  60  and a state of an ignition switch IG are taken into the microcomputer  52  through an input circuit  56 . 
     FIG. 3  is a flowchart showing an example of determination processing performed by the ECU  50  according to the embodiment. The determination processing is performed at a predetermined time interval by interrupt. As shown in  FIG. 3 , first in S 100  (S denotes Step), it is determined whether the ignition switch IG has been turned off. If the ignition switch IG has not been operated (S 100 : NO) and the operation of the internal combustion engine  1  is continued, the determination processing is ended once. 
   When it is determined that the operation of the internal combustion engine  1  is stopped by operating the ignition switch IG and that the ignition switch IG has been turned off (S 100 : YES), it is determined whether a provisional determination flag is set at 1 in S 110 . The provisional determination flag is set by provisional determination processing described later. When it is determined that the provisional determination flag is not set at 1 (S 110 : NO), the determination processing is ended once. 
   When it is determined that the provisional determination flag is set at 1 (S 110 : YES), determination energization processing described later is performed in S 120 . Then, it is determined whether determination of degradation of each glow plug  22  has been completed in S 130 . When it is determined that the determination has not been completed (S 130 : NO), the determination energization processing (S 120 ) is repeated. When it is determined that the determination has been completed (S 130 : YES), the energization of the glow plugs  22  of all the cylinders is ended in S 140 . After the energization is ended, the determination processing is ended once. 
     FIG. 4  is a flowchart showing an example of the determination energization processing (S 120 ) performed by the ECU  50  according to the embodiment. In the determination energization processing (S 120 ); first in S 200 ; processing of energizing the glow plugs  22  is performed. In the energization, the control circuit  61  brings the switching elements SW 1 -SW 4  into conduction states based on a command signal from the ECU  50  and applies, for example, rated voltage of the glow plugs  22  to the glow plugs  22  to energize the same. As a result, the glow plugs  22  produce heat. 
   After the glow plugs  22  are energized, it is determined in S 210  whether an energization time tj has elapsed after the start of energization. As shown in  FIG. 10 , the internal combustion engine  1  is warmed by the heat produced by energizing the glow plugs  22 . At this time, the internal combustion engine  1  is in an operation stoppage state, and therefore, air supply, gas exhaust, or fuel injection is not performed. Accordingly, heat radiation other than natural heat radiation does not occur, and change in the temperature THW of the coolant is small. As time passes, an equilibrium state in which the change in the temperature is small is established. Immediately after the ignition switch IG is turned off and the operation of the internal combustion engine  1  is stopped, the internal combustion engine  1  is still warm. The glow plugs  22  are also warm, and the coolant temperature THW of the coolant is also high. 
   Therefore, the time until the equilibrium state is established is short. The energization time tj until the equilibrium state is established is obtained by experiment or the like and set beforehand. A relationship between the coolant temperature THW of the coolant and the energization time tj may be stored, and the energization time tj may be set according to the coolant temperature THW sensed with the coolant temperature sensor  28 . Instead of applying the rated voltage to the glow plugs  22 , a voltage lower than the rated voltage may be applied to the glow plugs  22 . 
   When it is determined that the energization time tj has not elapsed (S 210 : NO), the process returns to the determination processing of  FIG. 3 . Then, it is determined in S 130  that the determination has not been completed (S 130 : NO), and the processing of S 200  and the following steps is repeated. 
   When it is determined that the energization time tj has elapsed (S 210 ; YES), it is determined that the temperature THG of the glow plugs  22  has substantially reached the equilibrium state and a determination condition is detected in S 220 . As mentioned above, the resistance value R of the glow plug  22  having a ceramic heater increases as the glow plug  22  is degraded more. If the resistance value R increases, the temperature THG of the glow plug  22  as the result of energization decreases. As shown in  FIG. 11 , a state in which the temperature THG is lower than a threshold value indicates a state in which the glow plug  22  has been degraded. The characteristic of the glow plug  22  changes in a direction shown by an arrow mark D in  FIG. 11  as the degradation of the glow plug progresses. 
   As the degradation determination condition, a value of current flowing through the glow plug  22  is sensed with the current detection circuit  62  as a sensing value when the glow plug  22  is energized by the processing of S 200 . A small sensing value indicates that the resistance value R is high and the glow plug  22  has been degraded. That is, if the current value is sensed as a sensing value when a constant voltage is applied to the glow plug  22 , the sensing value is inversely proportional to the resistance value R. 
   After the sensing, it is determined in S 230  whether the glow plug  22  is degraded (in interval DETERMINATION shown in  FIG. 10 ). It is determined that the resistance value R is high and the glow plug  22  is degraded if the sensing value is smaller than a predetermined threshold value. 
   When it is determined that the glow plug  22  is degraded, a degradation determination flag is set to 1 in S 240  and determination end processing is performed in S 250 . It is determined whether each glow plug  22  is degraded. In the determination end processing, the degradation determination flag is set to 1 for the glow plug  22  of the cylinder determined to be degraded and the number or the like indicating the cylinder having the degraded glow plug  22  is stored. When it is determined that the glow plugs  22  are not degraded, the determination end processing is performed and the process returns to the original determination processing. 0 is beforehand set as an initial value of the degradation determination flag. 
   As described above, in the embodiment, the glow plugs  22  are energized immediately after the operation of the internal combustion engine  1  is stopped, and the degradation of the glow plug  22  is determined based on the current value as a sensing value at that time. Immediately after the operation of the internal combustion engine  1  is stopped, the internal combustion engine  1  and the glow plugs  22  are still warm and the coolant temperature THW of the coolant is also high. The internal combustion engine  1  is in the stopped state and the air supply, the gas exhaust, or the fuel injection is not performed. Therefore, the equilibrium state in which the change in the temperature is small is reached in a short time. 
   Accordingly, the degradation determination of the glow plugs  22  can be performed correctly since the degradation determination is less prone to be influenced by the lowering of the coolant temperature THW. Since the internal combustion engine  1  and the glow plugs  22  are warm, the energization time tj to the equilibrium state may be short. Therefore, a load on the battery  30  can be reduced and determination can be performed with a low power. 
   Next, provisional determination processing for setting the above-mentioned provisional determination flag will be described with reference to  FIG. 5 . In the provisional determination processing, first in S 300 , it is determined whether the ignition switch IG is on. When the ignition switch IG is not operated and remains off (S 300 : NO), the provisional determination processing is immediately terminated. When the ignition switch IG is operated and switched on (S 300 : YES), it is determined in S 310  whether the degradation determination flag is set at 1. 
   The degradation determination flag is set to 1 by the above-mentioned processing of S 240  when the glow plug  22  is degraded. When the degradation determination flag is set at 1 (S 310 : YES), it is indicated in the indicator  53  that the glow plug  22  is degraded together with the number of the corresponding cylinder or the like in S 320 . 
   Since the above-mentioned determination processing is performed after the internal combustion engine  1  is stopped, the result of the determination processing cannot be reported to the driver by the indication on the indicator  53  or the like in the determination processing. In order to cope with this, the result of the determination is stored once by the above-mentioned processing of S 250  and the determination result is reported to the driver before the operation of the internal combustion engine  1  is started. Thus, the driver can be informed of the result of the determination before the driver starts the operation of the internal combustion engine  1 . 
   When the degradation determination flag is not set at 1 (S 310 ; NO) or after the degradation of the glow plug  22  is reported (S 320 ), it is determined whether the above-mentioned determination energization processing after the operation of the internal combustion engine  1  is stopped is in progress in S 330 . When the ignition switch IG is turned on again immediately after the internal combustion engine  1  is stopped, there is a possibility that the above-mentioned determination processing is in the progress. When the determination energization processing is in progress (S 330 : YES), the execution of the determination energization processing is continued in S 332 . 
   When the determination energization processing is not in progress (S 330 ; NO) or while the execution of the determination energization processing is continued, energization times of a preheating time tp and an afterglow time ta are calculated in S 340  based on the coolant temperature THW of the coolant sensed with the coolant temperature sensor  28  in S 340 . 
   As shown in  FIG. 12 , a relationship between the coolant temperature THW and each of the preheating time tp and the afterglow time ta is beforehand stored as a map. As shown in  FIG. 101  the preheating time tp is a time necessary for the glow plugs  22  to preheat the internal combustion engine  1  until starter drive is enabled when the engine  1  is started from a cold state. The afterglow time ta is a time for energizing the glow plugs  22  even after the internal combustion engine  1  is started until the operation is stabilized. 
   After the times tp, ta are calculated, it is determined whether the preheating time tp has elapsed in S 342 . When it is determined that the preheating time tp has not elapsed (S 342 : NO), the process proceeds to S 350 . In S 350 , it is determined whether an operation, in which the internal combustion engine  1  rotates, is in progress. When it is determined that the operation of the engine  1  is not in progress (S 350 : NO), the processing of S 350  and S 342  is repeated until the preheating time tp elapses. When it is determined that the preheating time tp has elapsed (S 342 . YES), it is determined that preparation for the start has been completed and a sign such as lamp indication is switched off in S 344 . 
   Then, it is determined whether a preheating guard time tpg has elapsed in S 346 . As shown in  FIG. 12 , the preheating guard time tpg is also calculated from the map stored beforehand based on the coolant temperature THW of the coolant sensed with the coolant temperature sensor  28 . Energization of the glow plugs  22  is continued until the preheating guard time tpg elapses even if the internal combustion engine  1  is not started. 
   When it is determined that the preheating guard time tpg has not elapsed (S 346 : NO), the process proceeds to S 350 . When the internal combustion engine  1  is started before the preheating guard time tpg elapses (S 350 : YES), provisional determination energization processing described later is performed in S 360 . Then, it is determined whether the afterglow time ta has elapsed in S 370 . When it is determined that the afterglow time ta has not elapsed (S 370 : NO), the processing of S 350  and the following steps is repeated. 
   The energization of the glow plugs  22  of all the cylinders is terminated in S 380  when it is determined that the preheating guard time tpg has elapsed (S 346 : YES) while the internal combustion engine  1  is not operated (S 350 : NO) or when the internal combustion engine  1  is operated (S 350 ; YES) and the afterglow time ta elapses (S 370 : YES). Then, the provisional determination processing is ended once. 
   In the provisional determination processing, when the internal combustion engine  1  is operated after the preheating time tp elapses as shown in  FIG. 10 , the provisional determination energization processing (S 360 ) is performed until the afterglow time ta elapses. 
   Next, the provisional determination energization processing of S 360  will be described with reference to  FIG. 6 . In the provisional determination energization processing, first in S 400 , the provisional determination flag is reset to 0 and then it is determined in S 410  whether the energization processing of the glow plugs  22  is in progress. For example, the energization of the glow plugs  22  has been completed by the processing of S 380  after the afterglow time ta elapses. Therefore, in such the case, the process returns to the original processing without performing the provisional determination energization processing (S 410 : NO). 
   When the energization is in progress (S 410 : YES), it is determined whether a provisional determination start time ts has elapsed in S 420 . As shown in  FIG. 13 , the provisional determination start time ts is calculated from a map stored beforehand based on the coolant temperature THW of the coolant sensed with the coolant temperature sensor  28 . The provisional determination start time ts is a time necessary for the temperature to be stabilized by energization of the glow plugs  22  after the internal combustion engine  1  is operated. The provisional determination start time ts is longer than the preheating time tp and shorter than the afterglow time ta. In  FIG. 13  or  10 , INTERVAL A represents an interval in which the provisional determination is possible. In  FIG. 13 , INTERVAL B represents an interval in which the provisional determination is prohibited. 
   The provisional determination is prohibited until the provisional determination start time ts elapses. When it is determined that the provisional determination start time ts has elapsed (S 420 : YES), it is determined whether the provisional determination has been completed in S 430 . When it is determined that the provisional determination has not been completed (S 430 : NO), it is determined whether the afterglow time ta has elapsed in S 440 . 
   When it is determined that the afterglow time ta has not elapsed (S 440 : NO), it is determined whether the rotation speed NE of the internal combustion engine  1  sensed with the rotation speed sensor  26  has been stabilized in S 450 . When fluctuation in the rotation speed NE is small, the operation of the internal combustion engine  1  is determined to be stable (S 450 : YES) and a condition for the provisional determination is detected in S 460 . The condition for the provisional determination according to the embodiment is the value of the current flowing through the glow plug  22  or the rotation speed NE of the internal combustion engine  1 . The current value is sensed with the current detection circuit  62 , and the rotation speed NE is sensed with the rotation speed sensor  26 . 
   After the sensing, based on the result of the sensing, it is determined whether there is a possibility of the degradation of the glow plug  22  in S 470 . When the current value of the glow plug  22  is equal to or lower than a predetermined threshold value, it is determined that there is a possibility of the degradation of the glow plug  22 . Since the internal combustion engine  1  is in operation, the glow plug  22  is cooled by the air supply, the gas exhaust or the fuel injection. Accordingly, the degradation of the glow plug  22  cannot be detected accurately. Therefore, the processing of S 460  and S 470  is required only to be able to determine whether there is a possibility of the degradation of the glow plug  22 . 
   When the glow plug  22  is degraded, the fluctuation in the rotation speed NE increases. Therefore, when the fluctuation in the rotation speed NE is greater than a preset fluctuation range, it is determined that there is a possibility of the degradation of the glow plug  22 . 
   When it is determined that there is a possibility of the degradation of the glow plug  22  (S 470 : YES), the provisional determination flag is set to 1 in S 480 . It is determined that there is a possibility of the degradation of the glow plug  22  and the provisional determination flag is set to 1 in S 480  also in the case where it is determined in S 450  that the rotation speed NE is not stable (S 450 ; NO), the provisional determination energization processing is repeatedly performed, and the rotation speed NE is not stabilized by the time when it is determined in S 440  that the afterglow time ta elapses (S 440 : YES). When it is determined that there is no possibility of the degradation of the glow plug  22  (S 470 : NO), the provisional determination is ended while the provisional determination flag is maintained in the initial state of 0. 
   It is determined whether there is a possibility of the degradation of the glow plug  22  by performing the provisional determination processing and the provisional determination energization processing. When there is a possibility of the degradation of the glow plug  22 , the provisional determination flag is set to 1. When there is no possibility of the degradation of the glow plug  22 , the provisional determination flag is kept at the initial state of 0. 
   When the above-mentioned determination processing is performed, it is determined in S 110  whether the provisional determination flag is set at 1. When the provisional determination flat is set at 1 to indicate that there is a possibility of the degradation, the processing of S 120  and the following steps is performed. Thus, the glow plugs  22  are energized immediately after the operation of the internal combustion engine  1  is stopped, and degradation of the glow plug  22  is accurately determined from the sensing value. 
   Since the provisional determination processing is performed, it is not necessary to perform the determination processing every time the operation is stopped. Accordingly, the load on the glow plugs  22  can be reduced. The provisional determination processing may be performed when necessary. The determination processing may be performed every time the operation is stopped. 
   In the above-mentioned determination processing and determination energization processing, energization of the glow plugs  22  is performed with the rated voltage for the preset energization time tj. Alternatively, the glow plugs  22  may be energized with an energization voltage V for an energization time tj corresponding to the coolant temperature THW of the coolant. 
     FIG. 7  is a flowchart showing an example of determination processing that varies the energization to the glow plugs  22  in accordance with the coolant temperature THW. The same processing as the processing in the above-mentioned determination processing will be marked with the same step number and the detailed description thereof will not be repeated below. 
   As shown in  FIG. 7 , when the provisional determination flag is set at 1 (S 110 : YES), the energization voltage V and the energization time tj are calculated in accordance with the coolant temperature THW sensed with the coolant temperature sensor  28  in S 112 . 
   As shown in  FIG. 14A , a relationship between the coolant temperature THW and the energization voltage V is stored beforehand as a map. The energization voltage V is calculated based on the sensed coolant temperature THW. As shown in  FIG. 14B , a relationship between the energization voltage V and the energization time tj is stored beforehand as a map. The energization time tj is calculated based on the calculated energization voltage V. When the glow plug  22  is energized with the thus calculated energization voltage V for the thus calculated energization time tj, the temperature THG of the glow plug  22  reaches an equilibrium state when the energization time tj elapses as shown in  FIG. 14C . 
   The determination energization processing (S 120 ) is performed after the energization voltage V and the energization time tj are calculated as mentioned above. Thus, the glow plug  22  is energized with the energization voltage V, which is lower than the rated voltage and corresponds to the coolant temperature THW, for the energization time tj corresponding to the coolant temperature THW. Accordingly, lifetime of the glow plug  22  can be improved and the power consumption can be reduced. 
   In some cases, the internal combustion engine  1  is restarted shortly after the operation of the internal combustion engine  1  is stopped. If the above-mentioned determination processing is performed in each of such the occasions the lifetime of the glow plug  22  and the power consumption can be adversely affected. 
     FIG. 8  is a flowchart showing an example of the determination processing of performing the determination at intervals. As shown in  FIG. 8 , when the provisional determination flag is set at 1 (S 110 : YES), it is determined in S 114  whether at least t minute/minutes have elapsed after the previous execution of the determination processing. 
   When it is determined that at least t minute/minutes have elapsed (S 114 : YES), the determination energization processing is performed in S 120  as mentioned above. When it is determined that t minute/minutes have not elapsed (S 114 : NO), the determination processing is ended once without performing the determination energization processing of S 120 . Thus, the lifetime of the glow plug  22  can be improved and the power consumption can be reduced. 
   An increasing number of vehicles perform idle stop in order to reduce emission from the internal combustion engine  1  and to improve fuel consumption of the same. If the above-mentioned determination processing is performed for each idle stop, the lifetime of the glow plug  22  and the power consumption will be adversely affected. 
     FIG. 9  is a flowchart showing an example of determination processing at idle stop. As shown in  FIG. 9 , it is determined whether the idle stop is in progress in S 102 . When it is determined that the idle stop is not in progress (S 102 : NO), the determination processing is ended once. When it is determined that the idle stop is in progress (S 102 : YES), the energization voltage V, the energization time tj, and the afterglow time ta are calculated according to the coolant temperature THW sensed with the coolant temperature sensor  28  in S 104 . 
   After the calculation, it is determined whether the determination by the determination energization processing is completed in S 106 . When it is determined that the determination is completed (S 106 : YES), the processing of ending the energization of all the cylinders is performed in S 140 . When it is determined that the determination is not completed (S 106 : NO), it is determined whether the provisional determination flag is set at 1 in S 110 . When it is determined that the provisional determination flag is not set at 1 (S 110 : NO), the processing of ending the energization of all the cylinders is performed in S 140 . When it is determined that the provisional determination flag is set at 1 (S 110 ; YES), the above-mentioned determination energization processing is performed in S 120 . 
   Thereafter, it is determined whether the operation of the internal combustion engine  1  is in progress in S 122 . When it is determined that the operation of the internal combustion engine  1  is not in progress (S 122 : NO), the processing of S 106  and the following steps is repeated. When it is determined that the internal combustion engine  1  has been restarted and the operation thereof is in progress (S 122 : YES), it is determined whether the afterglow time ta has elapsed in S 124 . 
   When it is determined that the afterglow time ta has not elapsed (S 124 : NO), the process waits until the time ta elapses. When it is determined that the afterglow time ta has elapsed (S 124 : YES), the processing of ending the energization of all the cylinders is performed in S 140  and the determination processing is ended. 
   As described above, after the determination energization processing and the degradation determination processing of the glow plug  22  are performed once in the idle stop, the determination energization processing is not performed in the repeated idle stop. Accordingly the lifetime of the glow plug  22  can be improved and the power consumption can be reduced. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.