Abstract:
A glow plug energization control apparatus is provided which includes a power supply working to supply electric power to a glow plug mounted in an internal combustion engine, an on-off switch working to produce a control trigger signal when turned on, and a controller including a microcomputer. The microcomputer is responsive to the control trigger signal to control supply of the electric power from the power supply to energize the glow plug. The microcomputer works to monitor an off-on interval from turning off to turning on of the on-off switch and control an amount of energization of the glow plug as a function of the off-on interval, thereby avoiding overheating of the glow plug.

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
   The present application claims the benefit of Japanese Patent Application No. 2005-76880 filed on Mar. 17, 2005, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to a glow plug energization controlling apparatus which may be employed in automotive vehicle to control energization of a glow plug working to assist in starting an internal combustion engine, and more particularly to such an apparatus equipped with a microcomputer designed to control an operation of a glow plug for avoiding overheating thereof. 
   2. Background Art 
   Usually, typical diesel automotive vehicles are equipped with a glow plug working to assist in heating inside a cylinder of a diesel engine up to an ignition temperature of an air-fuel mixture, especially when the outside air temperature is low, and the engine is cold. 
   In recent years, in order to meet requirements to improve the startability of the engine, engine control systems have been employed which are designed to apply a dc voltage to the glow plug to heat the glow plug to a target temperature quickly. Rough control of an on-duration for which the dc voltage is applied to the glow plug will, thus, result in increase in physical load on the glow plug. For instance, too long the on-duration of the glow plug will result in overheating of the glow plug, which may lead to wire breakage in the glow plug. 
   In recent years, for the purpose of increasing the accuracy in controlling the energization of the glow plug, glow plug energization control systems have been in widespread use which are equipped with a microcomputer designed to control power supply to the glow plug as well as other controls for the engine. When a key switch of the automotive vehicle is turned on, the electric power is supplied to the microcomputer. The microcomputer applies an effective voltage of 11V to the glow plug in the form of a PWM signal for a given initial energiation duration and then also applies an effective voltage of 7V to keep the glow plug at a target temperature of, for example, 900° C., for a given period of time (will also be referred to as a temperature-holding time below). 
   The above glow plug energization control systems, however, encounter a drawback in the following situation. 
   When the key switch is turned off by an operator in the course of energization of the glow plug and then turned on again immediately to reenergize the glow plug, the glow plug starts to be heated from a relatively high temperature. When the glow plug continues to be energized over the initial energization duration and the temperature-holding time, it may cause the glow plug to overheat. 
   In order to avoid the above problem, Japanese Patent First Publication No. 2004-108189 teaches a glow plug energization control system designed to keep the microcomputer activated after the key switch is turned off until the temperature of the glow plug decreases to a given lower level to calculate an optimum amount of energization of the glow plug which will not result in the overheating of the glow plug when the key switch is turned on immediately. This system, however, requires an electrical circuit which measures the voltage and current given to the glow plug to determine the optimum amount of energization, thus resulting in complexity and increase in production cost of the system. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide a simple and inexpensive structure of a glow plug energization control apparatus designed to ensure the stability in energizing a glow plug which is installed in, for example, a diesel engine. 
   According to one aspect of the invention, there is provided a glow plug energization controlling apparatus which may be employed in diesel automotive vehicle. The apparatus comprises: (a) a power supply working to supply electric power to a glow plug mounted in an internal combustion engine; (b) an on-off switch working to produce a control trigger signal when turned on; and (c) a controller including a microcomputer. The controller is responsive to the control trigger signal to control supply of the electric power from the power supply to energize the glow plug. The microcomputer works to monitor an off-on interval from turning off to turning on of the on-off switch and control an amount of energization of the glow plug as a function of the off-on interval, thereby avoiding overheating of the glow plug. 
   In the preferred mode of the invention, the microcomputer also monitors an switch on-duration for which the on-off switch is in an on-state and controls the amount of energization of the glow plug as a function of the switch on-duration. 
   The microcomputer also monitors a temperature parameter indicating a temperature of the engine and controls the amount of energization of the glow plug as a function of the temperature parameter. 
   The microcomputer stores therein a first map representing an initial energization duration, for which the glow plug is to be energized until a target temperature is reached, defined as a function of the temperature of the engine. The microcomputer determines the initial energization duration by look-up using the first map based on the temperature parameter and energizes the glow plug for the initial energization duration. 
   The microcomputer stores therein a second map representing a temperature-holding time, for which the target temperature is to be held constant after expiry of the initial energization duration, defined as a function the temperature of the engine. The microcomputer determines the temperature-holding time by look-up using the second map based on the temperature parameter and energizes the glow plug for the temperature-holding time after expiry of the initial energization duration. 
   If a temperature of the glow plug from which the glow plug is permitted to be energized for the initial energization duration without overheating is defined as an unoverheating temperature, the microcomputer stores therein a cooling time that is a time required by the glow plug to cool from the target temperature to the unoverheating temperature. When the on-off switch is turned off after the glow plug reaches the target temperature, the microcomputer is in a reset mode for the cooling time unless the on-off switch is turned on. 
   When the on-off switch is turned off and then on after the glow plug reaches the target temperature, the microcomputer calculates a reenergization duration tpr 2  for which the glow plug is to be energized according to an equation below
 
 tpr   2   =tp×t off/ ts   2 
 
where tp is the initial energization duration, toff is the off-on interval, and ts 2  is the cooling time, and wherein the microcomputer energizes the glow plug for the reenergization duration.
 
   When the on-off switch is turned off before the glow plug reaches the target temperature, the microcomputer calculates a second cooling time ts 1  according to an equation below
 
 ts   1   =ts   2  × t on/ tp  
 
where ts 2  is the cooling time required by the glow plug to cool from the target temperature to the unoverheating temperature, ton is a period of time for which the on-off switch is in the on-state during the initial energization duration, and tp is the initial energization duration, and wherein the microcomputer is in the reset mode for the second cooling time ts 1  unless the on-off switch is turned on.
 
   When the on-off switch is turned off and then on before the glow plug reaches the target temperature, the microcomputer calculates a reenergization duration tpr 1  for which the glow plug is to be energized according to an equation below
 
 tpr   1   =tp×t off/ ts   1 
 
where tp is the initial energization duration, toff is the off-on interval from turning off to turning on of the on-off switch, and ts 1  is the second cooling time, and wherein the microcomputer energizes the glow plug for the reenergization duration tpr 1 .
 
   According to the second aspect of the invention, there is provided a glow plug energization controlling method of controlling energization of a glow plug mounted in an internal combustion engine which comprises: (a) a first step of sampling a parameter indicating a temperature of the engine upon turning on of an on-off switch designed to produce a control trigger signal when turned on, finding an initial energization during for which the glow plug is to be energized until a target temperature is reached by look-up using a first map representing the initial energization duration defined as a function of the temperature of the engine, energizing the glow plug for the initial energization duration, and keeping the glow plug at the target temperature for a given temperature-holding time after expiry of the initial energization duration; (b) a second step of finding a cooling time ts 2  required by the glow plug to cool from the target temperature to an unoverheating temperature from which the glow plug is permitted to be energized for the initial energization duration without overheating, blocking supply of power to the glow plug when the on-off switch is turned off after expiry of the initial energization duration during the first step, and entering a rest mode to place the glow plug in an off-position until expiry of the cooling time ts 2  unless the on-off switch is turned on; and (c) a third step of, when the on-off switch is turned on again during the second step, calculating a reenergization duration tpr 2  for which the glow plug is to be energized as a function of an off-on interval from turning off to turning on of the on-off switch, energizing the glow plug for the reenergization duration tpr 2 , and keeping a temperature of the glow plug constant for the temperature-holding time after expiry of the reenergization duration tpr 2 . 
   In the preferred mode of the invention, the reenergization duration tpr 2  is given by an equation below
 
 tpr   2   =tp×t off/ ts   2 
 
where tp is the initial energization duration, toff is the off-on interval, and ts 2  is the cooling time.
 
   The method may further comprise a fourth step of, when the on-off switch is turned off during the first step before expiry of the initial energization duration, sampling the parameter indicating the temperature of the engine, calculating a second cooling time ts 1  required by the glow plug to cool to the unoverheating temperature, and entering a rest mode unless the on-off switch is turned off and a fifth step of, when the on-off switch is turned on again during the fourth step, calculating a reenergization duration tpr 1  for which the glow plug is to be energized as a function of an on-duration for which the on-off switch is in an on-state during the initial energization duration, energizing the glow plug for the reenergization duration tpr 1 , and holding the temperature of the glow plug for the temperature-holding time after expiry of the reenergization duration tpr 1 . 
   The second cooling time ts 1  is given by an equation below
 
 ts   1   =ts   2 × t on/ tp  
 
where ts 2  is the cooling time required by the glow plug to cool from the target temperature to the unoverheating temperature, ton is the on-duration for which the on-off switch is in the on-state during the initial energization duration, and tp is the initial energization duration.
 
   The reenergization duration tpr 1  is given by an equation below
 
 tpr   1   =tp×t off/ ts   1 
 
where tp is the initial energization duration, toff is a time interval from turning off to turning on of the on-off switch, and ts 1  is the second cooling time.
 
   The method may also include a fourth step of finding the cooling time ts 2  required by the glow plug to cool from the target temperature to the unoverheating temperature and, when the on-off switch is kept on after expiry of the temperature-holding time, deenergizing the glow plug until the cooling time ts 2  expires unless the on-off switch is turned off, a fifth step of, when the on-off switch is turned off during the fourth step, entering the rest mode during the cooling time ts 2  following the fourth step unless the on-off switch is turned on again, and a sixth step of, when the on-off switch is turned on again during the fifth step, calculating the reenergization duration tpr 2  for which the glow plug is to be energized as a function of the off-on interval from turning off to turning on of the on-off switch, and energizing the glow plug for the reenergization duration tpr 2 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a block diagram which shows a glow plug energization controller according to the present invention; 
       FIG. 2  is a block diagram which shows a modification of the glow plug energization controller of  FIG. 1 ; 
       FIG. 3  is a flowchart of a program to be executed by the glow plug energization controller of  FIG. 1  to control energization of a glow plug in a first control phase; 
       FIGS. 4(   a ) and  4 ( b ) show a flowchart of a second control phase to control reenegization of a glow plug after the glow plug reaches a target temperature; 
       FIGS. 5(   a ) and  5 ( b ) show a flowchart of a third control phase to control reenegization of a glow plug before the glow plug reaches a target temperature; 
       FIG. 6  shows a flowchart of a fourth control phase to control the state of a glow plug when a key switch is left on for power saving; 
       FIGS. 7(   a ) and  7 ( b ) are timecharts which show examples of operations of the glow plug energization controller of  FIG. 1 ; 
       FIGS. 8(   a ) and  8 ( b ) are timecharts which show examples of operations of the glow plug energization controller of  FIG. 1 ; 
       FIG. 9(   a ) is a graph showing an experimentally obtained relation between an on-duration of a glow plug and the temperature of the glow plug in terms of the temperature of coolant of an engine; 
       FIG. 9(   b ) is a graph plotting the on-duration of the glow plug, as illustrated in  FIG. 9(   a ), which changes as a function of the temperature of coolant of the engine; and 
       FIG. 10  is a graph which shows an experimentally obtained relation between a drop in temperature of a glow plug and the time (sec.) required for such a temperature drop. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a glow plug controller according to the invention. 
   The glow plug controller is mainly constructed by an engine electronic control unit (ECU)  4  equipped with a microcomputer usually installed in an automotive vehicle. The glow plug controller includes a glow plug on-off switching unit  5  equipped with a switch leading to a glow plug  6  installed in a diesel engine  2 . The engine ECU  4  is connected to a storage battery  3  and a key switch  1  implemented by an on-off switch such as a typical automotive ignition switch. The glow plug on-off switching unit  5  is so controlled by the ECU  4  that it is opened and closed cyclically for a short period of time and preferably implemented by an electronic switch such as a transistor, a power MOSFET, or a thyristor or a switching circuit including them. 
   The engine ECU  4  monitors the voltage, as developed by the battery  3 , the temperature of the engine  2 , and an on-off signal outputted by the key switch  1  and works to control an on-off operation of the glow plug on-off switching unit  5  at given times. When the glow plug on-off switching unit  5  is closed, it will cause the voltage to be applied from the battery  3  to the glow plug  6 . 
   The control of energization of the glow plug  6  is achieved by the engine ECU  4  generally installed in the automotive vehicle, thus eliminating the need for an additional microcomputer. In general, to typical engine ECUs, the battery voltage, the engine temperature, and the on-off state of the ignition switch are inputted to control the operation of the glow plug, thus permitting the glow plug controller of this embodiment to be made at a low cost to have a simple structure. 
   The glow plug on-off switching unit  5  may alternatively be, as illustrated in  FIG. 2 , assembled along with the engine ECU  4  or mounted in the same casing. 
     FIGS. 3 to 6  are flowcharts of logical steps or programs to be executed by the engine ECU  4  to control the operation of the glow plug  6 . The program of  FIG. 3  is to control initial energization of the glow plug  6 . The program of  FIGS. 4(   a ) and  4 ( b ) is to control immediate reenergization of the glow plug  6  after the glow plug  6  reaches a target temperature. The program of  FIGS. 5(   a ) and  5 ( b ) is to control immediate reenergization of the glow plug  6  before the glow plug  6  reaches the target temperature. The program of  FIG. 6  is to control the state of the glow plug  6  when the key switch  1  is left turned on. 
   Specifically, the control of energization of the glow plug  6  is broken down into four phases S 1 , S 2 , S 3 , and S 4 . When the key switch  1  is turned on by an operator, e.g., a driver of the vehicle, the engine ECU  4  is activated and enters the first control phase S 1 . When the engine ECU  4  has found the fact that the key switch  1  is turned on, the engine ECU  4  works to apply an effective voltage of, for example, 11V in the form of a PWM signal to the glow plug  6  to heat the glow plug  6  up to a target temperature T 1  of, for example, 900° C. quickly. The engine ECU  4  continues to apply an effective voltage of, for example, 7V to the glow plug  6  to keep the target temperature T 1  as it is and waits for a driver&#39;s start command, i.e., engine cranking. When the key switch  1  has been turned off after the glow plug  6  reaches the target temperature T 1 , the second control phase S 2  is entered. When the driver turns on the key switch  1  again before the temperature of the glow plug  6  drops completely, the engine ECU  4  calculates an optimum reenergization duration for which the glow plug  6  is to be kept on or energized again and then energizes the glow plug  6  for the calculated duration. When the key switch  1  has been turned off before the glow plug  6  reaches the target temperature T 1 , the third control phase S 3  is entered. When the driver turns on the key switch  1  again before the temperature of the glow plug  6  drops completely, the engine ECU  4  calculates an optimum reenergization duration for which the glow plug  6  is to be kept on or energized again and then energizes the glow plug  6  for the calculated duration. When the key switch  1  is kept on after expiry of a temperature-holding time during which the target temperature T 1  of the glow plug  6  is to be kept as it is, the fourth control phase S 4  is entered. The engine ECU  4  enters the power saving mode and turns off the glow plug  6  for power saving of the battery  3 . This also avoids overheating of the glow plug  6 . The first to fourth control phases S 1  to S 4  will be described below in detail. 
   The first control phase S 1  includes a sequence of steps, as illustrated in  FIG. 3 . 
   First, in step  101 , it is determined whether the voltage, as produced by the battery  3 , is lower than or equal to a permissible upper limit or not. For instance, the upper limit is 14.5V in the case where the rated voltage of the battery  3  is 12V. If a NO answer is obtained meaning that the voltage of the battery  3  is higher than the upper limit, and a power supply system is malfunctioning, then the routine proceeds to step  120  wherein the power is inhibited from being supplied the glow plug  6 , and this program is terminated. Alternatively, if a YES answer is obtained, then the routine proceeds to step  102  wherein a counter value t is reset to zero (0) and started to count the time. The routine proceeds to step  103  wherein the power to the glow plug on-off switching unit  5  is turned on. The routine proceeds to step  104  wherein an initial energization duration tp is calculated as a function of the temperature of coolant of the engine  2  using an initial energization map. Additionally, a temperature-holding time ta for which the target temperature T 1  is to be held as it is also calculated as a function the temperature of coolant of the engine  2  using a temperature-holding time map. The initial energization map represents a relation between the temperature of coolant of the engine  2  and the initial energization duration tp, as prepared experimentally, and is stored in a memory built in the engine ECU  4 . The temperature-holding time map represents a relation between the temperature of coolant of the engine  2  and the temperature-holding time ta, as prepared experimentally, and is stored in the memory of the engine ECU  4 . 
   The routine proceeds to step  105  wherein it is determined whether the temperature of coolant of the engine  2  is lower than a given level (e.g., 40° C.) or not. If a YES answer is obtained, then the routine proceeds to step  106 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  121  to keep the glow plug  6  off. If the current execution cycle of step  105  is the second or subsequent cycle, the glow plug  6  may be placed in the on-state. In such an event, the glow plug  6  is brought into the off-state. Specifically, when the temperature of coolant of the engine  2  is already high, the engine ECU  4  determines that the engine  2  is now running, and there is no need for heating the engine  2  using the glow plug  6  and terminates the control of energization of the glow plug  6 . 
   In step  106 , it is determined whether the key switch  1  is in the on-state or not. If a YES answer is obtained, then the routine proceeds to step  108 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  107 . In step  108 , it is determined whether there is a need for energizing the glow plug  6  or not. Specifically, a determination is made whether the counter value t is smaller than the sum of the initial energization duration tp and the temperature-holding time ta, as determined in step  104 , or not. If a YES answer is obtained, then the routine proceeds to step  110 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  109  wherein it is determined whether the engine is now running or not. If a YES answer is obtained meaning that there is no need for energizing the glow plug  6 , then the routine proceeds to step  122  wherein the control of energization of the glow plug  6  is terminated. Alternatively, if a NO answer is obtained, then the routine proceeds to step  123  wherein the fourth control phase S 4 , as will be described later in detail, is entered for interrupting the power supply to the glow plug  6  for power saving of the battery  3 . 
   In step  110 , it is determined whether there is a need for achieving initial energization of the glow plug  6  or not. Specifically, a determination is made whether the counter value t is smaller than the initial energization duration tp or not. If a YES answer is obtained, then the routine proceeds to step  111  wherein the engine ECU  4  applies, as described above, an effective voltage of, for example, 11V to the glow plug  6 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  112  wherein the engine ECU  4  continues to apply an effective voltage of, for example, 7V to the glow plug  6  to keep the target temperature T 1  as it is. After step  111  or  112 , the routine returns back to step  105 . 
   In step  107 , it is determined whether the counter value t is smaller than the initial energization duration tp, as derived in step  104 , or not. Specifically, a determination is made whether the glow plug  6  has not yet reached the target temperature T 1  or not. If a NO answer is obtained meaning that the glow plug  6  has reached the target temperature T 1 , the routine proceeds to step  125  wherein the second control phase S 2 , as will be described later in detail, is entered. Alternatively, if a YES answer is obtained meaning that the glow plug  6  has not yet reached the target temperature T 1 , the routine proceeds to step  124  wherein the third control phase S 3 , as will be described later in detail, is entered. 
     FIGS. 4(   a ) and  4 ( b ) show a sequence of steps of the second control phase S 2  to be entered in step  125  of  FIG. 3  when the key switch  1  has been turned off after the glow plug  6  reaches the target temperature T 1 . When the driver turns on the key switch  1  again before the temperature of the glow plug  6  drops completely, the engine ECU  4  calculates the optimum reenergization duration for which the glow plug  6  is to be kept on or energized again and then energizes the glow plug  6  for the calculated duration. 
   Specifically, in step  201 , a time ts 2  required until a brief reenergization control is unnecessary is read out of the memory of the engine ECU  4 . The time ts 2  is a fixed cooling time required by the glow plug  6  to cool from the target temperature T 1  to an unoverheating upper limit T 2 . The unoverheating upper limit T 2  is the temperature of the glow plug  6  (e.g., 550° C.) from which the glow plug  6  is permitted to be energized for the initial energization duration tp without overheating. The time ts 2  is a value depending upon a combination of the engine  2  and the glow plug  6  and may be found experimentally. 
   The routine proceeds to step  202  wherein the counter value t is reset to zero (0) and started to count the time. The routine proceeds to step  203  wherein the glow plug on-off switching unit  5  is turned off to block the power supply to the glow plug  6 . The routine proceeds to step  204  wherein it is determined whether the counter value t is smaller than the cooling time ts 2 , as derived in step  201 , or not. Specifically, the engine ECU  4  monitors an off-period of time during which the power supply to the glow plug  6  is cut. If a YES answer is obtained meaning that the counter value t is smaller than the cooling time ts 2 , then the routine proceeds to step  205 . Alternatively, if a NO answer is obtained meaning that the temperature of the glow plug  6  has decreased to a level which does not require the brief reenergization control, then the routine proceeds to step  224  to terminate the control of reenergization of the glow plug  6 . 
   In step  205 , it is determined whether the key switch  1  is turned on or not. If a NO answer is obtained, then the routine returns back to step  203 . Alternatively, if a YES answer is obtained, then the routine proceeds to step  207  wherein a reenergization duration tpr 2  is calculated according to an equation below. Additionally, the temperature-holding time ta for which the target temperature T 1  is held is also calculated as a function the temperature of coolant of the engine  2  using the temperature-holding time map.
 
 tpr   2   =tp×t off/ ts   2 
 
where tp is the initial energization duration, as derived in step  104 , toff is a time interval from turning off to turning on of the key switch  1 , that is, the counter value t, and ts 2  is the cooling time, as derived in step  201 .
 
   The routine proceeds to step  208  wherein the counter value t is reset to zero (0). 
   The routine proceeds to step  209 , as illustrated in  FIG. 4(   b ), wherein it is determined, like in step  101  of  FIG. 3 , whether the voltage, as produced by the battery  3 , is lower than or equal to, for example, 14.5V or not. If a NO answer is obtained meaning that the voltage of the battery  3  is higher than the permissible upper limit, and a power supply system is malfunctioning, then the routine proceeds to step  225  wherein the power is inhibited from being supplied the glow plug  6 , and this program is terminated. Alternatively, if a YES answer is obtained, then the routine proceeds to step  210  wherein it is determined whether the temperature of coolant of the engine  2  is lower than a given level (e.g., 40° C.) or not. If a NO answer is obtained, then the routine proceeds to step  226  to keep the glow plug  6  off. If the current execution cycle of step  210  is the second or subsequent cycle, the glow plug  6  may be placed in the on-state. In such an event, the glow plug  6  is brought into the off-state. Specifically, when the temperature of coolant of the engine is already high, the engine ECU  4  determines that the engine is now running, and there is no need for heating the engine using the glow plug  6  and terminates the control of reenergization of the glow plug  6 . 
   The routine proceeds to step  211  wherein it is determined whether there is a need for energizing the glow plug  6  or not. Specifically, a determination is made whether the counter value t is smaller than the sum of the reenergization duration tpr 2  and the temperature-holding time ta, as determined in step  207 , or not. If a YES answer is obtained, then the routine proceeds to step  213 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  212  wherein it is determined whether the engine is now running or not. If a YES answer is obtained meaning that there is no need for energizing the glow plug  6 , then the routine proceeds to step  227  wherein the control of reenergization of the glow plug  6  is terminated. Alternatively, if a NO answer is obtained, then the routine proceeds to step  228  wherein the fourth control phase S 4 , as will be described later in detail, is entered to interrupt the energization of the glow plug  6  for power saving of the battery  3 . 
   In step  213 , it is determined whether the counter value t is smaller than the reenergization duration tpr 2  or not, that is, there is a need for achieving initial reenergization of the glow plug  6  or not. If a NO answer is obtained, then the routine proceeds to step  215 . Alternatively, if a YES answer is obtained, then the routine proceeds to step  214  wherein the engine ECU  4  applies, as described above, an effective voltage of, for example, 11V to the glow plug  6 . In step  215 , the engine ECU  4  continues to apply an effective voltage of, for example, 7V to the glow plug  6  to keep the target temperature T 1  as it is. After step  214  or  215 , the routine proceeds to step  217  wherein it is determined whether the key switch  1  is in the on-state or not. If a YES answer is obtained, then the routine returns back to step  210 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  218  to check the temperature of the glow plug  6 . Specifically, a determination is made whether the counter value t is smaller than the reenergization duration tpr 2  or not. If a NO answer is obtained meaning that the glow plug  6  has already reached the target temperature T, then the routine returns back to step  201 . Alternatively, if a YES answer is obtained meaning that the key switch  1  has been turned off before the glow plug  6  reaches the target temperature T, then the routine proceeds to step  229  to initiate the third control phase S 3 . 
     FIGS. 5(   a ) and  5 ( b ) show a sequence of steps of the third control phase S 3  to be entered in step  124  of  FIG. 3  or step  229  of  FIG. 4(   b ) when the key switch  1  has been turned off before the glow plug  6  reaches the target temperature T 1 . When the driver turns on the key switch  1  again before the temperature of the glow plug  6  drops completely, the engine ECU  4  calculates the optimum reenergization duration for which the glow plug  6  is to be kept on or energized again and then energizes the glow plug  6  for the calculated duration. 
   Specifically, in step  301 , a time ts 1  is determined that is a cooling time required by the glow plug  6  to drop from the temperature upon turning off of the key switch  1  to the unoverheating upper limit T 2 . Specifically, the cooling time ts 1  is given by the following equation.
 
 ts   1   =ts   2 × t on/ tp  
 
where ts 2  is the cooling time, as derived in step  201  of  FIG. 4(   a ), ton is a period of time for which the key switch  1  is in the on-state during the initial energization duration tp, and tp is the initial energization duration, as derived in step  104  of  FIG. 3 .
 
   The routine proceeds to step  302  wherein the counter value t is reset to zero (0) and started to count the time. The routine proceeds to step  303  wherein the glow plug on-off switching unit  5  is turned off to block the power supply to the glow plug  6 . The routine proceeds to step  304  wherein it is determined whether the counter value t is smaller than the cooling time ts 1 , as derived in step  301 , or not. Specifically, the engine ECU  4  monitors an off-period of time during which the power supply to the glow plug  6  is cut. If a YES answer is obtained meaning that the counter value t is smaller than the cooling time ts 1 , then the routine proceeds to step  305 . Alternatively, if a NO answer is obtained meaning that the temperature of the glow plug  6  has decreased to a level which does not requires the reenergization control, then the routine proceeds to step  324  to terminate the reenergization control of the glow plug  6 . 
   In step  305 , it is determined whether the key switch  1  is turned on or not. If a NO answer is obtained, then the routine returns back to step  303 . Alternatively, if a YES answer is obtained, then the routine proceeds to step  307  wherein a reenergization duration tpr 1  is calculated according to an equation below. Additionally, the temperature-holding time ta for which the target temperature T 1  is held is also calculated as a function the temperature of coolant of the engine  2  using the temperature-holding time map.
 
 tpr   1   =tp×t off/ ts   1 
 
where tp is the initial energization duration, as derived in step  104 , toff is a time interval from turning off to turning on of the key switch  1 , that is, the counter value t, and ts 1  is the cooling time, as derived in step  301 .
 
   The routine proceeds to step  308  wherein the counter value t is reset to zero (0). 
   The routine proceeds to step  309 , as illustrated in  FIG. 5(   b ), wherein it is determined, like in step  101  of  FIG. 3 , whether the voltage, as produced by the battery  3 , is lower than or equal to, for example, 14.5V or not. If a NO answer is obtained meaning that the voltage of the battery  3  is higher than the permissible upper limit, and the power supply system is malfunctioning, then the routine proceeds to step  325  wherein the power is inhibited from being supplied the glow plug  6 , and this program is terminated. Alternatively, if a YES answer is obtained, then the routine proceeds to step  310  wherein it is determined whether the temperature of coolant of the engine  2  is lower than a given level (e.g., 40° C.) or not. If a NO answer is obtained, then the routine proceeds to step  326  to keep the glow plug  6  off. If the current execution cycle of step  310  is the second or subsequent cycle, the glow plug  6  may be placed in the on-state. In such an event, the glow plug  6  is brought into the off-state. Specifically, when the temperature of coolant of the engine is already high, the engine ECU  4  determines that the engine is now running, and there is no need for heating the engine using the glow plug  6  and terminates the control of reenergization of the glow plug  6 . 
   The routine proceeds to step  311  wherein it is determined whether there is a need for energizing the glow plug  6  or not. Specifically, a determination is made whether the counter value t is smaller than the sum of the reenergization duration tpr 1  and the temperature-holding time ta, as determined in step  307 , or not. 
   If a YES answer is obtained, then the routine proceeds to step  313 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  312  wherein it is determined whether the engine is now running or not. If a YES answer is obtained meaning that there is no need for energizing the glow plug  6 , then the routine proceeds to step  327  wherein the control of reenergization of the glow plug  6  is terminated. Alternatively, if a NO answer is obtained, then the routine proceeds to step  328  wherein the fourth control phase S 4 , as will be described later in detail, is entered to interrupt the energization of the glow plug  6  for power saving of the battery  3 . 
   In step  313 , it is determined whether the counter value t is smaller than the reenergization duration tpr 2  or not, that is, there is a need for achieving initial energization of the glow plug  6  or not. If a NO answer is obtained, then the routine proceeds to step  315 . Alternatively, if a YES answer is obtained, then the routine proceeds to step  314  wherein the engine ECU  4  applies, as described above, an effective voltage of, for example, 11V to the glow plug  6 . In step  315 , the engine ECU  4  continues to apply an effective voltage of, for example, 7V to the glow plug  6  to keep the target temperature T 1  as it is. After step  314  or  315 , the routine proceeds to step  317  wherein it is determined whether the key switch  1  is in the on-state or not. If a YES answer is obtained, then the routine returns back to step  310 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  318  to check the temperature of the glow plug  6 . Specifically, a determination is made whether the counter value t is smaller than the reenergization duration tpr 2  or not. If a NO answer is obtained meaning that the glow plug  6  has already reached the target temperature T, then the routine returns back to step  301 . Alternatively, if a YES answer is obtained meaning that the key switch  1  has been turned off before the glow plug  6  reaches the target temperature T, then the routine proceeds to step  329  to initiate the second control phase S 2 . 
     FIG. 6  shows a sequence of steps of the fourth control phase S 4  to be entered in step  123  of  FIG. 3 , step  228  of  FIG. 4(   b ), or step  328  of  FIG. 5(   b ), when the key switch  1  is kept on after expiry of the temperature-holding time ta for which the target temperature T 1  is to be held for saving the power of the battery  3 . 
   First, in step  401 , the time ts 2  required until the brief reenergization control is unnecessary is, like in step  201  of  FIG. 4(   a ), read out of the memory of the engine ECU  4 . 
   The routine proceeds to step  402  wherein the counter value t is reset to zero (0) and started to count the time. The routine proceeds to step  403  wherein the glow plug on-off switching unit  5  is turned off to block the electric communication with the glow plug  6 . The routine proceeds to step  404  wherein it is determined whether the counter value t is smaller than the cooling time ts 2 , as derived in step  401 , or not. Specifically, the engine ECU  4  monitors an off-period of time during which the power supply to the glow plug  6  is cut. If a YES answer is obtained meaning that the counter value t is smaller than the cooling time ts 2 , then the routine proceeds to step  405 . Alternatively, if a NO answer is obtained meaning that the temperature of the glow plug  6  has decreased to a level which does not requires the reenergization control, then the routine proceeds to step  424  to terminate the reenergization control of the glow plug  6 . 
   In step  405 , it is determined whether the key switch  1  is turned on or not. If a NO answer is obtained, then the routine returns back to step  203  in the second control phase S 2 . Alternatively, if a YES answer is obtained, then the routine returns back to step  403 . 
   As apparent from the above discussion, the glow plug controller is so designed that when the key switch  1  is turned off and then on again after expiry of the initial energization duration tp, the optimum reenergization duration tpr 2  may be derived as a function of a time interval between turning off and on of the key switch  1 , or when the key switch  1  is turned off and then on again before expiry of the initial energization duration tp, the optimum reenergization duration tpr 1  may be derived as a function of length of time the key switch  1  is in the on-state during the initial energization duration tp in order to avoid the overheating of the glow plug  6 . 
     FIGS. 7(   a ) to  8 ( b ) show examples of the above described operations of the engine ECU  4  to control the energization of the glow plug  6 . 
   In the example of  FIG. 7(   a ), when the key switch  1  is turned on at time t 0 , the engine ECU  4  supplies the power to the glow plug  6  until time t 1  to heat it quickly to the target temperature T 1 . After time t 1 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is until time tz. 
   In the example of  FIG. 7(   b ), when the key switch  1  is turned on at time t 0 , the engine ECU  4  supplies the power to the glow plug  6  until time t 1  (i.e., the initial energization duration tp) to heat it quickly to the target temperature T 1 . After time t 1 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is. When the key switch  1  is turned off at time t 2 , the engine ECU  4  deenergizes the glow plug  6 , so that the temperature of the glow plug  6  decreases. When the temperature of the glow plug  6  reaches the unoverheating upper limit T 2  that is, as described above, the temperature of the glow plug  6  (e.g., 550° C.) from which the glow plug  6  is permitted to be heated for the initial energization duration tp without overheating, and the key switch  1  is turned on at time t 3 , the engine ECU  4  supplies the power to the glow plug  6  until time t 4  (i.e., the initial energization duration tp) to heat it quickly to the target temperature T 1  again. After time t 4 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is until time tz. If a period of time required by the glow plug  6  to cool from the target temperature T 1  to the unoverheating upper limit T 2  is, as described above, defined as the cooling time ts 2 , the time interval toff between turning off (i.e., time t 2 ) and turning on (i.e., time t 3 ) of the key switch  1  is greater than the cooling time ts 2 . Specifically, at time t 3 , the temperature of the glow plug  6  has decreased sufficiently to a level which permits the glow plug  6  to be energized during a period of time tp that is identical with the initial energization duration tp without overheating. 
   In the example of  FIG. 8(   a ), when the key switch  1  is turned on at time t 0 , the engine ECU  4  supplies the power to the glow plug  6  until time t 1  (i.e., the initial energization duration tp) to heat it quickly to the target temperature T 1 . After time t 1 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is. When the key switch  1  is turned off at time t 2 , the engine ECU  4  deenergizes the glow plug  6 , so that the temperature of the glow plug  6  decreases. When the key switch  1  is turned on again at time t 3  before the temperature of the glow plug  6  reaches the unoverheating upper limit T 2 , the engine ECU  4  supplies the power to the glow plug  6  until time t 4  (i.e., the reenergization duration tpr 2 ) to heat it quickly to the target temperature T 1 . After time t 4 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is until time tz. In this example, the time interval toff between turning off (i.e., time t 2 ) and turning on (i.e., time t 3 ) of the key switch  1  is shorter than the cooling time ts 2 . Specifically, at time t 3 , the temperature of the glow plug  6  has not yet decreased to the level which permits the glow plug  6  to be energized for the initial energization duration tp without overheating. Therefore, the engine ECU  4 , as described above, calculates the reenergization duration tpr 2  as a function of the off duration toff to avoid the overheating of the glow plug  6 . 
   In the example of  FIG. 8(   b ), when the key switch  1  is turned on at time t 0 , the engine ECU  4  supplies the power to the glow plug  6  to heat it quickly. When the key switch  1  is turned off at time t 1 , the engine ECU  4  deenergizes the glow plug  6 , so that the temperature of the glow plug  6  decreases without reaching the target temperature T 1 . When the key switch  1  is turned on again at time t 2 , the engine ECU  4  supplies the power to the glow plug  6  until time t 3  (i.e., the reenergization duration tpr 1 ) to heat it quickly to the target temperature T 1 . After time t 3 , the engine ECU  4  keeps the glow plug  6  energized to hold the target temperature T 1  as it is until time tz. If a period of time required by the glow plug  6  to drop from the temperature upon turning off of the key switch  1  (i.e., time t 1 ) to the unoverheating upper limit T 2  is, as described above, defined as the cooling time ts 1 , the off-duration toff is shorter than the cooling time ts 1 . Specifically, at time t 2 , the temperature of the glow plug  6  has not yet decreased to the level which permits the glow plug  6  to be energized for the initial energization duration tp without overheating. Therefore, the engine ECU  4 , as described above, calculates the reenergization duration tpr 1  as a function of the on duration ton that is a period of time (i.e., t 0  to t 1 ) for which the key switch  1  is in the on-state in order to avoid the overheating of the glow plug  6 . 
     FIG. 9(   a ) is a graph showing an experimentally obtained relation between an on-duration (sec.) of the glow plug  6  and the temperature (° C.) of the glow plug  6  in terms of the temperature of coolant of the engine  2 . The line a represents for the case where the temperature of coolant of the engine  2  is constant at −25° C. The line b represents for the case where the temperature of coolant of the engine  2  is constant at 25° C. The graph shows that the higher the temperature of coolant of the engine  2 , the longer the on-duration of the glow plug  6  will be because the resistance of the glow plug  6  to current flow increases as the temperature of the engine  2  increases. 
     FIG. 9(   b ) is a graph plotting the on-duration of the glow plug, as illustrated in  FIG. 9(   a ), which changes as a function of the temperature of coolant of the engine  2 . The graph is stored in the ECU  4  as the initial energization map, as described above, for use in determining the initial energization duration tp for which the glow plug  6  is to be energized until the target temperature T 1  (900° C.) is reached. 
     FIG. 10  is a graph which shows an experimentally obtained relation between a drop in temperature (° C.)of the glow plug  6  and the time (sec.) required for such a temperature drop. The line a represents for the case where the temperature of coolant of the engine  2  is 25° C. meaning that the engine is cold. The line b represents for the case where the temperature of coolant of the engine  2  is 80° C. meaning that the engine is warmed up. The graph shows that the glow plug  6  cools from the target temperature T 1  (900° C.) at substantially the same rate regardless of the temperature of coolant of the engine  2 . A period of time required by the glow plug  6  to cool from the target temperature T 1  (900° C.) to the unoverheating upper limit T 2  (550° C.) is, as described above, stored as the cooling time ts 2  in the engine ECU  4 . 
   While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the glow plug controller may be engineered to control energization of a plurality of glow plugs and also be implemented by a typical microcomputer instead of the engine ECU  4 . In place of the temperature of coolant of the engine  4  used in determining the initial energization duration tp etc., the temperature of fuel, intake air, or exhaust air may be employed. The rated voltage of the battery  3  is not limited to 12V, but may be 24V.