Patent Publication Number: US-10316810-B2

Title: Vehicle control system

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
     The present application claims the benefit of priority of Japanese Patent Application No. 2015-220683 filed on Nov. 10, 2015, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1 Technical Field 
     The invention relates generally to a vehicle control system for a vehicle which operates in a preset mode to establish a mechanical engagement between a pinion and a ring gear before re-starting an engine mounted in a vehicle. 
     2 Background Art 
     In order to save fuel in vehicles such as automobiles, modern vehicle control systems execute an idle stop mode or a coasting mode. The idle stop mode is to stop an engine such as an internal combustion engine when a driver is braking to the vehicle or after the vehicle is stopped. The coasting mode is to stop the engine during traveling of the vehicle to perform a natural deceleration. After the engine is stopped in either of these modes, it is necessary to quickly restart the engine in response to a start request made by the driver of the vehicle. Particularly, in the coasting mode, the vehicle needs to be accelerated quickly in response to a driver&#39;s acceleration request, thus requiring a rapid start of the engine. 
     Usually, when it is required to restart the engine, the vehicle control system brings a pinion mounted on a starter motor into engagement with a ring gear fit on a rotational axis of the engine and then energizes the starter motor. In the case where the starter motor is a general type, the restarting of the engine while the speed of the engine is decreasing, however, results in an excessive decrease in service life of the starter motor or an increased level of mechanical noise arising from impact of the pinion on the ring gear. 
     In order to alleviate the above problem, some of the vehicle control systems are designed to control the actuation of the starter motor and the movement of the pinion to achieve the engagement with the ring gear independently from each other. Specifically, when the rotational speed of the engine is lower than a lower limit at which it is possible to start the engine without use of the starter motor and then higher than a given value, the vehicle control system actuates the starter motor before moving the pinion to bring the speed of the starter motor close to that of the ring gear and then moves the pinion to achieve the engagement with the ring gear. This achieves the restarting of the engine while the speed of the engine is dropping. However, when the pinion meshes the ring gear while the engine is rotating in the reverse direction, it will overload the starter motor. In order to eliminate an undesirable reduction in service life of the starter motor, the vehicle control system is inhibited from restarting the engine while the engine is reversing. 
     Japanese Patent No. 5251687 teaches a vehicle control system designed to avoid the above problems. The vehicle control system works to move the pinion to achieve engagement with the ring gear during the dropping of speed of the engine in the absence of an engine start request and then actuates the starter motor upon reception of the engine start request. This enables the engine to be started even in a swing (oscillation) period of time in which the engine alternately undergoes a normal rotation and a reverse rotation. The smaller a difference in rotational speed between the pinion and the ring gear, the longer the service life of the starter motor will be and also the smaller the noise arising from the impact of the pinion on the ring gear will be low. To this end, the vehicle control system, as taught in the above publication, is engineered to bring the pinion into engagement with the ring gear immediately before the engine is stopped. 
     The above vehicle control system is, as described above, capable of lowering the level of the noise emanating from the engagement of the pinion on the ring gear (which will also be referred to below as impact noise), but however, such noise occurs immediately before the engine is stopped in the idle stop mode or the vehicle is stopped, so that the level of the roadway noise or wind noise is very low, thus resulting in a high probability that the driver of the vehicle feels uncomfortable about the impact noise occurring in the preset mode in the course of the idle stop mode. 
     SUMMARY 
     It is therefore an object to provide a vehicle control system which is capable of alleviating a driver&#39;s perception about the impact noise arising from engagement of a pinion of an engine starter with a ring gear of an engine. 
     According to one aspect of the disclosure, there is provided a vehicle control system which is used in a vehicle equipped with an engine as a drive power source and works to establish engagement of a pinion of a starter motor with a ring gear mounted on a rotational axis of the engine and then actuate the starter motor to start the engine. The vehicle control system comprises: (a) an engine speed determiner which determines a rotational speed of the rotational axis of the engine; (b) a preset controller which executes a preset mode to move the pinion to achieve the engagement with the ring gear when the rotation speed, as determined by said engine speed determiner, has dropped below a given threshold value; and (c) a start controller which works to start the engine in response to a start request, the start controller. The preset controller determines the given threshold value as a function of a speed of the vehicle. 
     Usually, when the speed of the vehicle is high, the degree of noise arising from friction between wheels of the vehicle and the road surface (also called roadway noise) is high, so that the impact noise emanating from the pinion and the ring gear in the preset mode is overshadowed or drowned by the roadway noise. Conversely, when the speed of the vehicle is low, the roadway noise is usually low in level, so that the impact noise emanating from the pinion and the ring gear will be very audible. The continuous achievement of engagement between the pinion and the ring gear in the preset mode when the speed of the engine is still high will result in an increased length of time required to keep the engagement, in other words, an increased amount of electric power required to do it. Conversely, the execution of the preset mode when the speed of the engine is low will result in a decrease in length of time for which the engagement of the pinion with the ring gear is maintained, which leads to a reduction in consumption of the electrical power. The vehicle control system of this disclosure changes the threshold value based on the speed of the vehicle and initiates the preset mode when the rotational speed of the engine has dropped below the threshold value, in other words, executes the preset mode when the rotational speed of the engine is low when the level of the roadway noise is high, thereby causing the impact noise emanating from the pinion and the ring gear to be masked by the roadway noise and also resulting in a decrease in consumption of the electrical power in the preset mode. When the level of the road noise is low, the vehicle control system initiates the preset mode when the rotational speed of the engine is high, so that the impact noise generated upon the engagement of the pinion with the ring gear is overshadowed by the engine noise. The vehicle control system is, therefore, capable of minimizing the impact noise and the consumption of electrical power upon achievement of the preset mode in the vehicle and also quickly restarting the engine in response to the driver&#39;s restart request to the engine. 
     The preset controller may set the threshold value when the speed of the vehicle is higher to be smaller than that when the speed of the vehicle is lower. This enhances the above described beneficial effects provided by the vehicle control system. 
     The vehicle control system may also include a travel controller which executes a coasting mode to stop the engine to cut supply of drive power from the engine to a drive wheel of the vehicle or an idle stop mode to stop the engine before the vehicle is stopped based on the speed of the vehicle when a given operating condition is encountered. When one of the coasting mode and the idle stop mode is entered, the preset controller executes the preset mode. The given threshold value for use in the coasting mode is set smaller than that for use in the idle stop mode. 
     Usually, the speed of the vehicle in the coasting mode is higher than that in the idle stop mode. The impact noise emanating from the pinion and the ring gear is, thus, overshadowed by the roadway noise in the coasting mode. 
     Alternatively, when the idle stop mode is entered, the vehicle control system initiates the preset mode when the rotational speed of the engine is high, so that the impact noise is overshadowed by the engine noise. 
     According to the second aspect of the disclosure, there is provided a vehicle control system which is used in a vehicle equipped with an engine as a drive power source and works to establish engagement of a pinion of a starter motor with a ring gear mounted on a rotational axis of the engine and then actuate the starter motor to start the engine. The vehicle control system comprises: (a) a travel controller which executes a coasting mode to stop the engine to cut supply of drive power from the engine to a drive wheel of the vehicle or an idle stop mode to stop the engine before the vehicle is stopped based on the speed of the vehicle when a given operating condition is encountered; (b) an engine speed determiner which determines a rotational speed of the rotational axis of the engine; (c) a preset controller which executes a preset mode to move the pinion to achieve the engagement with the ring gear when the rotation speed, as determined by said engine speed determiner, has dropped below a given threshold value in execution of one of the coasting mode and the idle stop mode; and (d) a start controller which works to start the engine in response to a start request. The preset controller determines the given threshold value for use in the coasting mode to be smaller than that for use in the idle stop mode. 
     The above structure of the vehicle control system eliminates the need for monitoring or determining the speed of the vehicle for use in execution of the preset mode, thereby also obviating the need for, for example, a map representing a relation between the speed of the vehicle and the threshold value, which results in a simplified structure of the vehicle control system. 
     The vehicle control system in either of the first and second aspects may be designed so that before the engagement of the pinion with the ring gear is achieved, the starter motor is actuated to set a difference between a rotational speed of the pinion and a rotational speed of the rotational axis of the engine (i.e., a rotational speed of the ring gear) to be smaller than a given value. This results in a decreased level of the impact noise upon engagement of teeth of the pinion with teeth of the ring gear. 
     Additionally, the preset controller may also work to move the pinion after stopping rotation of the starter motor. When the starter motor continues to rotate after engagement of the pinion with the ring gear, it may cause the torque of the starter motor to be transmitted to the rotational axis of the engine, so that the engine is restarted. The vehicle control system of this disclosure starts moving the pinion after stopping the rotation of the starter motor, thus avoiding the above problem. 
     The vehicle control system in either of the first and second aspects may be designed so that the preset controller works to continue to energize a drive unit to maintain the engagement of the pinion with the ring gear, and the preset controller stops energizing the drive unit when the rotation of the engine is determined to have been stopped. 
     Before the rotation of the engine is completely stopped, there is, as described above, the swing period of time in which the engine alternately undergoes a normal rotation and a reverse rotation. In the swing period of time, the area of contact between the pinion and the ring gear, therefore, alternately increases and decreases, thereby resulting in instability of the engagement of the pinion with the ring gear unless the drive unit continues to be energized. Alternatively, after the rotation of the engine is stopped completely, the area of contact between the pinion and the ring gear is almost kept constant, so that the engagement of the pinion with the ring gear will be stable without need for energizing the drive unit. The vehicle control system of this disclosure stops actuating the drive unit when the rotation of the engine is found to have been stopped, thus resulting in a decreased length of time the drive unit continues to be energized without sacrificing the stability of engagement between the pinion and the ring gear. 
     The vehicle control system in either of the first and second aspects may also include an electrical rotating machine which is joined to the rotational axis of the engine. When the pinion is in engagement with the ring gear when the start request is made, the start controller starts the engine using the electrical rotating machine. 
     Before being stopped completely, the engine, as described above, alternately experiences normal and reverse rotation. When the engine is rotating in the reverse direction, an increased degree of torque is usually required to restart the engine. The preset engagement of the pinion and the ring gear, however, serves to suppress the reverse rotation of the engine, thus resulting in a decreased degree of torque for restarting the engine. For this reason, the vehicle control system of this disclosure may be, as described above, equipped with the electrical rotating machine in addition to the starter motor. The vehicle control system is capable of actuating the electrical rotating machine with less power to restart the engine because the reverse rotation of the engine is suppressed by the preset engagement of the pinion with the ring gear. 
     The vehicle control system in either of the first and second aspects may also include an electrical rotating machine which is joined to the rotational axis of the engine. When the pinion is in engagement with the ring gear when the start request is made, the start controller may start the engine using the starter motor, while when the pinion is in disengagement from the ring gear when the start request is made, the start controller may start the engine using the electrical rotating machine. Therefore, in response to the driver&#39;s start request, the vehicle control system is capable of immediately actuating the available starting machine (i.e., the starter motor or the electrical rotating machine) to achieve a quick restart of the engine. 
    
    
     
       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 schematic block diagram which illustrates a vehicle control system according to an embodiment; 
         FIGS. 2( a ) and 2( b )  are time charts which demonstrates operations of the vehicle control system of  FIG. 1 ; and 
         FIG. 3  is a flowchart of an engine starting control program executed by the vehicle control system of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is shown a vehicle control system according to an embodiment which is mounted in a vehicle, such as an automobile, equipped with an engine as a drive power source. 
     In  FIG. 1 , the engine  10  is a multi-cylinder internal combustion engine in which fuel, such as gasoline or diesel oil, is combusted and which is equipped with typical fuel injectors and a typical igniter. The engine  10  has the starter motor  11  mounted thereon. The starter motor  11  has a rotational axis (i.e., an output shaft) on which the pinion  12  is mounted. The pinion  12  is engageable with the ring gear  14  mounted on the output shaft  13  (i.e., a rotational axis) of the engine  10 . The starter motor  11  is equipped with the solenoid  15  which works to thrust the pinion  12  into engagement with the ring gear  14 . The solenoid  15  works as a drive unit for the pinion  12 . Specifically, when it is required to start the engine  10 , the solenoid  15  works to move the pinion  12  in the axial direction thereof to make engagement with the ring gear  14 , so that torque, as produced by the starter motor  11 , is transmitted to the output shaft  13  of the engine  10 . 
     To the output shaft  13  of the engine  10 , the ISG (Integrated Starter Generator)  17  is joined through the power transmitter  16 . The ISG  17  works as an electrical rotating machine. The power transmitter  16  includes a pulley and a belt. When it is required to supply the power to the output shaft  13  of the engine  10 , the ISG  17  operates as an electric motor. Alternatively, when it is required to convert torque, as produced by the engine  10 , into electrical power, the ISG  17  works as an electrical generator. 
     The output shaft  13  of the engine  10  is joined to the transmission  19  through the clutch  18 . The clutch  18  is implemented by, for example, a frictional clutch which includes a pair of clutch mechanisms equipped with a disc (i.e., a flywheel) joined to the output shaft  13  of the engine  10  and a disc (i.e., a clutch disc) joined to the input shaft  20  of the transmission  19 . When the discs of the clutch  18  are brought into contact with each other, the clutch  18  is placed in an engagement mode to transmit power between the engine  10  and the transmission  19 . Alternatively, when the discs of the clutch  18  are disengaged from each other, the clutch  18  is placed in a disengagement mode to block the transmission of power between the engine  10  and the transmission  19 . The clutch  18  of this embodiment is engineered as an automatic clutch which has, as described above, the engagement mode and the disengagement mode which are switched by an actuator such as an electrical motor. The clutch  18  may be installed inside the transmission  19 . 
     The transmission  19  is engineered as an automatic transmission equipped with a plurality of gear ratios. The transmission  19  works to change the speed of power, as produced by the engine  10  and inputted into the input shaft  20 , with a gear ratio selected as a function of the speed of the vehicle, the rotational speed of the engine  10 , and the position of a shift lever (also called a selector) of the transmission  19  and outputs it from the output shaft  21 . The shift lever is an operating lever of the transmission  19  and disposed near a driver&#39;s seat of the vehicle. The driver of the vehicle moves the shift lever to select one of a plurality of operating modes (i.e., the gear ratios) of the transmission  19 . The transmission  19  of this embodiment has, as the positions of the shift lever, a forward position (also called a D-range), a reverse position (also called a R-range), and a neutral position (also called a N-range). The transmission  19  is equipped with an automatic shift mechanism made of an actuator such as an electric motor or a hydraulic actuator. In the D-range, the gear ratios are automatically changed from one to another. To the output shaft  21  of the transmission  19 , the drive wheels  24  are joined through the differential gear  22  and the drive shaft  23 . 
     The vehicle control system of this embodiment also includes the ECU (Electronic Control Unit)  30 , the clutch controller  31 , the transmission controller  32 , and the motor controller  33 . The ECU  30  works to control an entire operation of the control system. The clutch controller  31  works to control an operation of the clutch  18 . The transmission controller  32  works to control an operation of the transmission  19 . The motor controller  33  works to control an operation of the ISG  17 . The ECU  30 , the clutch controller  31 , the transmission controller  32 , and the motor controller  33  are each realized by a typical electronic control device equipped with a microcomputer and monitor outputs from sensors installed in the control system to control the operations of the engine  10 , the clutch  18 , the transmission  19 , and the ISG  17 . The ECU  30 , the clutch controller  31 , the transmission controller  32 , and the motor controller  33  are joined together so that they are communicable with each other to share control signals or data signals with each other. The ECU  30  constitutes the vehicle control system, but however, the vehicle control system may alternatively be implemented by the ECU  30 , the clutch controller  31 , the transmission controller  32 , and the motor controller  33 . 
     The ECU  30  is electrically connected to the storage battery  34  and operates on power supplied from the battery  34 . The battery  34  is joined to the starter motor  11  through the first relay  35  and also to the solenoid  15  through the second relay  36 . The first relay  35  and the second relay  36  are closed, that is, connected in response to drive signals outputted from the ECU  30 , respectively. When the first relay  35  is closed, the starter motor  11  is actuated by the power delivered from the battery  34 . When the second relay  36  is closed, the solenoid  15  is actuated by the power from the battery  34  to thrust the pinion  12  into engagement with the ring gear  14 . 
     The above described sensors include the accelerator sensor  42 , the brake sensor  44 , the wheel speed sensor  45 , and the rotational speed sensor  46 . The accelerator sensor  42  measures an amount by which the accelerator pedal  41  is depressed, that is, the position of the accelerator pedal  41 . The brake sensor  44  measures an amount by which the brake pedal  43  is depressed, that is, the position of the brake pedal  43 . The wheel speed sensor  45  measures the speed of the drive wheels  24 . The rotational speed sensor  46  measure the rotational speed of the output shaft  13  of the engine  10 . Outputs from these sensors are inputted into the ECU  30 . The ECU  30  derives the position of the accelerator pedal  41 , as measured by the accelerator sensor  42 , as an accelerator position (i.e., an open position of a throttle valve). The ECU  30  also derives the speed of the drive wheels  24 , as measured by the wheel speed sensor  45 , as a vehicle speed. The vehicle control system also includes other sensors (not shown). The ECU  30  also serves as an engine speed determiner to determine the rotational speed of the output shaft  13  of the engine  10 . 
     The ECU  30  analyzes the outputs from the sensors and information inputted from the transmission controller  32  to perform control tasks such as control of the quantity of fuel to be sprayed by the fuel injectors and ignition timing of the igniter of the engine  10 . The clutch controller  31  performs a switching operation to engage or disengage the clutch  18  based on information inputted from the ECU  30 . Similarly, the transmission controller  32  works to change the gear ratios of the transmission  19  based on information inputted from the ECU  30 . 
     The ECU  30  also outputs control instructions to the motor controller  33 . The motor controller  33  then controls the operation of the ISG  17  according to the control instructions from the ECU  30 . Specifically, when the vehicle is accelerating, in other words, a greater degree of drive power or torque is required, the ISG  17  is actuated as an electrical motor. Alternatively, when the vehicle is decelerating or the amount of electric power remaining in the battery  34  is low, the IGS  17  is actuated as an electrical generator to charge the battery  34 . 
     When a given coasting condition is encountered while the vehicle of this embodiment is being driven by the drive power or torque produced by the engine  10 , the vehicle control system (i.e., the ECU  30 ) executes a coasting mode to stop the engine  10  and disengage the clutch  18 , thereby cutting supply of the torque produced by the engine  10  to the drive wheels  24 . Additionally, when the speed of the vehicle drops below a given value while the driver is depressing the brake pedal to decelerate the vehicle, the vehicle control system executes an idle stop mode (also called an automatic engine stop mode) to stop the engine  10 . The coasting mode and the idle stop mode are used for improving the fuel efficiency in the vehicle. In execution of either of the coasting and idle stop modes, the ECU  30  works as a travel controller. 
     When the coasting mode is entered to stop the engine  10  or the idle stop mode is entered to stop the engine  10 , the ECU  30  works as a preset controller to execute a preset mode which actuates the solenoid  15  to achieve engagement of the pinion  12  with the ring gear  14  before the speed of the engine  10  drops to zero. 
     In the preset mode, noise usually occurs when teeth of the pinion  12  impact teeth of the ring gear  14 . The electric power is required to actuate the solenoid  15 , so that the longer the length of time the solenoid  15  is energized, the greater the electric power will be. Usually, the engine  10  is in a swing (oscillation) mode where it alternately experiences a normal rotation and a reverse rotation before being stopped completely. In the swing mode, an area of contact between the pinion  12  and the ring gear  14  sequentially increases and then decreases. The deenergization of the solenoid  15  may, therefore, result in a risk that the pinion  12  disengages from the ring gear  14 . 
     After the speed of the engine  10  drops to zero, the area of contact between the pinion  12  and the ring gear  14  is almost kept constant, thus ensuring the stability in engagement of the pinion  12  and the ring gear  14 . It is, thus, advisable that the solenoid  15  be deenergized after the speed of the engine  10  becomes zero, and the stability of the engagement between the pinion  12  and the ring gear  14  is established. The achievement of engagement between the pinion  12  and the ring gear  14  when the speed of the engine  10  is still high will result in an increased length of time the solenoid  15  is energized, but it will cause the impact noise arising from the engagement between the pinion  12  and the ring gear  14  to be masked by noise of the engine  10 . 
     Accordingly, when the preset mode is required to be entered, the vehicle control system of this embodiment monitors the speed of the engine  10  and starts supplying the electric power to the solenoid  15  when the speed of the engine  10  has dropped below a preselected threshold value. Usually, when the speed of the vehicle is high, the degree of noise arising from friction between the wheels of the vehicle and the road surface (also called roadway noise) is high, so that the impact noise emanating from the pinion  12  and the ring gear  14  is overshadowed by the roadway noise. Conversely, when the speed of the vehicle is low, the roadway noise is usually low in level, so that the impact noise emanating from the pinion  12  and the ring gear  14  will be very audible. 
     In view of the above fact, when the speed of the vehicle is high at the time when it is required to start the preset mode, the vehicle control system decreases the threshold value used to be compared with the speed of the engine  10  as compared with when the speed of the vehicle is low. This causes the impact noise emanating from the pinion  12  and the ring gear  14  to be overshadowed by the roadway noise and also results in a decrease in period of time for which the solenoid  15  is energized to reduce the consumption of electrical energy in the battery  34 . Alternatively, when the speed of the vehicle is low at the time when it is required to start the preset mode, the vehicle control system increases the threshold value to mask the impact noise by the noise of the engine  10 . The threshold value may be switched between two discrete values when the speed of the vehicle reaches a given value or alternatively changed linearly in proportion to the speed of the vehicle or stepwise with a change in speed of the vehicle. 
     The preset mode will be described below in detail with reference to  FIGS. 2( a ) and 2( b ) .  FIG. 2( a )  demonstrates the case where the preset mode is entered when the speed of the vehicle is high.  FIG. 2( b )  demonstrates the case where the preset mode is entered when the speed of the vehicle is low. In the example of  FIG. 2( a )  where the speed of the vehicle is high, the engine  10  is stopped in the coasting mode, while in the example of  FIG. 2( b )  where the speed of the vehicle is low, the engine  10  is stopped in the idle stop mode. 
     In each of the examples of  FIGS. 2( a ) and 2( b ) , the speed of the engine  10  is lower than or equal to the threshold value at time t 1 , so that the preset mode is initiated. The threshold value is, as described above, set smaller when the speed of the vehicle is high than that when the speed of the vehicle is low. Specifically, in the example of  FIG. 2( a )  where the speed of the vehicle is high, the threshold value is selected to be 300 rpm, while in the example of  FIG. 2( b )  where the speed of the vehicle is low, the threshold value is selected to be 600 rpm. The threshold value in each of  FIGS. 2( a ) and 2( b )  is just one example. The threshold value when the speed of the vehicle is high only needs to be smaller than that when the speed of the vehicle is low. 
     After the preset mode is entered, the pinion  12  is moved to the ring gear  14  and then contacts it at time t 2 , so that impact noise occurs. At time t 3 , the pinion  12  meshes the ring gear  14 . Afterwards, the rotation of the engine  10  swings, that is, the engine  10  rotates in the normal direction and then reverses alternately until time t 4 . The pinion  12  is kept in engagement with the ring gear  14 , thus suppressing the swing motion of the output shaft  13  of the engine  10 . In  FIGS. 2( a ) and 2( b ) , a change in speed of the engine  10  in a period of time for which the engine  10  continues to swing is indicated by a solid line when the preset mode is entered and a broken line when the preset mode is not entered. When the speed of the engine  10  is found to have dropped to zero at time t 5 , the vehicle control system stops supplying the electric power to the solenoid  15  because the stability of engagement of the pinion  12  and the ring gear  14  is ensured by the friction therebetween. 
     After the preset mode is executed in the above way, the ECU  30  waits until a start request (i.e., an engine restart request) is made by the driver of the vehicle. When the vehicle is in the coasting mode, the engine restart request is provided by, for example, driver&#39;s depression of the accelerator pedal  41  which is detected by the accelerator sensor  42  or driver&#39;s release of the brake pedal  43  which is detected by the brake sensor  44 . When the driver&#39;s operation on the accelerator pedal  41  or the brake pedal  43  is made before the preset mode is entered after the fuel supply to the engine  10  is cut, the ECU  30  actuates the ISG  17  as an electric motor to start the engine  10  or resumes only injection of fuel to start the engine  10 . When it is required to start the engine  10  in the above way, the ECU  30  serves as a start controller. 
       FIG. 3  is a flowchart of a sequence of logical steps or program to be executed by the ECU  30 . 
     After entering the program, the routine proceeds to step S 101  wherein it is determined whether supply of fuel to the engine  10  is stopped or not. If a NO answer is obtained meaning that the supply of fuel to the engine  10  is not cut off, that is, there is no need for restarting the engine  10 , then the routine terminates. Alternatively, if a YES answer is obtained in step S 101 , then the routine proceeds to step S 102  wherein the threshold value for use in comparison with the rotational speed of the engine  10  is determined as a function of the speed of the vehicle. The routine then proceeds to step S 103  wherein it is determined whether a starter request (i.e., an engine restart request) has been made by the driver of the vehicle or not. For example, when the driver&#39;s depression of the accelerator pedal  41  is detected, the ECU  30  determines that the start request has been made by the driver to start the engine  10 . 
     If a YES answer is obtained in step S 103  meaning that the start request has been made, then the routine proceeds to step S 104  wherein the speed of the engine  10  is higher than or equal to a given value or not. The given value is a speed of the engine  10  which enables the engine  10  to be restarted by injecting fuel into the engine  10  and then igniting it without need for the power from the starter motor  11  or the ISG  17  and which is greater than an upper limit of the threshold value used to determine whether the preset mode should be started or not. If a YES answer is obtained in step S 104  meaning that the speed of the engine  10  is higher than or equal to the given value, then the routine proceeds to step S S 105  wherein the fuel is injected into the engine  10  and then ignited to restart the engine  10  without supplying the electric power to the starter motor  11  and the ISG  17 . 
     Alternatively, if a NO answer is obtained in step S 104  meaning that the speed of the engine  10  is not higher than the given value, then the routine proceeds to step S 106  wherein it is determined whether the speed of the engine  10  is higher than or equal to the threshold value, as derived in step S 102 , or not. If a YES answer is obtained meaning that the speed of the engine  10  is higher than or equal to the threshold value, it means that the preset mode is not yet entered. The routine then proceeds to step S 107  wherein the ISG  17  is actuated to restart the engine  10 . Alternatively, if a NO answer is obtained in step S 106  meaning that the speed of the engine  10  is lower than the threshold value, then the routine proceeds to step S 108  wherein it is determined whether the preset mode is being executed to bring the pinion  12  into engagement with the ring gear  14  or not. For instance, it is determined in step S 108  whether a given period of time has passed after the preset mode is started or not. If a YES answer is obtained meaning that the preset mode is being executed, then the routine proceeds to step S 109  wherein the starter motor  11  is energized to restart the engine  10 . Alternatively, if a NO answer is obtained in step S 108  meaning that the preset mode is not being executed, then the routine proceeds to step S 110  wherein the ISG  17  is actuated to restart the engine  10 . 
     If a NO answer is obtained in step S 103  meaning that the starter request is not made by the driver of the vehicle, then the routine proceeds to step S 111  wherein it is determined whether the speed of the engine  10  is lower than or equal to the threshold value or not. The speed of the engine  10 , as already described in  FIGS. 2 ( a   9  and  2 ( b ), does not decrease linearly, but lowers to zero while alternately increasing and decreasing. Therefore, once a YES answer is obtained in step S 111 , such determination is kept for a preselected period of time. Alternatively, a YES answer may be obtained in step S 111  when the speed of the engine  10  is kept lower than the threshold value for a given control cycle. 
     If a YES answer is obtained in step s 111  meaning that the speed of the engine  10  is lower than or equal to the threshold value, then the routine proceeds to step S 112  wherein it is determined whether the speed of the engine  10  is zero or not. If the determination of step S 112  is made in a swing period of time for which the engine  10  alternately undergoes a normal rotation and a reverse rotation, the engine  10  has not yet completely been stopped, but a YES answer may be obtained. In order alleviate this problem, when the speed of the engine  10  is kept zero for a given period of time, a YES answer is obtained in step S 112 . Alternatively, if a NO answer is obtained in step S 112  meaning that the speed of the engine  10  is not zero, then the routine proceeds to step S 113  wherein the solenoid  15  is actuated to move the pinion  12  for bringing the pinion  12  into engagement with the ring gear  14  or keeping the pinion  12  engaging the ring gear  14 . 
     Alternatively, if a NO answer is obtained in step S 111  meaning that the speed of the engine  10  is not lower than or equal to the threshold value or if a YES answer is obtained in step S 112  meaning that the speed of the engine  10  is zero and that the engagement of the pinion  12  with the ring gear can be kept without need for energizing the solenoid  15 , then the routine terminates without supplying the electric power to the solenoid  15 . 
     When the condition where a YES answer is obtained in step S 101  meaning that the engine  10  is undergoing a fuel cut, and a NO answer is obtained in step S 102  meaning that the start request is not made by the driver of the vehicle continues, it will cause the speed of the engine  10  to drop with time. The speed of the engine  10  will, therefore, become lower than the threshold value in the future, so that the preset mode is started to move the pinion  12 . 
     The vehicle control system of this embodiment offers beneficial advantages, as discussed below. 
     The vehicle control system is, as apparent from the above discussion, engineered to alter the threshold value as a function of the speed of the vehicle and enter the preset mode when the speed of the engine  10  has dropped below the threshold value. Specifically, when the level of the roadway noise is high, and the speed of the engine  10  is low, the vehicle control system executes the preset mode, thereby decreasing the consumption of electric power in the vehicle. Alternatively, when the level of the roadway noise is low, and the speed of the engine  10  is high, the vehicle control system executes the preset mode, thereby masking the impact noise emanating from the pinion  12  and the ring gear  14  with the noise of the engine  10 . The vehicle control system of this embodiment is capable of minimizing the impact noise and the consumption of electrical energy upon achievement of the preset mode in the vehicle and also quickly starting the engine  10  in response to the driver&#39;s restart request to the engine  10 . 
     The ISG  17  is joined to the output shaft  13  of the engine  10 . The restart of the engine  10  is, therefore, achieved by actuating the ISG  17  before the preset mode is executed, thus ensuring a quick restart of the engine  10 . The ISG  17  is joined to the output shaft  13  of the engine  10  through the power transmitter  26  made up of the pulley and the belt, thus minimizing mechanical noise when the ISG  17  is actuated to start the engine  10 . 
     Before the rotation of the engine  10  is completely stopped, there is, as described above, the swing period of time in which the engine  10  alternately undergoes a normal rotation and a reverse rotation. In the swing period of time, the area of contact between the pinion  12  and the ring gear  14 , therefore, alternately increases and decreases, thereby resulting in a difficulty in keeping the engagement of the pinion  12  with the ring gear  14  unless the drive unit (i.e., the solenoid  15 ) continues to be energized. Alternatively, after the rotation of the engine  10  is stopped completely, the area of contact between the pinion  12  and the ring gear  14  is almost kept constant, so that the engagement of the pinion  12  with the ring gear  14  is maintained without need for energizing the drive unit. The vehicle control system of this embodiment stops actuating the drive unit when the rotation of the engine  10  is found to have been stopped, thus resulting in a decreased length of time the drive unit continues to be energized without sacrificing the stability of engagement between the pinion  12  and the ring gear  14 . 
     Modifications 
     The vehicle control system of the above embodiment, as described above, determines the speed of the vehicle and then calculates the threshold value used for the preset mode as a function of the determined speed, but may alternatively be designed to determine the threshold value depending upon which of the coasting mode and the idle stop mode has been entered when the engine  10  is stopped. Specifically, the threshold value in the coasting mode is selected to be smaller than that in the idle stop mode, thereby eliminating the need for obtaining the speed of the vehicle when it is required to execute the preset mode and using the map listing the relation between the speed of the vehicle and the threshold value. This results in simplified operations of the vehicle control system. 
     The output shaft  13  of the engine  10  is, as described above, joined to the ISG  17 , but however, may instead be connected to a MG (Motor Generator). 
     When the preset mode is entered, the vehicle control system of this embodiment works to restart the engine  10  using the starter motor  11  to rotate the pinion  12  in response to the start request, but may alternatively be designed to use the ISG  17  to restart the engine  10  in the preset mode. Specifically, when the pinion  12  is in engagement with the ring gear  14  in the preset mode, the ECU  30  may start the engine  10  using the ISG  17  in response to the start request. As illustrated in  FIGS. 2( a ) and 2( b ) , the preset mode serves to suppress the reverse rotation of the engine  10 , thus resulting in a decreased degree of torque required to restart the engine  10 . 
     When it is required to execute the preset mode, the starter motor  11  may be actuated to rotate the pinion  12  before the solenoid  15  starts to be energized. In this case, a period of time in which the starter motor  11  is energized is preferably controlled to set a difference between a rotational speed of the pinion  12  and the threshold value to be smaller than a given value, e.g., fall in a given range for decreasing a relative speed of the teeth of the pinion  12  to those of the ring gear  14  when the solenoid  15  is actuated to mesh the pinion  12  with the ring gear  14  to reduce the level of noise generated when the pinion  12  meshes the ring gear  14 . When such a control mode is performed, the vehicle control system stops supplying the electric power to the starter motor  11  before energizing the solenoid  15  to move the pinion  12  or the pinion  12  impacts the ring gear  14 , thereby preventing the engine  10  from being started by the torque produced by the starter motor  11 . 
     The vehicle control system, as described above, uses the solenoid  15  as the drive unit for the pinion  12  and works to energize the solenoid  15  to bring the pinion  12  into engagement with the ring gear  14 , but however, may alternatively be designed to use a known means as the drive unit for the pinion  12  instead of the solenoid  15 . 
     The vehicle control system of the above embodiment is mounted, as an example, in the vehicle operated by the human driver, but may alternatively be employed with vehicles engineered to be at least partially accelerated or decelerated in an automatic mode. In this case, the vehicle control system may stop the engine  10  to execute the coasting mode or the idle stop mode when a distance between the vehicle and another preceding vehicle becomes short and then execute the preset mode to restart the engine  10  when the distance to the preceding vehicle becomes long. 
     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 embodiment which can be embodied without departing from the principle of the invention as set forth in the appended claims.