Abstract:
The engine control system, according to the present invention, for a hybrid vehicle, which has an internal combustion engine and an electric motor as driving force sources, and permits stopping and starting of the engine in accordance with predetermined drive conditions, the engine control system comprises: power-consumption detector for detecting an amount of power consumption by predetermined device; first power storage unit for supplying electric energy to the motor and storing energy generated when the electric motor is operated as a generator by the internal combustion engine and energy regenerated by regeneration; second power storage unit for supplying electric energy to the predetermined device; remaining-amount detector for detecting a remaining charge of said first power storage unit; stop/start determination device for determining whether to permit stopping or starting of the engine in accordance with the amount of power consumption by the predetermined device and the remaining charge of the first power storage unit; engine stopper for stopping the engine based on a result of a decision by the stop/start determination device; and engine starter for starting the engine based on the result of the decision by the stop/start determination device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an engine control system for a hybrid vehicle, and, more particularly, to an engine control system which stops or restarts the engine in consideration of the remaining charge of a high-voltage battery and the amount of power consumption by accessories or other devices. 
     This application is based on Japanese Patent Application No. 11-62414, the contents of which are incorporated herein by reference. 
     2. Description of the Related Art 
     Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In the series hybrid vehicles, the engine drives a generator whose electric power output is used to drive the motor, which in turn drives the wheels. In the parallel hybrid vehicles, the motor coupled to the engine assists the rotation of the drive shaft of the engine while charging a high-voltage battery with electric energy using the motor itself as a generator or using a separate generator. 
     Because the engines of such hybrid vehicles can constantly run within the engine-speed range of a high fuel mileage and a low emissions level or their drive loads can be reduced, the hybrid vehicles can achieve lower fuel consumption and lower emissions levels than conventional engine vehicles. 
     As disclosed in, for example, Japanese Unexamined Patent Application, First Publication Hei 8-317505, some of those hybrid vehicles are designed to be capable of stopping the operation of the engine in accordance with predetermined driving conditions. 
     Because of the ability to stop the operation of the engine, such a hybrid vehicle is superb in preventing over-charging of the battery or achieving a further improvement on the fuel consumption by stopping the operation of the engine in accordance with predetermined operational conditions, such as the remaining charge of the battery, but has the following problem. 
     For example, while a certain amount of energy remains in the battery when fuel supply cut is carried out during deceleration, it is not adequate to keep fuel supply cut and stop the engine when the 12-V system consumes a large amount of power and energy should be supplemented from the high voltage battery to the 12-V battery, e.g., when turning on head lights while driving the vehicle in a night. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an engine control system for a hybrid vehicle, which can stop the engine in accordance with the power consumption of the 12V system. 
     To achieve this object, according to one aspect of this invention, an engine control system for a hybrid vehicle, which is equipped with an internal combustion engine (e.g., an engine E in one embodiment) and an electric motor (e.g., a motor M in the embodiment) as driving force sources and a power storage unit, and permits stopping and starting of the engine in accordance with predetermined drive conditions, comprises: power-consumption detector (e.g., the 12V system power-consumption sensor S 8  in the embodiment) for detecting the amount of power consumption by a predetermined device (e.g., accessories such as lights and an air conditioner, or an ECU); first power storage unit (e.g., a high-voltage battery BAT in the embodiment) for supplying electric energy to said motor and storing energy generated when the electric motor is operated as a generator by the internal combustion engine and energy regenerated by regeneration; second power storage unit (e.g., a 12-volt (low-voltage) auxiliary battery 12BAT in the embodiment) for supplying electric energy to the predetermined device; remaining charge detector (e.g., a current sensor S 7  or the like in the embodiment) for detecting a remaining charge of the first power storage unit (e.g., a remaining charge QBAT in the embodiment); stop/start determination device (e.g., step Sill, step S 216  and step S 316  in the embodiment) for determining whether to permit stopping or starting of the engine in accordance with the amount of power consumption by the predetermined device (e.g., the amount of 12V system power-consumption VELIDLn in the embodiment) and the remaining charge of the power storage unit; engine stopper (e.g., step S 129  and step S 320  in the embodiment) for stopping the engine based on a result of a decision by the stop/start determination device; and engine starter (e.g., step S 209  and step S 323  in the embodiment) for starting the engine based on the result of the decision by the stop/start determination device. 
     With the above structure, because stopping and starting of the engine can be permitted in accordance with the power consumed by the predetermined device and the remaining charge of the first power storage unit, the engine can be stopped according to the actual conditions based not only on the remaining charge of the first power storage unit but also on the amount of power consumption of the predetermined device. This can prevent the occurrence of the situation that while the engine is stopped because of certain amount of charge remains in the power storage unit, a large amount of power consumption by the predetermined device actually reduces the remaining charge of the first power storage and thus disables restarting of the engine. 
     The predetermined drive conditions for stopping the engine are, for example, such that on the premise that the engine can be started by the motor (e.g., when a motor-start enable/disable flag F_MOTSTB is “1” in step S 106  in the embodiment), the amount of the remaining battery charge (or the state of charge) SOC is equal to or above an over-discharge area (e.g., an energy-storage-zone flag F_ESZONE is “0” in step S 107  in the embodiment), and the engine coolant temperature TW is equal to or higher than a predetermined level (e.g., the coolant temperature TW is equal to or higher than a coolant-temperature lower limit TWFCMG in step S 108  in the embodiment). 
     According to another aspect of this invention, the first power storage unit supplies electric energy to the second power storage unit via a downverter, and is capable of supplying power to drive the predetermined device via the downverter, the stop/start determination device determines to permit stopping of the engine when the remaining charge of the power storage unit is equal to or greater than an upper threshold value of the remaining charge of the power storage unit (e.g., an engine-stop disable high-voltage battery capacity upper limit QBATIDHN in the embodiment) set in association with the amount of power consumption by the predetermined device, and determines to permit starting of the engine when the remaining charge of the power storage unit is equal to or smaller than a lower threshold value of the remaining charge of the power storage unit (e.g., an engine-stop enable high-voltage battery capacity lower limit QBATIDLN in the embodiment) set in association with the amount of power consumption by the predetermined device. 
     The stopping of the engine operation can include fuel supply cut when the vehicle decelerates. This can permit suppression of fuel consumption when the vehicle is decelerating. In this case, returning from the fuel cutting state or restart of fuel supply is implemented in accordance with the remaining charge of the power storage unit and the amount of power consumption by predetermined device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing the overall structure of one embodiment of this invention 
     FIG. 2 is a flowchart showing a routine for determining the operation mode of a motor; 
     FIG. 3 is a flowchart illustrating a routine for determining whether or not to stop the engine of an MT vehicle; 
     FIG. 4 is a flowchart illustrating a routine for determining whether or not to restart the MT vehicle; and 
     FIG. 5 is a flowchart showing a routine for a determination of engine stop/engine restart of a CVT vehicle. 
     FIG. 6 is a flowchart for monitoring the power consumption of a 12-V system. 
     FIG. 7 is a graph illustrating the relationship between the remaining charge of a battery and the power consumption of the 12V system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be explained with reference to the accompanying drawings. 
     FIG. 1 is a schematic diagram showing the overall structure of a parallel hybrid vehicle, specifically, a hybrid vehicle having a manual transmission system installed therein (including a vehicle in which a CVT (Continuously Variable Transmission) is installed). Driving forces from both an engine E and a motor M are transmitted via a transmission T, such as an automatic transmission or a manual transmission, to front wheels Wf which are the drive wheels. When the hybrid vehicle decelerates and the driving force is transmitted from the front wheels Wf to the motor M, the motor M acts as a generator to generate so-called regenerative braking force, so that the kinetic energy of the vehicle body is stored as electric energy. 
     A power drive unit PD performs the driving and regeneration of the motor M in response to control commands from a motor ECU  3 . A high-voltage battery BAT for sending and receiving electric energy to and from the motor M is connected to the power drive unit PD. The battery BAT includes a number of modules connected in series, each module having a number of cells connected in series. The hybrid vehicle includes a 12-volt auxiliary battery 12BAT for driving various accessories. The auxiliary battery 12BAT is connected to the battery BAT via a downverter DV. The downverter DV, controlled by an engine ECU  1 , reduces the voltage from the battery BAT and charges the auxiliary battery 12BAT. 
     A battery ECU  2 , for example, computes the remaining charge (or the state of charge) SOC of the battery BAT. In FIG. 1, reference numeral “4”) denotes a brake vacuum control unit, and the broken-line block  5  is a CVTECU. 
     The engine ECU  1  controls fuel supply amount controller F 1  for controlling the amount of fuel supplied to the engine E, a starter motor ST, an ignition timing, etc. in addition to the motor ECU  3  and the downverter DV. 
     Therefore, the engine ECU  1  receives SOC information on the battery remaining charge from the battery ECU  2 , motor information from the motor ECU  3  and signals from various sensors and various switches. The sensors include a vehicle speed (V) sensor S 1 , a throttle-opening-state (TH) sensor S 2 , engine coolant temperature (TW) sensor S 3 ,an engine intake-air temperature (TA) sensor S 4 , an engine speed (NE) sensor S 5  and a brake-vacuum sensor S 6  for detecting the brake-master-power negative pressure. There are also a 12-V system power-consumption sensor S 8  for detecting the amount of power consumption by the accessories and a current sensor S 7  which, together with the battery ECU  2 , detects the remaining charge of the battery BAT. Those switches, though not illustrated, include an ignition switch, a reverse switch, a brake switch, a neutral switch and a clutch switch. 
     Based on the signals from the various sensors and the various switches, the SOC information and the motor information, the engine ECU  1  performs fuel supply cut and starting of the engine. 
     For a CVT-installed vehicle, N (Neutral), P (Parking) and R (Reverse) position switches are provided in addition to the neutral switch, reverse switch and clutch switch. 
     Determination of Motor Operation Model 
     Referring to the flowchart of FIG. 2, the control mode for the motor of this hybrid vehicle will be explained. 
     In step S 1 , it is determined whether the vehicle is a manual transmission (MT) vehicle or a CVT vehicle. When it is determined that the vehicle is an MT vehicle, an engine stop determination (for MT vehicle) which will be discussed later is carried out in step S 2 . Then, a restart determination is performed in step S 3  after which the flow proceeds to step S 5 . 
     When it is determined in step S 1  that the vehicle is a CVT vehicle, an engine stop/engine restart determination (for CVT vehicle) which will be discussed later is performed in step S 4 . Then, the flow proceeds to step S 5 . 
     In step S 5 , an assist trigger determination is executed to determine whether or not to carry out assist by the motor. There are various assisting methods; for example, the determination can be made based on parameters such as the throttle opening state (e.g., throttle opening degree, or throttle opening amount) and the vehicle speed. Then, it is determined in step S 6  whether the throttle is completely closed based on a throttle-completely closed-state flag F_THIDLMG. 
     When in step S 6  the throttle-completely-closed-state flag F_THIDLMG is “0”, that is, when it is determined that the throttle valve is completely closed, and when it is determined in step S 7  that the vehicle speed V is “0”, that is, the vehicle is not running, the “idle mode” is selected in step S 8 , so that the engine E is maintained in an idling state. 
     When in step S 6  the throttle-completely-closed-state flag F_THIDLMG is “0”, that is, when it is determined that the throttle valve is completely closed, and when it is determined in step S 7  that the vehicle speed V is not “0”, the “deceleration mode” is selected in step S 9  and regenerative braking by the motor M is carried out. Further, the regeneration energy is stored to charge the battery. 
     When in step S 6  the throttle-completely-closed-state flag F_THIDLMG is “1”, that is, when it is determined that the throttle valve is open, the flow proceeds to step S 10 . In this step S 10 , it is determined based on a motor assist flag F MAST whether to select the “acceleration mode” or the “cruise mode”. 
     When it is determined in step S 5  that the assist by the motor M is required, and when it is determined in step S 10  that the motor assist flag F_MAST is “1”, the “acceleration mode” is selected in step Sil, and the motor M assists the driving force of the engine E. When it is determined in step S 10  that the motor assist flag F_MAST is “0”, the “cruise mode” is selected in step S 12 , and the motor M is not driven so that the vehicle runs only on the driving force from the engine E. Accordingly, step S 13  provides the output from the motor corresponding to each mode. 
     The following describes the “engine stop determination (MT)”, the “restart determination” and the “engine stop/engine restart determination (CVT)” in the above-described flowchart. 
     Engine Stop Determination (MT) 
     Now, the engine stop determination for an MT vehicle will be discussed. In the engine stop determination, specifically, whether or not to permit the engine to stop is determined according to the running conditions of the engine and the driver&#39;s operation, basically on the condition that stopping the engine is permitted only when the start of the engine can be assisted by the motor. 
     FIG. 3 is a flowchart illustrating a routine for determining whether or not to stop the engine of an MT vehicle. The state of a start-switch-ON start flag F_MGST is determined in step S 101  in this flowchart. When the start-switch-ON start flag F_MGST is “0”, that is, when it is the first running, a restart flag FJIDLREST for restarting the engine by opening the throttle is set to “0” in step S 102 . Then, the engine-stop control execution flag F_FCMG is set to “0” in the next step S 103 . In the next step S 104 , “0”) is set to a flag F_FCBRK which indicates that the brake is released while fuel supply cut originated from deceleration is continuing, after which the flow returns. 
     That is, at the time of first running, the individual flags are initialized by the operation from the ON action of the ignition switch to the ON action of the start switch. 
     When it is determined in step S 101  that the start-switch-ON start flag F_MGST is “1” or it is not the first running, the state of the engine-stop control execution flag F_FCMG is checked in step S 105 . The decision here is made to distinguish the condition for stopping the engine from the condition for restarting the engine after the engine has stopped. When the flag F_FCMG was already “1”, this step is passed and the flow returns to the operation-mode determining routine to execute the restart determination. 
     When it is determined that the engine-stop control execution flag F_FCMG is “0”, it is then determined in step S 106  if the motor start enable flag F_MOTSTB is “1”, that is, if the starting of the engine by the motor is possible. Whether or not the start of the engine can be performed by the motor is checked by signals from the motor ECU  3 . When it is determined that the flag F_MOTSTB is “0”, the starting of the engine by the motor is not possible so that the flow goes to step S 104 . When it is determined that the flag F_MOTSTB is “1”, the flow goes to step S 107 . Thus, if the starting of the engine by the motor is not possible, therefore, the engine will not be stopped. 
     In step S 107 , it is determined if the energy-storage-zone flag F_ESZONE is “1”, that is, the battery&#39;s remaining charge SOC is in an over-discharge zone (e.g., 20% or smaller). This prevents the engine from being stopped when the battery&#39;s remaining charge is not much. 
     When the energy-storage-zone flag F_ESZONE is “1”, that is, when it is determined that the battery&#39;s remaining charge SOC is in the over discharge zone, the flow proceeds to step S 104 . When it is determined that this flag F_ESZONE is “0”, i.e., that the battery&#39;s remaining charge SOC is not in the over-discharge zone, the coolant temperature TW is compared with the coolant-temperature lower limit TWFCMG (e.g., 40° C.) for stopping the engine in step S 108 . 
     When it is determined that the coolant temperature TW is lower than the coolant-temperature lower limit TWFCMG, the flow goes to step S 104 . This prevents the engine from being stopped when it is not in a fully warmed-up state. When it is determined that the coolant temperature TW is equal to or higher than the coolant-temperature lower limit TWFCMG, the intake-air temperature TA is compared with an intake-air temperature upper limit TAFCMG (e.g., 40° C.) for stopping the engine in step S 109 . 
     When it is determined that the intake-air temperature TA is higher than the intake-air temperature upper limit TAFCMG, the flow proceeds to step S 104 . Accordingly, when the intake-air temperature is high, the engine is not stopped in consideration of the deterioration of the startability and the air conditioning performance that should be secured. When it is determined that the intake-air temperature TA is equal to or lower than the intake-air temperature upper limit TAFCMG for stopping the engine, the flow proceeds to step S 110 . 
     In step S 110 , it is determined whether or not the shift position is the position R (Reverse). When a reverse switch flag F_RVSSW is “1”, that is, when it is determined that the shift position is the position R, the flow proceeds to step S 104 . This prevents the engine from being stopped when the vehicle starts or stops at a low speed in the reverse mode, thus improving the operability. When it is determined that the shift position is other than the position R, the state of a large-consumption-in-12V-system and restart enable flag F_FCMGBAT is determined in step Sill. The control which determines the value of this flag F_FCMGBAT will be discussed later. 
     When it is determined that this flag F_FCMGBAT is “0”, that is, when restarting the engine is not possible due to large consumption in the 12V system, the flow advances to step S 104 . When it is determined that the flag F_FCMGBAT is “1”, that is, when restarting the engine is possible even with large consumption in the 12V system, the flow advances to step S 112 . 
     In step S 112 , it is determined whether or not the throttle-completely closed-state flag F_THIDLMG is “1”. When the flag F_THIDLMG is “1”, that is, when it is determined that the throttle is not completely closed, the flow advances to step S 104 . As a result, the engine is not stopped. When the flag F_THIDLMG is “0”, that is, when it is determined that the throttle is completely closed, the flow advances to step S 113 . 
     In step S 113 , the state of a neutral-switch-OK flag F_OKNSW is determined, and then the state of a clutch-switch-OK flag F_OKCLSW is determined in step S 114 . In the next step S 115 , the state of a brake-switch-OK flag F_OKBRKSW is determined. 
     Those steps S 113 , S 114  and S 115  determine if their associated switches are functioning adequately. When the switches are not failing, the respective flags are set to “1”. When it is determined that the individual switches are normal, the flow advances to step S 116 . When each flag value is “0” because the associated switch is abnormal or is not checked as normal, the flow goes to step S 104 . 
     In step S 116 , the vehicle speed V is compared with an engine-stop-at low-speed determining vehicle speed VIDLST (e.g., 3 km/h). This comparison allows fuel supply cut while the vehicle is running (at and after step S 122 ) and the engine to be stopped when the vehicle is stopped (at and after step S 117 ). This can improve the fuel consumption. 
     When it is determined in step S 116  that the vehicle speed V is lower than the engine-stop-at-low-speed determining vehicle speed VIDLST, the state of the restart flag F_IDLREST for restarting the engine by opening the throttle is determined in step S 117 . When the flag F_IDLREST is “1” or when it is determined that the throttle is opened, the flow returns. Accordingly, when the engine is restarted by turning the ignition switch ON or opening the throttle while the engine is stopped, this flag value becomes “1” and the engine will not be stopped again until the vehicle starts moving. 
     When it is determined that the restart flag FIDLREST is “0”, the flow advances to step S 118 . In this step S 118 , the state of a neutral keeping flag F NDLY is determined. When the neutral keeping flag F_NDLY is “1”, that is, when it is determined that the neutral state is maintained for a predetermined time, the flow goes to step S 129 . When it is determined that this flag F_NDLY is “0”, the flow goes to step S 119  which determines the state of a flag F_FCMGV for checking if the vehicle speed has exceeded a predetermined vehicle speed after starting of the engine by the starter. The expression “after starting of the engine by the starter” means after restarting (the same is true in the following description). 
     When it is determined that the flag F_FCMGV is “0”, the flow returns. When it is determined that the flag F_FCMGV is “1”, the flow advances to step S 120  to check the state of a flag F_NGRMG for determining if the gear in use is other than the first gear. When this flag F_NGRMG is “0”, that is, when it is determined that the vehicle has stopped during or after running on the first gear, the engine is not stopped because there may be a mode in which the vehicle starts right after stopping, such as a temporary stop mode. In this case, therefore, the flow returns. When it is determined that this flag F_NGRMG is “1”, the state of a clutch-thrust keeping flag F_CLON is determined in step S 121 . When the clutch-thrust keeping flag F_CLON is “0”, the flow returns. When it is determined that the clutch-thrust keeping flag F_CLON is “1”, the flow advances to step S 129  to set the engine-stop control execution flag F_FCMG to “1” after which the flow returns. This causes the engine to stop when the vehicle is stopped without gear change with the clutch kept thrust at a gear position other than the first gear and reverse gear. 
     When it is determined in step S 116  that the vehicle speed V is equal to or higher than the engine-stop-at-low-speed determining vehicle speed VIDLST, the flag F_FCMGV for checking if the vehicle speed has exceeded a predetermined vehicle speed after starting of the engine by the starter is set to “1”, and the restart flag F_IDLREST for restarting the engine by opening the throttle is set to “0” in step S 122 . Then, the flow advances to step S 123 . 
     In step S 123 , the vehicle speed V is compared with a vehicle speed upper limit VFCMGST (e.g., 20 km/h) for maintaining fuel supply cut during deceleration. When it is determined that the vehicle speed V is equal to or higher than the vehicle speed upper limit VFCMGST, the flow advances to step S 104 . When it is determined that the vehicle speed V is lower than the vehicle speed upper limit VFCMGST, the flow advances to step S 124  to determine the state of the neutral keeping flag F_NDLY. 
     When it is determined that the neutral keeping flag F_NDLY is “1”, the flow advances to step S 129 . When it is determined that the neutral keeping flag F_NDLY is “0”, the flow moves to step S 125 . In step S 125 , the state of the flag F_NGRMG is checked to determine if a gear other than the first gear has been used. When this flag F_NGRMG is “0”, the flow returns. When the flag F_NGRMG is “1”, the flow advances to step S 126  to determine the state of a flag F_CLNE indicating that the engine speed NE at the time the clutch is thrust is greater than a predetermined value. The predetermined value here is, for example, 700 rpm. 
     When the engine speed NE at the time the clutch is thrust is greater than the predetermined value, it is determined that the flag F_CLNE is “0”. In this case, the flow returns. When it is determined that the flag F CLNE which indicates the result of the comparison of the engine speed NE at the time of thrusting the clutch with the predetermined value is “1”, that is, when the clutch is free at an engine speed lower than the predetermined engine speed NE, fuel supply cut during deceleration is maintained. Then, the state of a flag F_FCBRK indicating the brake having been released while fuel supply cut in deceleration is kept is determined in step S 127 . 
     When it is determined that this flag F_FCBRK is “1”, the flow returns. When it is determined that this flag F_FCBRK is “0”, the state of the brake switch is determined in step S 128 . When it is determined that the brake switch is “OFF”, the flow returns. When it is determined that the brake switch is “ON”, the engine-stop control execution flag F_FCMG is set to “1”) in step S 129 , after which the flow returns. 
     Restart Determination 
     FIG. 4 is a flowchart illustrating a routine for determining whether or not to restart the engine of an MT vehicle. 
     In the first step S 201 , the state of the engine-stop control execution flag F_FCMG is checked. Because the restart determination is performed when fuel supply cut by stopping the engine continues, when the engine-stop control execution flag F_FCMG is “0”, this flowchart is passed, a restart inhibiting delay timer tmFCMG when the remaining charge SOC of the battery goes below the lower limit is set in step S 210 , and the flow returns. 
     When the engine-stop control execution flag F_FCMG is “1”, it is determined in step S 202  whether the clutch switch is “ON” or “OFF”. When in step S 202  the clutch switch is “ON”, that is, when it is determined that the clutch is free, it is then determined in step S 203  if the throttle-completely-closed-state flag F_THIDLMG is “1”. 
     When in step S 203  the throttle-completely-closed-state flag F_THIDLMG is “1”, that is, when it is determined that the throttle is not completely closed, the flow advances to step S 206 . In step S 206 , engine restart or returning from the fuel supply cut state is performed by opening the throttle and the restart flag F_IDLREST is set to “1”. Stopping the engine or keeping fuel supply cut is therefore avoided by setting the flag this way. 
     In step S 207 , the flag F_FCMGV for checking if the vehicle speed has exceeded the predetermined vehicle speed after starter-oriented starting, the flag F_NGRMG for checking if the gear in use is other than the first gear, the clutch-thrust keeping flag F_CLON and the flag F_CLNE for checking if the engine speed NE at the time of thrusting the clutch is greater than the predetermined value are set to “0” In step S 208 , the flag F_FCBRK which indicates that the brake has been released during deceleration fuel supply cut is set to“1 ” In step S 209 , the engine-stop control execution flag F_FCMG is set to “0” after which the flow advances to step S 210 , and then the flow returns. This scheme prevents stopping of the engine, or fuel supply cut from being carried out again in other cases than the case where the gear is set to the neutral position again. 
     When in step S 203  the throttle-completely-closed-state flag F_THIDLMG is “0”, that is, when it is determined that the throttle is completely closed, the flow advances to step S 204  to determine if the gear has previously been in the neutral position. When it is determined in step S 204  that the previous gear position is the neutral one, it is then determined in step S 205  if the transmission is currently in gear. When it is determined that the transmission is currently in gear, the flow advances to step S 207  to restart the engine or return from the fuel supply cut state. 
     When it is determined in step S 204  that the transmission has previously been in gear or it is determined in step S 205  that the transmission is currently in neutral, the flow moves to step S 214 . 
     When in step S 202  the clutch switch is “OFF”, that is, it is determined that the clutch is engaged, it is then determined in step S 211  if the transmission is currently in neutral. When it is determined in step S 211  that the transmission is currently in gear, the flow returns after setting the restart-inhibiting delay timer tmFCMG in step S 212 . When it is determined in step S 211  that the transmission is currently in neutral, it is determined in step S 213  if the throttle-completely-closed-state flag F_THIDLMG is “1”. 
     When it is determined in step S 213  that the throttle-completely closed-state flag F_THIDLMG is “1”, the flow advances to step S 206 . If it is determined in step S 211  that the transmission is in neutral and it is determined in step S 213  that the throttle is thrust, restarting the engine or returning from the fuel supply cut state is carried out. When it is determined that the throttle-completely-closed-state flag F_THIDLMG is not “1” or the throttle is completely closed, the flow goes to step S 214 . 
     In step S 214 , the vehicle speed V is compared with the engine-stop-at low-speed determining vehicle speed VIDLST. When it is determined that the vehicle speed V is lower than the engine-stop-at-low-speed determining vehicle speed VIDLST, the flow advances to step S 216 . When it is determined that the vehicle speed V is equal to or higher than the engine stop-at-low-speed determining vehicle speed VIDLST, it is determined in step S 215  if the brake switch is “ON” or “OFF”. When in step S 215  the brake switch is “OFF”, the flow goes to step S 208 . When the brake is released during fuel supply cut while running, therefore, the engine returns from the fuel supply cut state. When the brake switch is “ON”, on the other hand, the flow goes to step S 216 . 
     In step S 216  is determined the flag F_FCMGBAT for determining whether or not to restart the engine based on the reduction in the remaining charge of the battery. When the restart flag F_FCMGBAT is “0”, that is, when it is determined that the engine should be restarted because the battery&#39;s remaining charge has dropped, the flow advances to step S 217 . 
     When in step S 216  the restart flag F_FCMGBAT is “1”, that is, it is determined that restarting of the engine is unnecessary, the restart-inhibiting delay timer tmFCMG is set in step S 218 , and the flow then goes to step S 219 . 
     In step S 217 , the state of the restart-inhibiting delay timer tmFCMG is checked. 
     When in step S 217  the restart-inhibiting delay timer tmFCMG is “0”, the flow advances to step S 207 . When it is determined that the restart inhibiting delay timer tmFCMG is not equal to “0”, the flow advances to step S 219 . 
     In step S 219 , the absolute value of the brake-master-power negative pressure MPGA is compared with the absolute value of the brake-master power negative pressure upper limit #MPFCMG (e.g., −430 mmHg). It is desirable that the brake-master-power negative pressure upper limit #MPFCMG be −465 mmHg, for example, when the vehicle is stopped, and be −540 mmHg when the vehicle is running in which state the negative pressure is easily secured. 
     When it is determined in this step that the brake-master-power negative pressure MPGA is equal to or lower than the brake-master-power negative pressure upper limit #MPFCMG (“YES”), a sufficient negative pressure is secured so that the flow returns without restarting the engine. 
     When it is determined that the brake-master-power negative pressure MPGA is closer to the atmospheric pressure than the brake-master-power negative pressure upper limit #MPFCMG (“NO”), the flow advances to step S 208  to restart the engine. 
     In the case that the negative pressure of the brake-master-power is likely to be gone due to pumping of the brake applied during stopping of the engine or the fuel supply cut process, the engine is restarted or the engine returns from the fuel supply cut state in order to secure the brake force, so that the brake-master-power negative pressure MPGA is secured. 
     Engine Stop/Engine Restart Determination 
     The following describes the engine stop/engine restart determination for a CVT vehicle. FIG. 5 is a flowchart showing a routine for the determination of engine stop of a CVT vehicle. 
     In step S 301  in this figure, the state of the start-switch-ON start flag F_MGST is checked. When the start-switch-ON start flag F_MGST is “0”, that is, when it is determined that it is the first running, a timer tmSFTR for waiting for stabilization of a change in shift range is set in step S 302 . In step S 322 , both the flag F_FCMGV for checking if the vehicle speed has exceeded a predetermined vehicle speed after starting of the engine by the starter and an engine-stop request flag F_FCMGSTB for the CVT are set to “0” Then, the engine-stop control execution flag F_FCMG is set to “0” in step S 323  and the flow returns. 
     When in step S 301  the start-switch-ON start flag F_MGST is “1”, that is, when it is determined that it is not the first running, it is determined in step S 303  whether or not communication information P — MOTSTB from the motor ECU  3  is “1”. This communication information P — MOTSTB indicates that the motor-assisted starting of the engine is possible when it is “1”, and indicates the motor-assisted starting of the engine is not possible when it is When in step S 303  the communication information P — MOTSTB is “1”, the coolant temperature TW is compared with the coolant-temperature lower limit TWFCMG below which the engine is to be stopped in the next step S 304 . 
     When it is determined that the coolant temperature TW is lower than the coolant-temperature lower limit TWFCMG, the flow advances to step S 302 . This prevents the engine from being stopped when the engine is not fully warmed up. When it is determined that the coolant temperature TW is equal to or higher than the coolant-temperature lower limit TWFCMG, the intake-air temperature TA is compared with the intake-air temperature upper limit TAFCMG above which the engine is to be stopped in step S 305 . 
     When it is determined that the intake-air temperature TA is higher the intake-air temperature upper limit TAFCMG, the flow advances to step S 302 . As a result, the engine is not stopped in consideration of the deterioration of the startability and the air conditioning performance that should be secured. When it is determined that the intake-air temperature TA is equal to or lower than the intake-air temperature upper limit TAFCMG, the flow proceeds to step S 306 . 
     In the next step S 306 , the state of the brake-switch-OK flag F_OKBRKSW is checked. This step checks if the brake switch is functioning properly, and a flag value of “1” is set when the switch is not failing. When it is determined that the brake switch is normal, the flow advances to step S 307 . When the brake switch is abnormal and the flag value is “0”, the flow goes to step S 302 . 
     In step S 307 , it is determined if the shift position is in the N (Neutral) range, P (Parking) range or in another range. 
     When it is determined that the shift position is in a range other than the N range and P range, the state of a drive-range flag F_CVTEDX is determined in step S 308 . This drive-range flag F_CVTEDX indicates the D range when the decision value is “0” and indicates the R range or other ranges when the decision value is“1 ”) 
     When in step S 308  the drive-range flag F_CVTEDX is “1”, the flow goes to step S 310  to stop the engine. When the drive-range flag F_CVTEDX is “0”, on the other hand, it is determined in step S 309  if the shift-range stabilization waiting timer tmSFTR is “0” When in S 309  the shift-range stabilization waiting timer tmSFTR is “0”, the flow advances to step S 322 . When the shift-range stabilization waiting timer tmSFTR is not “0”, the flow returns. 
     As the shift-range stabilization waiting timer tmSFTR is provided, even when the shift position passes the R range at the time the gear is shifted between the D range and the P range, the number of times of the engine is stopped by releasing the stopping of the engine is not reduced. 
     When it is determined in step S 307  that the shift range is in the N range or the P range, the shift-range stabilization waiting timer tmSFTR is set in the next step S 310  to stop the engine. 
     In the subsequent step S 311  is checked the state of the flag F_FCMGV for checking if the vehicle speed has exceeded a predetermined vehicle speed after starting of the engine by the starter. 
     When it is determined that the flag F_FCMGV is “0”, the flow advances to step S 312  where vehicle speed V is compared with an engine-stop-at-low-speed determining vehicle speed VIDLSTC (e.g., 15 km/h). 
     When it is determined in step S 312  that the vehicle speed V is lower than the engine-stop-at-low-speed determining vehicle speed VIDLSTC, the flow advances to step  322 . When it is determined that the vehicle speed V is equal to or higher than the engine-stop-at-low-speed determining vehicle speed VIDLSTC, the flag F_FCMGV is set to “1” in step S 313 . 
     Those steps S 311 , S 312  and S 313  do not set the flags, that have been initialized (step S 322 ) by restarting after the engine was stopped, to “1” until the vehicle speed V exceeds the engine-stop-at-low-speed determining vehicle speed VIDLSTC. 
     In a case of heavy traffic, temporary stopping and restarting or the like, stopping and restarting are likely to be frequently repeated. As in step S 119  for an MT vehicle, therefore, once the engine is restarted, it will not be stopped again until it runs for a certain distance. 
     In step S 321 , the state of the brake switch is determined. When it is determined that the brake switch is “ON”, the flow advances to step S 315  to check the state of the throttle-completely-closed-state flag F_THIDLMG. When the throttle-completely-closed-state flag F_THIDLMG is “1”, that is, when it is determined that the throttle is not completely closed, the flow goes to step S 322 . As a result, the engine is not stopped. When the throttle completely-closed-state flag F_THIDLMG is “0”, that is, when it is determined that the throttle is completely closed, the flow goes to step S 316  to check the state of the flag F_FCMGBAT for determining whether or not to restart the engine based on the reduction in the remaining charge of the battery. 
     When this flag F_FCMGBAT is “0”, that is, when it is necessary to restart the engine as a result of the battery&#39;s remaining charge having fallen, the flow goes to step S 322 . When the flag F_FCMGBAT is “1”, that is, when restarting the engine originating from the reduction in the battery&#39;s remaining charge is unnecessary, the flow goes to step S 317 . 
     In step S 317 , the absolute value of the brake-master-power negative pressure MPGA is compared with the absolute value of the brake-master power negative pressure upper limit #MPFCMG for determining whether to stop the engine. 
     When it is determined in this step that the brake-master-power negative pressure MPGA is equal to or lower than the brake-master-power negative pressure upper limit #MPFCMG (“YES”), the flow advances to step S 318  to stop the engine. 
     When it is determined that the brake-master-power negative pressure MPGA is closer to the atmospheric pressure than the brake-master-power negative pressure upper limit #MPFCMG (“NO”), the flow goes to step S 322  to restart the engine. 
     In the case that the negative pressure of the brake-master-power negative pressure is likely to be gone due to pumping of the brake applied during stopping of the engine or the fuel supply cut process, the engine is restarted or the engine returns from the fuel supply cut state in order to secure the brake force, so that the brake-master-power negative pressure MPGA is secured. This can relieve the burden that would otherwise be put on the driver due to an insufficient brake-master-power negative pressure MPGA. 
     Then, the engine-stop request flag F_FCMGSTB is set to “1” in step S 318 , and the state of a CVT-engine stop-OK flag F_CVTOK is checked in step S 319 . When the CVT-engine stop-OK flag F_CVTOK is “1”, that is, when it is determined that the engine is ready to be stopped, the engine-stop control execution flag F_FCMG is set to “1”, and then the flow returns. When the CVT-engine stop-OK flag F_CVTOK is “0”, that is, when it is determined that the engine is not ready to be stopped, the engine-stop control execution flag F_FCMG is set to “0”, after which the flow returns. 
     In the step S 321 , the state of the brake switch is checked. When it is determined that the brake switch is “OFF”, both the flag F_FCMGV for checking if the vehicle speed has exceeded a predetermined vehicle speed after starting of the engine by the starter, and the engine-stop request flag F_FCMGSTB to the CVT are set to “0”. Then, the engine-stop control execution flag F_FCMG is set to “0”, after which the flow returns. 
     Monitoring Power Consumption by 12-V System 
     Referring now to FIGS. 6 and 7, a description will be given of the control that determines the value of the large-consumption-in-12-V-system and restart enable flag F_FCMGBAT in step Sill in the engine stop determining routine (MT), in step S 216  in the restart determining routine and in step S 316  in the engine stop determining routine (CVT). It is to be noted that the power consumption of the 12-V system is power that is consumed by accessories such as head lamps. The power is supplied from the 12-V battery 12BAT. When the remaining charge of the 12-V battery 12BAT decreases, the power that is lost is supplied to the 12V battery 12BAT via the downverter DV from the high-voltage battery BAT. 
     In step S 401  in FIG. 6, the state of a high-voltage battery remaining charge-based engine stop lower limit flag F_FCBATL is checked. When the battery&#39;s remaining charge is less than a predetermined value to be discussed later, this flag F_FCBATL is set to “1”, whereas when the vehicle speed V is equal to or higher than the engine-stop-at-low-speed determining vehicle speed VIDLST (e.g., 5 km/h), the flag F_FCBATL is set to “0”) When it is determined in step S 401  that the state of the flag F_FCBATL is “1”, the vehicle speed V is compared with the engine-stop-at low-speed determining vehicle speed VIDLST. When it is determined that the vehicle speed V is lower than the vehicle speed VIDLST, “0” is set to the large-consumption-in-12V-system and restart enable flag F_FCMGBAT in step S 412 , inhibiting the stopping of the engine (step S 209  and step S 323 ). That is, when the remaining charge of the high-voltage battery BAT is low and the vehicle speed is low, the remaining charge of the battery BAT cannot become unrecoverable so that stopping the engine is inhibited. 
     When it is determined in step S 402  that the vehicle speed V is lower than the vehicle speed VIDLST, the high-voltage battery remaining-charge based engine stop lower limit flag F_FCBATL is set to “0” in step S 403  after which the flow then proceeds to step S 404 . When this flag F_FCBATL is “1”) in step S 401 , the flow also proceeds to step S 404 . 
     In step S 404 , the state of the engine-stop control flag F_FCMG is determined. When it is determined that the flag F_FCMG is “1”, the flow goes to step S 405 . When it is determined that the flag F_FCMG is “0”, on the other hand, the flow goes to step S 409 . 
     In step S 405 , the state of the flag F_FCMGBAT is determined. When the determined value is “1”, the flow goes to step S 406 , whereas when the determined value is “0”, the flow goes to step S 409 . 
     That is, when idling is not stopped (“NO” in step S 404 ) and when stopping the engine is not permitted by the flag F_FCMGBAT while idling is stopped (“0” in step S 405 ), an engine-stop disable high-voltage battery capacity upper limit QBATIDHN table is searched based on 12-V system average power-consumption VELAVE in step S 409  as shown in FIG.  7 . 
     When idling is stopped (“YES” in step S 404 ) and when stopping the engine is permitted by the flag F_FCMGBAT (“1” in step S 405 ), an engine stop enable high-voltage battery capacity lower limit QBATIDLN table is searched based on the 12V system average power-consumption VELAVE in step S 406  as shown in FIG.  7 . 
     The QBATIDHN table is searched in step S 409 , and the battery&#39;s remaining charge QBAT (the same as the battery&#39;s remaining charge SOC) is compared with the engine-stop disable high-voltage battery capacity upper limit QBATIDHN in step S 410 . That is, this discrimination is made to check whether or not the engine can be stopped. 
     When it is determined in this step S 410  that the battery&#39;s remaining charge QBAT is equal to or greater than the upper limit QBATIDHN, the large-consumption-in-12V-system and restart enable flag F_FCMGBAT is set to “1” in step S 411 , thus permitting the engine to be stopped, and then the flow returns. 
     The QBATIDLN table is searched in step S 406 , and the battery&#39;s remaining charge QBAT is compared with the engine-stop enable high voltage battery capacity lower limit QBATIDLN in step S 407 . That is, this discrimination is made to check whether or not the engine can be started (or stopping the engine can be permitted). 
     When it is determined in this step S 407  that the battery&#39;s remaining charge QBAT is equal to or greater than the lower limit QBATIDLN, the flow returns. When it is determined that the battery&#39;s remaining charge QBAT is smaller than the lower limit QBATIDLN, on the other hand, the high-voltage battery remaining-charge-based engine stop lower limit flag F_FCBATL is set to “1” and the flow proceeds to step S 412 . In step S 412 , the large-consumption-in-12V-system and restart enable flag F_FCMGBAT is set to “0”, thus inhibiting the stopping of the engine. Then, the flow returns to the operation-mode determining routine. 
     In FIG. 7, the horizontal scale represents the amount of power consumption (e.g., the average value) VELIDLn (ampere) of the 12V system (accessories), and the vertical scale the battery&#39;s remaining charge QBAT. The starting point of the engine-stop disable high-voltage battery capacity upper limit QBATIDHN is 25.4% and the starting point of the engine-stop enable high-voltage battery capacity lower limit QBATIDLN is 25%. It is to be noted that the battery&#39;s remaining charge QBAT of 25% is the lower limit at which the engine can be restarted. 
     The line of the upper limit QBATIDHN gradually moves away from the line of the lower limit QBATIDLN so as to set the time constant between the line of the upper limit QBATIDHN that indicates the lower limit of the engine-stop enable range to the line of the lower limit QBATIDLN that indicates the upper limit below which restarting the engine is needed, for example, when the battery&#39;s remaining charge decreases with constant power consumption of the 12V system. This prevents the engine-stop sustaining period from varying depending on the amount of the power consumption of the 12V system, and thus eliminates the situation wherein with a large amount of the power consumed by the 12V system, the engine should be restarted immediately after it is stopped, which would make the driver feel uncomfortable. 
     Even when a certain amount of the remaining charge of the battery is secured in step S 107 , for example, the engine stop control can be executed in consideration of the power consumption of the accessories by checking the state of the large-consumption-in-12V-system and restart enable flag F_FCMGBAT or checking if it is the engine-stop enable state or the engine restart enable state in consideration of the power consumption of the 12V system in step S 111 . 
     This invention is not limited to the above-described embodiment; although the foregoing description has been given of the case where the operation voltage of accessories is 12 V, it may be set to other voltages than 12V. 
     According to this invention, as described above, because stopping and starting of the engine can be permitted in accordance with the consumed power of accessories as well as the remaining charge of the power storage unit, it is possible to prevent the occurrence of such an even that while the engine is stopped because of a certain amount of charges remaining in the power storage unit, a large amount of power consumption by the accessories actually reduces the remaining charge of the power storage and thus disables the restarting of the engine. 
     Further, this invention has such the advantage of making it possible to determine whether or not stopping the engine should be permitted and restarting the engine should be permitted in accordance with the amount of power consumption by the accessories.