Abstract:
A vehicle equipped with a brake booster that uses the negative pressure in an intake pipe of an internal combustion engine includes an automatic stop-start apparatus that performs an automatic stop-start control of the engine while securing a sufficient brake booster negative pressure in accordance with changes in the atmospheric pressure. The apparatus detects the pressure in the brake booster. If the detected pressure exceeds a predetermined reference value during the automatically stopped state of the engine, the apparatus outputs warning information or automatically restarts the engine. The reference value is changed in accordance with the magnitude of the atmospheric pressure around the vehicle.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 11-183062 filed on Jun. 29, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a technology for automatically stopping and starting an internal combustion engine installed in a motor vehicle. 
     2. Description of Related Art 
     In order to reduce the fuel consumption of and the emissions and noise from internal combustion engines installed in motor vehicles and the like, automatic stop-start apparatus for automatically stopping the operation of a vehicle-installed internal combustion engine during a stoppage of the vehicle (e.g., during a red traffic light or the like) and automatically starting the engine when the vehicle begins to move have been pursued. 
     There is a widely known automotive brake mechanism that has a brake booster for boosting the brake operating force from a driving person by using a negative pressure that occurs in an intake passage of an internal combustion engine (intake pipe negative pressure). 
     In this brake mechanism, an intake pipe negative pressure occurring in the intake passage is constantly supplied to the brake booster when the internal combustion engine is in operation. Therefore, if a negative pressure in the brake booster is consumed for a braking operation, the amount of pressure consumed is offset by an intake pipe negative pressure from the intake passage. 
     When the internal combustion engine is in an automatically stopped state, no intake pipe negative pressure occurs in the intake passage, so that no negative pressure is supplied from the intake passage to the brake booster. Therefore, if an automatic engine stop-start technology is applied to a motor vehicle equipped with the above-described brake mechanism, the following problem may occur. For example, when the motor vehicle is stopped on a slope and a negative pressure in the brake booster is consumed by a braking operation, the amount of pressure consumed is not compensated for. In that case, the amount of assist force that can be produced by the brake booster decreases, and the amount of brake-operating force that needs to be provided by a driving person correspondingly increases. 
     Japanese Patent Application Laid-Open No.  58-35245  discloses an automatic engine stop-start apparatus for coping with the aforementioned problem. If the pressure in the brake booster exceeds a predetermined value (that is, if the degree of negative pressure in the brake booster becomes less than a predetermined value) due to a driving person&#39;s braking operation while the engine is in the automatically stopped state, the apparatus warns the driving person, or automatically starts the engine while warning the driving person, to produce an intake pipe negative pressure and supply the intake pipe negative pressure to the brake booster. In this manner, the apparatus secures a brake booster negative pressure needed to boost the brake operating force. 
     Since the negative pressure in the brake booster is a relative pressure between the pressure in the brake booster and the atmospheric pressure, the brake booster negative pressure changes in magnitude with changes in the atmospheric pressure even if the pressure in the brake booster remains unchanged. 
     The above-described automatic engine stop-start apparatus uses a fixed criterion (predetermined value) to evaluate the brake booster negative pressure, and does not take changes in the atmospheric pressure into consideration. Therefore, in the apparatus, it is difficult to accurately evaluate the degree of brake booster negative pressure when the engine is in the automatically stopped state. 
     More specifically, at locations where the atmospheric pressure is relatively low, for example, at high altitudes or the like, it is difficult to accurately evaluate the degree of brake booster negative pressure. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a technology for automatically stopping and automatically starting an internal combustion engine in a vehicle equipped with a brake booster that uses the negative pressure in an intake pipe of the engine, while securing a sufficient brake booster negative pressure in accordance with changes in the atmospheric pressure. 
     To achieve the aforementioned and/or other objects, a control apparatus for an internal combustion engine in accordance with one aspect of the invention includes a pressure detector that detects a pressure in the brake booster, and a controller that automatically stops the internal combustion engine when a predetermined stop condition is met, and that automatically starts the internal combustion engine when a predetermined start condition is met. The controller also outputs a warning and/or performs an engine restart operation when the pressure detected by the pressure detector exceeds a reference value while the internal combustion engine is automatically stopped. The controller also can change the reference value in accordance with an atmospheric pressure. 
     Therefore, if the actual degree of negative pressure in the brake booster decreases during the automatically stopped state of the engine, the apparatus is able to immediately start the engine or advise a driving person to start the engine. As a result, the intake pipe negative pressure that occurs in the intake pipe of the engine is supplied to the brake booster, so that a sufficient negative pressure is reliably provided in the brake booster. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
     FIG. 1 is a schematic diagram of a construction of an internal combustion engine to which a first embodiment of the invention is applied; 
     FIG. 2 is a diagram of an internal construction of an ECU; 
     FIG. 3 is a flowchart illustrating an automatic stop-start control routine; 
     FIG. 4 is a diagram for illustration of a related-art method for setting a reference value used for determining a brake booster negative pressure; 
     FIG. 5 is a flowchart illustrating a reference value setting control routine; 
     FIG. 6 is a diagram for illustration of a method for setting a reference value used for determining a brake booster negative pressure according to the first embodiment; 
     FIG. 7 is a schematic diagram of a construction of an internal combustion engine to which an automatic stop-start apparatus according to a second embodiment of the invention is applied; 
     FIG. 8 is a diagram of an internal construction of an ECU; 
     FIG. 9 is a diagram indicating a brake booster negative pressure determining reference value in a case where the initial value of a learned atmospheric pressure value is set to a value corresponding to a relatively low altitude above sea level; and 
     FIG. 10 is a diagram indicating a brake booster negative pressure determining reference value in a case where the initial value of a learned atmospheric pressure value is set to a value corresponding to a relatively high altitude above sea level 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of an automatic stop-start apparatus for an internal combustion engine according to the invention will be described hereinafter with reference to the accompanying drawings. 
     FIG. 1 is a schematic diagram of a construction of a vehicle having an internal combustion engine to which a first embodiment of the invention is applied. 
     An internal combustion engine  1  shown in FIG. 1 is a water-cooled four-cylinder gasoline engine. The engine  1  is connected to a transmission (T/M)  200  via a clutch mechanism (or torque converter)  100 . 
     The transmission  200  is connected to drive wheels via a propeller shaft, a differential gear and the like. 
     In the power transmission system described above, torque is transmitted from an output shaft or crankshaft (not shown) of the engine  1  to the transmission  200  via the clutch mechanism  100 , when the clutch mechanism  100  is engaged. The rotating speed is reduced or increased by the transmission  200  . Torque is then transmitted from the transmission  200  to the drive wheels via the propeller shaft, the differential gear and the like. 
     An intake manifold  2  is connected to the engine  1 . Branch pipes of the intake manifold  2  communicate with combustion chambers (not shown) of corresponding cylinders via intake ports. The intake manifold  2  is connected to a surge tank  3 . The surge tank  3  is connected to an air cleaner box  5  via an intake pipe  4 . 
     A throttle valve  7  for adjusting the flow of air through the intake pipe  4  is provided in a pathway of the intake pipe  4 . The throttle valve  7  is provided with an actuator  8  that is formed by a stepper motor or the like and drives the throttle valve  7  in opening and closing directions in accordance with the magnitude of current applied thereto. The throttle valve also is provided with a throttle position sensor  9  that outputs an electric signal corresponding to the extent of opening of the throttle valve  7 . 
     The throttle valve  7  is connected to an accelerator lever (not shown) that is turned in association with an accelerator pedal  10  provided in a passenger compartment. The accelerator lever is provided with an accelerator position sensor  11  that outputs an electric signal corresponding to the amount of rotation of the accelerator lever (that is, an electric signal corresponding to the amount of depression of the accelerator pedal  11 ). 
     In the intake system constructed as described above, air drawn into the air cleaner box  5  is filtered by an air filter disposed in the air cleaner box  5  to remove dust or the like from air. After air is then led into the intake pipe  4  the rate of air flow is adjusted by the throttle valve  7  After that, air is led to the surge tank  3  and then to the intake manifold  2 . Air is then distributed into the intake ports of the engine  1  via the corresponding branch pipes. 
     An air flow meter  6  that outputs an electric signal corresponding to the mass of air flowing through the intake pipe  4  is provided at a site in the intake pipe  4  upstream of the throttle valve  7 . 
     A negative pressure passage  38  is connected to the surge tank  3 . The negative pressure passage  38  is connected to a brake booster  39  that serves as a power source for a mechanism for braking the vehicle having the engine  1 . Provided in a pathway of the negative pressure passage  38  is a one-way valve  45  that allows air to flow from the brake booster  39  toward the surge tank  3  and that prevents air from flowing from the surge tank  3  toward the brake booster  39 . 
     The brake booster  39  is disposed between a master cylinder  40  and a brake pedal  40 . The brake booster  39  is designed so as to add to the operating force on the brake pedal  41  and transmit the increased force to the master cylinder  40 . 
     More specifically, the brake booster  39  has a diaphragm that is provided in a box so as to be easily movable in opposite directions, a pushrod that extends through the diaphragm and that is supported by a casing so as to advance and withdraw in cooperation with reciprocating movements of the diaphragm, and a return spring that urges the diaphragm in such a direction as to withdraw the pushrod backward. 
     Therefore, the space inside the casing of the brake booster  39  is divided into two spaces by the diaphragm. Hereinafter, the space on the pushrod advancing side of the diaphragm is termed negative pressure chamber, and the space on the pushrod withdrawing side of the diaphragm is termed atmospheric chamber. 
     The negative pressure chamber communicates with the negative pressure passage  38  and receives an intake pipe negative pressure that occurs in the surge tank  3 . 
     The atmospheric chamber communicates with the negative pressure passage  38  when the diaphragm and the pushrod are held at a normal position by the force from the return spring. When the diaphragm and the pushrod are moved in the advancing direction, a communication path between the atmospheric chamber and the negative pressure passage  38  is closed, and the atmospheric chamber communicates with a predetermined atmosphere introducing passage. 
     The master cylinder  40  has a reservoir tank for storing brake fluid, and a pressure chamber for pressurizing brake fluid. 
     A master cylinder piston is disposed for easy reciprocating movements in the pressure chamber of the master cylinder  40 . 
     A base end of the master cylinder piston is connected to a distal end of the pushrod of the brake booster  39  so that the master cylinder piston is movable back and forth in concert with reciprocating movements of the pushrod. 
     The pressure chamber of the master cylinder  40  communicates with wheel cylinders of brake calipers provided for individual wheels of the vehicle, via brake lines  43 , so that brake fluid pressurized in the pressure chamber is delivered to the wheel cylinders. 
     The brake pedal  41 , provided in the compartment, is connected to a base end of the pushrod of the brake booster  39  via an operating rod and the like. 
     In the brake system constructed as described above, the diaphragm of the brake booster  39  is urged to the normal position by the return spring when the brake pedal  41  is not operated. During the non-operated state of the brake pedal  41 , the intake pipe negative pressure is delivered to both the atmospheric chamber and the negative pressure chamber, so that the pressures in the atmospheric chamber and the negative pressure chamber become equal. The diaphragm is thus held at the normal position. 
     When the brake pedal  41  of the braking system is operated, the operating force on the brake pedal  41  is transmitted to the pushrod of the brake booster  39  via the operating rod and the like to advance the pushrod. 
     When the pushrod is advanced, the negative pressure-introducing path to the atmospheric chamber of the brake booster  39  is closed, and the atmosphere introducing passage to the atmospheric chamber is opened, so that the atmospheric chamber receives the atmospheric pressure. Since the intake pipe negative pressure continues to be delivered to the negative pressure chamber of the brake booster  39 , the pressure in the atmospheric chamber becomes higher than the pressure in the negative pressure chamber, thereby generating a force that moves the diaphragm in the pushrod advancing direction (hereinafter, referred to as “assist force”). The assist force acts on the pushrod via the diaphragm. 
     As a result, the pushrod is advanced by the operating force from the brake pedal  41  combined with the assist force generated by the brake booster  39 . The thus-boosted operating force is transmitted from the pushrod to the master cylinder piston of the master cylinder  40  to pressurize brake fluid in the pressure chamber. 
     The pressure of brake fluid pressurized in the pressure chamber of the master cylinder  40  is delivered to the wheel cylinders via the brake lines  43 . 
     The brake booster  39  is provided with a brake booster pressure sensor  42  that outputs an electric signal corresponding to the pressure in the negative pressure chamber. 
     Fuel injection valves  13   a ,  13   b ,  13   c ,  13   d  (hereinafter, collectively referred to as “fuel injection valves  13 ”) are disposed in the branch pipes of the intake manifold  2  so that an inject-on port of each fuel injection valve  13  faces the corresponding intake port. The fuel injection valves  13  are connected to a fuel distribution pipe  14  that is connected to a fuel pump (not shown). 
     The fuel injection valves  13  are connected to drive circuits  15   a ,  15   b ,  15   c ,  15   d  (hereinafter, collectively referred to as “drive circuits  15  ”), whereby the corresponding fuel injection valves  13  are opened. 
     In the fuel injection system constructed as described above, fuel ejected from the Fuel pump is supplied to the fuel distribution pipe  14 . Fuel is then distributed from the fuel distribution pipe  14  to each fuel injection valve  13 . When a drive circuit  15  applies a drive current to the corresponding fuel injection valve  13  the fuel injection valve  13  opens to inject fuel supplied from the fuel distribution pipe  14 , into the corresponding intake port. After being injected into the intake port, fuel is supplied into the corresponding combustion chamber of the engine  1  while mixing with air flowing from the intake manifold  2  into the intake port. 
     An exhaust manifold  16  is connected to the engine  1 . Branch pipes of the exhaust manifold  16  are connected to exhaust ports of the combustion chambers of the corresponding cylinders. The exhaust manifold  16  is connected to an exhaust pipe  17  that is connected at a downstream end thereof to a muffler (not shown). 
     Provided in a pathway of the exhaust pipe  17  is an exhaust gas control catalyst device  18  for substantially removing harmful gas components, such as carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HC) and the like, from exhaust gas discharged from the engine  1 . The exhaust gas control catalyst device  18  may be formed by a three-way catalyst, an oxidizing catalyst, a selective reducing NOx catalyst, an absorbing-reducing NOx catalyst, or the like. 
     In the exhaust system constructed as described above, exhaust gas is discharged from the combustion chambers of the engine  1  into the exhaust ports after air-fuel mixture is burned in the combustion chambers. After being discharged into the exhaust ports, exhaust gas is led from the exhaust ports through the branch pipes, and then is led from the exhaust manifold  16  into the exhaust pipe  17 . 
     After being introduced into the exhaust pipe  17 , exhaust gas is introduced into the exhaust gas control catalyst device  18 , whereby harmful gas components are substantially removed from the exhaust gas. After that, exhaust gas is emitted from the muffler into the atmosphere. 
     An air-fuel ratio sensor  19  is provided at a site in the exhaust pipe  17  upstream of the exhaust gas control catalyst device  18 . The air-fuel ratio sensor  19  outputs an electric signal corresponding to the air-fuel ratio of exhaust gas flowing through the exhaust pipe  17 . 
     The engine  1  also has a power generator mechanism  400  that is connected to a crank pulley (not shown) mounted on a base end of the crankshaft via a belt (not shown). The power generator mechanism  400  is formed by, for example, an alternator, a regulator, a controller, and the like. 
     The engine  1  is also provided with a crank position sensor  20  that outputs a pulse signal every time the crankshaft (not shown) turns a predetermined angle (e.g., 10 degrees), and a water temperature sensor  21  that outputs an electric signal corresponding to the temperature of cooling water that flows in a water jacket formed in the engine  1 . 
     The engine  1  or the clutch mechanism  10  is provided with a starter motor  300  having a pinion gear that meshes with a ring gear provided on a circumferential portion of a flywheel or drive wheel (not shown) that is connected to a distal end of the crankshaft. 
     The starter motor  300  is provided with a battery  500  for supplying drive power to the starter motor  300 , and a starter switch (ST.SW)  26  for changing between the supply of drive power to the starter motor  300  and the discontinuation of the power supply. 
     When the starter switch  26  is switched from an OFF state to an ON state by a driving person&#39;s operation, drive current is supplied from the battery  500  to the starter motor  300 , so that a rotating shaft of the starter motor  300  turns. 
     The torque of the rotating shaft of the starter motor  300  is transmitted to the crankshaft via the pinion gear and the flywheel, so that the cranking of the engine  1  is performed. 
     The transmission (T/M)  200  is provided with a pickup type rotational speed sensor  12  that outputs a pulse signal every time an output shaft (not shown) of the transmission  200  turns a predetermined angle. 
     The engine  1  constructed as described above is provided with an electronic control unit (ECU)  22  for controlling the engine  1 . 
     Input ports of the ECU  22  are connected, via electric wiring, to the air flow meter  6 , the throttle position sensor  9 , the accelerator position sensor  11 , the rotational speed sensor  12 , the air-fuel ratio sensor  19 , the crank position sensor  20 , the water temperature sensor  21 , the starter switch  26  and the power generator mechanism  400  and, furthermore, to a shift position sensor  23  that detects the position of a shift lever disposed in the compartment, a brake switch  24  that detects the operated/non-operated states of the brake pedal, an ignition switch (IG.SW)  27  that is operated by a driving person, an SOC controller  28  that calculates a state of charge of the battery  500  from an integrated value of the amount of current discharged from and the amount of current charged into the battery  500 , an atmospheric pressure sensor  44  that outputs an electric signal corresponding to the atmospheric pressure, and the like. Thus, the output signals of the various sensors and the like are input to the ECU  22 . 
     Output ports of the ECU  22  are connected to the actuator  8 , the drive circuits  15 , the starter motor  300 , the power generator mechanism  400 , and the like, via electric wiring. Using the signals from the various sensors as parameters, the ECU  22  is able to send control signals to the actuator  8 , the drive circuits  15 , the starter motor  300 , the power generator mechanism  400 , and the like. 
     As shown in FIG. 2, the ECU  22  has a CPU  29 , a ROM  30 , a RAM  31  and a backup RAM  32 , that are interconnected by a bidirectional bus  37 . The bidirectional bus  37  is also connected to a first input interface circuit  33  via an A/D converter  36 , and is also connected to a second input interface circuit  34  and an output interface circuit  35 . 
     The first input interface circuit  33  is connected to the air flow meter  6 , the throttle position sensor  9 , the accelerator position sensor  11 , the air-fuel ratio sensor  19 , the water temperature sensor  21 , the SOC controller  28 , the brake booster pressure sensor  42 , the atmospheric pressure sensor  44 , and the power generator mechanism  400 , via electric wiring. The first input interface circuit  33  receives the output signals of the sensors, voltage generated by the power generator mechanism  400 , and the like, and inputs the signals, the voltage and the like to the A/D converter  36 . 
     The A/D converter  36  converts the various signal inputs from the first input interface circuit  33  from analog forms into digital forms, and then sends the signals to the CPU  29  or to the RAM  31  via the bidirectional bus  37 . 
     The second input interface circuit  34  is connected to the rotational speed sensor  12 , the crank position sensor  20 , the shift position sensor  23 , the brake switch  24 , the starter switch  26 , and the ignition switch  27 , via electric wiring. The second input interface circuit  34  inputs the signals from the various sensors and the like, and sends the signals to the CPU  29  or to the RAM  31  via the bidirectional bus  37 . 
     The output interface circuit  35  is connected to the actuator  8 , the drive circuits  15 , the starter motor  300 , and a controller of the power generator mechanism  400 , via electric wiring. The output: interface circuit  35  sends various control signals from the CPU  29  to the actuator  8 , the drive circuits  15 , the starter motor  300 , or the controller of the power generator mechanism  400 . 
     The ROM  30  stores various application programs of a fuel injection amount control routine for determining an amount of fuel to be injected from each fuel injection valve  13  a fuel injection timing control routine for determining a timing of injecting fuel from each fuel injection valve  13  an ignition timing control routine for determining an ignition timing of each cylinder, a throttle opening amount control routine for determining an extent of opening of the throttle valve  7 , and the like, and various control maps. 
     Examples of the control maps stored in the ROM  30  include a fuel injection control map that indicates a relationship between the operation state of the engine  1  and the fuel injection amount, a fuel injection timing control map that indicates a relationship between the operation state of the engine  1  and the fuel injection timing, an ignition timing control map that indicates a relationship between the operation state of the engine  1  and the ignition timing, and the like. 
     The RAM  31  stores signals from the various sensors, results of computations by the CPU  29 , and the like. The results of computations include, for example, an engine revolution speed calculated based on the output signal of the crank position sensor  20 , a vehicle traveling speed (vehicle speed) or a vehicle travel distance calculated based on the output signal of the rotational speed sensor  12 , and the like. The signals from the various sensors, the results of computations by the CPU  29 , and the like are updated to the latest data every time the crank position sensor  20  outputs a pulse signal. 
     The backup RAM  32 , is a non-volatile memory that retains data even after the engine  1  is stopped. 
     The CPU  29  operates in accordance with the application programs stored in the ROM  30 . Using the signals from the various sensors, the CPU  29  executes the fuel injection control, the ignition control, the throttle control, and the like, and also executes an automatic stop-start control, which is a main focus of the invention. 
     The automatic stop-start control according to this embodiment will be described below. 
     To perform the automatic stop-start control, the CPU  29  executes an automatic stop-start control routine as illustrated in FIG.  3 . The automatic stop-start control routine is repeatedly executed at every predetermined time interval when the ignition switch  27  is in the ON state. 
     In the automatic stop-start control routine, the CPU  29  first determines in S 301  whether the ignition switch  27  is in the ON state. 
     If it is determined in S 301  that the ignition switch  27  is in the OFF state, the CPU  29  ends the execution of the routine. 
     If the determination in S 301  is affirmative (YES), the CPU  29  proceeds to S 302 , in which the CPU  29  determines whether the automatic stop-start control is being performed, that is, whether the engine  1  is in the automatically stopped state. 
     If the determination in S 302  is negative (NO), the process proceeds to S 303 , in which the CPU  29  determines whether a condition for automatically stopping the engine  1  is met. 
     The automatic stop condition includes, for example: 
     a condition that the vehicle traveling speed is zero; 
     a condition that the value of the output signal of the shift position sensor  23  indicates a NEUTRAL position; 
     a condition that the engine revolution speed calculated based on the value of the output signal of the crank position sensor  20  is less than or equal to a predetermined revolution speed; and 
     a condition that the value of the output signal of the accelerator position sensor  11  indicates that the amount of depression of the accelerator pedal  10  is zero. 
     If the determination in S 303  is NO, the CPU  29  temporarily ends the execution of the routine. 
     If the determination in S 303  is YES, the process proceeds to S 304 , in which the CPU  29  reads a reference value (that is determined by a different routine of setting a reference value) from the backup RAM  32 , or from the RAM  31 , and inputs the output signal of the brake booster pressure sensor  42  (brake booster pressure), and compares the brake booster pressure with the reference value. 
     The reference value is set as an upper limit value of the pressure that is needed for the brake booster  39  to produce a desired assist force, for the following reason. The negative pressure in the brake booster  39  is a relative pressure of the pressure (absolute pressure) in the negative pressure chamber of the brake booster  39  relative to the atmospheric pressure. Therefore, if the pressure in the negative pressure chamber becomes higher than a predetermined pressure, the pressure therein relative to the atmospheric pressure decreases and makes it impossible for the brake booster  39  to produce a desired assist force. 
     It is preferable that the reference value be variable in accordance with the atmospheric pressure occurring at the location where the vehicle travels. 
     The brake booster pressure sensor  42  is a sensor for detecting the absolute pressure in the negative pressure chamber of the brake booster  39 , and the degree of negative pressure in the negative pressure chamber is determined by a relative pressure based on the output signal of the brake booster pressure sensor  42  and the atmospheric pressure. Therefore, if the reference value is a fixed value, a problem is likely to occur at locations where the atmospheric pressure is relatively low, for example, at high altitudes. That is, at such a location, the pressure in the negative pressure chamber may be less than the reference value even though the pressure in the negative pressure chamber relative to the atmospheric pressure is actually lower than an allowable range. 
     FIG. 4 indicates an example case where the reference value is fixed at about 400 mHg. At 0 m of altitude above sea level, the atmospheric pressure is about 760 mmHg, and the relative pressure based on the reference value and the atmospheric pressure becomes higher than 300 mmHg. Therefore, as long as the pressure in the negative pressure chamber is less than the reference value, the brake booster  39  is allowed to produce a sufficient assist force. 
     Above h [m] (indicated by the broken vertical line in FIG. 4) of altitude above sea level, the atmospheric pressure is less than 600 mmHg, and the relative pressure based on the reference value and the atmospheric pressure becomes less than 200 mmHg. Therefore, there is a danger that the brake booster  39  may become unable to produce a sufficient assist force even though the pressure in the negative pressure chamber is lower than the reference value. 
     In this embodiment, however, the reference value used to evaluate the degree of negative pressure in the brake booster  39  is corrected based on the actual atmospheric pressure occurring at a location where the vehicle travels, so as to improve the determination precision. 
     More specifically, the CPU  29  sets a reference value through a reference value setting control routine as illustrated in FIG.  5 . The reference value setting control routine is pre-stored in the ROM  30  or the like, and is executed repeatedly at every predetermined time interval when the ignition switch  27  is in the ON state. 
     In this routine, the CPU  29  first inputs the output signal of the atmospheric pressure sensor  44  (atmospheric pressure Pa) in S 501 . 
     In S 502 , the CPU  29  calculates a reference value Prs by subtracting a negative pressure PrN needed for the brake booster  39  to produce a minimum required assist force, from the atmospheric pressure Pa input in S 501 . 
     In S 503 , the CPU  29  stores the reference value Prs calculated in S 502  into a predetermined area of the RAM  31  or of the backup RAM  32 . The CPU  29  then ends the execution of the FIG. 5 routine. 
     Due to the reference value setting control routine, the reference value is set taking the actual magnitude of atmospheric pressure into consideration, as indicated in FIG.  6 . As a result, it becomes possible for the brake booster  39  to produce a minimum required assist force at any altitude above sea level as long as the pressure in the  25 , negative pressure chamber of the brake booster  39  is less than the reference value. 
     Referring back to the automatic stop-start control routine in FIG. 3, if it is determined in S 304  that the value of the output signal of the brake booster pressure sensor  42  (brake booster pressure) is less than the reference value, it is considered that a negative pressure needed for the brake booster  39  to produce a minimum required assist force is already provided in the negative pressure chamber of the brake booster  39  and, therefore, the operation of the engine  1  does not need to be continued in order to supply the intake pipe negative pressure into the negative pressure chamber. Then, the CPU  29  proceeds to S 305 . 
     In S 305 , the CPU  29  executes the automatic stop control of the engine  1 . In the automatic stop control, the CPU  29  stops the operation of the engine  1  by, for example, discontinuing the supply of the drive power from the drive circuits  15  to the fuel injection valves  13  (generally termed fuel cut control) and/or controlling the throttle valve actuator  8  so as to completely close the throttle valve  7 . 
     After executing the processing of S 305 , the CPU  29  temporarily ends the execution of the routine. 
     Conversely, if it is determined in S 304  that the value of the output signal of the brake booster pressure sensor  42  (brake boost pressure) is greater than or equal to the reference value, it is considered that the negative pressure needed for the brake booster  39  to produce the minimum required assist force is not provided in the negative pressure chamber of the brake booster  39  and, therefore, the operation of the engine  1  needs to be continued in order to supply the intake pipe negative pressure into the negative pressure chamber. Then, the CPU  29  temporarily ends the execution of the routine (i.e., without executing the automatic stop control). 
     If it is determined in S 302  that the engine  1  is in the automatically stopped state, the process proceeds to S 306 , in which the CPU  29  determines whether a condition for automatically starting the engine  1  is met. 
     The automatic start condition includes, for example: 
     a condition that the value of the output signal of the shift position sensor  23  indicates the NEUTRAL position; 
     a condition that the engine revolution speed calculated based on the value of the output signal of the crank position sensor  20  is 0 rpm; and 
     a condition that a clutch pedal is depressed and the clutch is disengaged. 
     If it is determined in S 306  that the condition for automatically starting the engine  1  is met, the process proceeds to S 307 , in which the CPU  29  executes the control of automatically starting the engine  1 . Then, the CPU  29  temporarily ends the execution of the routine. 
     In the automatic start control, the CPU  29  executes, for example, the activation of the starter motor  300 , the activation of the fuel injection valves  13  and the activation of ignition plugs (not shown). 
     If it is determined in S 306  that the condition for automatically starting the engine  1  is not met, the process proceeds to S 308 . In S 308 , the CPU  29  reads the reference value from the RAM  31  or from the backup RAM  32 , and inputs the value of the output signal of the brake booster pressure sensor  42  (brake booster pressure), and compares the brake booster pressure with the reference value as in S 304 . 
     If it is determined in S 308  that the brake booster pressure is less than the reference value, it is considered that the negative pressure needed for the brake booster  39  to produce the minimum required assist force is already provided in the negative pressure chamber of the brake booster  39  and, therefore, the engine  1  does not need to be started in order to supply the intake pipe negative pressure into the negative pressure chamber. The CPU  29  then temporarily ends the execution of the routine. 
     Conversely, if it is determined in S 308  that the brake booster pressure is greater than or equal to the reference value, it is considered that the negative pressure needed for the brake booster  39  to produce the minimum required assist force is not provided in the negative pressure chamber of the brake booster  39  and, therefore, the engine  1  needs to be started in order to supply the intake pipe negative pressure into the negative pressure chamber. Then, the CPU  29  proceeds to S 309 . 
     In S 309 , the CPU  29  informs a driving person that the engine  1  will be forcibly started. More specifically, the CPU  29  informs a driving person that the engine  1  is to be forcibly started, by, for example, turning on a warning light provided in the passenger compartment, or by producing a warning sound from a speaker provided in the compartment, or by displaying a message on a display device provided in the compartment, or by producing an announcement from a speaker provided in the compartment, or the like. 
     After executing the processing of S 309 , the CPU  29  proceeds to S 307 , in which the CPU  29  executes the control of automatically starting the engine  1  in order to forcibly start the engine  1  as described above. 
     Due to the restart of the engine  1 , an intake pipe negative pressure occurs in the surge tank  3  and is supplied to the negative pressure chamber of the brake booster  39  via the negative pressure passage  38  so that the degree of negative pressure in the negative pressure chamber of the brake booster  39  increases. 
     Thus, in this embodiment, the reference value used to evaluate the degree of negative pressure in the brake booster  39  is set based on the actual atmospheric pressure. As a result, it becomes possible to accurately evaluate the degree of negative pressure in the brake booster  39  even in a situation where the atmospheric pressure fluctuates. 
     Therefore, when the negative pressure in the brake booster  39  becomes insufficient during the automatically stopped state of the engine  1 , the engine  1  is immediately started automatically to supply the brake booster  39  with the intake pipe negative pressure that occurs in the surge tank  3 . In this manner, the embodiment is able to reliably provide a negative pressure that is needed for the brake booster  39  to boost the brake operating force. 
     In the foregoing embodiment, a dedicated sensor for detecting the atmospheric pressure (atmospheric pressure sensor) is employed since the internal combustion engine to which the invention is applied is represented by a mass flow-type internal combustion engine equipped with an air flow meter or a similar sensor for directly detecting an amount of intake air in the foregoing embodiment. However, if the invention is applied to a speed density-type internal combustion engine wherein an amount of intake air is estimated from the intake pipe pressure and the engine revolution speed, the atmospheric pressure may be detected by using a sensor that detects the intake pipe pressure, instead of using the dedicated atmospheric pressure sensor. 
     In that case, the atmospheric pressure may be determined based on a signal output from the intake pipe pressure sensor immediately before the engine is started (for example, when the ignition switch is on and the starter switch is off), or a signal output from the intake pipe pressure sensor when the engine is in a steady state. 
     A second embodiment of an automatic stop-start apparatus for an internal combustion engine according to the invention will be described below. Constructions and portions that distinguish the second embodiment from the first embodiment will be described below, and constructions and portions substantially the same as those of the first embodiment will not be described again. 
     Whereas the first embodiment evaluates the degree of negative pressure in the brake booster  39  based on the atmospheric pressure detected directly by the atmospheric pressure sensor, the second embodiment evaluates the degree of negative pressure in the brake booster  39  based on an estimated atmospheric pressure. 
     FIG. 7 is a schematic diagram of a construction of an internal combustion engine  1  to which an automatic stop-start apparatus according to the second embodiment is applied. A surge tank  3  is provided with an intake air temperature sensor  46  that outputs an electric signal corresponding to the temperature of intake air flowing through the surge tank  3  (atmospheric temperature). 
     As indicated in FIG. 8, the output signal of the intake air temperature sensor  46  is input to a CPU  29  or to a RAM  31  of an ECU  22  via a first input interface circuit  33  and an A/D converter  36 . 
     Pre-stored in a ROM  30  of the ECU  22  is a charging efficiency control map that indicates a relationship among the throttle opening amount TA, the engine revolution speed Ne and the mass of intake air per revolution of the engine  1  GNo occurring in a condition where the atmosphere (atmospheric pressure and the atmospheric temperature) is in a predetermined standard state (e.g., atmospheric pressure Po=99 kPa, atmospheric temperature To=298K). The charging efficiency control map is empirically determined. 
     The CPU  29  of the ECU  22  estimates an actual atmospheric pressure Pa based on the charging efficiency control map and the output signals of various sensors including the intake air temperature sensor  46 , and corrects the fuel injection amount based on the estimated atmospheric pressure Pa. 
     An example of a specific method for estimating an atmospheric pressure will be described below. 
     The CPU  29  first reads from the RAM  31  the value of the output signal of the air flow meter  6  (mass of intake air) Ga, the engine revolution speed Ne, the value of the output signal of the throttle position sensor  9  (throttle opening amount) TA, and the output signal (atmospheric temperature) Ta of the atmospheric pressure sensor  44 . 
     Subsequently, the CPU  29  calculates a mass of intake air per revolution of the engine  1  GNa by dividing the mass of the intake air Ga by the engine revolution speed Ne. The CPU  29  also calculates a mass of intake air per revolution of the engine  1  GNo corresponding to the standard state of the atmosphere and the throttle opening amount TA and the engine revolution speed Ne, by using the throttle opening amount TA and the engine revolution speed Ne as parameters and accessing the charging efficiency control map stored in the ROM  30 . 
     The mass of intake air per revolution of the engine  1  GNa can be expressed as in equation (1). 
     
       
         GNa=Va×K×(Pa/Ta) 
       
     
     where: 
     Va=volume of atmospheric air actually taken into the engine  1  per revolution; 
     K=constant 
     Pa=pressure of atmospheric air actually taken into the engine  1 ; 
     Ta=temperature of atmospheric air actually taken into the engine  1 ; 
     Vs=total piston displacement of the engine  1 ; and 
     ηv=as volumetric efficiency of the engine  1 . 
     The mass of intake air per revolution of the engine  1  GNo corresponding to the standard state of the atmosphere can be expressed as in equation (2). 
     
       
         GNo=Vao×K×(Po/Tc) =Vs×ηv×K×(Po/To)  (2) 
       
     
     where: 
     Vao=volume of atmospheric air actually taken into the engine  1  per revolution; 
     K=constant 
     Po=pressure of atmospheric air actually taken into the engine  1 ; 
     To=temperature of atmospheric air actually taken into the engine  1 ; 
     Vs=total piston displacement of the engine  1 ; 
     ηv=volumetric efficiency of the engine  1 . 
     Under a condition that the throttle opening amount TA and the engine revolution speed Ne remain the same, the volumetric efficiency ηv remains constant regardless of the magnitude of the atmospheric pressure, so that the actual atmospheric pressure Pa can be determined as in equation (3) based on equations (1) and (2). 
     
       
         Pa=(GNa×Po×Ta)/(GNo×To)  (3) 
       
     
     The atmospheric pressure Pa is stored into a predetermined area of the RAM  31  or of the backup RAM  32 , and is updated by a learning control. 
     The above-described method makes it possible to minimize the error of the estimated value of atmospheric pressure from the actual atmospheric pressure. 
     Subsequently, in order to evaluate the degree of negative pressure occurring in the brake booster  39  during the automatic stop-start control of the engine  1  and, in particular, during the automatically stopped state of the engine  1 , the CPU  29  reads the atmospheric pressure Pa estimated by the above-described method from the RAM  31  or the backup RAM  32 . Then, the CPU  29  evaluates the degree of negative pressure in the brake booster  39  in a procedure substantially the same as that in the first embodiment. 
     According to the fuel injection control of the engine, if the power supply to the ECU  22  is discontinued due to disconnection of a battery or the like, the learned value of atmospheric pressure stored in the RAM  31  or the backup RAM  32 , is cleared. In that case, an initial value pre-stored in the ROM  30  is used as a basis to restart the fuel injection control. However, if the initial value of atmospheric pressure pre-stored in the ROM  30  is immediately used, without any modification, in the automatic stop-start control, drawbacks as stated below may occur. 
     For example, if an initial value of the learned atmospheric pressure value is pre-set for the fuel injection control assuming a relatively low altitude above sea level (a location where the atmospheric pressure is relatively high), and is used in the automatic stop-start control, the reference value for evaluating the degree of negative pressure in the brake booster  39  is set as a value corresponding to an atmospheric pressure at a low altitude above sea level as indicated in FIG.  9 . Therefore, when the vehicle is at a high altitude above sea level, there is danger of false evaluation of the degree of negative pressure in the brake booster  39 . 
     Therefore, in a case where the learned value of atmospheric pressure calculated in the fuel injection control or the like is used in the automatic stop-start control, it is preferable that a storage medium (e.g., EEPROM) that is freely readable-programmable and capable of retaining the stored content even after discontinuation of the power supply thereto (e.g., a non-volatile memory) be provided in the ECU  22  and that the learned value of atmospheric pressure calculated in the fuel injection control be stored into the storage medium. If the learned value of atmospheric pressure provided in the fuel injection control is set as an initial value, it is preferable that the learned value of atmospheric pressure stored in the storage medium be used to perform the automatic stop-start control. 
     In a case where an initial value dedicated to the automatic stop-start control is pre-set separately from an initial value for the fuel injection control and where the learned value of atmospheric pressure for the fuel injection control is reset to the initial value due to disconnection of the battery or the like, the initial value dedicated to the automatic stop-start control may be used to evaluate the degree of negative pressure in the brake booster. 
     In that case, it is preferable that the initial value dedicated to the automatic stop-start control be set assuming a relatively high altitude above sea level (a location where the atmospheric pressure is relatively low) as indicated in FIG.  10  . 
     The second embodiment described above is able to accurately evaluate the degree of negative pressure in the brake booster by using existing hardware constructions even in an internal combustion engine that is not equipped with an atmospheric pressure sensor, an intake pipe pressure sensor or the like. 
     In the above-described automatic stop-start apparatus for an internal combustion engine according to the invention, when it is determined whether sufficient negative pressure needed for the brake booster to boost the brake operating force is provided in the brake booster, the reference value used for the determination is corrected based on the atmospheric pressure occurring at the (current) location of the vehicle. Therefore, the degree of negative pressure in the brake booster can be accurately evaluated even in a situation where the atmospheric pressure fluctuates. 
     Therefore, when the actual degree of negative pressure in the brake booster decreases during the automatically stopped state of the engine, the automatic stop-start apparatus according to the invention is able to immediately start the internal combustion engine or advise a driving person to start the engine. As a result, the intake pipe negative pressure occurring in the intake passage of the engine is supplied to the brake booster, so that a sufficient negative pressure is reliably provided in the brake booster. 
     In the illustrated embodiment, the controller (ECU  22 ) is implemented as a programmed general purpose computer. It will be appreciated by those skilled in the art that the controller can be implemented using a single special purpose integrated circuit (e.g., ASIC) having a main or central processor section for overall, system-level control, and separate sections dedicated to performing various different specific computations, functions and other processes under control of the central processor section. The controller also can be a plurality of separate dedicated or programmable integrated or other electronic circuits or devices (e.g., hardwired electronic or logic circuits such as discrete element circuits, or programmable logic devices such as PLDs, PLAs, PALs or the like). The controller can be implemented using a suitably programmed general purpose computer, e.g., a microprocessor, microcontroller or other processor device (CPU or MPU), either alone or in conjunction with one or more peripheral (e.g., integrated circuit) data and signal processing devices. In general, any device or assembly of devices on which a finite state machine capable of implementing the flowcharts shown in FIGS. 3 and 5 can be used as the controller. A distributed processing architecture can be used for maximum data/signal processing capability and speed. 
     While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments or constructions. On the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single embodiment, are also within the spirit and scope of the invention.