Abstract:
In order to provide a brake booster negative pressure controller which reduces deterioration in the fuel consumption due to operation for closing a throttle to assure a brake booster negative pressure while assuring the braking capability required according to driving conditions and can safely assure a braking capability even when a failure occurs in a negative pressure sensing system, said brake booster negative pressure controller comprises a brake booster for assuring a master vac negative pressure by using a negative pressure of an engine, a throttle valve whose opening angle can be operated independently from the stroke of an accel pedal, and a brake operation sensing means, wherein the negative pressure controller has a speed sensing means and a means for closing the throttle valve by a predetermined amount when the vehicle speed sensed by the vehicle speed sensing means is equal to or higher than a predetermined value and application of the brake is sensed by the brake operating sensing means.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a brake booster negative pressure controller and, more particularly, to a brake booster negative pressure controller for use in an engine operated in a state where a suction negative pressure is low. 
     In recent years, a vehicle is often provided with a brake booster in order to reduce the stepping force when a brake pedal of the vehicle is stepped on. The brake booster is partitioned into two chambers A and B by a diaphragm therein. An assist force upon braking is obtained by the difference between pressures in the chambers. The chamber A as one of the two chambers is connected to an intake manifold of a suction system of an engine via a one-way valve by piping. When the negative pressure in the intake manifold is higher than that in the brake booster, the one-way valve is opened and the negative pressure in the intake manifold is introduced into the chamber A. In a state where the brake is not applied, the pressures in the two chambers are equal. When the brake is applied, atmosphere is introduced into one of the chambers, causing the pressure difference between the two chambers, and the assist force upon braking is consequently obtained. 
     In lean-burn engine, cylinder injection engine, or the like, however, operation is performed while taking a large volume of new air, so that only a negative pressure lower than a conventional one is obtained in the intake manifold. Consequently, the negative pressure in the brake booster cannot be sufficiently increased and a sufficient assist force cannot be obtained upon braking. 
     In order to solve such a problem, the technique (refer to Japanese Patent Application Laid-Open No. 7-247866) of always monitoring the pressure in the brake booster, closing a throttle valve by a predetermined amount while the pressure value is on the atmosphere side more than a predetermined threshold, and increasing the negative pressure in the intake manifold has been proposed. 
     On the other hand, there is provided the technique (refer to Japanese Patent Application Laid-Open No. 8-164840) of a brake booster negative pressure controller comprising: pressure sensing means for sensing a pressure acting on a brake booster; throttle valve closing means for closing the throttle valve by a predetermined amount when a pressure sensed by the pressure sensing means is lowered below a predetermined threshold pressure; brake operation sensing means for sensing the operation of the brake; and throttle valve closing means for closing the throttle valve by a predetermined amount in the case where the pressure sensed by the pressure sensing means is lowered below the predetermined threshold pressure when the brake is operated by brake operating means. 
     The braking performance required of the brake is largely influenced by the driving state of the vehicle. When the speed is high, the higher braking performance is necessary. The above-mentioned two techniques of the negative pressure controller intend to always obtain the brake booster negative pressure of a predetermined amount or more at an arbitrary speed. As a result, sufficient braking performance can be always obtained at an arbitrary speed of the vehicle. For this purpose, however, the negative pressure threshold has to be determined on the basis of a case where the highest braking performance is required (at the time of high speed of the vehicle or the like) as a reference. 
     Consequently, even when the sufficient braking performance is assured at the time of low speed (sufficient negative pressure is assured in the brake booster), the throttle valve is closed by a predetermined amount at the time point when the negative pressure in the brake booster becomes lower than the threshold. Because of the useless operation, there are problems that pumping loss of the engine is increased and the fuel consumption deteriorates. 
     When the brake is applied while the accel is stepped on, the braking performance higher than that in an ordinary state is required since the braking is already applied. In the conventional brake booster negative pressure controller, however, no measure is taken against the case. 
     Further, when the negative pressure in the brake booster cannot be normally sensed due to a failure in a sensor or the like, even if the negative pressure in the brake booster is insufficient, there is a case that such a state cannot be detected. There is consequently the possibility that the sufficient braking performance cannot be obtained. 
     SUMMARY OF THE INVENTION 
     The present invention is made in consideration of the problem and it is an object of the invention to provide a brake booster negative pressure controller which reduces deterioration in the fuel consumption caused by throttle closing operation for assuring the brake booster negative pressure while assuring a braking capability required according to driving conditions and which can safely assure the braking capability even when a failure occurs in the negative pressure sensing system. 
     In order to achieve the object, according to the invention, there is provided a brake booster negative pressure controller comprising a brake booster for assuring a master vac negative pressure by using a negative pressure of an engine, a throttle valve whose opening angle can be operated independently from a stroke of an accel, and a brake operation sensing means, characterized in that the negative pressure controller has a vehicle speed sensing means and a means for closing the throttle valve by a predetermined amount when the vehicle speed sensed by the vehicle speed sensing means is equal to or higher than a predetermined value and application of a brake is sensed by the brake operation sensing means. 
     According to another fundamental embodiment of the invention, there is provided a brake booster negative pressure controller comprising a brake booster for assuring a master vac negative pressure by using a negative pressure of an engine, a throttle valve whose opening angle can be operated independently from a stroke of an accel, a negative pressure sensing means for sensing a negative pressure in the brake booster, a means for closing the throttle valve by a predetermined amount when the detection value obtained by the negative pressure sensing means becomes a value on an atmosphere side more than a predetermined value, and a brake operation sensing means, characterized in that the negative pressure controller has a vehicle speed sensing means and a means for closing the throttle valve by a predetermined amount when the vehicle speed sensed by the vehicle speed sensing means is equal to or larger than a predetermined value and application of a brake is sensed by the brake operation sensing means. 
     According to an embodiment of the invention, a brake booster negative pressure controller is characterized by comprising a means for changing the throttle valve closing amount on the basis of the vehicle speed sensed by the vehicle speed sensing means or a brake booster negative pressure sensed by the negative pressure sensing means. 
     Further, according to another preferable embodiment, the brake booster negative pressure controller of the invention is characterized by comprising an accelator hereinafter, accel stroke estimating means for estimating an accel stroke, and that the throttle valve is closed by a predetermined amount when the estimation value of the accel stroke is equal to or larger than a predetermined value and application of the brake is sensed by the brake operation sensing means. 
     According to one of embodiments, the accel stroke estimating means comprises an engine negative pressure sensing means and an engine negative pressure determining means for determining that the driver performs an operation for continuing the driving when the engine negative pressure is equal to or lower than a predetermined value. According to another embodiment, the accel stroke estimating means comprises an intake air flow rate sensing means and an intake air flow rate determining means for determining that the driver performs an operation for continuing the driving when the intake air flow rate is equal to or larger than a predetermined value. According to further another embodiment, the accel stroke estimating means comprises an accel operation sensing means and an accel operation determining means for determining that the driver performs an operation for continuing the driving when the accel operation amount is equal to or larger than a predetermined value or the detection value indicates that the accel is stepped on. 
     Further, according to another embodiment of the invention, the brake booster negative pressure controller comprises a means for changing the amount of closing the throttle valve on the basis of an engine negative pressure sensed by the engine negative pressure sensing means, an intake air flow rate sensed by the intake air flow rate sensing means, an accel stroke sensed by the accel operation sensing means, or a brake stroke sensed by the brake operation sensing means. 
     Further, according to another preferable embodiment of the invention, there is provided a brake booster negative pressure controller comprising a brake booster, a throttle valve whose opening angle can be operated independently from a stroke of an accel, a negative pressure sensing means for sensing a negative pressure in the brake booster, a negative pressure determining means for determining that the negative pressure sensed by the negative pressure sensing means is equal to or lower than a threshold, and a means for closing the throttle valve by a predetermined amount when it is determined by the negative pressure determining means that the negative pressure is equal to or lower than the threshold, 
     wherein the controller has: 
     a means for closing the throttle valve by a predetermined amount when it is determined that the negative pressure is equal to or lower than the threshold in any of the one or more other negative pressure determining means for determining that the negative pressure sensed by the negative pressure sensing means is equal to or lower than a predetermined value; two or more negative pressure determining means for determining that the negative pressure in any of one or more other brake booster negative pressure sensing means is equal to or lower than a predetermined value; and a means for closing the throttle valve by a predetermined amount when the negative pressure is equal to or lower than the threshold in any of the two or more negative pressure determining means. 
     According to the brake booster negative pressure controller of the invention constructed as mentioned above, when an arbitrary part in the negative pressure monitoring system is failed or when any of the negative pressure determining means and the like makes erroneous determination due to a bug, insufficiency of the negative pressure in the brake booster is sensed and the throttle valve can be closed, so that the necessary braking performance can be obtained. The malfunction indicator light or the like is turned on, thereby enabling the driver to be urged to repair the failure before a new failure occurs. 
     While obtaining the braking capability according to the driving conditions on the basis of information such as vehicle speed, brake operation, and accel operation, the fuel consumption deterioration caused by the operation for closing the throttle valve necessary for the brake capability can be minimized. 
     Further, when both of the accel and brake are stepped on, the throttle valve is closed irrespective of the detection value of the negative pressure sensor. Consequently, the negative pressure higher than an ordinary threshold is assured and the braking force higher than an ordinary one can be obtained, so that the possibility of runaway due to erroneously operation of the pedal can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the whole configuration of an engine control system having a brake booster negative pressure controller of an embodiment of the invention. 
     FIG. 2 is a block diagram showing the control of a control unit A in the brake booster negative pressure controller in FIG.  1 . 
     FIG. 3 is a block diagram showing the control of a control unit B in the brake booster negative pressure controller in FIG.  1 . 
     FIG. 4 is a flowchart of a control executed by a negative pressure determining means in the brake booster negative pressure controller in FIG.  1 . 
     FIG. 5 is a diagram illustrating the contents of a means for inhibiting lean burn according to driving conditions of the brake booster negative pressure controller in FIG.  1 . 
     FIG. 6 is a diagram showing a negative pressure threshold switching means in the brake booster negative pressure controller in FIG.  1 . 
     FIG. 7 is a diagram of an actual example in which the negative threshold is tabulated and obtained from the relation between the booster negative pressure and the vehicle speed. 
     FIG. 8 is a diagram of another actual example in which the negative pressure threshold is tabulated and obtained from the relation between the booster negative pressure and the vehicle speed. 
     FIG. 9 is a diagram showing the whole configuration of an engine control system having a brake booster negative pressure controller of another embodiment of the invention. 
     FIG. 10 is a partial control block diagram of a brake stroke sensor replacing a brake switch in the brake booster negative pressure controller in FIG.  9 . 
     FIG. 11 is a partial control block diagram of an accel stroke sensor replacing an accel switch in the brake booster negative pressure controller in FIG.  9 . 
     FIG. 12 is a partial control block diagram of an accel step-on determining means replacing the accel switch in the brake booster negative pressure controller in FIG.  9 . 
     FIG. 13 is a partial control block diagram of an intake manifold negative pressure sensing means replacing the accel switch in the brake booster negative pressure controller in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of a brake booster negative pressure controller of the invention will be described in detail hereinbelow with reference to the drawings. 
     FIG. 1 shows a whole configuration of an engine control system provided with a brake booster negative pressure controller of a first embodiment of the invention. In FIG. 1, air introduced into an engine  507  is taken from the inlet  502   a  of an air cleaner  502 , passes through an intake air meter  503  and an electronic throttle  505  in which a throttle valve  505   a  for controlling an intake air flow is disposed, and enters an intake manifold  506 . 
     The intake air led to the intake manifold  506  is distributed to intake pipes connected to cylinders  507   b  of the engine  507  and is led to a combustion chamber  507   c  in the cylinder  507   b.    
     On the other hand, fuel such as gasoline is sucked from a fuel tank  514  and pressurized by fuel pumps  510  and  511 , and the pressurized fuel is supplied to a fuel system in which a fuel injection valve  509  and variable fuel pressure regulators  512  and  513  which control the fuel to be within a predetermined range are disposed. The fuel pressure is measured by a fuel pressure sensor  523  and is injected from the fuel injection valve  509  whose fuel injection port is open to the combustion chamber  507   c  in each of the cylinders  507   b  into the combustion chamber  507   c . The air flowed into the combustion chamber  507   c  and the injected fuel are mixed, ignited by a spark plug  508  by piezo electricity from an ignition coil  522 , and are burned. The exhaust gas burned in the combustion chamber  507   c  of the engine  507  is led to an exhaust pipe  519  and is discharged to the outside of the engine  507  via a catalyst  520 . 
     A signal indicative of the intake air flow is outputted from the air flow meter  503  and is supplied to a control unit A  515 . A crank angle sensor  516  is rotated by the camshaft (not shown in the diagram) of the engine  507  and generates a signal indicative of a rotation position of the crankshaft. This signal is also supplied to the control unit A  515 . By the signals, fuel injection timing, ignition timing, an opening angle of SCV, and the like are controlled. An A/F sensor  518  provided for the exhaust pipe  519  senses an actual operation air-fuel ratio from components of the exhaust gas and outputs a signal of the ratio. This signal is also supplied to the control unit A  515 . 
     The control unit A  515  executes a fuel supply control, an ignition timing control, an intake air volume control, an emission regulation adapted control, and the like by fetching signals from various sensors and the like which sense the operating conditions of the engine  507  as inputs, executing a predetermined computing process, outputting various control signals derived as results of the computing process, and outputting predetermined control signals to the fuel injection valve  509 , the ignition coil  522 , and the like. In the control unit A  515 , by processing a signal of the crank angle sensor  516 , fluctuation in the rotation of the engine  507  is calculated. 
     Further, an accel pedal  540  and a brake pedal  527  are arranged. An accel switch  541  for detecting the operation of the accel pedal  540  is attached to the accel pedal  540 . A brake switch  532  for detecting the operation of the brake pedal  527  is attached to the brake pedal  527 . A speed sensor  542  is mounted in the transmission (not shown). Detection values detected by those components are supplied to the control unit A  515  and a control unit B  535 . 
     A brake booster  525  constructing the brake booster negative pressure controller of the embodiment is operated by the brake pedal  527  and two negative pressure sensors, namely, a sensor A  526  and a sensor B  531  for sensing the negative pressure of the brake booster  525  are attached to the brake booster  525 . A power is supplied from a power supply circuit A  534  to the sensor A  526  and a power is supplied from a power supply circuit B  533  to the sensor B  531 . Detection values obtained by the negative pressure sensors A  526  and B  531  are supplied to the control units A  515  and B  535 . A communication line  536  is provided between the control units A  515  and B  535  and data is transferred between the control units A  515  and B  535  through the communication line  536 . 
     The opening angle of the electronic throttle  505  is controlled by the control unit A  515  and the power are supplied from a power source  539  for the electronic throttle. The power source  539  can be turned on/off by operating relays  537  and  538  from the control units A  515  and B  535 . In a normal state where there is no fault in the brake booster negative pressure controller, the switches of the relays  537  and  538  are kept in an ON state. In the electronic throttle  505 , when the power is not supplied, the throttle valve  505   a  is returned to the fully closed position by a spring. 
     FIG. 2 is a block diagram of a control showing the contents of processes performed in the control unit A  515 . An accel stroke as an intention of the driver is inputted together with the engine rotational speed to a target torque computing means  105  where a target torque necessary to realize the acceleration/deceleration corresponding to the operation of the driver is calculated. In a target air-fuel ratio computing means  104 , among a stoichiometric ratio and air-fuel ratios corresponding to a homogeneous lean burn and a stratification lean burn in addition to the engine speed, a maximum air-fuel ratio (target air-fuel ratio) which can realize the target torque is usually selected. A target cylinder intake volume computing means  106  calculates a target intake volume necessary to realize both of the target air-fuel ratio and a target torque from the target air-fuel ratio and torque, a throttle opening angle computing means  107  calculates a target throttle opening angle which can realize the target intake volume, and an intake volume correcting means  108  controls so that the electronic throttle  505  is opened at the target throttle opening angle. In this instance, the target intake volume and an intake volume obtained from an airflow sensor (not shown) or an intake manifold pressure sensor (not shown) are compared with each other. When the difference between them is equal to or larger than a predetermined value, the electronic throttle power source relay  537  is turned off, the power of the electronic throttle  505  is cut, and the throttle valve  505   a  is fully closed. 
     While the air-fuel ratio corresponding to the homogeneous lean burn or the stratification lean burn is selected in the target air-fuel ratio computing means  104 , the air flow rate is higher than that at the time of stoichiometric ratio, so that the negative pressure in the intake manifold  506  becomes lower than that at the time of stoichiometric ratio. Consequently, a case where a sufficient negative pressure is not assured in the brake booster  525  occurs. 
     On the other hand, a detection value obtained by the brake booster negative pressure sensor A  526  is supplied to both of a brake booster negative pressure determining means A  103   a  and a negative pressure sensor A sampling value diagnosing means  109 . The negative pressure sensor sampling value diagnosing means  109  obtains the absolute value of the difference between a sampling value of the negative pressure sensor A  526  in the control unit A  515  and a sampling value of the negative pressure sensor A  526  in the control unit B  535  transferred from the control unit B  535  via the communication line  536 . When the result is equal to or larger than a predetermined value, the diagnosing means  109  determines that a fault occurs in the A/D converter in either one of the control units and sets a flag FBRKNGA requesting to set an abnormal time lean burn inhibition flag FBRKNG. 
     A similar diagnosis is performed in a negative pressure sensor B sampling value diagnosing means  110  with respect to a sampling value of the negative pressure sensor B  531  and a value of a flag FBRKNGB requiring to set the lean burn inhibition flag FBRKNG at the time of occurrence of abnormality is obtained. In the negative pressure determining means A  103   a , a process shown in the flowchart of FIG. 4 is executed every predetermined time. 
     In FIG. 4, the voltage of the negative pressure sensor A  526  is converted into a negative pressure in step  301  which is substituted for a variable PMBAVRA. After that, in step  302 , whether or not the value of the variable PMBAVRA is equal to or smaller than a negative pressure PLA necessary for braking is determined. When it is determined that it is equal to or smaller than the negative pressure (insufficient negative pressure determining threshold) PLA necessary for braking, the processing routine advances to step  303  and a flag FSTLNA requiring to set a flag FSTLN for inhibiting the lean burn is set to “1”. When it is determined that the value of the variable PMBAVRA is equal to or larger than the negative pressure PLA necessary for braking, the routine advances to step  304 , and further, whether or not the variable PMBAVRA is equal to or larger than a braking assuring negative pressure (negative pressure assurance determining threshold) PHA (&gt;PLA) is determined. When it is determined to be equal to or larger than the negative pressure assurance determination threshold PHA, the flag FSTLNA for requiring to set the flag FSTLN inhibiting the lean burn is cleared to “0”in step  305 . When it is determined in step  304  that the variable PMBAVRA is equal to or smaller than the negative pressure assurance determining threshold PHA, the process is finished without performing anything. 
     A similar process is performed also to a voltage obtained by the negative pressure sensor B  531  by a negative pressure determining means B  103   b . The detected negative pressure is substituted for a variable PMBAVRB. When it is determined that the value of the variable PMBAVRB is equal to or smaller than a negative pressure PLB necessary for braking, a flag FSTLNB requiring to set the flag FSTLN which inhibits the lean burn is set to “1”. When it is determined that the variable PMBAVRB is equal to or larger than a braking assuring negative pressure (negative pressure assurance determining threshold) PHB (&gt;PLB), the flag FSTLNB is cleared to “0”. 
     The values of the flags FSTLNA and FSTLNB inhibiting the lean burn obtained as mentioned above are inputted to a lean burn inhibition determining means  112 . Even one of the flags is “1”, the lean burn inhibition determining means  112  sets the value of the flag FSTLN inhibiting the air-fuel ratio corresponding to lean burn from being selected by the target air-fuel ratio computing means  104  is set to “1”. 
     The above-described processes in the negative pressure determining means A  103   a  and B  103   b , the negative pressure sensor sampling value diagnosing means  109 , the negative pressure sensor sampling value diagnosing means  110 , and the lean burn inhibition determining means  112  are performed also in the control unit B  535  as shown in FIG.  3 . 
     Sampling values of the negative pressure sensors A  526  and B  531  obtained in the control unit B  535  have sampling errors different from those included in the sampling values of the negative pressure sensors in the control unit A  515 . Consequently, in order to avoid that the determination results in negative pressure determining means A  203   a  and B  203   b  are different from those in the control unit A  515 , the determination is carried out to negative pressure recognition values PMBAVRA and PMBAVRB in the control unit A  515 . 
     The negative value recognition values PMBAVRA and PMBAVRB are transferred to the control unit B  535  via the communication line  536 . The determination results in the negative pressure determining means A  203   a  and B  203   b  are supplied to a lean burn inhibition determining means  212  and a lean burn inhibition determination flag FSTLN#S is obtained in a manner similar to the case in the control unit A  515 . 
     The values of the lean burn inhibition determination flags FSTLN and FSTLN#S obtained as mentioned above are inputted to both of the negative pressure determination result diagnosing means  111  in the control unit A  515  and a negative pressure determination result diagnosing means  211  in the control unit B  535 . The negative pressure determination result diagnosing means  111  in the control unit A  515  compares the lean burn inhibition determination flags FSTLN and FSTLN#S. When they do not coincide with each other, a flag FNG requesting to set the abnormal time lean burn inhibition flag FBRKNG is set. A similar process is carried out also in the negative pressure determination result diagnosing means  211  in the control unit B  535 . When the flags do not coincide with each other, a flag FNG#S requesting to set the abnormal time lean burn inhibition flag FBRKNG#S is set. 
     When either the abnormal time lean burn inhibition flag FBRKNGA, FBRKNGB, or FNG is set to “1”, an abnormal time lean burn inhibition determining means  113  in the control unit A  515  sets the abnormal time lean burn inhibition flag FBRKNG to “1”. Similarly, in an abnormal time lean burn inhibition determining means  213  in the control unit B  535  as well, when either FBRKNGA#S, FBRKNGB#S, or FNG#S is equal to “1”, the abnormal time lean burn inhibition flag FBRKNG#S is set to “1”and, simultaneously, a malfunction indicator light  543  (FIG. 1) is turned on. 
     In the target air-fuel ratio computing means  104 , since the selection of the air-fuel ratio corresponding to the lean burn is inhibited when the flag FSTLN, FBRKNG, or FBRKNG#S is set to “1”, the target air-fuel ratio is set to a low value. As a result, the value of the target intake volume calculated by the target intake volume computing means  106  becomes low, the throttle opening angle is controlled to become small, the negative pressure in a collector  506  increases, and the negative pressure in the brake booster  525  accordingly increases, thereby obtaining a necessary brake assist force. 
     By constructing the brake booster negative pressure controller of the embodiment as mentioned above, when an arbitrary one part in the negative pressure monitoring system (the negative pressure sensor A  526 , the power supply circuit  534  for the sensor  526 , the negative pressure sensor B  531 , the power supply circuit  533  for the sensor  531 , the A/D A/D converter (not shown) in the control unit A  515 , the A/D converter (not shown) in the control unit B  535 , and harnesses) is failed or when either the negative pressure determining means  103   a ,  103   b , the sampling value diagnosing means  109 ,  110 , or the negative pressure determination result diagnosing means  111  determines erroneously due to a bug, insufficiency in the negative pressure in the brake booster is detected and the throttle valve  505   a  can be closed, so that the necessary braking performance can be obtained. The malfunction indicator light  543  is turned on, thereby enabling the driver to be urged to repair the trouble before a new failure occurs. 
     Since the braking capability necessary for the brake differs according to the speed of a vehicle and the operation of the driver, in order to obtain a sufficient braking capability under any driving conditions of the vehicle, the value of the negative pressure insufficiency determining threshold PLA has to be determined on the basis of the case where the highest braking capability is required as a reference. As a result, even when a high braking capability is not required so much in a driving state such that the vehicle speed is low and when a negative pressure sufficient to perform braking from such a state is still assured in the brake booster, the operation for closing the throttle valve  505   a  to assure the negative pressure is executed at the time point when the negative pressure becomes to an atmosphere side more than the negative pressure insufficiency determining threshold PLA. Because of the unnecessary operation, the pumping loss increases and the fuel ratio deteriorates accordingly. 
     The brake booster negative pressure controller of the embodiment, however, has the function of obtaining the braking performance for assuring the negative pressure necessary according to the operating conditions, thereby enabling the increase in the pumping loss caused by the operation for closing the throttle valve which assures the negative pressure more than necessary to be suppressed and improving the fuel consumption. The construction for realizing the function will be described hereinbelow. 
     In FIG. 1, the detection values obtained by the accel switch  541 , the brake switch  532 , and the speed sensor  542  are inputted to both of the control units A  515  and B  535 . In the control units A  515  and B  535 , the lean burn inhibition is determined and the negative pressure insufficiency determination threshold PLA is switched to/from the negative pressure assurance determination threshold PHA on the basis of the detection values. 
     FIG. 5 shows a means for inhibiting the lean burn according to driving conditions. The determination of the lean burn inhibition or permission based on the detection values of the accel switch  541 , the brake switch  532 , and the speed sensor  542  is performed by the means for inhibiting the lean burn according to the travel conditions. When both of the brake and accel pedals are stepped on (painted part in FIG.  5 ), the braking capability higher as compared with that in an ordinary state is required in order to maintain the vehicle in a stationary state. Consequently, it is necessary to increase the negative pressure in the brake booster by inhibiting the lean burn and closing the throttle valve. In the control to perform the above operation, therefore, the flag FLNSTP requiring to set the flag FSTLN inhibiting the lean burn is set. 
     Since a higher braking capability is required at the time of high vehicle speed, when the vehicle speed is equal to or higher than a predetermined value VSPLIM (high vehicle speed) and the brake is applied (the part of oblique lines in FIG.  5 ), the flag FLNSTP requiring to set the flag FSTLN inhibiting the lean burn is set. 
     When the flag FLNSTP is set to “1”, the lean burn inhibition flag FSTLN is set in the lean burn inhibiting means  112  in FIG.  2 . In a manner similar to the case of insufficient negative pressure, the air-fuel ratio corresponding to the stoichiometric state is selected in the target air-fuel ratio computing means  104  and the throttle valve  505   a  is operated in the closing direction. With the increase in the negative pressure in the intake manifold  506 , therefore, the negative pressure in the brake booster  525  also increases, so that the necessary braking capability can be obtained. 
     The means for inhibiting lean burn (FIG. 5) according to the driving conditions is also provided in the control unit B  535 . Determination is made in a manner similar to the case in the control unit A  515 , the result FLNSTP#S of the determination is supplied to the lean burn inhibition determining means  212 , and a process is executed similarly. 
     On the other hand, the negative pressure insufficiency determination threshold PLA and the negative pressure assurance determination threshold PHA are switched by the negative pressure threshold switching means shown in FIG.  6  and are obtained by searching a table with a vehicle speed. From the table, a value of the brake booster negative pressure necessary to obtain the brake assist force required to stop the vehicle from an arbitrary speed is obtained. At the time of low vehicle speed which does not require a high braking capability, therefore, the execution of closing operation of the throttle valve for assuring the negative pressure more than necessary is avoided, the increase in the pumping loss is suppressed, and the fuel consumption is improved. 
     FIG. 7 illustrates the above operation. (1) When the negative pressure threshold is fixed, in order to obtain a sufficient brake performance with an arbitrary vehicle speed, the negative pressure insufficiency determination threshold has to be set to a value at which the brake capability required at the time of the highest vehicle speed is obtained, so that the negative pressure threshold is inevitably set to a high value (negative pressure side). On the contrary, (2) when the negative pressure threshold tabulated in the present embodiment is employed, setting is carried out according to the required braking capability. Consequently, an insufficient negative pressure is not determined even in the region of (3), so that the increase in pumping loss caused by the operation for closing the throttle valve to assure the negative pressure can be suppressed by such an amount and the fuel consumption can be improved. 
     When the function of the means for determining lean burn inhibition according to the driving conditions is considered, the threshold table as shown in FIG. 8 can be also set and the fuel consumption can be improved in a wider region. 
     According to the brake booster negative pressure controller of the embodiment as mentioned above, even in the case where an arbitrary part of the negative pressure monitoring system is failed, insufficiency in the negative pressure can be detected, the brake booster negative pressure which enables safe braking can be assured, and deterioration in the fuel consumption caused by the operation for closing the throttle valve necessary for the braking performance can be minimized while obtaining the braking capability according to the travel conditions on the basis of the information of the vehicle speed, brake operation, accel operation, and the like. 
     Although the embodiment of the invention has been described above in detail, the invention is not limited by the foregoing embodiment but can be variably changed in designing without departing from the spirit of the invention described in the scope of the claims. 
     For example, FIG. 9 shows an engine control system having the brake booster negative pressure controller of another embodiment of the invention. According to the embodiment, the brake booster is provided with only the single negative pressure sensor  526  and the single control unit  515 . 
     In the system having only one negative pressure sensing mechanism of the present embodiment, in cases such that the negative pressure sensor is failed and the insufficiency in the negative pressure cannot be detected, the means for inhibiting lean burn (FIG. 5) according to the driving conditions described in the first embodiment can be used as an auxiliary negative pressure assuring means for assuring the minimum negative pressure. 
     In the present embodiment, a negative pressure switch (not shown) can be substituted for the functions of the negative pressure sensor B  531  and the negative pressure determining means  103   b  (or  203   b ) of the first embodiment. 
     In the present embodiment, the function of the brake switch  532  can be replaced by a sensor  544  for sensing the stepping force of the brake as shown in FIG.  10 . Detection can be also performed by a brake step-on determining means  545  which determines that the brake is applied when the detection value of the sensor  544  is equal to or larger than a predetermined value. 
     Further, in the embodiment, the function of the accel switch  541  is replaced by a sensor  546  for sensing the accel stroke as shown in FIG.  11  and an accel stroke estimating means having an accel step-on determining means  547  for determining that the accel pedal is stepped on when the detection value of the sensor  546  is equal to or larger than a predetermined value can be constructed. 
     Furthermore, as shown in FIG. 12, the accel stroke estimating means can be constructed by comprising an air flow rate sensing means  548  and an accel step-on determining means  549  for determining that the accel pedal is stepped on when the detection value of the sensor  548  is equal to or larger than a predetermined value. As shown in FIG. 13, the accel stroke estimating means can be constructed by comprising an intake manifold negative pressure detecting means  550  and an accel step-on determining means  551  for determining that the access pedal is stepped on when a detection value of the intake manifold negative pressure detecting means  550  is equal to or lower than a predetermined value. 
     Further, the detection value obtained from the speed sensor mechanism  542  in the foregoing two embodiments can be replaced by the product of the engine rotational speed and the gear ratio in a transmission (not shown). 
     As will be understood from the above description, according to the brake booster negative pressure controller of the invention, since the negative pressure is monitored by two independent negative pressure monitoring systems, even if an arbitrary part in the monitoring system is failed, the insufficiency of negative pressure can be certainly detected. 
     Since the negative pressure determination threshold is switched according to the vehicle speed, the execution of closing operation of the throttle valve for assuring the negative pressure more than necessary is avoided at the time of low vehicle speed when the high braking capability is not required, and the increase in the pumping loss of the engine is suppressed, so that the fuel consumption can be improved. 
     Furthermore, when both of the accel and brake are applied, the throttle valve is closed irrespective of the detection value of the negative pressure sensor. Consequently, the negative pressure higher than an ordinary threshold is assured, the braking force higher than the ordinary one can be obtained, and the possibility of runaway due to an erroneous operation of the pedal can be reduced. 
     Further, in the case where the brake is applied when the vehicle speed is equal to or higher than a predetermined value, the operation for closing the throttle valve is carried out irrespective of the detection value of the negative pressure sensor. Consequently, the negative pressure higher than the ordinary threshold can be assured, the braking force higher than the ordinary one can be obtained, and the sufficient braking performance can be obtained also at the time of applying the brake when the vehicle speed is high.