Patent Publication Number: US-9902386-B2

Title: Method and system for reducing vacuum consumption in a vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 13/943,189, entitled “METHOD AND SYSTEM FOR REDUCING VACUUM CONSUMPTION IN A VEHICLE,” filed on Jul. 16, 2013, the entire contents of which are hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND/SUMMARY 
     Vacuum may be used in a vehicle to apply motive force in vehicle systems. For example, vacuum may be used to apply vehicle brakes, operate a turbocharger waste gate, and adjust valve positions in heating and ventilation ducts. However, vacuum in vehicle systems is becoming a less available resource due to the trend of engine downsizing and variable valve timing to improve vehicle fuel economy. 
     One of the more significant consumers of vacuum in a vehicle is the vehicle brake system. Vacuum is used in a brake booster to apply brakes. In particular, vacuum is applied to both sides of a brake booster diaphragm when brakes are not applied. Pressure equalization across the diaphragm allows the diaphragm to return to a position where a piston in the master cylinder does not increase brake line pressure. When the brakes are applied, vacuum on a working side of the diaphragm is displaced with ambient air while vacuum remains present on the vacuum side of the diaphragm. Consequently, a pressure differential is produced across the diaphragm that motivates the diaphragm to apply force to the piston in the master cylinder, thereby increasing brake pressure and applying the brakes. 
     During vehicle braking, a driver receives visual and physical cues that allow the driver to know whether or not a proper amount of force is being applied to the brake pedal to provide the desired braking amount or level. However, when the vehicle is stopped, the driver receives much less information regarding whether or not braking force is adequate or more than is desired to keep the vehicle from moving. Consequently, the driver may apply more brake force than is desired to keep the vehicle from moving. As a result, more vacuum than is desired may be consumed when the vehicle is stopped. 
     The inventors herein have recognized the above-mentioned disadvantages and have developed a method for conserving vacuum, comprising: limiting a brake line pressure increase at a wheel brake in response to an increasing in brake pedal force when a vehicle is stopped. 
     By limiting a brake line pressure increase, it may be possible to reduce vacuum consumption in a vehicle. Specifically, pressure in brake lines supplying wheel brakes may be limited via closing a valve located between a master cylinder piston and wheel brakes. Closing the valve limits master cylinder piston motion because brake fluid between the master cylinder piston and valve is nearly incompressible, thereby limiting master cylinder piston motion when the valve is closed and the brake is applied. The master cylinder piston is also mechanically coupled to a diaphragm in the brake booster that separates a brake booster working chamber from a brake booster vacuum chamber. Consequently, brake booster diaphragm motion is limited when master cylinder piston motion is limited. The brake booster diaphragm defines one side of the brake booster working chamber, and brake booster working chamber volume is substantially fixed (e.g., changes by less than 10% of total brake booster volume) when motion of the diaphragm is limited via the master cylinder piston. As a result, a driver may only decrease vacuum in the working chamber to an extent determined by the volume of the brake booster working chamber, which is related to the brake booster diaphragm position. In this way, vehicle brakes may be applied to provide a desired amount of braking force while brake booster vacuum consumption is limited. 
     The present description may provide several advantages. In particular, the approach may conserve vacuum in a vehicle so that the vehicle&#39;s engine operates for less time at low intake manifold pressures. The approach may also conserve fuel since the engine may be able to operate more efficiently at higher intake manifold pressures for longer periods of time. Additionally, the approach conserves vacuum responsive to vehicle operating conditions such as road grade and vehicle mass. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic depiction of an engine and a portion of a braking system; 
         FIG. 2  shows an example vehicle braking system where the method of  FIG. 4  may be applied to conserve vacuum; 
         FIG. 3  shows an example operating sequence where vacuum of a vacuum system is conserved; and 
         FIG. 4  shows an example method for conserving vacuum. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to conserving vacuum for a vehicle.  FIGS. 1 and 2  show an example system for providing vacuum for a vehicle.  FIG. 3  shows an example sequence where vacuum is conserved while operating a vehicle.  FIG. 4  shows a method for conserving vacuum for use in vehicle systems. 
     Referring to  FIG. 1 , internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in  FIG. 1 , is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Each intake and exhaust valve may be operated by an intake cam  51  and an exhaust cam  53 . The position of intake cam  51  may be determined by intake cam sensor  55 . The position of exhaust cam  53  may be determined by exhaust cam sensor  57 . 
     Fuel injector  66  is shown positioned to inject fuel directly into cylinder  30 , which is known to those skilled in the art as direct injection. Alternatively, fuel may be injected to an intake port, which is known to those skilled in the art as port injection. Fuel injector  66  delivers liquid fuel in proportion to the pulse width of signal FPW from controller  12 . Fuel is delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). Fuel injector  66  is supplied operating current from driver  68  which responds to controller  12 . In addition, intake manifold  44  is shown communicating with optional electronic throttle  62  which adjusts a position of throttle plate  64  to control air flow from intake boost chamber  46 . 
     Compressor  162  draws air from air intake passage  42  to supply boost chamber  46 . Exhaust gases spin turbine  164  which is coupled to compressor  162  via shaft  161 . Compressor bypass valve  158  may be electrically operated via a signal from controller  12 . Compressor bypass valve  158  allows pressurized air to be circulated back to the compressor inlet to limit boost pressure. Similarly, waste gate actuator  72  allows exhaust gases to bypass turbine  164  so that boost pressure can be controlled under varying operating conditions. 
     Vacuum is supplied to vehicle systems via vacuum providing device  24  (e.g. an aspirator/ejector/venturi pump). In this example, the aspirator is placed between the compressor outlet and the compressor inlet. In some examples, the aspirator may also be placed between the filtered air inlet and the intake manifold. Further, the aspirator can be placed across any two differing pressure potentials. Compressor  162  provides compressed air as a motive fluid via converging section duct  31  to converging section  35  of vacuum providing device  24  (e.g., an ejector). The motive fluid is combined with air from vacuum reservoir  138  via vacuum port duct  37  and check valve  60 . Check valve  60  allows flow when the pressure produced via the ejector within vacuum port duct  37  is lower than the pressure within reservoir  138 . Mixed air exits at diverging section  33 . In some examples, vacuum reservoir  138  may be referred to as a vacuum system reservoir since it can supply vacuum throughout the vacuum system and since brake booster  140  may contain a vacuum reservoir too. Pressure in reservoir  138  may be monitored via vacuum reservoir pressure sensor  193 . Vacuum system reservoir  138  provides vacuum to brake booster  140  via check valve  65 . Check valve  65  allows air to enter vacuum system reservoir  138  from brake booster  140  and substantially prevents air from entering brake booster  140  from vacuum system reservoir  138 . Vacuum system reservoir  138  may also provide vacuum to other vacuum consumers such as turbocharger waste gate actuators, heating and ventilation actuators, driveline actuators (e.g., four wheel drive actuators), fuel vapor purging systems, engine crankcase ventilation, and fuel system leak testing systems. Check valve  61  limits air flow from secondary vacuum consumers (e.g., vacuum consumers other than the vehicle braking system) to vacuum system reservoir  138 . Brake booster  140  may include an internal vacuum reservoir, and it may amplify force provided by foot  152  via brake pedal  150 . Brake booster  140  is coupled to master cylinder  148  for applying vehicle brakes (not shown). Brake booster  140  and brake pedal  150  are part of vehicle braking system  101 . In this example, brake booster  140  is an active brake booster where vacuum within a working side of brake booster  140  is based on a position of brake pedal  150 . Closing a valve between master cylinder and wheel cylinder may also be employed for conventional boosters (not active) that have a mechanical valve that allows atmospheric air to enter the working chamber. Brake pedal  150  may be mechanically coupled to brake booster  140  so that during some conditions brake pedal  140  directly operates master cylinder  148 . During other conditions, brake fluid pressure produced by master cylinder  148  is based on the net force of the booster force and brake pedal force, but brake pedal  140  does not directly operate master cylinder  148 . If brake pedal  140  is not directly operating on master cylinder  148  and the brake pedal is applied, pressure in a working chamber of brake booster  140  is adjusted via adjusting valves described in  FIG. 2 . 
     The operator&#39;s foot and the brake booster may apply a high force on the master cylinder resulting in a high master cylinder fluid pressure, but since an isolation valve(s) is closed, the wheel cylinders see a reduced line pressure as compared to if the valve were open. Closing the valve reduces stroke of the master cylinder and the brake booster which conserves vacuum. If a conventional brake booster is used, vacuum conservation comes only from the reduced brake stroke. If an active brake booster is present where valves control air flow into and out of the brake booster working chamber, vacuum is additionally conserved via limiting the atmospheric air entering the working chamber. 
     Distributorless ignition system  88  provides an ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . 
     Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. 
     Controller  12  is shown in  FIG. 1  as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random access memory  108 , keep alive memory  110 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  134  coupled to an accelerator pedal  130  for sensing accelerator position adjusted by foot  132 ; a position sensor  154  coupled to brake pedal  150  for sensing brake pedal position; a knock sensor for determining ignition of end gases (not shown); a measurement of engine manifold pressure (MAP) from pressure sensor  121  coupled to intake manifold  44 ; a measurement of boost pressure from pressure sensor  122  coupled to boost chamber  46 ; an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position; a measurement of air mass entering the engine from sensor  120  (e.g., a hot wire air flow meter); and a measurement of throttle position from sensor  58 . Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . Engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
     In some examples, the engine may be coupled to an electric motor/battery system in a hybrid vehicle. The hybrid vehicle may have a parallel configuration, series configuration, or variation or combinations thereof. Further, in some examples, other engine configurations may be employed, for example a diesel engine. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  54  closes and intake valve  52  opens. Air is introduced into combustion chamber  30  via intake manifold  44 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  52  and exhaust valve  54  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  54  opens to release the combusted air-fuel mixture to exhaust manifold  48  and the piston returns to TDC. Note that the above is described merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
     Referring now to  FIG. 2 , a first example braking system where the method of  FIG. 4  may be applied is shown. Braking system  101  of  FIG. 2  may be included with the engine shown in  FIG. 1 . Hydraulic lines are shown solid, electrical connections are shown as dashed, and pneumatic connections are shown as dash-dot. 
     Braking system  101  includes a brake pedal  150  and a brake position sensor  154 . In some examples, brake system  101  may also include a brake pedal force sensor  251 . Brake pedal  150  may be operated by foot  152  to move rod  213 . Rod  213  is mechanically coupled to diaphragm  245 . Diaphragm  245  is also mechanically coupled to piston  297  of master cylinder  148 . The position of diaphragm  245  is adjusted via brake pedal force, vacuum levels in working chamber  247  and vacuum chamber  248 , and return spring  270  when hydraulic control valves  295  and  294  are open. By changing the position of diaphragm  245  the volume of working chamber  247  may be adjusted. In particular, when hydraulic control valves  294  and  295  are open, volume (and pressure) in working chamber  247  may be increased when force applied to rod  213  allows air enter working chamber  247  and displace diaphragm  245 . However, if hydraulic control valves  294  and  295  are closed, an increase in volume of working chamber  247  may be limited even when additional force is applied to rod  213 . Closing hydraulic valves  294  and  295  fixes the volume of brake fluid between master cylinder piston  297  and valves  294  and  295 , thereby limiting motion of piston  297 , even if additional force is applied to brake pedal  150  or diaphragm  245  after hydraulic control valves  294  and  295  are closed. 
     Working chamber  247  selectively receives air from a high pressure source (e.g., atmospheric pressure) via a port to atmosphere  219  when rod  213  moves to allow a valve  218  to vent brake booster working chamber  247  to atmosphere. Valve  218  also allows air to pass from working chamber  247  to vacuum chamber  248  when brake pedal  150  is released. Valve  218  does not allow air into working chamber  247  from atmosphere when air passes from working chamber  247  to vacuum chamber  248 . In this way, vacuum in working chamber  247  may be displaced or added so that additional force is applied or removed from diaphragm  247 . 
     Pressure sensor  235  senses pressure in first brake line  232  downstream of hydraulic control valve  295 . Pressure sensor  237  senses pressure in second brake line  231  downstream of hydraulic control valve  294 . Controller  12  operates hydraulic control valves  294  and  295  in response to output of pressure sensors  235  and  237 , vehicle speed, and transmission operating state. 
     Vacuum reservoir  138  supplies vacuum to brake booster  140  via check valve  65 . Pressure in vacuum reservoir  138  is sensed via pressure sensor  193 . In some examples, vacuum reservoir  138  may be incorporated into brake booster  140 . Vacuum is supplied to vacuum reservoir  138  via check valve  60 . Vacuum is supplied to check valve  60  via the engine intake manifold or a device such as an ejector. 
     Master cylinder  148  may supply pressurized brake fluid to brakes  290  for stopping rotation of wheels  291 . Brake lines  231  and  231  allow fluidic communication between master cylinder  148  and brakes  290 . The front left vehicle wheel is designated FL, the front right wheel is designated FR, the right rear wheel is designated RR, and the rear left wheel is designated RL. 
     Thus, the system of  FIGS. 1 and 2  provides for conserving vacuum, comprising: a transmission; vehicle brakes; a brake pedal; a vacuum brake booster coupled to the brake pedal and in communication with the vehicle brakes; a master cylinder including a piston, the master cylinder coupled to the vacuum brake booster and in fluidic communication with the vehicle brakes; and a controller including executable instructions stored in non-transitory memory to limit motion of the piston while the transmission is being shifted and while the brake pedal is applied. The system includes where the transmission is shifted from neutral or park into a forward gear. The system includes where motion of the piston is limited via closing a valve positioned along a brake line extending from the master cylinder to the vehicle brakes. The system further comprises an engine and additional instructions to automatically stop the engine and piston motion while the engine is stopped. The system further comprises additional instructions to control brake line pressure at the vehicle brakes based on road grade. The system includes where limiting motion of the piston limits brake booster working chamber volume expansion. 
     Referring now to  FIG. 3 , operating characteristics of the vacuum conservation method of  FIG. 4  are shown. The sequence of  FIG. 3  may be provided by the method of  FIG. 4  being performed in the system of  FIGS. 1 and 2 . Vertical markers T 0 -T 10  represent times of particular interest in the sequence. 
     The first plot from the top of  FIG. 3  represents brake pedal force versus time. Alternatively, brake position may be substituted for brake pedal force. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents brake pedal force and the brake pedal is at a base position when the trace is at the X axis. The brake pedal force increases in the direction of the Y axis arrow. The brake pedal is not applied when brake pedal force is zero or at the X axis. 
     The second plot from the top of  FIG. 3  represents a command to hydraulic brake control valves (e.g., hydraulic control valves  294  and  295  of  FIG. 2 ). The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents hydraulic brake control valve command and the hydraulic brake control valve is commanded open when the hydraulic brake valve command is a low value near the X axis. The hydraulic brake control valve is commanded closed when the hydraulic brake valve command is a high value near the Y axis arrow. By commanding the hydraulic brake control valve closed, the positions of master cylinder piston  297  and diaphragm  245  of  FIG. 2  are limited from moving. 
     The third plot from the top of  FIG. 3  represents brake booster working chamber volume versus time. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents brake booster working chamber volume and the brake booster working chamber volume increases in the direction of the Y axis arrow. Trace  302  is a solid line and it represents a brake booster working chamber volume according to the method of  FIG. 4 . Trace  304  is a dashed line and it represents brake booster working chamber volume that is based solely on brake pedal force/position. Where only the solid line is visible, both traces  302  and  304  are at the same level. 
     The fourth plot from the top of  FIG. 3  represents brake booster working chamber vacuum versus time. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents brake booster working chamber vacuum and brake booster working chamber vacuum increases (e.g., pressure decreases) in the direction of the Y axis arrow. Trace  306  is a solid line and it represents brake booster working chamber vacuum when the method of  FIG. 4  in the system of  FIGS. 1 and 2  controls vacuum in the brake booster working chamber. Trace  308  is a dashed line and it represents brake booster working chamber vacuum that is based solely on brake pedal position. Where only the solid line is visible, both traces  306  and  308  are at the same level. When deployed in the conventional booster, lines  308  and  306  are coincident. When deployed in an active booster, less atmospheric air is allowed into the working chamber thus, the vacuum is deeper in the active booster than in the conventional booster. While one system conserves more vacuum than the other, both may conserve vacuum over the state of the art. 
     The fifth plot from the top of  FIG. 3  represents selected transmission gear versus time. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents transmission gear. P represents park, R represents reverse, N represents neutral, D represents drive, and L represents low. The vehicle&#39;s transmission is in the gear represented by the level of the trace. 
     The sixth plot from the top of  FIG. 3  represents vehicle speed versus time. The X axis represents time and time increases from the left side of  FIG. 3  to the right side of  FIG. 3 . The Y axis represents vehicle speed and vehicle speed increases in the direction of the Y axis arrow. Vehicle speed is zero when the trace is near the X axis. Horizontal line  335  represents a threshold vehicle speed below which the hydraulic control valves may be closed responsive to brake pedal position and vehicle speed. In other words, if the brake pedal is applied and vehicle speed is less than threshold  335 , the hydraulic control valves may be closed to limit brake line pressure and to limit vacuum consumption to a threshold level that stops the vehicle but that does not continue to consume vacuum as additional force is applied to the brake pedal. 
     At time T 0 , the vehicle is stopped and the transmission is in park. The hydraulic control valve command (e.g., the command for valves  294  and  295  of  FIG. 2 ) is low indicating the vehicle brake pressure is not being limited. The brake pedal is not applied and the transmission is in park. The brake booster working chamber volume is at a lower level indicating that the brake booster diaphragm is not deflecting significantly due to brake pedal force and the pressure differential across the diaphragm. The brake booster working chamber vacuum is at a higher level. Such conditions are indicative of conditions when a vehicle is parked and the engine is operating or stopped. 
     At time T 1 , the brake pedal is applied and the engine is started via engaging a pushbutton or starter switch while the transmission is still in park. Applying the brake may be a requirement for engaging an engine starter and starting the vehicle when the vehicle is stopped and in park. The brake booster working chamber volume begins to increase and the vacuum chamber volume decreases (not shown) in response to air entering the working chamber and brake pedal position (e.g., the brake is being applied). Brake booster working chamber vacuum decreases in response to air entering the brake booster working chamber and brake pedal position (e.g., brake pedal position determines air flow rate into the brake booster working chamber). The transmission remains in park and the hydraulic control command remains not asserted so that the hydraulic control valve remains open. The vehicle braking force increases when the hydraulic valve command is not asserted and brake pedal application force increases. The vehicle also remains in a stopped state. Such conditions may be present when a person subconsciously applies the vehicle brake while the vehicle is parked. 
     At time T 2 , the hydraulic valve command is asserted and the hydraulic brake valve closes in response to vehicle braking force reaching a threshold level (not shown). In one example, vehicle braking force may be determined from pressures within brake lines that provide brake fluid to vehicle brakes. For example, a brake line pressure may be used to index a table or function that outputs a braking force. The threshold vehicle braking force may be determined based on vehicle mass, road grade, and a predetermined braking offset. The method of  FIG. 4  limits brake booster working chamber volume at the level indicated by trace  302  in response to closing the hydraulic control valve. For systems that adjust brake booster working chamber vacuum based on brake pedal position, the brake booster working chamber volume increases to the level indicated by trace  304 . Thus, the method of  FIG. 4  limits brake booster working chamber volume so that less air may enter the brake booster, thereby reducing vacuum consumption. 
     The brake pedal force continues to increase and the brake booster working chamber vacuum decreases as air enters the brake booster working chamber. The brake booster working chamber vacuum operating according to the method of  FIG. 4  decreases at a faster rate as indicated by trace  306 , but not as to the reduced vacuum level of trace  308 , since the brake booster working chamber volume is limited. The brake booster working chamber vacuum operating according to the method that adjusts brake booster working chamber vacuum based solely on brake pedal position decreases at a slower rate as indicated by trace  308  since the brake booster working chamber volume is increasing. Additionally, trace  308  goes to a lower vacuum level since the brake pedal allows brake booster vacuum control valve  218  of  FIG. 2  to open further since motion of the brake pedal is not limited by the hydraulic control valve. Note the difference in the systems. The operator may apply the same brake force in both cases, but in the one case (where the hydraulic control valve is closed at T 2 ) the brake line pressure is limited, yet sufficient to hold the vehicle stopped even if the vehicle were not in park. Thus, vehicle controls prevent needless extra brake booster stroke that would consume additional vacuum. 
     At time T 3 , the brake pedal is released as indicated by the brake pedal position transitioning to a lower level. The hydraulic control valve command remains asserted so that the hydraulic control valve remains closed. The hydraulic control valve remains closed so that the master cylinder piston movement is limited when the vehicle brakes are applied, thereby limiting vacuum consumption by the brake booster. The brake booster working chamber volume decreases for the method of  FIG. 4  (e.g., trace  302 ) and for the method where brake booster working chamber vacuum varies with brake pedal position and application force (e.g., trace  304 ) in response to the brake pedal being released. Additionally, brake booster vacuum increases for the method of  FIG. 4  (e.g., trace  306 ) and for the method where brake booster working chamber vacuum varies with brake pedal position and application force (e.g., trace  308 ) in response to the brake pedal being released. The transmission remains in park and the vehicle remains stopped. 
     At time T 4 , the brake pedal is applied for a second time as a pre-requisite to changing the transmission PRNDL selection. The brake is applied to allow the transmission to be shifted into drive as is shown shortly thereafter. The hydraulic control valve remains closed to limit movement of the brake booster diaphragm, thereby limiting vacuum consumption. The brake booster working chamber volume increases for the method of  FIG. 4 , trace  302 , and for the method where brake booster working chamber vacuum is adjusted based on brake pedal position, trace  304 , but the method of  FIG. 4  limits brake booster working chamber volume, whereas the method where brake booster working chamber vacuum is adjusted based on brake pedal position is allowed to increase even further. The brake booster working chamber vacuum decreases an additional amount for the method that adjusts brake booster vacuum solely based on brake pedal position, trace  308 , because additional brake force moves the brake pedal and allows more air into the brake booster working chamber. On the other hand, for the system that operates according to the method of  FIG. 4 , trace  306 , the brake booster working chamber vacuum is limited since the master cylinder piston is limited from moving by the hydraulic control valve being closed. Limiting the master cylinder piston motion limits brake booster diaphragm motion and opening of the brake booster vacuum valve  218 . Consequently, the method where brake booster working chamber vacuum is adjusted based solely on brake pedal position consumes more vacuum than the method of  FIG. 4 . The transmission is shifted from park to drive while the brake pedal is applied and vehicle speed remains at zero. 
     At time T 5 , the brake pedal is released by the driver and the hydraulic control valve command changes state to a not asserted state in response to the brake pedal being released. However, in some examples, the hydraulic control valve may remain closed until a driver or engine demand torque is increased. The brake booster working chamber volume for the method of  FIG. 4 , trace  302 , and for the method where brake booster vacuum is adjusted based on brake pedal position, trace  304 , both decrease as the brake booster diaphragm deflection is reduced in response to the pressure differential across the brake booster diaphragm being reduced. The brake booster working chamber vacuum also increases for both the method of  FIG. 4 , trace  306 , and for the method where brake booster working chamber vacuum is adjusted based on brake pedal position, trace  308 , in response to the brake pedal being released. The vehicle remains in drive and vehicle speed begins to increase between time T 5  and time T 6 . 
     At time T 6 , vehicle speed is decreased and the vehicle brakes are applied. The hydraulic control valve command remains not asserted so that the driver may apply full braking force while the vehicle is moving. The brake booster working chamber volume for the method of  FIG. 4 , trace  302 , and for the method where brake booster working chamber volume is adjusted based on brake pedal position, trace  304 , both increase in response to increasing brake pedal force. Thus, vacuum is consumed equally by the system that operates according to method  4  and the system that operates solely based on brake pedal position. The vehicle remains in drive and the vehicle begins to decelerate. Since the vehicle is moving, the brakes operate normally and no vacuum conservation measures are provided. The vehicle stops just prior to time T 7 . 
     At time T 7 , the driver applies additional force and the brake pedal position is further displaced from the base brake pedal position after the vehicle has stopped. The hydraulic control valve command is asserted to close the hydraulic control valve in response to the increasing brake force. Alternatively, the hydraulic control valve may be asserted in response to vehicle speed reaching zero vehicle speed. The brake booster working chamber volume for the method of  FIG. 4 , trace  302 , becomes limited in response to the hydraulic control valve closing. The brake booster working chamber volume for the method that adjusts brake booster working chamber vacuum based solely on brake pedal position, trace  304 , continues to increase in response to the increasing brake pedal position (not shown) as brake pedal force increases. The brake booster working chamber vacuum stays at a same value as prior to time T 7  for the system that applies the method of  FIG. 4 . The brake booster working chamber vacuum decreases for the system that adjusts brake booster vacuum solely responsive to brake pedal position since the brake pedal is allowed to travel further when brake pedal force is increased. The vehicle remains in drive and vehicle speed remains at zero. 
     At time T 8 , the brake pedal is released by the driver and the hydraulic control valve command changes state to a not asserted state in response to the brake pedal being released. However, in some examples, the hydraulic control valve may remain closed until a driver or engine demand torque is increased. The brake booster working chamber volume for the method of  FIG. 4 , trace  302 , and for the method where brake booster vacuum is adjusted solely based on brake pedal position, trace  304 , both decrease as the brake booster diaphragm deflection is reduced in response to the pressure differential across the brake booster diaphragm being reduced. The brake booster working chamber vacuum also increases for both the method of  FIG. 4 , trace  306 , and for the method where brake booster working chamber vacuum is adjusted based on brake pedal position, trace  308 , in response to the brake pedal being released. The vehicle remains in drive and vehicle speed begins to increase between time T 8  and time T 9 . 
     At time T 9 , the vehicle is moving at a speed below threshold speed  335  and the driver applies the brake pedal as indicated by increasing brake pedal force. The brake booster working chamber volume for the method of  FIG. 4 , trace  302 , and the method that adjusts brake booster working chamber vacuum solely based on brake pedal position, trace  304 , increase in response to the increasing brake pedal force. Further, the brake booster working chamber vacuum for the method of  FIG. 4 , trace  306 , decreases in response to increasing force applied to the brake pedal. The brake booster working chamber vacuum for the method that adjusts brake booster working chamber vacuum in response to brake pedal position also decreases in response to the brake pedal position changing as the brake force is increased. The vehicle remains moving and the transmission remains in drive. 
     At time T 10 , the hydraulic control valve command changes state to close the hydraulic control valve in response to braking force reaching a threshold braking force. In one example, the threshold braking force is based on brake line pressure. The brake line pressure is used to index a function or table of empirically determined values of vehicle brake force based on brake line pressure. The brake booster working chamber volume is limited in response to the hydraulic control valve closing as indicated by trace  302  for a system that operates according to the method of  FIG. 4 . For the system that adjusts brake booster vacuum in response to brake pedal position, the brake booster working chamber volume continues to increase as brake force increases and as the brake pedal is displaced further from its base position as indicated by trace  304 . The brake booster working chamber vacuum for the system operating according to the method of  FIG. 4  decreases as shown by trace  302 , but the decrease is limited since the diaphragm position limits opening of valve  218  shown in  FIG. 2 . The brake booster working chamber vacuum for the system that adjusts brake booster working chamber vacuum based on brake pedal position is reduced to a lower level than the brake booster working chamber vacuum based on the method of  FIG. 4  as indicated by trace  304 . Thus, the method of  FIG. 4  limits brake booster working chamber vacuum reduction and braking force in response to vehicle speed, transmission gear, and brake pedal application force, and desired braking force. The vehicle decelerates to a stop and the transmission remains in drive. 
     Referring now to  FIG. 4 , an example method for conserving vacuum is shown. The method of  FIG. 4  may be stored as executable instructions in non-transitory memory of the system shown in  FIGS. 1 and 2 . Further, the method of  FIG. 4  may provide the operating sequence shown in  FIG. 3 . 
     At  402 , method  400  judges whether or not the vehicle&#39;s transmission is in park or neutral. The transmission gear may be determined from output of a gear selector sensor. If method  400  determines that the transmission is in park or neutral the answer is yes and method  400  proceeds to  404 . Otherwise, the answer is no and method  400  proceeds to  410 . In addition, the engine may not be started or may be in the process of being started in response to a request to start the engine at  402 . For example, the engine may be being started in response to a driver depressing a push button start device. 
     At  404 , method  400  determines whether or not the brake pedal is applied. The brake pedal position is an indication of whether or not the brake pedal is applied. If method  400  judges that the brake pedal is applied the answer is yes and method  400  proceeds to  406 . Otherwise, the answer is no and method  400  proceeds to exit. The vehicle engine may be stopped or rotating at  404 . 
     At  406 , method  400  limits brake boost assist and brake booster working chamber volume. In one example, brake boost assist is limited based on road grade, atmospheric pressure, and vehicle mass. Road grade may be determined via in inclinometer or an accelerometer. Vehicle mass may be estimated via the following equation: 
             Mv   =       (       Tw   ⁢           ⁢   1     -     T   ⁢           ⁢   w   ⁢           ⁢   2       )     +       (       Tr   ⁢           ⁢   2     -     Tr   ⁢           ⁢   1       )       Rrr   ·   g   ·     (       sin   ⁢           ⁢   Θ   ⁢           ⁢   1     -     sin   ⁢           ⁢   Θ   ⁢           ⁢   2       )                 
Where My is mass of the vehicle, Tw1 is torque at the vehicle wheel for grade 1, Tw2 is torque at the vehicle wheel for grade 2, Rrr is driven wheel rolling radius, g is gravity constant, Trl1 is road load at driven wheel on grade 1, Trl2 is road load at driven wheel grade 2, Θ1 is road 1 angle, and Θ2 is road 2 angle.
 
     In one example, a desired braking force may be used to index a table or function that outputs a brake line pressure that provides the desired braking force. In particular, brake line pressure is used to index a function or table stored in memory that holds empirically determined valves of vehicle brake force based on brake line pressure. If the vehicle is parked or in neutral on a flat road, the table or function outputs a desired brake line pressure to hold the vehicle stopped (e.g., a pressure increase that corresponds to 5 N-m). In one example, the base force to hold the vehicle stopped may include an additional force amount to keep the vehicle stopped due to unforeseen conditions (e.g., 5 N-m). 
     In addition, braking force may be added to the base amount of brake force based on vehicle mass and road grade. If vehicle mass is greater than a base vehicle mass, the braking force is increased as a function of vehicle mass. The increase in brake force due to vehicle mass may be empirically determined and stored in memory as a function of vehicle mass. The increase in brake force due to road grade may be empirically determined and stored in memory as a function of road grade. The increases in brake force are converted to increases in brake line pressure, and the brake line pressure is increased or decreased to the desired pressure via opening or closing the hydraulic control valve (e.g., valves  294  and  295  in  FIG. 2 ). The brake line pressure may be increased when the brake pedal is depressed with an increasing amount of force while the hydraulic control valves are open. The brake line pressure may be held at a desired pressure by closing the hydraulic control valves when brake line pressure reaches a desired pressure. Brake line pressure may be reduced when brake pedal force is reduced and when the hydraulic control valves are open. Additionally, brake line pressure may be adjusted via adjusting output of a pump supplying brake fluid to vehicle brakes. 
     By opening and closing the hydraulic control valves, the brake booster working chamber volume and brake boost assist may be limited. For example, the hydraulic control valves may be closed when brake line pressure reaches a desired brake line pressure based on road grade, engine torque, transmission gear, and vehicle mass. The brake line pressure is held at the brake line pressure present when the hydraulic valves were closed. Closing the hydraulic control valves limits master cylinder piston motion by not allowing brake fluid between the master cylinder and hydraulic control valve to be displaced. Limiting master cylinder piston motion also limits brake booster diaphragm motion, brake booster working chamber volume, and brake pedal motion since the master cylinder piston is coupled to the brake booster diaphragm and the brake pedal. 
     Additionally, in some examples a valve may be placed between atmosphere and the brake booster working chamber to limit air flow into the brake booster working chamber when the brake pedal is applied. For example, if the brake line pressure reaches a desired pressure, air flow to the brake booster working chamber may be stopped along with closing the hydraulic control valve. 
     The brake line pressure is adjusted to supply the base braking force and braking force for road grade and vehicle mass in response to the brake pedal being initially applied; however, brake force is not adjusted proportionally with brake pedal position so that vacuum may be conserved. In one example, if the vehicle is in park or neutral, the brake line pressure is maintained until the vehicle is shifted into reverse or a forward gear. Method  400  returns to  404  after brake boost assist is limited. If the transmission is in park, no compensation for road grade or vehicle mass is provided. 
     At  410 , method  400  judges whether or not the vehicle&#39;s transmission is in neutral or in a gear. In one example, method  400  may judge that the transmission is being shifted based on a position of a gear selector. Additionally, the vehicle brake must be applied to shift from park or neutral into a gear. If method  400  judges that the transmission is being shifted from neutral or park into a gear, the answer is yes and method  400  proceeds to  412 . Otherwise, the answer is no and method  400  proceeds to  430 . 
     At  412 , method  400  estimates vehicle mass and road grade. In one example, vehicle mass is determined as described at  406 . Road grade is determined via an inclinometer. Method  400  proceeds  414  after vehicle mass and road grade are determined. 
     At  414 , method  400  limits brake booster assist and brake line pressure to limit vehicle motion while the vehicle brake pedal is applied allowing the transmission to be shifted. A base brake force to hold the vehicle stopped is estimated based on engine torque delivered to vehicle wheels, vehicle mass, road grade, and barometric pressure. Torque at the vehicle wheels produced by the engine is estimated by indexing a table or function using engine speed and load. The table outputs an engine torque and the engine torque is multiplied by factors for gear ratios between the engine and the wheels as well as for torque converter torque multiplication to determine torque at the wheels produced by the engine. Torque at the wheels from the engine is added to torque at the wheels due to road grade. Torque the wheels due to road grade is mass of the vehicle multiplied by the gravity constant multiplied by the sine of the road angle. 
     The desired braking force is increased to provide a braking force that is equivalent to the engine torque produced at the vehicle wheels plus the torque due to vehicle mass and road grade plus a predetermined additional amount of torque. The braking force is produced via increasing the brake line pressure to a pressure that produces the desired braking force. The brake line pressure is increased by the driver applying the brake pedal and opening the hydraulic control valve. In one example, the desired braking force is input to an empirically determined function or table that outputs a desired brake line pressure and the hydraulic control valve is closed when the desired brake line pressure is achieved by the driver applying force to the brake pedal. The brake line pressure may be measured and compared against the desired brake line pressure to adjust the brake line pressure via closed-loop control. Method  400  proceeds to  416  after the brake line pressure is adjusted and limited to a desired brake line pressure. Additionally, limiting brake line pressure via closing the hydraulic control valve limits brake booster working chamber volume since master cylinder piston motion is limited when the hydraulic control valve are closed as previously described. It should be noted that the brake line pressure is not adjusted proportional to brake pedal position when the vehicle is stopped and the hydraulic control valves are closed. In this way, vacuum consumption may be decreased. 
     At  416 , method  400  judges whether or not the vehicle brake pedal is applied. The vehicle brake pedal may be judged applied or not applied based on brake pedal position or brake pedal force. If method  400  judges that the brake pedal is applied, method  400  returns to  414 . Otherwise, the answer is no and method  400  proceeds to  418 . 
     At  418 , method  400  increases vacuum in the brake booster working chamber and opens the hydraulic control valve. The pressure differential across the brake booster diaphragm is also reduced when the brake is release. Thus,  418  shows normal brake system operation since the brakes are not applied and the vehicle is in gear. Method  400  exits after opening the hydraulic control valve and reducing the pressure differential across the brake booster diaphragm. 
     At  430 , method  400  judges whether or not vehicle speed is less than a threshold vehicle speed (e.g., 2 KPH). If method  400  judges that vehicle speed is less than a threshold vehicle speed, method  400  proceeds to  434 . Otherwise, the answer is no and method  400  proceeds to  432 . In some examples, if vehicle motion is detected, brake assist is provided and vacuum consumption by the brake booster is not limited. The threshold speed in such cases may be used to reduce the uncertainty of detecting zero vehicle speed. 
     At  432 , method  400  adjusts brake booster working chamber vacuum and/or pressure differential across the brake booster diaphragm in proportion to the position of the vehicle brake pedal. Additionally, the hydraulic control valve is opened and the brake booster working chamber is allowed to achieve full capacity volume when the brake is applied. At  432 , the brakes are operating normally since the vehicle is in motion. Pneumatic boost assist is not limited nor is hydraulic brake pressure, with the exception of anti-lock braking conditions. For example, if the brake is being applied and brake pedal position is moving away from a base brake pedal position, the pressure differential across the brake booster is increased to increase braking force. The pressure differential across the brake booster diaphragm is increased via applying the brake pedal. The pressure in the brake lines increases as the brake pedal force increases. Braking force may be increased or decreased at  432  depending on brake pedal position. Method  400  proceeds to exit after the hydraulic control valve is opened and brake booster working pressure and/or the pressure differential across the brake booster is adjusted. 
     At  434 , method  400  judges whether or not the vehicle brake is applied. The vehicle brake may be judged applied or not applied based on brake pedal position. If method  400  judges that the brake pedal is applied, the answer is yes and method  400  proceeds to  436 . Otherwise, the answer is no and method  400  proceeds to  418 . 
     At  436 , method  400  estimates vehicle mass and road grade as described at  412  and  406 . Method  400  proceeds to  438  after vehicle mass and road grade are estimated. In some examples, the vehicle&#39;s engine may be automatically stopped based on vehicle operating conditions without a driver directly stopping the engine. For example, the engine may be automatically stopped when the vehicle speed is zero and engine load is less than a threshold load. 
     At  438 , method  400  limits brake boost assist, brake booster working chamber volume, and brake line pressure to limit vehicle motion as described at  414 . Method  400  proceeds to exit after brake boost assist, brake booster working chamber volume, and brake line pressure are limited. Thus, even if the vehicle is moving at a slow speed, the brake boost amount may be limited so as to conserve vacuum. 
     Thus, the method of  FIG. 4  provides for a method for conserving vacuum, comprising: limiting a brake line pressure increase at a wheel brake in response to an increasing in brake pedal force when a vehicle is stopped. The method includes where the brake line pressure increase is limited via closing a valve located along a brake line, the brake line extending from a master cylinder to the wheel brake. The method further comprises holding the brake line pressure at a pressure based on road grade. The method further comprises holding the break line pressure at a pressure based on vehicle mass. The method further comprises decreasing vacuum in a brake booster working chamber while limiting the brake line pressure increase. 
     In some examples, the method further comprises limiting the brake line pressure increase in response to a transmission of the vehicle being in park or neutral. The method further comprises limiting the brake line pressure increase in response to the increasing brake pedal force while an engine of the vehicle is being stopped or being started. The method further comprises automatically stopping an engine of the vehicle when the vehicle is stopped and holding a substantially constant brake line pressure at the wheel brake while the engine is stopped. 
     The method of  FIG. 4  also provides for conserving vacuum, comprising: limiting volume expansion of a brake booster working chamber in response to a speed of a vehicle. The method includes where the speed of the vehicle is less than a threshold speed or zero speed, and where the limiting of volume expansion occurs while an engine of the vehicle is being started via a pushbutton. The method includes where volume expansion of the brake booster working chamber is limited via closing a valve located along a brake line between a master cylinder and a wheel brake, and further comprising limiting air entry into the brake booster working chamber. The method further comprises not constraining volume expansion of the brake booster working chamber in response to the speed of the vehicle exceeding a threshold speed. The method also includes where the volume expansion of the brake booster occurs in response to deflection of a brake booster diaphragm. The method also includes where the speed of the vehicle is zero, and further comprising starting the engine via a pushbutton while a brake pedal is being applied. 
     As will be appreciated by one of ordinary skill in the art, the methods described in  FIG. 4  may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the objects, features, and advantages described herein, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. In addition, the terms aspirator or venturi may be substituted for ejector since the devices may perform in a similar manner. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, single cylinder, I2, I3, I4, I5, V6, V8, V10, V12 and V16 engines operating in natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage.