Abstract:
A hydraulic brake assist vacuum maintenance system includes a vacuum source, a vacuum booster/master cylinder interfacing with the vacuum source, a brake pedal interfacing with the vacuum booster/master cylinder, a brake control unit interfacing with the vacuum booster/master cylinder, wheel brakes interfacing with the brake control unit and a hydraulic brake assist interfacing with the brake control unit and adapted to hydraulically operate the wheel brakes upon reduction or cessation of vacuum pressure from the vacuum source to the vacuum booster/master cylinder.

Description:
FIELD 
       [0001]    The disclosure generally relates to vehicle braking systems. More particularly, the disclosure relates to a hydraulic brake assist vacuum maintenance system and method which maintains vacuum pressure for braking using Hydraulic Brake Assist (HBA) during auto stop engine shutdown. 
       BACKGROUND 
       [0002]    Modern vehicles are increasingly using an auto stop feature in which the vehicle engine is shut down to conserve fuel when the vehicle stops. During auto stop, vacuum pressure may not be available for operation of the vehicle brake system. If the vacuum level is low or becomes low while the engine is off, an expensive electric vacuum pump or engine startup may be necessary to achieve an adequate level of vacuum pressure for braking. 
         [0003]    Accordingly, a hydraulic brake assist vacuum maintenance system and method which maintains hydraulic pressure for braking using Hydraulic Brake Assist (HBA) during auto stop engine shutdown is needed. 
       SUMMARY 
       [0004]    The disclosure is generally directed to a hydraulic brake assist vacuum maintenance system. An illustrative embodiment of the system includes a vacuum source, a vacuum booster/master cylinder interfacing with the vacuum source, a brake pedal interfacing with the vacuum booster/master cylinder, a brake control unit interfacing with the vacuum booster/master cylinder, wheel brakes interfacing with the brake control unit and a hydraulic brake assist interfacing with the brake control unit and adapted to hydraulically operate the wheel brakes upon reduction or cessation of vacuum pressure from the vacuum source to the vacuum booster/master cylinder. 
         [0005]    The disclosure is further generally directed to a hydraulic brake assist vacuum maintenance method for a vehicle having hydraulic brake assist. An illustrative embodiment of the method includes terminating operation of a vehicle engine of the vehicle and providing hydraulic pressure for braking of the vehicle using the hydraulic brake assist. 
         [0006]    In some embodiments, the hydraulic brake assist vacuum maintenance method may include terminating operation of a vehicle engine of the vehicle; providing hydraulic pressure for braking of the vehicle using the hydraulic brake assist; determining whether braking of the vehicle is occurring; determining whether braking demand is being met if the braking of the vehicle is occurring; determining whether the vehicle is moving if the braking demand is not being met; determining braking pressure required to hold the vehicle on a grade if the vehicle is not moving; and applying braking pressure up to the braking demand but limited to a predetermined minimum level or level required to hold the vehicle on the grade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The disclosure will now be made, by way of example, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a block diagram of an illustrative embodiment of the hydraulic brake assist vacuum maintenance system. 
           [0009]      FIG. 2  is a flow diagram of an illustrative embodiment of the hydraulic brake assist vacuum maintenance method. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the appended claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
         [0011]    Referring initially to  FIG. 1 , an illustrative embodiment of a hydraulic brake assist vacuum maintenance system, hereinafter system, is generally indicated by reference numeral  100 . The system  100  may include a vehicle engine  102  of a vehicle. The vehicle engine  102  may be controlled by an engine electronic control unit  118 . The vehicle engine  102  may provide vacuum pressure  103  to a vacuum booster/master cylinder  104 . A brake pedal  106  may interface with the vacuum booster/master cylinder  104 . Responsive to depression of the brake pedal  106 , the vacuum booster/master cylinder  104  may provide hydraulic pressure  105  to a brake control unit or hydraulic control block  108 . The brake control unit  108  may provide hydraulic pressure  109  to wheel brakes  110  of the vehicle to actuate the wheel brakes  110  and decelerate or stop the vehicle. 
         [0012]    A Hydraulic Brake Assist (HBA)  120  may interface with the brake control unit  108 . The HBA  120  may be conventional and may include a pressure sensor  112  which is adapted to measure the magnitude of the hydraulic pressure  105 . The pressure sensor  112  may provide pressure sensor data  113  which indicates the hydraulic pressure  105  to a brake electronic control unit  116 . Vehicle motion/orientation sensors  114  may be adapted to measure the motion and/or orientation of the vehicle and may provide motion/orientation data  115  to the brake electronic control unit  116 . Through the brake control unit  108 , the brake electronic control unit  116  may control the magnitude of the hydraulic pressure  109  responsive to the pressure sensor data  113  and the motion/orientation data  115 . The brake electronic control unit  116  of the HBA  120  may interface with the engine electronic control unit  118 . 
         [0013]    In exemplary application of the system  100 , under driving conditions, the vehicle engine  102  is operated responsive to input from the engine electronic control unit  118 . The vehicle engine  102  provides vacuum pressure  103  to the vacuum booster/master cylinder  104 . Upon depression of the brake pedal  106 , the vacuum booster/master cylinder  104  provides hydraulic pressure  105  to the brake control unit  108 . In turn, the brake control unit  108  controls the magnitude of the hydraulic pressure  109  which is applied to the wheel brakes  110 . Depending on the magnitude of the hydraulic pressure  109 , the wheel brakes  110  decelerate or stop the vehicle. 
         [0014]    During auto stop of the vehicle, the engine electronic control unit  118  terminates further operation of the vehicle engine  102 . In the event of subsequent braking during the auto stop mode of the vehicle, the brake pedal  106  is depressed. In response, the vacuum booster/master cylinder  104  provides hydraulic pressure  105 , reduced in magnitude due to the lack or absence of vacuum pressure  103  from the vehicle engine  102 , to the brake control unit  108 . The brake control unit  108  provides hydraulic pressure  109  of correspondingly reduced magnitude to the wheel brakes  110  of the vehicle. 
         [0015]    The pressure sensor  112  of the HBA  120  monitors the hydraulic pressure  109  which is delivered from the brake control unit  108  to the wheel brakes  110 . Simultaneously, the vehicle motion/orientation sensors  114  monitor the motion and orientation of the vehicle, including but not limited to motion which is imparted to the vehicle by any gradient on which the vehicle may rest. The brake electronic control unit  116  uses the pressure sensor data  113  from the pressure sensor  112  and the motion/orientation data  115  from the vehicle motion/orientation sensors  114  to augment the magnitude of the hydraulic pressure  109  which is applied to the wheel brakes  110  depending on the braking needs of the vehicle. Accordingly, the HBA  120  provides boosted hydraulic pressure  109  via a hydraulic pump and motor (not illustrated) and the brake control unit  108 . Upon resumption of driving conditions, the engine electronic control unit  118  operates the vehicle engine  102 . The vehicle engine  102  provides vacuum pressure  103  to the vacuum booster/master cylinder  104  and the brake control unit  108  actuates the wheel brakes  110  of the vehicle in the conventional manner responsive to depression of the brake pedal  106 . 
         [0016]    In some applications, NVH (Noise, Vibration and Harshness) may result due to the pump noise which is inherent in operation of the HBA  120 . The NVH can be attenuated or managed through the use of limited pressure control which could be limited as a function of grade detection, rolling information or open loop max pressure control, for example and without limitation. 
         [0017]    Referring next to  FIG. 2 , a flow diagram  200  of an illustrative embodiment of the hydraulic brake assist vacuum maintenance method is shown. The method  200  begins at block  202 . In block  204 , a determination may be made as to whether the vehicle engine is on or off. If the vehicle engine is not off, the method may end at block  222 . If the vehicle engine is on, a determination may be made as to whether vehicle braking is occurring in block  206 . If braking is not occurring, the method may end at block  222 . If braking is occurring, a determination may be made as to whether braking demand or pressure is being met (block  208 ). If braking demand or pressure is being met, the method may end at block  222 . If braking demand or pressure is not being met, a determination may be made as to whether the vehicle is moving (block  210 ). In some applications, this may include a determination of vehicle speed  210   a.    
         [0018]    If the outcome of the determination made at block  210  indicates that vehicle is not moving, the brake pressure which is required to hold the vehicle on a grade may be determined (block  212 ) based on a grade calculation  212   a . In block  214 , brake pressure may be applied using an HBA (Hydraulic Brake Assist) of the vehicle up to the braking demand but limited to a predetermined minimum level or a level required to hold the grade. In block  214   a , a determination may be made as to whether the braking demand up to a predetermined minimum level which is required to hold the grade is being met. If the braking demand or pressure is being met, the method may end at block  222 . If the braking demand or pressure is not being met, the vehicle engine may be turned on at block  218  and the method may then end at block  222 . 
         [0019]    If the outcome of the determination made at block  210  indicates that the vehicle is moving, brake pressure up to the braking demand may be applied using the HBA (Hydraulic Brake Assist) of the vehicle in block  220 . In block  216 , a determination may be made as to whether the braking demand or pressure is being met. If the braking demand or pressure is being met, the method may end at block  222 . If the braking demand or pressure is not being met, the vehicle engine may be turned on at block  218   a  and the method may then end at block  222 . 
         [0020]    Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.