Abstract:
A control system for evaluating a brake booster system is provided. The control system includes an engine evaluation module that detects an engine off condition. A pressure evaluation module monitors hydraulic brake line pressure and detects changes in brake booster pressure during the engine off condition. A fault reporting module selectively detects a brake booster fault based on the brake line pressure and the changes in brake booster pressure.

Description:
FIELD 
       [0001]    The present disclosure relates to brake booster systems for hybrid vehicles. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    As an alternative to the internal combustion engine, automotive manufacturers have developed hybrid powertrains that include both an electric traction machine and an internal combustion engine. During operation, vehicles including the hybrid powertrain use one or both of the power sources to improve efficiency. 
         [0004]    Hybrid vehicles include either a parallel drivetrain configuration or a series drivetrain configuration. In the parallel hybrid vehicle, the electric machine works in parallel with the engine to combine the power and range advantages of the engine with the efficiency and the electrical regeneration capability of the electric machine. In the series hybrid vehicle, the engine drives a generator to produce electricity for the electric machine, which drives a transaxle. This allows the electric machine to assume some of the power responsibilities of the engine, thereby permitting the use of a smaller and more efficient engine. Additionally, for either described hybrid configuration, the engine may be turned off while the vehicle is stopped and the drivers foot remains on the brake pedal. This is done to conserve fuel—increasing the duration of engine stoppage while the vehicle is at rest increases the hybrid fuel economy benefit. 
         [0005]    Some hybrid vehicles include a vacuum driven brake booster that reduces the brake pedal effort required to achieve a desired vehicle braking force. These hybrid vehicles use the engine&#39;s intake manifold as a source for the vacuum when the engine is turned off for hybrid operation, a finite level of vacuum is held in the system which is depleted as the brake pedal is modulated. Hybrid vehicles require sufficient brake booster vacuum levels during engine off operation to maintain brake assist. If brake booster vacuum falls below a threshold during engine off hybrid operation, the engine will start so that brake booster vacuum can be replenished. Normally brake booster vacuum is depleted via brake modulation, but a leaky brake booster system can also cause brake vacuum to fall below the engine start threshold preventing or shortening hybrid engine off operation. Since this failure mode results in an impact to emissions and fuel economy, the brake booster system should be diagnosed for leaks. 
       SUMMARY 
       [0006]    Accordingly, a control system for evaluating a vacuum assisted brake booster system is provided. The control system includes an engine evaluation module that detects an engine off condition. A pressure evaluation module monitors hydraulic brake line pressure and detects changes in brake booster pressure during the engine off condition. A fault reporting module selectively detects a brake booster fault based on the brake line pressure and the changes in brake booster pressure. 
         [0007]    In other features, a method of monitoring a brake booster system for leaks is provided. The method includes: detecting an engine off condition; during the engine off condition, monitoring brake line pressure and determining changes in brake booster pressure; and selectively detecting a brake booster fault based on the monitoring brake line pressure and the determining the changes in brake booster pressure. 
         [0008]    In still other features, a hybrid vehicle that includes an engine is provided. The hybrid vehicle includes a brake booster vacuum system in fluid communication with a vehicle braking system and in vacuum pressure communication with the engine and that provides braking assistance to the braking system of the hybrid vehicle. A first pressure sensor generates a brake line pressure signal based on a brake line pressure of the braking system. A second pressure sensor generates a brake booster pressure signal based on a brake booster pressure in the brake booster vacuum system. A control module detects a leak in the brake booster vacuum system based on the brake line pressure signal and the brake booster pressure signal. 
         [0009]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0010]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0011]      FIG. 1  is a block diagram illustrating a hybrid vehicle including a brake booster leak detection system according to various aspects of the present disclosure. 
           [0012]      FIG. 2  is a dataflow diagram illustrating a brake booster leak detection system according to various aspects of the present disclosure. 
           [0013]      FIG. 3  is a flowchart illustrating a brake booster leak detection method according to various aspects of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0015]    Referring now to  FIG. 1 , an exemplary hybrid vehicle  10  is shown. As can be appreciated, the brake booster leak detection methods and systems of the present disclosure can be used in various series and parallel hybrid vehicles. For exemplary purposes, the brake booster leak detection methods and systems of the present disclosure will be discussed in the context of a parallel hybrid vehicle. The vehicle  10  shown in  FIG. 1  includes an engine  12  that drives a transmission  14 . The transmission  14  can be either an automatic or a manual transmission that is driven by the engine  12  through a corresponding torque converter or clutch  16 . The engine  12  includes N cylinders  18 . Although  FIG. 1  depicts four cylinders (N=4), it can be appreciated that the engine  12  may include additional or fewer cylinders  18 . For example, engines having 4, 5, 6, 8, 10, 12 and 16 cylinders are contemplated. Air flows into the engine  12  through a throttle  20  and is combusted with fuel in the cylinders  18 . 
         [0016]    The vehicle  10  further includes an electric machine  22  and a battery  24 . The electric machine  22  operates in one of a motor mode and a generator mode. When operating in the motor mode, the electric machine  22  is powered by the battery  24 . When in motor mode, the electric machine  22  provides positive torque which assists the engine  12  or drives the transmission  14 . When operating in the generator mode, the electric machine  22  generates electrical energy to charge the battery  24 . The electric machine  22  may be driven by the engine  12  or by the transmission  14 . As can be appreciated, the battery  24  can power other vehicle accessories in addition to the electric machine  22 . 
         [0017]    A vehicle operator manipulates a brake pedal  32  to regulate vehicle braking. More particularly, a braking system  36  adjusts vehicle braking based on a force applied to the brake pedal  32  to regulate vehicle speed. A vacuum assisted brake booster  38  receives negative pressure supplied by the manifold (not shown) of the engine  12 . The vacuum assisted brake booster  38  uses the negative pressure as a vacuum to provide extra force to assist driver braking. 
         [0018]    A first pressure sensor  40  generates a brake booster pressure signal  42  based on a pressure supplied to the vacuum assisted brake booster  38 . A second pressure sensor  44  generates a brake line pressure signal  46  based on a line pressure in the braking system  36 . The control module  26  receives the pressure signals  42  and  46  and detects leaks in the brake booster vacuum as will be discussed further below. 
         [0019]    Referring now to  FIG. 2 , a dataflow diagram illustrates various embodiments of a brake booster leak detection system that may be embedded within the control module  26 . Various embodiments of brake booster leak detection systems according to the present disclosure may include any number of sub-modules embedded within the control module  26 . As can be appreciated, the sub-modules shown may be combined and/or further partitioned to similarly detect a leak in the vacuum assisted brake booster  38 . Inputs to the system may be sensed from the vehicle  10  ( FIG. 1 ), received from other control modules (not shown) within the vehicle  10  ( FIG. 1 ), and/or determined by other sub-modules (not shown) within the control module  26 . In various embodiments, the control module  26  of  FIG. 2  includes an engine evaluation module  50 , a pressure evaluation module  52 , and a fault reporting module  54 . 
         [0020]    The engine evaluation module  50  monitors engine evaluation parameters  56  to determine when the engine  12  ( FIG. 1 ) is off or the vehicle  10  ( FIG. 1 ) is off but the control module  26  ( FIG. 1 ) is still powered (i.e. extended engine off power mode). If one or more of the engine off conditions are met, the engine evaluation module  50  sets a diagnostic enable flag  58  to TRUE. Otherwise, the diagnostic enable flag  58  remains set to FALSE. The pressure evaluation module  52  begins evaluating the pressure signals  42  and  46  once the diagnostic enable flag  58  is TRUE. More specifically, the pressure evaluation module  52  monitors brake line pressure  46  for a predetermined time. If the brake line pressure  46  remains substantially constant for that predetermined time, the pressure evaluation module determines a change in brake booster vacuum pressure  60  over that predetermined amount of time. 
         [0021]    The fault reporting module  54  evaluates the change in brake booster vacuum pressure  60  to determine if a leak is present. If the change in brake booster vacuum pressure  60  indicates a vacuum decay has occurred, a report status  62  is set that indicates a leak is present or the test has failed. Otherwise, if the change in brake booster vacuum pressure  60  indicates no decay or not enough decay (i.e. based on a predetermined decay threshold) has occurred, the report status  62  is set to indicate a leak is not present or the test has passed. In various embodiments, the fault reporting module  54  applies a statistical filter such as an Exponential Weighted Moving Average (EWMA) to the change in brake booster vacuum pressure  60 . The fault reporting module  54  then evaluates a result of the statistical filter to determine whether a leak is present. 
         [0022]    Referring now to  FIG. 3 , a flowchart illustrates a brake booster leak detection method that can be performed by the control module  26  of  FIG. 2  according to various aspects of the present disclosure. As can be appreciated, the order of execution of the steps of the brake booster leak detection method can vary without altering the spirit of the method. The method may be performed periodically during control module operation or scheduled to run based on certain events. The method may begin at  100 . Engine evaluation parameters are monitored at  110  and  120 . If the engine is stopped at  110  or the engine is in an extended engine off power mode at  120 , an initial brake booster vacuum pressure (BBV 0 ) is captured at  130 . An initial brake line pressure (BLP 0 ) is captured at  140  and a time parameter (T) is initialized to zero at  150 . Otherwise, if the engine is not stopped and the engine is not operating in an extended engine off power mode, engine evaluation parameters are continually monitored at  110  and  120 . 
         [0023]    Once the initial brake line pressure (BLP 0 ) and the initial brake booster vacuum pressure (BBV 0 ) are captured at  130  and  140  and the time (T) is initialized at  150 , the current brake line pressure (BLP) is evaluated at  160 . If the current brake line pressure (BLP) is less than or equal to the initial brake line pressure (BLP 0 ) plus a predetermined offset, the time (T) is incremented at  170 . Otherwise, if the current brake line pressure (BLP) is greater than the initial brake line pressure (BLP 0 ) plus the predetermined offset, the method may end at  220 . 
         [0024]    At  180 , if the time (T) is greater than or equal to a predetermined time threshold, a change in brake booster vacuum pressure is computed at  190 . The change in brake booster vacuum pressure can be computed as a ratio (R) based the initial brake booster vacuum pressure (BBV 0 ) and a current brake booster vacuum pressure (BBV) and based on the following equation: 
         [0000]        R =( BBV   0   −BBV )/ BBV   0 .  (1) 
         [0000]    The brake booster vacuum ratio (R) is then processed at  200  to determine if a leak is present. For example, a statistical filter such as EWMA is applied to the ratio (R). If the result of the statistical filter is zero or below a predetermined threshold, a leak is not present and the test has passed. If the result of the statistical filter is greater than a predetermined threshold, a leak is present and the test has failed. The status of the leak is reported at  210 . If a leak is detected, the report status  62  ( FIG. 2 ) indicates Test Fail. If a leak is not detected, the report status  62  ( FIG. 2 ) indicates Test Pass. The method may end at  220 . 
         [0025]    As can be appreciated, once the report status  62  ( FIG. 2 ) is set to Test Fail, additional steps can be performed to notify other systems and users of the failure. In various embodiments, a diagnostic code is set based on the report status  62  ( FIG. 2 ). The diagnostic code can be retrieved by a service tool or transmitted to a remote location via a telematics system. In various other embodiments, an indicator lamp is illuminated based on the report status  62  ( FIG. 2 ). In various other embodiments, an audio warning signal is generated based on the report status  62  ( FIG. 2 ). 
         [0026]    As can be appreciated, all comparisons discussed above can be implemented in various forms depending on the selected values for comparison. For example, a comparison of “greater than or equal to” may be implemented as “greater than” in various embodiments. Similarly, a comparison of “less than or equal to” may be implemented as “less than” in various embodiments 
         [0027]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while this disclosure has been described in connection with particular examples thereof, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and the following claims.