Abstract:
A vehicle control system reduces vehicle rollback upon brake release. The control system includes a brake system, a vehicle grade measurement device and a controller that modulates applied brake pressure of the brake system based on a grade measurement of the grade measurement device. The controller actuates brake-hold device communicating with the brake system based on the grade measurement through pulse width modulation. The control system communicates with a motor generator and an engine to provide a start power to the engine upon brake release based on the grade measurement. Fuel injectors of the engine are enabled upon brake release based on the grade measurement. The control system further communicates with a transmission forward clutch to provide selective rotational communication between the transmission and the engine based on the grade measurement.

Description:
FIELD OF THE INVENTION 
     The present invention relates to vehicle control systems, and more particularly to vehicle control systems for minimizing vehicle rollback. 
     BACKGROUND OF THE INVENTION 
     Vehicle rollback may occur when a vehicle is stopped on an inclined road surface. Hybrid powertrains typically turn off the engine when the vehicle is stopped and restart the engine when the brakes are released. The vehicle rollback may occur between the time that the brakes are released and the time that sufficient output torque is present. 
     Some conventional powertrain control systems employ a neutral idle control strategy. For this type of vehicle, the transmission is automatically moved to neutral while idling to reduce fuel consumption and/or idle vibration. The vehicle is held on uphill inclines using the vehicle braking system, and hardly at all by the drivetrain. Vehicle rollback may be more pronounced in vehicles with relatively high vehicle mass and relatively low engine torque. In these vehicles, the drive axle is not preloaded with a sufficient amount of torque following brake release while the vehicle is stopped to overcome the effects of gravity. Current solutions include increasing idle speed to increase available drive torque when the brakes are released. However, increasing idle speed requires increased idle speed for all conditions, even when the increased idle speed is not necessary. The increased idle speed reduces fuel economy and increases noise. 
     SUMMARY OF THE INVENTION 
     A control system and method in a vehicle reduces vehicle rollback upon brake release. The control system includes a brake system and a vehicle grade measurement device that generates a grade signal. A controller modulates applied brake pressure of the brake system based on the grade signal of the grade measurement device. 
     In other features the control system further comprises an engine having fuel injectors and a motor generator. The motor generator provides a start power to the engine upon brake release based on the grade signal. The fuel injectors are enabled some time after brake release when certain parameters are met, at a rate based on the grade signal. The controller actuates a brake-hold device communicating with the brake system based on the grade signal. The brake-hold device can include solenoids to hold brake pressure. 
     The control system further includes a transmission forward clutch disposed between a transmission and the engine. The transmission forward clutch provides selective rotational communication between the transmission and the engine based on the grade signal. 
     A method for reducing rollback in a vehicle upon brake release includes determining if the vehicle is stopped. A grade signal is produced based on a grade measurement of the vehicle. The idle RPM of the engine is increased as a function of the grade signal. 
     A method for reducing vehicle rollback upon brake release in a vehicle includes determining if the vehicle is stopped. A grade signal is produced based on a grade measurement of the vehicle. The engine is stopped based on the grade signal. A brake system of the vehicle is actuated based on the grade signal. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a control system that minimizes vehicle rollback according to the present invention; 
         FIG. 2 . illustrates predetermined grade thresholds according to the present invention; and 
         FIG. 3  is a flowchart illustrating steps for controlling vehicle rollback according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring now to  FIG. 1 , an engine control system  10  according to the present invention includes a controller  12  and an engine  16 . The engine  16  includes a plurality of cylinders  18  each with one or more intake valves and/or exhaust valves (not shown). The engine  16  further includes a fuel injection system  20  and an ignition system  24 . An electronic throttle controller (ETC)  26  adjusts a throttle area in an intake manifold  28  based upon a position of an accelerator pedal  30  and a throttle control algorithm that is executed by the controller  12 . It will be appreciated that ETC  26  and controller  12  may include one or more controllers. One or more sensors  30  and  32  such as a manifold pressure sensor and/or a manifold air temperature sensor sense pressure and/or air temperature in the intake manifold  20 . 
     A position of the accelerator pedal  30  is sensed by an accelerator pedal sensor  40 , which generates a pedal position signal that is output to the controller  12 . A position of a brake pedal  44  is sensed by a brake pedal sensor  48 , which generates a brake pedal position signal that is output to the controller  12 . Emissions system sensors  50  and other sensors  52  such as a temperature sensor, a barometric pressure sensor, and other conventional sensor and/or controller signals are used by the controller  12  to control the engine  16 . An output shaft of the engine  16  is coupled by a torque converter  54  and transmission forward clutch  56  to a transmission  60  to front and/or rear wheels. The transmission  60  is preferably a continually variable transmission but may alternatively be a conventional transmission. The transmission forward clutch  56  is preferably engaged with hydraulic fluid supplied by an electro-hydraulic controller (not shown) under control of the controller  12  depending on the operating conditions. The transmission forward clutch  56  couples the engine  16  to the transmission  60  when the vehicle is moving and can disconnect the engine  16  from the transmission  60  when the vehicle is at rest. When the operator commands an engine restart, the motor generator  62  is rotated as a motor so that the engine  16  can be rotated at a speed sufficient to cause starting thereof. The transmission forward clutch  56  is engaged at a predetermined rate. As will be described, the controller  12  of the present invention controls the rate at which the transmission forward clutch  56  is engaged based on the vehicle grade. 
     The controller  12  communicates with a motor generator  62  that is coupled to the engine  16  using a drive  64  such as a belt drive, a chain drive, a clutch system or any other device. The vehicle can be driven forward either by the engine  16 , the motor generator  62  or a combination of both. During vehicle braking, the motor generator  62  is driven as a generator to charge a complement of electrical storage members  66 . The vehicle uses electro-dynamic braking as well as conventional friction braking. The electrical storage members  66  supplies power to the motor generator  62  when it is operated as a motor. The motor generator  62  can also be driven as a generator during normal vehicle operation to maintain a specific minimum charge at the electrical storage members  66 . 
     A grade sensor  68  generates a grade signal, which represents the grade of the vehicle (the angle of a longitudinal axis of the vehicle relative to horizontal). The controller  12  communicates with a brake-hold device  70  incorporated on a brake system  72  to selectively retain brake pressure after the brake pedal  44  has been released as will be described in greater detail. The brake-hold device  70  may include solenoids, motors, and/or other devices. 
     With continued reference to  FIG. 1  and further reference to  FIG. 2 , the grade sensor  68  outputs the grade signal to the controller  12 . The controller  12  may define predetermined grade ranges representing a low slope (L), a medium slope (M), a high slope (H) and a greatest slope (G). In the present invention, the controller  12  performs corrective steps to minimize vehicle rollback during times when rollback is likely based on the measured grade. 
     The present invention employs vehicle speed and acceleration correction parameters according to the signal measured by the grade sensor  68 . In this regard, the controller  12  communicates with the brake-hold device  70  of the brake system  72 , the motor generator  62 , the transmission forward clutch  56  and the fuel injection  20  as needed. The brake-hold device  70  of the present invention may be incorporated into the hydraulic brake system  72  as additional components or alternatively may be included as components of a conventional braking system. The brake-hold device  70  according to the present invention are used to facilitate a hill-hold condition for a vehicle starting on an uphill grade to selectively retain brake pressure after the brake pedal has been released. As the uphill grade is increased ( FIG. 2 ), an increased brake pressure is favorable. The brake-hold device  70  may be actuated through pulse width modulation (PWM) with an increasing duty cycle as a function of the grade. 
     The controller  12  communicates with the transmission forward clutch  56  to provide a corrected modulation rate. With a hybrid vehicle with an engine-stop-start functionality, when the engine  16  is restarted from 0 RPM to idle speed, the transmission forward clutch  56  is typically modulated to smoothly creep the vehicle forward. For flat or downhill grades, the transmission forward clutch  56  is engaged at a nominal modulation profile. According to the present invention, the transmission forward clutch  56  is closed at a more aggressive rate based on the measured incline to more quickly counteract the uphill grade load. 
     The controller  12  communicates with the fuel injection  20  to provide a corrected re-fire threshold. In this way, the rate at which the cylinders  18  are re-fired during restart is increased according to the grade measured by the grade sensor  68 . 
     Referring now to  FIG. 3 , steps for minimizing vehicle rollback for a hybrid vehicle are shown generally at  100 . Control begins with step  102 . In step  104 , control determines if the brake pedal  44  is applied. If not, control loops to step  104 . If the brake pedal is applied, control determines if the vehicle is stopped in step  106 . If not, control loops to step  104 . If the vehicle is stopped, a vehicle grade measurement is communicated by the grade sensor  68  to the controller  12  in step  108 . In step  112  control determines if the grade measurement is greater than (G). If the grade measurement is greater than (G), idle speed is increased in step  114 . In this way, the controller  12  identifies a predetermined greatest rate for modulating the brake-hold device  70  and a most aggressive rate for closing the transmission forward clutch  56  or maintaining its closure. In step  118 , the brake-hold device  70  of the brake system  72  is actuated through pulse width modulation (PWM) at the greatest rate for maximum brake pressure retention. In step  120  control determines if the brake pedal  44  is released. If not, control loops to step  120 . If the brake pedal is released, the brake-hold device is released at a predetermined low rate in step  124 , and control ends in step  174 . 
     If the grade measurement is not greater than (G), control determines if the grade measurement is greater than high (H) in step  130 . If the grade measurement is greater than (H), the engine  16  is stopped in step  132 . As such, the controller  12  identifies a predetermined high value for modulating the brake-hold device  70  and a high rate for closing the transmission forward clutch  56 . Because the engine  16  is stopped, the momentum parameters also include a predetermined high rate for enabling the fuel injection  20  and a predetermined high power for initiating the motor generator  62 . In step  138 , the brake-hold device  70  of the brake system  72  is actuated through PWM at a high rate. In step  140 , control determines if the engine  16  is restarted. If not, control loops to step  140 . If the engine  16  is restarted, the motor generator  62  is started at a high power, the transmission forward clutch  56  is closed at the high rate, the fuel injection  20  is initiated at the high rate, and the brake pressure is released at a medium rate in step  142 , and control ends in step  174 . 
     If the grade measurement is not greater than (H), control determines if the grade measurement is greater than medium (M) in step  146 . If the grade measurement is greater than (M), the engine  16  is stopped in step  148 . As such, the controller  12  identifies a predetermined medium rate for modulating the brake-hold device  70  and a medium rate for closing the transmission forward clutch  56 . In addition, a predetermined medium rate is defined for enabling the fuel injection  20 . In step  154 , the brake-hold device  70  of the brake system  72  is actuated through PWM at a medium rate. In step  156  control determines if the engine  16  is restarted. If not, control loops to step  156 . If the engine  16  is restarted, the transmission forward clutch  56  is closed at the medium rate, the fuel injection  20  is initiated at the medium rate, and the brake pressure is released at a fast rate in step  158 , and control ends in step  164 . 
     If the grade measurement is not greater than (M), control determines if the grade measurement is greater than low (L) in step  160 . If the grade measurement is greater than (L), the engine  16  is stopped in step  164 . As such, the controller  12  identifies a predetermined low rate for modulating the brake-hold device  70  and a low rate for closing the transmission forward clutch  56 . In step  168 , the brake-hold device  70  is actuated through PWM at a low rate. In step  170  control determines if the engine  16  is restarted. If not, the control loops to step  170 . If the engine  16  is restarted, the transmission forward clutch  56  is closed at the lowest rate and the fuel injection  20  is initiated at the lowest rate in step  172 . Control ends in step  174 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. For example, while the grade information as described above is performed by a separate grade measurement device, the grade information may likewise be obtained through calculation by the engine controller based on engine load, transmission ratio and other powertrain variables. Furthermore, while parameters are described as being associated with predetermined grade conditions, these parameters may similarly applied to other grade conditions. For example, although the motor generator  62  is started at a high power for grade condition (H), it is appreciated that the motor generator  62  may be started at increments of increased power for each grade condition. In addition, while the preceding discussion is adapted for implementation with a hybrid vehicle, the same may be applied for a conventional vehicle such as those employing a neutral idle transmission. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.