Abstract:
A method for restarting an engine of a vehicle stopped on a grade, comprising the steps of engaging a gear of a transmission through which the engine and wheels of the vehicle are driveably connected mutually, using brake pressure to engage wheel brakes and produce a road gradient wheel torque that holds the vehicle stationary on the grade, initiating an engine restart, operating the engine to produce wheel torque equal to or greater than the road gradient wheel torque, and releasing the brake pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a vehicle powertrain applicable to a hybrid electric vehicle (HEV). More particularly, the invention pertains to controlling the stopping and restarting of an engine on a road grade. 
     2. Description of the Prior Art 
     A HEV is a vehicle that combines a conventional propulsion system, which includes an internal combustion engine and a step-change automatic transmission, a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle. 
     Motor vehicles can be designed to employ certain aspects of hybrid electric technology, but without use of a hybrid electric powertrain. Certain vehicles having a conventional powertrain, but no electric machine for driving the wheels, called micro-HEVs, shutdown the engine during conditions where the engine operates at idle speed to reduce fuel consumption and reduce emissions while the vehicle is stopped. 
     During normal vehicle operation many instances arise where the vehicle must stop: at traffic signals, cross-walks, stop signs and the like. In micro-HEVs the engine is shut down if no power is required, e.g. while waiting at a traffic light. As soon as power is requested, the engine is automatically restarted. By avoiding an unnecessary engine idling event, the vehicle&#39;s fuel economy is improved. For this purpose, it is desirable to shut down the engine function as much as possible when certain engine stop conditions are satisfied. 
     A vehicle stopped on a surface that has a sufficient grade or slope and whose powertrain includes an automatic transmission, can experience a vehicle roll-back event while the engine is idling. A conventional automatic transmission is driven by the engine through a torque converter. With the vehicle on a flat surface and the engine at idle, torque transmitted to the transmission is generally sufficient to enable slight forward rolling of the vehicle, i.e., vehicle creeping. When the vehicle is on a slight grade of positive slope (3%-7%), torque transferred to the transmission is generally sufficient to hold the vehicle stationary preventing roll-back. On higher grades (7% and greater), however, vehicle roll-back can occur, causing reverse torque transfer through the transmission. 
     In a micro-HEV with the engine shutdown and the vehicle stationary on an uphill road grade, vehicle roll-back and reverse torque transfer effects can be worse because there is no torque output from the engine before the engine startup and insufficient traction torque during engine restart. 
     During an engine startup in gear process, the gradient load torque T RL  is transferred from the wheels by the driveline to the transmission. T RL =mg sin e*Rw, wherein (m) is vehicle mass and (e) is the road gradient angle and Rw is the effective tire radius. T RL  is transmitted to the engine as additional load during an engine restart. When the hill gradient is 3% and higher, the following three problems can be observed during micro-hybrid vehicle starts on hill. First, a torque surge caused by the engine restart may bring unexpected vehicle forward motion, which is both undesirable for the vehicle and uncomfortable for the driver. Such a torque spike phenomenon will be more substantial on a descending hill. Second, additional negative torque load on the engine caused by T RL  can stall the engine during engine restart, because the initial engine restart torque and the starter may not be large enough to drive the additional load. Third, after the engine restart, the creep torque at engine idle speed may not be sufficient to counteract the road gradient torque load on the vehicle before the driver applies the accelerator pedal. As a result, the vehicle will roll back on the incline before the drive actively depresses the accelerator pedal to power the vehicle. 
     SUMMARY OF THE INVENTION 
     A method for restarting an engine of a vehicle stopped on a hill, comprising the steps of using brake pressure to engage wheel brakes and produce reactive friction wheel torque that holds the vehicle stationary for the current road gradient, initiating an engine restart, operating the engine to produce wheel torque equal to or greater than the road gradient wheel torque, and releasing the brake pressure. 
     The method prevents a micro-HEV during an engine automatic restart and drive-off maneuver from rolling in a direction opposite to the direction of the driver request. The method also helps avoiding engine stall during hill restarting and unexpected vehicle acceleration jerk as the engine restarts. These advantages are realized by use of a service brake to build-up and maintain brake friction torque capability on at lease one of the driven axle wheels, as well as the undriven wheels, while the vehicle is stationary. Brake pressure may be applied and maintained either by the operator depressing the brake pedal or automatically through operation of a brake control system. 
     After the operator releases the service brake, the Hill Start Brake Assist (HSBA) control system retains the applied pressure in the brake system to a magnitude that is necessary to maintain the vehicle stationary on the hill and to suppress powertrain torque disturbances during the engine restart. During vehicle acceleration, the HSBA control reduces brake pressure in balance with the increasing driving torque. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a micro-HEV powertrain; 
         FIG. 2  is schematic diagram showing a portion of a HSBA controller; 
         FIG. 3  contains graphs of powertrain variables during an engine restart under HSBA control on a hill whose gradient is in a medium range of road gradient. 
         FIG. 4  contains graphs of powertrain variables during an engine restart under HSBA control on a hill whose gradient is in a high range of road gradient. 
         FIG. 5  contains graphs of powertrain variables during an engine restart under an alternative HSBA control to that of  FIG. 4 ; 
         FIG. 6  contains graphs of powertrain variables during an engine restart under HSBA control on a downhill; and 
         FIGS. 7A and 7B  illustrate a logic flow diagram of the steps of an algorithm for controlling the engine restart. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, the micro-HEV powertrain  10  of  FIG. 1  includes a power source  12 , such as an internal combustion engine; an enhanced engine starter motor  14 ; automatic transmission  16 ; input shaft  18 ; impeller  20 , driveably connected by shaft  18  to the engine; turbine  22 , hydrokinetically driven by the impeller; a torque converter lockup clutch  36 ; transmission output  24 ; final drive mechanism  26 , connected to the output; an electric auxiliary hydraulic pump (EAUX)  28 , whose output pressurizes the hydraulic system of the transmission; an electric storage battery  30 , which supplies electric power to the pump  28  and starter  14 ; and axle shafts  32 ,  33 , driveably connect to the driven wheels  34 ,  35  through the output  24  and final drive mechanism  26 ; brakes  78 ,  79 ; and brake lines  76 ,  77 . 
     A gear selector  40  is moved manually by the vehicle operator among P, R, N and D positions in an automatic mode channel  42  and between upshift (+) and downshift (−) positions in a manual range mode channel  44 . 
     Accelerator and brake pedals  50 ,  52 , controlled manually by the vehicle operator, provide input demands to a controller  46  for changes in engine wheel torque and changes in brake force at wheel brakes  78 ,  79 , respectively. 
     Located within transmission  16  are friction control elements, i.e., clutches and brakes, whose state of coordinated engagement and disengagement produce the forward gears and reverse gear. The first forward gear, low gear, is produced when at least one, but preferably two of the control elements  54 ,  56  are engaged concurrently. The transmission control elements, whose engagement produces the desired gear in which the vehicle will be launched, are referred to as launch elements  54 ,  56 . Hydraulic line pressure produced by the electric auxiliary pump  28  while the engine  12  is shutdown is used to fill and stroke the launch elements  54 ,  56 , thereby preparing the transmission  16  for responsive torque transmission once the engine restart is completed. Stroking the launch control elements  54 ,  56  takes up clearances between the servo pistons and a pack of friction plates in the control elements, and clearances among the friction plates. The launch elements  54 ,  56  have substantially no torque transmitting capacity when stroke pressure is present in the servo cylinders that actuate the launch elements. 
     Transmission  16  also contains a hydraulic pump  53 , such as a gerotor pump or vane pump, whose output is used to produce pressure in the transmission&#39;s hydraulic circuit, through which the control elements  54 ,  56  are pressurized to a state of full engagement in coordination with the engine restart method. 
     The microprocessor-based controller  46 , accessible to a hill start brake assistance (HSBA) engine restart control algorithm  70 , communicates through electronic signals transmitted on a communication bus with the engine  12 , starter  14 , transmission  16 , battery  30 , auxiliary pump  28 , gear shifter  40 , and the accelerator and brake pedals  50 ,  52 . 
       FIG. 2  schematically illustrates controller  46 . Controls  60  for the brake system, which include a HSBA brake actuator  62 , receive input signals from and transmit commands to brake actuators and sensors  64 . The engine control module (ECM)  50  includes an engine start-stop scheduler  66 , which transmits engine start/stop requests  67  to a chassis and powertrain coordinator (CPTC)  68 , which contains a HSBA engine restart control algorithm  70 . The brake system controls  62  and CPTC  68  communicate through a high speed controller area network (HS-CAN)  72 . 
     HSBA control is preferably entered when the micro-HEV is stationary, brake pedal  52  is depressed, the magnitude of the road gradient is determined, engine  12  is stopped, accelerator pedal  50  is released, the park brake is released, and gear shift selector  40  is in a Drive range position. 
     Four use cases are illustrated in  FIGS. 3-6  to demonstrate the function of the HSBA engine restart control.  FIG. 3  shows graphs of powertrain variables during an engine restart under HSBA control on a hill whose gradient has a positive slope in a low road gradient range (e.g. between 3 percent and 7 percent). During period A, the vehicle is stopping and becomes stationary before the engine restart is initiated at  80  by a restart request. During period B, an engine restart is initiated and combustion becomes sustained. During period C, the vehicle launches ahead as vehicle speed on the uphill slope increases. 
     Graph  82  represents the application and subsequent gradual release of the brake pedal  52  while the vehicle is stopped. 
     Graph  83  indicates that the gear selector  40  is continually in the Drive or Low positions. While the vehicle is stopped, the magnitude of the road gradient is identified, driver applies brake pedal  52  to keep the vehicle stationary, and HSBA control is enabled. 
     Graph  84  represents the application of the accelerator pedal  50  following the engine restart. 
     The road gradient torque load T RL , which is transmitted from the wheels  34 ,  35  though the driveline to the transmission  16 , is T RL =mg sin θ*Rw, wherein (m) is vehicle mass and θ is the road gradient angle, positive for uphill slope and negative for downhill slope, and Rw is the effective tire radius. 
     When the driver releases the brake pedal  52  and prepares to accelerate the vehicle, brake pressure  86  in lines  76 ,  77  is held constant through HSBA control at pressure  90  if the master cylinder pressure P_MC drops to or below a predefined pressure level P_HSBA  90 . Brake pressure  90  enables the service brakes  78 ,  79  to counteract the road gradient torque load T RL  and to suppress a torque spike  94  in the powertrain  10 . In general, P_HSBA pressure  90  has the maximum value of either the hill hold wheel torque for the gradient load or the torque spike suppression wheel torque. Graph  88  represents the sufficiently high brake pressure level. 
     Graph  94  shows that HSBA control is enabled after the magnitude of the road gradient T RL  is determined. Graph  96  represents active HSBA control after release of the brake pedal  52 . Graph  98  represents active HSBA control becoming inactive when sufficient wheel torque occurs or a HSBA timer  100  expires at  106 . 
     The engine restart is initiated at  80  by using starter  14  to crank engine  12  as brake pedal  50  is released while the gear selector  40  is in a forward drive position, i.e., the DRIVE or LOW position. 
     Controller  46  sets an engine starting flag  106  when the engine restart is initiated at  80 , an engine speed peak passed flag at  108 , and an engine running flag at  110  when sustained engine combustion occurs. 
     When the engine restart is initiated, the HSBA timer  100  is set to a first calibrated level  102  and begins to count down during Phase I control. If a peak  104  in engine restart speed is observed before the HSBA timer  100  counts down to zero, brake pressure  90  is maintained and timer is reset to a second calibrated level  105 . If the first calibrated level  102  of HSBA timer  100  expires before the engine speed peak  104  occurs, brake pressure  90  is reduced immediately upon a release of the brake pedal  50 . 
     If the first level  102  timer  100  does not expire before the engine running flag is set at  110 , HSBA timer  100  is extended during HSBA Phase II control to the second calibrated level  105  and counts down. During HSBA Phase II control, HSBA controller  46  holds elevated brake line pressure  90  until either HSBA timer  100  expires or powertrain wheel torque is sufficient to counteract the road gradient torque load, i.e., T CRANK   _   WHL ≧T RL . If the vehicle creep torque is larger than T RL , HSBA, controller  46  reduces brake pressure  90  upon release of brake pedal  50  immediately after Phase I control. The driveline torque at the wheels T CRANK WHL  is estimated based on engine speed, displacement of accelerator pedal  52  and driveline gear information. The proposed brake pressure control does not exclude the case where individual brake circuit pressure or wheel chamber pressure will be used for control action determination rather than the brake master cylinder pressure. 
     Graph  112  represents vehicle speed increasing from zero after sustained engine combustion occurs at  110 . 
     Graph  114  represents the state of the engine restart request, which occurs at  80 . 
     Graph  116 , which represents engine speed, shows an increase in engine speed beginning at the start of the engine restart  80  when the starter  14  cranks the engine  12 . Engine speed continues to increase following the first engine combustion  118 , remains relatively steady at idle speed  120  during the period while engine combustion is sustained, and increases further  122  as engine torque increases. 
     Graph  124  represents wheel torque T WHL , which is the sum of engine crankshaft torque at the wheels  34 ,  35  in the current gear T CRANK   _   WHL , brake torque T BRK ; and road load at the wheels T RL  (T WHL =T CRANK   _   WHL +T BRK +T RL . Graph  126  represents crankshaft torque T CRANK   _   WHL  at the wheels  34 ,  35  in the current gear. Graph  128  represents brake torque T BRK . Graph  130  represents road gradient torque load T RL  which is a small negative torque produced by the road gradient. 
     Similar to first use case of  FIG. 3 , the second HSBA use case shown in  FIG. 4 , illustrates the HSBA control function when the engine is stopped automatically and vehicle is stationary on a medium to high gradient hill with a positive slope in the range of seven percent to thirty five percent. In this case, the P_HSBA brake pressure  140  is set at pressure that is greater than brake pressure  90  corresponding to the increased road gradient torque load. 
     HSBA control is terminated later at  142  than in the first use case, i.e., when sufficient powertrain propulsion torque is present at wheel  34 ,  35  as required to counteract the larger negative road gradient torque load  144  after the driver depresses the accelerator pedal  52 . The HSBA control is enabled and active even without engine  12  automatically stopped. In this situation, HSBA control goes directly to phase II and is terminated according to phase II conditions. 
       FIG. 5  illustrates an optional control applicable to the first and second use cases, when the brake line pressure for powertrain torque disturbance suppression  144 , P_HSBA, is much larger than that of the pressure level to counteract road gradient torque load  146 , P_ARL. The only difference here is the brief pressure decrease from P_HSBA to P_ARL after control phase I is completed at  148 . By doing this, the brake line pressure level is kept at as low as possible to minimize the drag to vehicle drive-off motion during brake pressure release. 
       FIG. 6  illustrates HSBA control applied to a vehicle on a downhill, i.e., having negative slope. In this instance, the brake line pressure  90  is held at P_HSBA, i.e., at a magnitude large enough to suppress the addition of both the powertrain disturbance torque and the gradient load torque. HSBA control is terminated immediately after phase I is completed at  150 , because the creep torque or road gradient torque load is expected to drive the vehicle immediately after the engine restart. 
       FIGS. 7A and 7B  illustrate a logic flow diagram of the steps of algorithm  70  for controlling the engine restart. At step  160  a test is made to determine whether the brake system is operative. If the result of test  160  is logically false, at step  162  the HSBA control is inhibited, i.e., turned off. 
     If the result of test  160  is logically true, at step  164  a test is made to determine whether the vehicle is stopped on a grade or hill using information produced at step  170 . If the result of test  164  is logically false, control returns to step  164 . If the result of test  164  is true, at step  166  HSBA control is in enabled. 
     At step  168  a test is made to determine whether engine  12  is automatically stopped. 
     At step  170 , the road gradient θ and the road gradient torque load T RL  are determined and used as input data in steps  172 ,  174  and  202 . 
     If the result of test  168  is true, at step  172  the brake pressure required for the engine restart control is determined with reference to the road gradient torque load from step  170 . If the result of test  168  is false, control advances to step  174 . 
     At step  176  a test is made to determine whether the brake system pressure is equal to or greater than pressure in the brake system master cylinder. If the result of test  176  is false control returns to step  176 . 
     If the result of test  176  is true, at step  178  HSBA control is activated and brake pressure is held at the P_HSBA to keep the vehicle stationary. 
     At step  180  a test is made to determine whether an engine restart command has issued from controller  46 . 
     If the result of test  180  is false, at step  182  a test is made to determine whether the brake pedal  50  has been reapplied. If the result of test  182  is false, control returns to step  178 . 
     If the result of test  182  is true, at step  184  HSBA control is deactivated, brake pressure P_HSBA is released, and control returns to step  176 . 
     At step  186 , an engine restart is initiated. 
     At step  188 , the HSBA timer  100  is set to level one, and at step  190  the timer counts down. 
     At step  192  a test is made to determine whether the HSBA timer  100  has not expired. If the result of test  192  is false, i.e., the timer has expired, control advances to step  194 . 
     If the result of test  192  is true, i.e., the timer has not expired, at step  195  a test is made to determine whether the brake pedal  50  has been reapplied. If the result of test  195  is false, at step  196  a test is made to determine whether a peak in engine speed has occurred. 
     If the result of test  196  is false, at step  197  a test is made to determine whether engine  12  has stalled requiring another engine restart. If the result of test  197  is false, control returns to step  190 . If the result of test  197  is true, control returns to step  188 . 
     If the result of test  196  is true, at step  198  the HSBA timer  100  is set to level two, and at step  200  the timer begins to count down. 
     At step  202  a test is made to determine whether (i) the timer  100  has not expired or (ii) whether wheel torque produced by the engine  12  is greater than the road load torque load (T CRANK   _   WHL &gt;T RL ). 
     If the result of test  202  is true, at step  204  brake pressure is released, allowing the vehicle to accelerate. 
     At step  206  HSBA control is deactivated and disabled, and control returns to step  160 , whereupon algorithm  70  is reexecuted. 
     If the result of test  202  is false, at step  208  a test is made to determine whether brake pedal  50  has been reapplied. If the result of test  208  is false, control returns to step  200 . 
     If the result of test  208  is true, control advances to step  194 , where brake pressure is released, and to step  206 . 
     If the result of either of tests  192  and  195  is true, control advances to step  194 , where brake pressure is released, and to step  206 . 
     If the result of test  168  is false, at step  174  the magnitude of HSBA brake pressure required for the current road gradient is determined. 
     At step  209 , a test is made to determine whether the vehicle is stationary and whether master cylinder pressure P_MC is equal to or less than P_HSBA brake pressure. If the result of test  209  is false, control returns to step  166 . 
     If the result of test  209  is true, at step  210  the HSBA brake pressure is applied and held constant, and timer  100  is set to level two. Then control advances to step  200 , where the timer counts down. 
     If the result of test  196  is true, an optional logic path, which begins at step  212 , addresses the condition in which brake line pressure for powertrain torque disturbance suppression P_HSBA is much larger than that of the pressure required to counteract road gradient torque load P_ARL. At step  212 , brake line pressure P_HSBA is reduced to road gradient torque load pressure P_ARL, and control advances to step  198 . 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.