Abstract:
A method is provided for optimizing the cycling frequency between engine on/off states in a vehicle having a controller and auto start/auto stop functionality. The method includes detecting an engine state cycling event, measuring a plurality of vehicle operating values, and using the controller to optimize the cycling frequency via at least one of: automatically adjusting an interval between an auto start event and an auto stop event when each of a first set of the vehicle operating values exceeds a corresponding threshold, and temporarily inhibiting the auto start/auto stop functionality when any value in a second set of the vehicle operating values falls outside of a hysteresis band created around the second set. A vehicle includes an engine and a controller having an algorithm for optimizing the cycling frequency between engine on/off states as set forth above.

Description:
TECHNICAL FIELD 
     The present invention relates generally to electrical power flow control aboard a vehicle, and more particularly to a method and a system for optimizing the start/stop cycling frequency of an engine in a vehicle having auto stop/auto start functionality. 
     BACKGROUND OF THE INVENTION 
     Certain vehicle designs, such as hybrid electric vehicles (HEV), are able to selectively utilize different energy sources in order to optimize fuel efficiency. An HEV having a full hybrid powertrain can use either or both of an internal combustion engine and a high-voltage energy storage system (ESS) for propulsion. That is, a typical full HEV can be electrically propelled, usually immediately upon starting the HEV and during vehicle speeds up to a relatively low threshold speed. One or more high-voltage motor/generator units (MGU) may alternately draw power from and deliver power to the ESS as needed. Above the threshold speed, the engine can be started and engaged with a transmission to provide the required propulsive torque. 
     By way of contrast, the powertrain of a mild HEV typically lacks the capability of propelling the HEV via purely electrical means, but nevertheless retains certain key design features of the full HEV, e.g., the regenerative braking capability used for recharging the ESS using the MGU, as well as the capability of selectively shutting down or powering off the engine. The capability of an HEV to selectively shut off and restart the engine when the vehicle is at a standstill, and/or when operating in a stabilized low-speed drive mode, is of particular fuel-saving benefit relative to conventional idling vehicle designs. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method is provided for optimizing engine start/stop cycling frequency in a vehicle having engine start/stop functionality as noted above. Such a vehicle may be configured as a hybrid electric vehicle (HEV) as described above, and may include a high-voltage motor generator unit (MGU) adapted to assist the automatic starting of the engine after an auto stop event. The method may be embodied in algorithmic form, and may be automatically executed via an onboard controller to optimize auto start/auto stop cycling frequency. 
     Within the scope of the invention, the algorithm may include two sub-processes, each approaching the engine start/stop cycling frequency optimization function in a different manner after first detecting a cycling event. In the first sub-process, the algorithm may make threshold comparisons between a first set of vehicle operating values, e.g., vehicle output speed and an accumulator value, with the algorithm automatically adjusting the time or interval between auto start/auto stop events based on these operating values. When the elapsed time since an immediately preceding event falls within a calibrated window, the algorithm may increase the accumulator value, and may set the accumulator value to zero when the duration of an engine-on or engine-off state exceeds a calibrated threshold. 
     In the second sub-process, which may be executed separately from the first sub-process or concurrently therewith, a dead band or a hysteresis band, as that term is well understood in the art, may be created around a second set of operating values, e.g., both an accelerator pedal position and the output speed. Auto stop functionality may be automatically inhibited or temporarily disabled or delayed if either value remains within the hysteresis band. The size of the hysteresis band may be adjusted based on how much time has elapsed since the last auto start event was executed. 
     According to one embodiment, the algorithm may include detecting a cycling event, i.e., sequential auto start/auto stop events, and then reducing the cycling frequency based in part on the current operating state of the vehicle. That is, the time between future or subsequent auto start and auto stop events may be automatically adjusted to a more acceptable value. The algorithm embodying the method may be executed by the controller to track the number of times or the duration since the engine has cycled from an engine-off state to engine-on state, or vice versa. Each time such a cycling event occurs outside of an allowable threshold cycling interval or window, an accumulator or other counter may be incremented. Additional time may be added between auto stop events based on the accumulator count and vehicle speed, or based on other suitable operating values. 
     In particular, a method or algorithm is provided herein for optimizing a frequency of a cycling event occurring between an auto start and an auto stop event in a vehicle. The algorithm includes detecting the cycling event, measuring a plurality of vehicle operating values, and at least one of a first sub-process and a second sub-process. The first sub-process automatically adjusts an interval between auto start/auto stop events when each of a first set of the values are determined to exceed corresponding thresholds, and the second sub-process inhibits auto stop functionality when any of a second set of the vehicle operating values falls outside of a hysteresis band around the second set of the values. 
     A vehicle is also provided herein, and includes an engine and a controller. The controller selectively shuts off or powers down the engine during an auto stop event, and commands a restart of the engine during an auto start event. The controller includes the algorithm described above, which is executed by the controller to optimize the cycling frequency between auto start/auto stop events. The algorithm detects the cycling event, measures the vehicle operating values, and adjusts an interval between a subsequent auto start/auto stop event when each of a first set of the values are greater than a corresponding threshold, and/or inhibits auto stop when any of a second set of the values falls outside of a hysteresis band around the second set of the values. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a vehicle having auto stop/auto start functionality and a controller with an auto stop/auto start cycling frequency optimization algorithm; and 
         FIG. 2  is a graphical flow chart describing the algorithm usable with the vehicle of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures,  FIG. 1  shows a vehicle  10  having auto stop/auto start functionality as explained above. The vehicle  10 , which may be configured as a hybrid electric vehicle (HEV) as shown, includes a controller (C)  37  adapted for executing an auto stop event during vehicle idle or when operating below a threshold speed, and for executing an auto start event when engine propulsion is required. The controller  37  includes an algorithm  100  for optimizing the frequency of engine on/off state cycling, hereinafter referred to as the cycling frequency, with the algorithm explained below with reference to  FIG. 2 . The vehicle  10  includes an accelerator pedal  15  having a detectable pedal position (arrow P X ), with the pedal position transmitted to and/or read by the controller  37  as set forth below. 
     The vehicle  10  includes an internal combustion engine (E)  12  having a crankshaft  13  and an output member  20 . The vehicle  10  includes a transmission (T)  14  having an input member  22  and an output member  24 . Output member  20  of the engine  12  may be selectively connected to input member  22  via a torque transfer mechanism or clutch device  18 . The transmission  14  may be configured as an electrically variable transmission (EVT) or any other suitable transmission capable of transmitting propulsive torque to a set of road wheels  16  via output member  24 . Output member  24  of the transmission  14  rotates at an output speed (N O ) in response to an output speed request ultimately determined by the controller  37 . 
     The vehicle  10  may include a high-voltage (HV) electric motor/generator unit (MGU)  26 , such as a multi-phase electric machine of approximately 60 volts to approximately 300 volts or more depending on the design. MGU  26  is electrically connected to an HV battery or an energy storage system (ESS)  25  via an HV DC bus  29 , a voltage inverter or power inverter module (PIM)  27 , and an HV alternating current (AC) bus  29 A. The ESS  25  may be selectively recharged using the MGU  26  when the MGU is operating in its capacity as a generator, for example by capturing energy during a regenerative braking event. 
     During normal operation of the vehicle  10 , the MGU  26  may be used to selectively rotate a belt  23  of the engine  12 , or another suitable portion thereof, thereby cranking the engine during an auto start event as set forth above. The vehicle  10  may also include an auxiliary power module (APM)  28 , e.g., a DC-DC power converter, which is electrically connected to the ESS  25  via the DC bus  29 . The APM  28  may also be electrically connected to the auxiliary battery  41 , e.g., a 12-volt DC battery, via a low-voltage (LV) bus  19 , and adapted for energizing one or more auxiliary systems  45  aboard the vehicle  10 . 
     Still referring to  FIG. 1 , the controller  37  may be configured as a single or a distributed control device that is electrically connected to or otherwise in hard-wired or wireless communication with each of the engine  12 , the MGU  26 , the ESS  25 , the APM  28 , the PIM  27 , and the auxiliary battery  41  via a control channel  51 , as illustrated by dashed lines. Control channel  51  may include any required transfer conductors, e.g., a hard-wired or wireless control link(s) or path(s) suitable for transmitting and receiving the necessary electrical control signals for proper power flow control and coordination aboard the vehicle  10 . The controller  37  may include such control modules and capabilities as might be necessary to execute all required power flow control functionality aboard the vehicle  10  in the desired manner. 
     The controller  37  may be configured as a general purpose digital computer generally comprising a counter or accumulator  50 , a microprocessor or central processing unit, read only memory (ROM), random access memory (RAM), electrically-erasable programmable read only memory (EEPROM), high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O), as well as appropriate signal conditioning and buffer circuitry. Any algorithms resident in the controller  37  or accessible thereby, including the auto stop/auto start cycling frequency optimizing algorithm  100  in accordance with the invention as described below with reference to  FIG. 2 , can be stored in ROM and executed to provide the respective functionality. 
     As used herein, the term auto stop refers to the ability of the vehicle  10  to selectively shut down or power off the engine  12  whenever the HEV is idle or at a standstill, such as while waiting at an intersection, in low-speed traffic, or when otherwise determined by the control logic resident within the controller  37 . In this manner, the vehicle  10  is able to minimize idle fuel consumption. After an auto stop event, the MGU  26  may be used to rapidly restart the engine  12 , with this process referred to herein as an auto start event. 
     Within the scope of the invention, the controller  37  includes or has access to the algorithm  100  mentioned above and described below in detail with reference to  FIG. 2 . The controller  37  executes the algorithm  100  to automatically monitor and adjust the auto stop/auto start cycling frequency. 
     Referring to  FIG. 2 , the algorithm  100  may be read in conjunction with the structure shown in  FIG. 1  and described above. The algorithm  100  begins with steps  102  and  103  concurrently in one embodiment. In another embodiment, steps  102  and  103  may be executed individually as two different sub-processes as determined by the controller  37 , or optionally by an operator of the vehicle  10  if so configured. As such, the even steps outlined hereinafter may be referred to as the first sub-process of algorithm  100 , and the odd steps as the second sub-process of the same algorithm. 
     At step  102 , it is determined whether an auto stop/auto start cycling event has occurred. Within the scope of the invention, such a cycling event occurs when the engine  12  transitions from an engine-on state to an engine-off state, or vice versa. If such a cycling event is affirmatively detected or its presence otherwise determined, the algorithm  100  proceeds to step  104 , otherwise the algorithm is finished. 
     At step  103 , the algorithm  100  determines the effective pedal position (P X ) of the accelerator pedal  15 , and the corresponding vehicle output speed (N O ). Once determined, whether via direct measurement, calculation, or otherwise, the algorithm  100  proceeds to step  105 . 
     At step  104 , the vehicle output speed (N O ) and a value [A] of the counter or accumulator  50  are compared to corresponding calibrated threshold values. If the vehicle output speed (N O ) and the accumulator value [A] exceed their corresponding threshold values, the algorithm  100  proceeds to step  106 , otherwise it proceeds to step  108 . 
     At step  105 , the algorithm  100  creates a dead band or hysteresis box or band around the values of the pedal position (P X ) and vehicle output speed (N O ) determined at step  103 , and then it proceeds to step  107 . 
     At step  106 , the algorithm  100  modifies the time between auto start/auto stop events. The amount of the modification may vary depending on the variance of either or both of the values of the vehicle output speed (N O ) and the accumulator value (A) from their corresponding thresholds. That is, given a low vehicle output speed (N O ) and a high accumulator value [A], the time or interval between immediately subsequent or future auto start/auto stop events may be increased a relatively large amount, while given a high vehicle output speed (N O ) and a low accumulator value [A] the same time or interval may be increased a relatively small amount. The algorithm  100  then proceeds to step  108 . 
     At step  107 , the algorithm  100  determines whether the corresponding value of either of the pedal position (P X ) or the vehicle output speed (N O ) is within the hysteresis band created at step  105 . If so, the algorithm  100  proceeds to step  109 , and is otherwise finished. 
     At step  108 , the algorithm  100  determines the amount of elapsed time that has elapsed since the last auto stop/auto start event, e.g., by referencing a timer, and then compares this value to a threshold interval or window. The window may be relatively short in duration, approximately equal to duration considered reasonable or unobtrusive by a typical driver. If the elapsed time falls within the window, the algorithm  100  proceeds to step  110 , otherwise the algorithm is finished. 
     At step  109 , auto stop capability may be temporality prevented or inhibited, e.g., during a stabilized low-speed drive mode. The algorithm  100  then proceeds to step  111 . Step  109  may allow a higher opportunity cost to be applied to the engine-off state when the vehicle output speed (N O ) and effective pedal position (P X ) determined at step  103  are within the calibrated window of step  107 . 
     At step  110 , the algorithm  100  increments the accumulator value and proceeds to step  112 , having determined at step  108  that the time since the last auto stop event falls within too short of a time period relative to the threshold interval or window. 
     At step  111 , the algorithm  100  measures the elapsed time since the last auto stop event, e.g., by referencing a timer, and then proceeds to step  113 . 
     At step  112 , the algorithm  100  determines whether the duration that the engine  12  has been in an engine-on state or in an engine-off state exceeds a calibrated threshold. The duration may be the same or different for the two engine states, i.e., on and off, depending on the design of the vehicle  10 . If the engine  12  has been on or off for longer than the calibrated threshold(s), the algorithm  100  proceeds to step  114 , and is otherwise finished. 
     At step  113 , the algorithm  100  automatically adjusts the hysteresis band previously created at step  105  based on the elapsed time since the last auto stop event (see step  111 ). For example, the size of the hysteresis box may be automatically narrowed if a threshold amount of time has passed since the last auto start. The algorithm  100  then returns to step  107  as set forth above. 
     At step  114 , the algorithm  100  sets the accumulator value (A) to zero, i.e., resets or zeroes the accumulator  50 . The algorithm  100  is then finished. When the algorithm  100  resumes with step  102 , it may do so with a zero value. In this manner, the algorithm  100  specifically detects heavy auto start/auto stop cycling and then attempts to reduce this frequency in order to improve performance of the vehicle  10 , while at the same time avoiding accumulation counts under normal driving conditions. 
     As will be understood by those of ordinary skill in the art, and as noted elsewhere above, the algorithm  100  may be executed in whole or in part depending on the design of the vehicle  10 . For example, the evenly numbered steps  102 - 114  may be executed as a first sub-process to provide an engine cycle accumulator mode. In this mode, the algorithm  100  examines the number of times the engine  12  cycles from an engine-on to an engine-off state, or vice versa. Cycles are only accumulated if the engine start occurs within a short period of time after the engine stop, i.e., rapid cycling of the type experienced in relatively heavy traffic. 
     Each time a specific cycling event occurs, accumulator  50  is incremented as noted above. Based on the count or value (A) of the accumulator  50  and the output speed of the HEV  10 , additional time may be added to inhibit an auto stop event, and to reduce auto start cycling in traffic. The accumulator  50  is then cleared (see step  114 ) when the engine  12  is in an off or on state for an extended period of time. In this manner, the algorithm  100  in steps  102 - 114  may be used to detect heavy auto start/auto stop cycling, with the algorithm reducing the frequency of such cycling to improve drivability and avoid accumulation counts under normal driving conditions. 
     Likewise, the odd steps  103 - 113  may be executed as a second sub-process to provide a stability-based inhibit logic mode. In this mode, the algorithm  100  creates boundaries in the form of a hysteresis band or box around the pedal position (Px) and the vehicle output speed (N O ). The longer the engine  12  runs, the smaller the hysteresis box may become, and the more likely the engine has stabilized. 
     Engine run time may be monitored, and a multiplier may be added to the amount of hysteresis for both the pedal position (P X ) and the vehicle output speed (N O ). Cycling may be reduced when the pedal position (P X ) or the vehicle output speed (N O ) are insufficiently varied. An example of such cycling may occur when driving the vehicle  10  around a low-speed sweeping corner or curve, where the driver of the vehicle does not substantially tip out the throttle, but the engine  12  may nevertheless cycle off and on due to the slight reduction in pedal request. The algorithm  100  may be calibrated to avoid such a cycling event. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.