Abstract:
A hydraulic regenerative drive system for a vehicle is disclosed. An electronic controller ( 28 ) receives a nominal engine throttle signal ( 35 ) and generates a time-variant torque signal ( 26 ) therefrom. A hydraulic control circuit ( 18 ) receives controlling signals ( 15 ) from the electronic controller ( 14 ). A reservoir ( 62 ) is in fluid communication with the hydraulic control circuit ( 18 ) for storing hydraulic fluid. A pump/motor unit ( 60 ) has a controlled element ( 61 ) providing variable displacement, and is in fluid communication with the hydraulic control circuit ( 18 ). The pump/motor unit ( 60 ) is adapted for connection to the drive train ( 12 ) of the vehicle. An accumulator ( 64 ) is in fluid communication with the hydraulic control circuit ( 18 ). The controller ( 28,14 ) controls a controlled-torque retard mode of operation, in which the controlled element ( 61 ) of the pump motor unit ( 60 ) is controlled by the torque signal ( 26 ) to impart a dynamically calculated retarding torque to the drive train ( 12 ), and the pump/motor unit ( 60 ) pumps fluid from said reservoir to the accumulator ( 64 ) via the hydraulic control circuit ( 18 ). The controller ( 28,14 ) also controls a controlled-torque propulsion mode of operation in which the controlled element ( 61 ) is controlled by the torque signal ( 26 ) to impart a dynamically calculated propelling torque to the drive train ( 12 ), and the pump/motor unit ( 60 ) motors under influence of fluid from the accumulator ( 64 ) passing to the reservoir ( 62 ) via the hydraulic control circuit ( 18 ).

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the field of hydraulic regenerative drive systems for vehicles. It relates also the controls for such systems.  
       BACKGROUND  
       [0002]     Regenerative drive systems act in a first manner to retard a vehicle such that motive energy is extracted from the vehicle&#39;s drive train and provides rotary (kinetic) energy to a pump/motor unit, which pumps a fluid from a reservoir to a higher pressure accumulator. Such drive systems also act to propel a vehicle by the reverse process, in which the stored (potential) energy of the fluid is released from the accumulator and drives the pump/motor unit to impart energy to the drive train. Two examples of such a system are described in U.S. Pat. No. 4,986,383 to Evans, issued on Jan. 22, 1991, and in U.S. Pat. No. 5,024,489 to Tanaka et al, issued on Jun. 18, 1991.  
         [0003]     Regenerative drive systems have demonstrated the ability to cut fuel consumption in vehicles by 10-25%, although this is strongly dependent upon drive cycle and vehicle type (especially weight). There remains considerable interest in further improving such performance, either in the mechanical and hydraulic components themselves, or in control systems for the circuits constituted by these components.  
       SUMMARY  
       [0004]     In general terms, there is disclosed a hydraulic regenerative drive system for a vehicle including an electronic controller receiving a nominal engine throttle signal and generating a time-variant torque signal therefrom. The controller controls (i) a controlled-torque retard mode of operation, in which a dynamically calculated retarding torque is imparted to the drive train of said vehicle, and (ii) a controlled-torque propulsion mode of operation, in which a dynamically calculated propelling torque is imparted to said drive train.  
         [0005]     There is further disclosed a hydraulic regenerative drive system for a vehicle comprising: 
        an electronic controller receiving a nominal engine throttle signal, generating a time-variant torque signal therefrom;     a hydraulic control circuit receiving controlling signals from the electronic controller;     a reservoir in fluid communication with said hydraulic control circuit for storing hydraulic fluid;     a pump/motor unit having a controlled element providing variable displacement, said pump/motor unit being in fluid communication with said hydraulic control circuit and adapted for connection to the drive train of a vehicle; and     an accumulator in fluid communication with said hydraulic control circuit;     and wherein said controller controls (i) a controlled-torque retard mode of operation, in which said controlled element is controlled by said torque signal to impart a dynamically calculated retarding torque to said drive train, and said pump/motor unit pumps fluid from said reservoir to said accumulator via said hydraulic control circuit, and (ii) a controlled-torque propulsion mode of operation, in which said controlled element is controlled by said torque signal to impart a dynamically calculated propelling torque to said drive train, and said pump/motor unit motors under influence of fluid from said accumulator passing to said reservoir via said hydraulic control circuit.        
 
         [0012]     There is yet further disclosed an electronic controller for a hydraulic regenerative drive system, said controller receiving a nominal engine throttle signal, generating a time-variant torque signal therefrom, and wherein said controller controls (i) a controlled-torque retard mode of operation of said drive system, in which a controlled element of a pump/motor unit is controlled by said torque signal to impart a dynamically calculated retarding torque to a vehicle drive shaft; and (ii) a controlled-torque propulsion mode of operation, in which said pump/motor unit is controlled by said torque signal to impart a dynamically calculated propelling torque to said drive shaft.  
         [0013]     There is yet further disclosed a method for controlling operation of a hydraulic regenerative drive system comprising the steps of: 
        generating a time-variant torque signal from a nominal engine throttle signal; and     instructing either (i) a controlled-torque retard mode of operation, in which a controlled element of a pump/motor unit is controlled by said torque signal to impart a dynamically calculated retarding torque to a vehicle drive shaft, or (ii) a controlled-torque propulsion mode of operation, in which said controlled element is controlled by said torque signal to impart a dynamically calculated propelling torque to said drive shaft.        
 
         [0016]     Preferably, the electronic controller or a further method step outputs a modified engine throttle signal, and said torque signal and the vehicle&#39;s engine torque corresponding to the modified throttle signal are equal to the torque corresponding to the nominal engine throttle signal.  
         [0017]     There additionally is disclosed a computer program product comprising a computer program stored on a storage medium, the program including code means for performing the method steps given above  
         [0018]     Other aspects of the system, controller and method are disclosed in the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic block diagram of a Regenerative Drive System (RDS) and interfaces with vehicle management systems.  
         [0020]      FIG. 2  is a schematic block diagram of the RDS of  FIG. 1  in a stand-by mode of operation.  
         [0021]      FIG. 3  is a schematic block diagram of the RDS of  FIG. 1  in a retard mode of operation.  
         [0022]      FIG. 4  is a schematic block diagram of the RDS of  FIG. 1  in a propulsion mode of operation.  
         [0023]      FIG. 5  is a schematic block diagram of the RDS of  FIG. 1  in a dump mode of operation.  
         [0024]      FIGS. 6 and 7  are graphs used to derive a measure of actual torque.  
         [0025]      FIG. 8  is a schematic block diagram of a low-level control.  
         [0026]      FIGS. 9-26  are block flow diagrams for the states of  FIG. 8 .  
         [0027]      FIG. 27  shows a torque characteristic for a propulsion fuel saving mode of operation.  
         [0028]      FIG. 28  shows a torque characteristic for a propulsion boost mode of operation.  
         [0029]      FIG. 29  is a flow diagram showing how throttle position is modified.  
         [0030]      FIG. 30  shows a typical engine torque/speed characteristic.  
         [0031]      FIG. 31  is a schematic block diagram of a vehicle drive train.  
         [0032]      FIG. 32  shows a torque characteristic for a retard fuel saving mode of operation.  
         [0033]      FIG. 33  shows a torque characteristic for a retard dash mode of operation. 
     
    
     DETAILED DESCRIPTION  
       [0034]     Overview  
         [0035]      FIG. 1  shows a schematic representation of a RDS  10  having connection with the drive train  12  of a vehicle. The drive train  12  also connects the vehicle&#39;s engine  11  with the vehicle&#39;s driving wheels  13 . The RDS  10  has a low-level strategy control unit  14 , embodied in a programmed microprocessor. The low-level strategy control unit  14  interfaces, by various b-directional signals  15  with various sensors and actuators  16  associated with a hydraulic circuit  18 , a pump motor unit  20  and a clutch  22 . The cooperative function of the low-level strategy control  14 , the hydraulic circuits  16  and the sensors and actuators  18  is to provide, in the most general sense, torque-controlled retard and propulsion modes of operation. In the ‘Retard’ mode, energy is drawn-off the drive train  12  by the pump/motor unit  20  to give a braking effect, and stored. In the ‘Propulsion’ mode, stored energy is imparted to the drive train  12  by the pump/motor unit  20  to supplement or replace vehicle engine motive force. Other modes/states of operation are also supported (as will be described), including ‘Standby’ and ‘Disengaged’ (i.e. unclutched).  
         [0036]     The low-level control unit  14  is concerned with the control of retarding or propelling torque, and in that sense is provided with a time-variant torque value  26  (‘commanded torque’) provided by a high-level strategy control unit  28 . The high-level control unit  28  also receives drive shaft speed signal  29 , an ‘available torque’ signal  30 , and an ‘actual torque’ value  32  from the low-level strategy control unit  14 . The high-level control unit  28  also receives an engine speed signal  38 , and interfaces with the vehicle&#39;s throttle system  34 .  
         [0037]      FIGS. 2-5  show details of the sensors and actuators  16 , the hydraulic circuits  18 , the pump/motor unit  20 , and the clutch  22 , with reference to various states of operation. The low- and high-level strategy control units  14 ,  28  are embodied in an Electronic Control Unit (ECU)  90 , as will be described.  
         [0038]     Beginning with  FIG. 2 , a torque input/input (take-off) point  50  (also referred to as the ‘drive shaft’) represents the drive train  12  of the vehicle shown in  FIG. 1 . A mechanical clutch  52  is controlled by a clutch piston  54  and, in turn, by a clutch actuator  56 . The clutch  52  serves to connect a pump/motor unit  60  (i.e.  20  in  FIG. 1 ) to the drive shaft  50 . A hydraulic fluid is circulated between an accumulator  62  and a reservoir  64  according to whether the pump/motor unit  60  is motoring to provide torque to the drive shaft  50 , or pumping under torque imparted by the drive shaft  50 . The specific hydraulic circuits and actuators will be described below with reference to the various modes of operation.  
         [0039]     For the purposes of providing an example, consider a vehicle of 16,000 kg mass, a pump/motor unit of capacity 250 cc/rev, a 180 litre accumulator and maximum flow rate of 400 l/min.  
         [0040]     The pump/motor unit  60 , in the preferred embodiment, is a variable displacement axial piston pump, and in the present example is a Bosch Rexroth model A4VSO. The controlling element of the pump/motor unit  60  is a swash plate  61  which is adjustable in terms of angular displacement to give varying degrees of pumping or motoring action. Negative swash plate angle (−15 to 0 degrees) represents retarding (pumping) operation, whereas positive swash plate angle (i.e. 0 to +15 degrees) represents propulsion (motoring) operation.  
         [0041]     The specific hydraulic circuits arrangements of  FIGS. 2-5  should be read in conjunction with the state diagram of  FIG. 8 . The hydraulic circuits arrangements of  FIGS. 2-5  represent steady state conditions, and in the terms of  FIG. 8  are the Standby state  206 , the Retard state  212  and the Propulsion state  224 .  FIG. 8  includes many other states additional to these ‘stable’ states, as will presently be described.  
         [0042]     Standby Mode  
         [0043]     Returning then to  FIG. 2 , which represents the hydraulic fluid flow in the Standby mode. In this mode of operation, the pump/motor unit  60  is maintained in a charged state in the sense that it is slightly retarding (i.e. pumping), and thus drawing energy from the drive shaft  50 . The purpose of the Standby mode is to ensure there is sufficient hydraulic pressure for the pump/motor unit  60  to permit control of the swash plate  61 , and secondarily to provide lubrication and cooling.  
         [0044]     At a −2 degrees swash plate angle, the pump/motor unit  60  is acting to slightly retard the drive shaft  50 . The hydraulic fluid is drawn from reservoir  64  from a centrifugal pump assembly  66  passing a check valve  70  and then to the pump/motor unit  60 . The direction of flow is indicated by the closely-spaced arrowheads. The hydraulic flow rate is approximately 50 litres per minute. The fluid flow is enabled by a load/standby solenoid  72  acting on a relief valve  74 , such that when the load solenoid  72  is deactivated there is a 0 bar pressure drop across the relief valve  74 . A 20 bar valve  76  induces a pressure drop of that amount, after which the fluid passes a cooling circuit  78  and a filter circuits  80  before returning to the reservoir  64 .  
         [0045]     The centrifugal pump unit  66  receives approximately 5 litres per minute of fluid and is self-latching by a hydraulic latching circuit  82  under control of a suction (air) charge solenoid  84 . The function of the centrifugal pump unit  66  is to maintain suction pressure above a minimum specified value, e.g. 0.8 bar absolute for the example pump/motor unit.  
         [0046]     The swash plate angle as set by an actuator  86 . The load/standby solenoid  72 , the suction (air) charge solenoid  84  and the swash angle control actuator  86  are all electrically connected to the ECU  90  which implements the low-level control strategy, in the sense of sequencing the various solenoids and actuators.  
         [0047]     In summary then, the solenoid states for the Standby mode are:  
                                                   Solenoid   State                           Standby solenoid (72)   De-energised (OFF)           Propulsion solenoid (100)   De-energised (OFF)           Dump solenoid (110)   Energised (ON)           Air charge solenoid (84)   Energised (ON)                      
 
         [0048]     Retard Mode  
         [0049]      FIG. 3  shows the Retard mode of operation. In this mode, the pump/motor unit  60  is pumping fluid from the reservoir  64  to the accumulator  62 , drawing kinetic energy off the drive shaft  50 . The swash plate  61  is set by the actuator  86  to the chosen angular setting (i.e. between −2 and −15 degrees). The fluid flows from the reservoir  64  via the centrifugal pump unit  66  and the check valve  70  through the pump/motor unit  60 . The standby solenoid  72 , on this occasion, is energized such that the relief valve  74  presents a 350 bar pressure drop. The standby solenoid  72  remains in a state such that the relief valve  74  allows fluid flow with a 350 bar pressure drop, and that fluid flows via the standby valve  76 , through the cooler  78  and the filter  80 , returning to the reservoir  64 .  
         [0050]     The main body of fluid proceeds from the pump/motor unit  60  through the check valve  92  and then accumulates in the accumulator  62 . This is because the check valve  92  presents only a 5 bar pressure drop, whereas the relief valve  74  presents a 350 bar pressure drop. For the swash plate  61  at a setting of −15 degrees, fluid flow of up to 400 litres per minute will be generated. Accumulator pressure is measured by a pressure sensor  94  and is used to control the swash angle actuator  86  to complete retarding operation when the accumulator  62  is full. If a situation is reached where the accumulator  62  is full and the retarding operation continues, then the fluid will prefer to flow via the relief valve  74 , and the 350 bar pressure drop will result in heat being generated.  
         [0051]     In summary then, the solenoid states for the Retard mode are:  
                                                   Solenoid   State                           Standby solenoid (72)   Energised (ON)           Propulsion solenoid (100)   De-energised (OFF)           Dump solenoid (110)   Energised (ON)           Air charge solenoid (84)   Energised (ON)                      
 
         [0052]     Propulsion Mode  
         [0053]      FIG. 4  shows the Propulsion mode of operation, in which accumulated fluid under pressure is used to drive the pump/motor unit  60  to impart kinetic energy to the drive shaft  50 .  
         [0054]     The propulsion solenoid  100  is energised to allow the fluid in the accumulator  62  to pass the check valve  92 . The swash angle control actuator  86  sets a swash plate  61  position in the range 0 to +15 degrees to control the rate of fluid being sourced from the accumulator  62  and thus control the torque applied to the drive shaft  50  by the pump/motor unit  60 . The fluid typically will have a flow rate of 400 litres per minute and is blocked by the check valve  70 , rather flowing through a further check valve  102  then the standby valve  76 , resulting in a 20 bar pressure drop and, again, passing via the cooling circuit  78  and the filter circuit  80  to the reservoir  64 . The standby solenoid  72  is in an energized state so that the relief valve  74  presents a 350 bar pressure drop, and is therefore blocking to the flow of fluid from the accumulator  62 .  
         [0055]     A cooling solenoid  104  causes a valve  106  to open such that pilot fluid flow also proceeds to the coiling fans  108  associated with the cooling circuit  80 . The pilot flow typically is of the order of 12 litres per minute.  
         [0056]     In summary then, the solenoid states for the Propulsion mode are:  
                                                   Solenoid   State                           Standby solenoid (72)   Energised (ON)           Propulsion solenoid (100)   Energised (ON)           Dump solenoid (110)   Energised (ON)           Air charge solenoid (84)   Energised (ON)                      
 
         [0057]     Dump Mode  
         [0058]      FIG. 5  shows an arrangement where accumulated fluid in the accumulator  62  is required to be discharged. This may occur in situations where maintenance is required to be done and it would be dangerous to have a pressure of fluid present in the accumulator  62 . The path the fluid follows is achieved by a dump solenoid  110  being activated such that the associated valve  112  opens to allow the path of fluid which otherwise is blocked by the check valve  92 . The fluid passes through a controlling orifice  111  then via the cooling element  76  and the filter unit  78 , returning to the reservoir  64 .  
         [0059]     Determination of Torque  
         [0060]     As indicated in relation to  FIG. 1 , the high-level strategy control unit  28  determines a commanded torque value  26 . This torque value must be converted into a time-variant signal representing swash plate angle. For variable displacement axial piston pumps, torque is proportional to the working fluid pressure, swash plate displacement and hydro-mechanical efficiency. Displacement, in turn, is proportional to swash plate angle. The conversion between torque and angle (and vice versa) is achieved of a process of interpolation.  
         [0061]     A data set is obtained by experimentation to determine, for a given class of pump/motor unit (and gearbox if applicable), the pressure and rotational speed values for given swash plate angles. A characteristic pump/motor unit will operate in conditions of between 0-350 bar at speeds between 0-2,200 rpm for swash plate angles of −15 degrees to +15 degrees. Conveniently, 35 bar increments, 200 rpm increments and 3 degree increments are adopted. The data set thus can be thought of as 11 ‘angle tables’ (ie. − 15 , − 12 , . . . ,  0 , + 3 , . . . ,+ 15  degrees), each having 11 pressure×11 speed values.  
         [0062]     The data sets thus require a measurement of pump/motor unit speed, which is provided to the ECU  90  by a pump/motor unit speed sensor  120 . The pump/motor unit pressure is determined from a pressure sensor  122 .  
         [0063]     Consider now the conversion of torque (z) to angle (θ). For a commanded torque value (z comm ), the actual/nominal pressure and speed values are ascertained, and for each angle table the adjacent pressures (y 1 , y 2 ) and speeds (x, x 2 ) are identified. As shown in  FIG. 6 , for a given angle table there will be a set of combinations (x 1 , y 1 ), (x 1 , y 2 ), (x 2 , y 1 ), (x 2 , y 2 ) nearest the nominal value (x nom , y nom ) giving respective torque values of z 11 , z 12 , z 21 , z 22 . The process is to solve, for each angle table, for a torque value z result , being a linear interpolation between z 11 , z 12 , z 21 , z 22 . There will now be a set of torque values for each angle (θ): z result,θ . Two such torque values will be nearest the commanded torque, z comm  in a ±sense (i.e. a ‘just above’ value and a ‘just below’ value), designated as z result, above  and z result,below .  
         [0064]     A process of linear interpolation is performed, as shown in  FIG. 7 , between z result,above  (for angle m) and z result,below  (for angle n) and z comm , to derive a value of angle θ comm  lying between θ m  and θ n . This is the swash plate angle provided to the swash angle control actuator  86 . Clearly swash plate angle (θ) is a dynamic variable, responding to changes in commanded torque.  
         [0065]     A swash angle feedback sensor  150  provides a feedback signal to the low-level control strategy unit  14  (embodied in the ECU  90 ). The conversion from measured angle to delivered torque follows the reverse process. The thusly calculated delivered torque is supplied to the high-level control unit  28  as the value ‘actual torque’  32 .  
         [0066]     Clutch Operation  
         [0067]     The clutch actuator  56  is shown only in general terms. An appropriate configuration is a pneumatic over hydraulic self-latching type. A pneumatic supply  130  is provided, under the control of a pneumatic clutch supply solenoid  132 . A pilot hydraulic line  134  is also shown, providing sufficient pressure in Standby mode to operate the clutch piston  54 . The clutching movement is controlled by a modulation solenoid  136 .  
         [0068]     It is usual that clutch slip protection will be provided in the event that the pump/motor unit  60  seizes. This is achieved by the mechanical rating of the clutch plates  52  and the operating pressure applied by the modulation solenoid  136 .  
         [0069]     A drive line speed and direction sensor  140  is also provided. The signal  29  derived from the sensor  140  is used in operation and protection schemes implemented by both the low-level and the high-level strategy control units  14 ,  28 , as will be described.  
         [0070]     Low-level Strategy Control  
         [0071]     Referring now to  FIG. 8 , it can be seen that there are a number of discrete states with an overall strategy  200  governing operation of the RDU  10 . Each state represents a set of conditions that must be satisfied in order to pass safely to another state. The states can be thought of as rules designed to ensure safe and correct operation of the hydraulic circuits in particular. The Standby state  206 , the Retard state  212  and the Propulsion state  224  have already been generally described with reference to  FIGS. 2-4 . The Dump mode is not shown in the state diagram. The remaining states (except the Disengaged state  236  and the Reverse state  238 ) can be thought of as transitions.  
         [0072]     Start-up State  
         [0073]     Referring then to  FIG. 9 , the start-up state occurs when the ECU  90  is first powered up (step  300 ), waiting for a period of 500 ms (steps  302 ,  304 ). The process determines whether a test mode (i.e. Full Manual) should be entered (steps  306 ,  308 ), and if not then the process turns all of the solenoids  72 ,  84 ,  100 ,  110  off, and drives the swash plate  61  to the standby angular position (step  310 ). The process then waits for an indefinite period for the commanded angle to be set to the standby position and for the accumulator to be empty (i.e. less than 20 bar) (steps  312 ,  314 ). Once this has occurred, the process waits for the state timer to decrement to 0 (step  316 ) before proceeding to the Pending Standby state  204  (step  318 ). Any error condition will cause entry of the Standby Error state  208  (steps  320 ,  322 ).  
         [0074]     Pending Standby State  
         [0075]     Referring then to  FIG. 10 , on entering the Pending Standby state  204  (step  330 ), the solenoids  72 ,  84 ,  100 ,  110  are set to standby conditions as mentioned above (step  332 ) causing the pump/motor unit  60  to unload, if necessary. The swash plate  61  is commanded to the standby position (step  332  also). A minimum state timer is set to 100 ms (steps  334 ,  336 ). Up to 6 seconds is allowed for the swash plate  61  to move into the standby angular window and for the pump to unload to less than 45 bar (i.e. the standby pressure) (steps  338 ,  346 ). Once this has occurred and the state timer has expired (step  342 ), the flow progresses to the Standby state (step  344 ). If swash plate  61  has not moved into the standby window within 6 seconds, or the pump/motor unit pressure remains high, the process flags a condition of ‘pump pressure over standby threshold’ (step  340 ). This leads to the Standby Error state  208  (steps  350 ,  352 ).  
         [0076]     Standby State  
         [0077]     Referring then to  FIG. 11 , on entering the Standby state  206  (step  360 ), the solenoids  72 ,  84 ,  100 ,  110  are set to the standby conditions (step  362 ). The process then checks the positional stability of the swash plate  61  by measuring the time that it may be outside the swash angle window (steps  364 ,  366 ,  368 ). If not stable, then an error occurs (steps  388 ,  390 ). Once the swash plate is stabilised, the process checks the direction of rotation of the drive shaft  50  by the sensor  140 . If a “reverse” condition is detected, and the accumulator pressure is at 50% capacity, the process proceeds to the Reverse state  238  (steps  374 ,  376 ,  378 ). Otherwise, if the shaft direction is forward, the process then checks if the shafts&#39;s speed is greater than a minimum high speed threshold (step  380 ), and if the pump/motor unit pressure is less than a maximum standby pump pressure (i.e. 45 bar) (step  384 ). If yes, then the process proceeds to check the commanded swash plate position (step  392 ). If the position is less than a Standby low-level (step  392 ), then the process leads to the Pre-Retard state  210 . If the commanded position of the swash plate  61  is in a Start Propulsion condition, and the accumulator as at 50% capacity (step  394 ), then the process leads to the Pre-Propulsion Stage 1 state  218  (step  398 ). Any errors lead to the Standby Error state  208  (steps  400 ,  402 ). There is additionally a transition to the Disengaged state  236  that is not specifically shown in  FIG. 11 . If no commanded torque value above or below the standby value arises within a predetermined period of time, then the pump/motor unit  60  should be disengaged from the drive train  12  by the operation of the clutch  22 .  
         [0078]     Standby Error State  
         [0079]     Referring now to  FIG. 12 , on entering the Standby Error state  208  (step  410 ), the solenoids  72 ,  84 ,  100 ,  110  are set to standby conditions (step  412 ). The swash plate solenoid  86  is commanded to the standby position (step  412  also). A minimum state timer is set to three seconds, meaning that up to three seconds are allowed for the swash plate  61  to move into the standby window (steps  414 ,  416 ). Once this has occurred, or if three seconds expires (step  418 ), there is an unconditioned transition to the Error state  234  (step  420 ).  
         [0080]     Pre-Retard State  
         [0081]     Referring now to  FIG. 13 , when the Pre-retard state  210  is entered (step  430 ), the standby solenoid  72  is switched on, as are the dump and air charge solenoids  84 ,  100 ,  110 . This allows the pump/motor unit  60  to load, if not already loaded. The swash plate solenoid  86  is commanded to the standby position (step  432  also). A minimum state timer is set to 200 ms (steps  434 ,  436 ). Up to six seconds are allowed for the pump/motor unit  60  to load (steps  438 ,  448 ), or a pre-retard pressure error flag is raised (step  440 ), and the process proceeds to the Retard Error state  214  (step  458 ). If the pump loads within six seconds (step  442 ), the process proceeds to the Retard state  212  (step  446 ). If the drive shaft is rotating in the reverse direction (step  452 ), the process raises an error flag (step  454 ) and proceeds to the Retard Error state  214  (step  458 ).  
         [0082]     Retard State  
         [0083]     Referring now to  FIG. 14 , when the Retard state  212  is entered (step  460 ), the solenoids  72 ,  84 ,  100 ,  110  are set to loaded conditions (step  462 ). The swash solenoid  86  moves the swash plate  61  to the calculated commanded retard angle (step  462  also). A continuous check is made of the drive shaft speed to determine that it is above a minimum threshold speed (step  464 ), and also to determine that the command has not returned to “standby” (step  468 ). If either conditions are true, then the process proceeds to the Terminate Retard state  216  (steps  466 ,  470 ). The pump/motor unit pressure is also continually checked (step  472 ), and any low pressure will cause a minimum low pressure error flag to be raised (step  474 ), and the process proceeds to the Retard Error state  214  (steps  480 ,  482 ). If the drive shaft is rotating in the reverse direction, then a reverse error is flagged, and the process to proceed to the Retard Error state  214  (step  482 ).  
         [0084]     Retard Error State  
         [0085]     Referring now to  FIG. 15 , on entering the Retard Error state  214  (step  490 ), the solenoids  72 ,  84 ,  100 ,  110  are set to the loaded conditions, meaning that the standby solenoid  72  is activated. The swash plate  61  is limited to standby angle conditions (step  490  also). A minimum state timer is set to three seconds, giving up to three seconds for the swash plate to move in to the standby window (steps  494 ,  496 ,  498 ). Once this has occurred, or the three second period expires, the process unconditionally proceeds to the Error state  214  (step  500 ). If the shaft is rotating in the reverse direction, then the process raises a reverse error flag (step  504 ).  
         [0086]     Terminate Retard State  
         [0087]     Referring now to  FIG. 16 , on entering the Terminate Retard state  216  (step  510 ), the solenoids  72 ,  84 ,  100 ,  110  are set such that, in particular, the propulsion solenoid  100  is deactivated. With the pump/motor unit  60  still loaded, the swash plate  61  is commanded to the min/max swash terminate mode angle (step  512 ). A state timer is set to 300 ms (steps  514 ,  516 ), and therefore waits until the swash plate  61  moves into the window (step  518 ) before deciding if the high-level command has moved directly to propulsion. If not, or if the drive shaft speed has dropped below a minimum speed threshold (step  520 ), then the process proceeds to the Pending Standby state  204  (steps  522 ,  530 ). If, on the other hand, the drive shaft speed is above the minimum threshold, the command has changed to propulsion, the shaft is rotating in the forward direction and there is more than 50% accumulator capacity (step  524 ), then the process moves to the Pre-propulsion Stage 1 state  218  (step  526 ). Any errors if detected result in progress to the Retard Error state  214  (steps  532 ,  534 ,  536 ,  538 ).  
         [0088]     Pre-Propulsion Stage 1 state  
         [0089]     Referring now to  FIG. 17 , on entering the Pre-Propulsion Stage 1 state  218  (step  540 ), the solenoids  72 ,  84 ,  100 ,  110  are set to loaded conditions, causing the pump/motor unit  60  to load, if not already so (step  542 ). The swash plate is commanded to an angle relative to the shaft speed (step  552 ) as previously described with reference to  FIGS. 6 and 7 ). The process ensures that the minimum load pressure is achieved even at very low speeds (steps  544 ,  546 ,  548 ,  550 ). If it is the first time in the state, the state timer is set to 100 ms (steps  554 ,  556 ). Up to six seconds is allowed for the recovery according to the following checks: speed higher than threshold speed (step  562 ); and the pump is loaded (step  570 ). If these checks are satisfied, then the process proceeds to Pre-propulsion Stage 2 state  220  (step  568 ). If not, then the process proceeds to the Propulsion Error 2 state  222  (steps  578 ,  580 ). If the drive shaft is rotating in the reverse direction, then the process flags an error (steps  574 ,  576 ). If the shaft speed is less than the threshold value, the process proceeds to the Pending Standby state  204  (steps  562 ,  564 ).  
         [0090]     Pre-Propulsion Stage 2 State  
         [0091]     Referring then to  FIG. 18 , on entry into the Pre-Propulsion Stage 2 state  220  (step  590 ), all of the solenoids  72 ,  84 ,  100 ,  110  are switched on. The swash plate  61  is commanded to an angle relative to the drive shaft speed (step  602 ). The process ensures that the minimum load pressure is achieved even at very low drive shaft speeds (steps  594 ,  596 ,  598 ,  600 ). If it is the first time in this state, then a state timer is set to 200 ms (steps  604 ,  606 ). Up to one second (step  608 ) is allowed for the recovery according to the following checks: commanded angle remains above standby (step  612 ), drive shaft speed is higher than a threshold speed (step  616 ), accumulator capacity is higher than 10% (step  612 ), the pump/motor unit is still loaded (step  620 ), and the shaft is rotating in the forward direction (step  628 ). If all of these checks are satisfied, then the process proceeds to the Propulsion state  224  (step  626 ). Otherwise, the process proceeds Propulsion Error 1 state  226  (steps  614 ,  632 ,  634 ).  
         [0092]     Propulsion Error 2 State  
         [0093]     Referring then to  FIG. 19 , on entering the Propulsion Error 2 state  222  (step  640 ), the solenoids  72 ,  84 ,  100 ,  110  are set to loaded conditions and the swash plate  61  is commanded to the standby position (step  642 ). A minimum state timer is set to three seconds, thus allowing up to three seconds for the measured swash plate angle to move into the standby window (steps  644 ,  646 ,  648 ,  650 ). Once this has occurred, or the time has expired, the process proceeds unconditionally to the Error state  234  (step  652 ). If the drive shaft is rotating in the reverse direction, then a reverse error is flagged (steps  654 ,  656 ).  
         [0094]     Propulsion State  
         [0095]     Referring then to  FIG. 20 , once the Propulsion state  224  is entered (step  660 ), all solenoids  72 ,  84 ,  100 ,  110  are switched on, and the swash plate  61  is allowed to move to the commanded propulsion angle (step  662 ). A series of propulsion checks are made: is the drive shaft speed above the minimum threshold speed (step  672 ), has the command not returned to the “standby” or is the accumulator capacity greater than 10% (step  668 ), is the pump/motor unit still loaded (step  664 ), and is the shaft still rotating in the forward direction (step  678 ). If any of these conditions are not satisfied, then the process will proceed to the Terminate Propulsion state 1 stage  228  (steps  670 ,  674 ). Any other errors cause the process to proceed to the Propulsion Error 1 stage  226  (steps  666 ,  678 ,  680 ,  682 ).  
         [0096]     Propulsion Error 1 State  
         [0097]     Referring then to  FIG. 21 , when the Propulsion Error 1 state  226  is entered (step  690 ), all the solenoids  72 ,  84 ,  100 ,  110  are switched on, and the swash plate  61  is commanded to the standby position (step  692 ). A minimum state timer is set to three seconds, allowing up to three seconds for the swash plate  61  to move into the standby window (steps  694 ,  696 ,  698 ). Once this has occurred, or the three second period expires, the process proceeds to Propulsion Error 2 state  222  (step  700 ), unconditionally. If the shaft is rotating in the reverse direction, then an error flag is raised (step  704 ).  
         [0098]     Terminate Propulsion Stage 1 State  
         [0099]     Referring then to  FIG. 22 , when the Terminate Propulsion Stage 1 state  228  is entered (step  710 ), all the solenoids  72 ,  84 ,  100 ,  110  will be activated, and the swash plate  61  is limited to the terminate mode angle value (step  714 ). A state timer is set to six seconds, allowing that period of time for the swash plate  61  to move into the standby window (steps  714 ,  716 ,  718 ,  724 ). If the swash plate  61  fails to move into the standby window within the six second period, then the process proceeds to the Propulsion Error 1 state  226  (step  720 ). If the condition is satisfied, however, the process proceeds to the Terminate Propulsion Stage 2 state  230  (step  726 ). Any other error, including shaft rotating in reverse, results in the process proceeding to the Propulsion Error 1 state  226  (steps  720 ,  728 ,  730 ,  732 ,  734 ).  
         [0100]     Terminate Propulsion Stage 2 State  
         [0101]     With reference to  FIG. 23 , when the Terminate Propulsion Stage 2 state  230  is entered (step  736 ), the propulsion solenoid  100  is deactivated, however the standby solenoid  72  remains activated, and the pump/motor unit  60  is still loaded (step  738 ). The swash plate  61  is commanded to the terminate mode angle (step  738  also) and a state timer is set to 100 ms (steps  740 ,  742 ). When the timer has decremented to 0 without any errors occurring (step  744 ), the process will proceed to the Terminate Propulsion Stage 3 state  232  (step  746 ). Any errors detected, including the drive shaft rotating in the reverse direction, will cause the process to proceed to the Propulsion Error 2 state  222  (steps  748 ,  750 ,  752 ,  754 ).  
         [0102]     Terminate Propulsion Stage 3 State  
         [0103]     Referring then to  FIG. 24 , on entry into the Terminate Propulsion Stage 3 state  232  (step  760 ), the solenoids  72 ,  84 ,  100 ,  110  are set to loaded conditions, the pump/motor unit  60  will already be loaded, and the swash plate  61  is commanded to the terminate mode angle (step  762 ). A minimum state timer is set to six seconds. A check is made of whether the drive shaft speed is above a minimum threshold (step  776 ), then if the commanded mode has proceeded to Retard (step  780 ). If so, the process checks the drive shaft speed and that the minimum transition time has passed (steps  776 ,  786 ,  788 ). If these conditions are met, then the process proceeds to the Pre-retard state  210  (step  782 ). If the commanded mode has not changed, the process checks that the commanded state has returned to standby (step  780 ), and that the swash plate has moved to the standby position (step  784 ), by the elapse of time, and then proceeds to the Pending Standby state  204  (step  774 ). If none of the conditions are meeting within the period of six seconds or if any general or reverse drive shaft errors occur, then the process proceeds to the Propulsion Error 2 state  222  (steps  770 ,  790 ,  792 ,  794 ,  796 ).  
         [0104]     Error State  
         [0105]     Referring then to  FIG. 25 , when the Error state  234  is entered (step  800 ) the solenoids  72 ,  84 ,  100 ,  110  are set to the Standby conditions and the swash plate  61  to the standby angle (step  802 ). The process then decrements a recovery time measure (step  804 ). The process is concerned with providing recovery times for errors and looking for swash plate and standby pressure stability (step  806 ), to move to the Pending Standby state  204  (step  808 ).  
         [0106]     Disengaged State  
         [0107]     The Disengaged state  236  is the default position for the clutch  52 . The objective is to disengage the RDS  10  whenever possible, to avoid wear and slow drawing off of stored energy during Standby mode. It is required that both electric power and hydraulic pressure be present in order to move to the Pending Standby state  204 .  
         [0108]     Reverse State  
         [0109]     Referring then to  FIG. 26 , when the Reverse state  238  is entered (step  900 ), the solenoids  72 ,  84 ,  100 ,  110  are set to standby conditions and limits are placed on the swash plate actuated (step  902 ). If the drive shaft is detected rotating in the reverse direction, or if the accumulator has a capacity of less than 5% (step  904 ), then the process proceeds to the Pending Standby state  204  (step  906 ). Otherwise, the process proceeds to the Standby Error state  208  (steps  908 ,  910 ).  
         [0110]     High-level Strategy Control  
         [0111]     The high-level control strategy and low-level control strategy, in a preferred embodiment, are implemented as separate computer programs that pass variables between each other, but otherwise act autonomously. The low-level control strategy has responsibility of ensuring safe operation of the RDS  10 , in the form of absolute rules. The high-level control strategy also operates on rules concerned with safe vehicle operation. There thus is a two-tier approach to safe operation.  
         [0112]     The high-level strategy control unit  28  received inputs from the vehicle throttle system  34 , an engine speed sensor  38 , and the drive shaft signal  29 , the available torque signal  30  and the actual torque signal  32  as shown in  FIG. 1 . The principal control variable is throttle position.  
         [0113]     Throttle  
         [0114]     In some classes of vehicle, the throttle operation will provide that the first, say, 0-18% of throttle position is a form of engine braking (such as exhaust braking). The range of 20%-60% may represent constant speed of the vehicle, and it is only throttle positions in excess of 60% that represent vehicle acceleration. Of course, in other vehicles, any throttle position &gt;0% may represent propulsion.  
         [0115]     Propulsion Mode  
         [0116]     As described previously, in the Propulsion mode the RDS  10  will be used as a source of energy for the vehicle  11 .  
         [0117]     The high-level strategy control unit  28  performs a conversion from a (input) ‘nominal throttle’ signal  35  to a (output) ‘commanded torque’ signal  26  and a (output) ‘reduced/modified throttle’ signal  37 . This is expressed as ‘torque split’, being the relative contributions of the vehicle&#39;s engine  11  and the RDS  10 .  
         [0118]     There are two basic approaches/modes to torque splitting: ‘fuel saving’ mode and ‘boost’ mode.  
         [0119]     Propulsion—Fuel Saving Mode  
         [0120]     The approach of the fuel saving mode is to replace some portion of engine torque by the RDS  10  operating in Propulsion mode, and between the engine  11  and the RDS  10 , providing the appropriate torque for the throttle setting selected by the driver.  
         [0121]     One benefit of this mode (as the name suggests) is to save on the consumption of fuel by recovering and utilising the vehicle&#39;s kinetic energy. Any reduction in fuel usage has a concomitant reduction in greenhouse gas emissions.  
         [0122]     Referring now to  FIG. 27 , a torque splitting arrangement is shown. It is assumed a constant propulsion torque is to be imparted to the drive train  12 . The value T drive  is provided entirely by the engine in the period 0-t 1 . At time t 1 , the RDS  10  moves from standby into propulsion mode, and steps to a constant commanded torque value T comm . At the same time, the torque contributed by the engine  11  steps down such that the value T drive  remains constant. In the period t 2 -t 3 , the engine  11  and the RDS  10  provide a respective constant torque contribution. In the period t 3 -t 4 , a similar stepping occurs such that from t 4  onwards, the only contribution is from the engine  11 . Typically this represents the situation where the available torque signal  30  has reduced to or below 10% of full capacity. For all time periods, the relation: T drive =T engine +T comm =constant, holds true, in this example. However, engine torque is never constant as a function of speed. Thus, T engine  will rarely be able to be held constant in a propulsion event, meaning that the RDS  10  torque component will not be piecewise linear, but at all times attempting to maintain the driving torque to be constant.  
         [0123]     Propulsion-Boost Mode  
         [0124]     The basic idea behind the boost mode is to supplement engine torque to give an additional short term power capacity on propulsion, and to over-work an engine during retardation to—as quickly as possible—charge the accumulator  62  to be ready for the next propulsion event. In other words, the consumption of fuel is not a concern.  
         [0125]      FIG. 28  shows graphically the relation: T drive =T engine +T comm , where, in the period t 1 -t 4 , the engine torque is supplemented by the RDS torque.  
         [0126]     Throttle Modification  
         [0127]     Referring now to the flow diagram of  FIG. 29 , the vehicle&#39;s throttle position is continuously monitored (step  1000 ). A calculation is performed to give a driving torque value (step  1002 ). This process requires the engine speed signal  38 .  FIG. 30  shows a representative diagram of engine torque versus engine speed for a 100% throttle setting. The characteristic typically needs to be measured. It is assumed that there is a linear relationship between throttle position and torque for any given speed. Thus, for say, a 50% throttle position, then for the relevant engine speed, the nominal engine torque T 50%  can be determined.  
         [0128]     The nominal engine torque is known, and needs to be referenced to a torque value at the drive shaft  50 , where the RDS  10  acts (step  1004 ). Referring now to  FIG. 31 , a block diagram of the mechanical components of the drive train are shown. It is therefore necessary to mathematically model the torque as it passes a torque converter  1020 , a gearbox  1022  and a transfer case  1024 . Torque variously will be a function of speed, gear losses, torque split between front wheels  1026  and rear wheels  13 , etc. The mathematical model can be developed based on measurements that provide data sets forming the basis of look-up tables.  
         [0129]     Now that the torque at the drive shaft (equivalent to the throttle position) is known, the torque split is determined (step  1006 ). To do this, the operational mode is firstly read (step  1008 ). Assume for the purposes of the discussion that Fuel Saving mode is selected, meaning that a constant torque approach is adopted (see  FIG. 28 ). The instantaneous available RDS torque  30  is read (step  1010 ), and a target torque (i.e. commanded) value is selected to be less than the available torque. The target torque value might typically be 65% of the available torque. The available torque will decrease over time for any propulsion event, and it may be necessary to reduce the target torque to track the reducing available torque.  
         [0130]     The (time variant) commanded/target torque value  26  thus is provided to the low-level strategy control unit  14  (step  1012 ). The corresponding engine torque component must be converted back to a modified throttle position (step  1014 ), following a reverse process according to the drive train model as discussed above. The thus-derived modified throttle position  37  is returned to the throttle system  34  (step  1016 ).  
         [0131]     There will be situations where there is a form of throttle-related engine braking, meaning that only a partial range of throttle position represents propulsion. In such a case an appropriate offset will need to be provided to ensure the RDS mode of operation matches that intended by the driver operating the throttle.  
         [0132]     The high-level strategy control unit  28  implements a PID control algorithm that uses the actual torque signal  32  as a feedback variable to be compared with the commanded torque value  26 .  
         [0133]     Retard-Fuel Saving Mode of Operation  
         [0134]     During retard, the objective is to absorb drive shaft torque at a constant level to give a constant deceleration, and to charge the accumulator  62  to a full condition within a time period characteristic of a deceleration episode.  
         [0135]      FIG. 32  shows the RDS torque characteristic in a retard mode of operation. The high-level strategy control unit  28  seeks to draw-off energy from the drive train at a constant torque. The commanded target torque value, T target , is maintained to be within a range bounded by maximum and minimum values, T max , T min . The real-time value of T target  as a function of throttle position is determined empirically, in consideration of giving the driver a natural ‘feel’ of deceleration. T max  will be set to achieve a maximum braking effect, typically 0.15 g.  
         [0136]     Retard-Boost Mode  
         [0137]      FIG. 34  graphically shows the same relationship T drive =T engine +T comm =constant. In other words, the driving (retarding) torque remains constant, and the recharging of the accumulator  62  occurs by the engine working harder during the period t 1 -t 4 .  
         [0138]     Throttle Modification  
         [0139]     During Retard mode operation, there is no need to be concerned with engine throttle setting, save where engine braking is provided, in which case the target retard torque may be adjusted to ensure a constant deceleration to account for the engine braking contribution. Once, again, a PID feedback control algorithm will be used to control the target retard torque against the actual torque  32 .  
         [0140]     Transmission Considerations  
         [0141]     The foregoing description contemplates a vehicle having an automatic transmission. Of course, many vehicles will have manual transmissions, which means the throttle and vehicle clutch pedals are constantly operated. It is thus necessary to discriminate a clutching event over a braking/deceleration event. This can be done by detecting near-simultaneous operation of the clutch pedal and reduction in throttle (typically to a zero setting), so that the RDS  10  might continue in its current mode regardless of a clutching event taking place.