Abstract:
This invention is a control system for a clutch for connecting an engine to the powertrain of an HEV. The system includes a controller programmed to determine a filtered speed error of the engine and a starter/motor and to determine an engine run command. Monitoring devices operatively connected to the engine and the starter/motor are connected to output data representing the engine and starter/motor speeds to the controller. The controller is programmed to generate a clutch position command, dependent on the data, to a servo-actuator connected to the clutch. The invention, further, provides methods for controlling such a clutch including the steps of determining an engine run command, determining a filtered speed error of the engine and a starter/motor and generating a clutch position command.

Description:
FIELD OF INVENTION 
   The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a strategy to control engaging and disengaging a clutch used to connect an engine to the powertrain of an HEV. 
   BACKGROUND OF INVENTION 
   The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky. 
   The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set. 
   Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE. 
   A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a parallel/series “split” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed. 
   The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or driveability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV&#39;s potential benefits. 
   One such area of HEV development is controlling the engagement and disengagement of the engine from the HEV powertrain. Frequently, this is done using a two-way clutch in parallel HEV&#39;s. A two-way clutch allows the engine to drive the motor, and allows the engine and motor to drive the vehicle. Clutch control strategies for HEVs are known in the art. See generally, U.S. Pat. No. 5,979,257 to Lawrie and U.S. Pat. No. 5,943,918 to Reed, Jr. et al. Nevertheless, none are designed to control engaging and disengaging a two-way clutch to connect the engine from a parallel HEV. 
   SUMMARY OF INVENTION 
   Accordingly, an object of the present invention is to provide a strategy to control engaging and disengaging a clutch used to connect an engine to the powertrain of an hybrid electric vehicle (HEV). 
   Briefly, the invention provides a system for clutch control in an HEV. The system, which controls a clutch for connecting an engine to the powertrain of the HEV includes a controller programmed to determine a filtered speed error of the engine and a starter/motor and to determine an engine run command. Monitoring devices operatively connected to the engine and the starter/motor are connected to output data representing the engine and starter/motor speeds to the controller. The controller is programmed to generate a clutch position command, dependent on the data, to a servo-actuator connected to the clutch. 
   The invention, further, provides methods for controlling such a clutch including the steps of determining an engine run command, determining a filtered speed error of the engine and a starter/alternator (or starter/motor) and generating a clutch position command. The step of determining an engine run command may include the steps of determining whether the clutch is engaged, determining whether the engine is at least spinning at a predetermined idle speed, and commanding a fuel request to the engine when the clutch is engaged and the engine is spinning at least at the predetermined idle speed. The step of determining a filtered speed error may include the steps of determining a speed error, determining a scaled speed error; and inputting the scaled speed error to a digital lowpass filter. 
   Other features and advantages of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing advantages and features, as well as other advantages and features will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which: 
       FIG. 1  illustrates a general parallel hybrid electric powertrain configuration. 
       FIG. 2  illustrates a clutch control operation logic of the present invention. 
       FIG. 3  illustrates a 20 second simulation of the present invention. 
       FIG. 4  illustrates an expanded view of the 3 to 5 second period of the  FIG. 3  simulation. 
       FIG. 5  illustrates an expanded view of the 16 to 18 second period of the  FIG. 3  simulation. 
       FIGS. 6A-C  illustrates a control strategy using the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention relates to hybrid electric vehicles (HEVs) and, more particularly, a strategy to control engaging and disengaging a clutch used to connect an engine to the powertrain of an HEV. The preferred embodiment of the present invention uses a controller for engaging and disengaging a dry two-way clutch used for connecting an engine to a powertrain in a parallel hybrid electric vehicle (PHEV). 
     FIG. 1  illustrates a possible PHEV powertrain to demonstrate the present invention and is generally indicated at  18 . This powertrain  18  has an engine  20  (such as a conventional 2.0 L spark-ignited, internal combustion engine (ICE)) and a combination starter/motor  24  to supply motive torque for the vehicle. The starter/motor  24  is configured and sized to not only provide motive torque, but also to spin the engine  20  for starting purposes. For the present invention a 60 horse power (HP) starter/motor  24  can be used. The vehicle powertrain also has a disconnect clutch (“clutch”)  22  positioned between the engine  20  and starter/motor  24 . The clutch  22  can be a two-way dry disconnect clutch known in the art. The clutch  22  can be connected to the engine  20  on an engine flywheel and can connect to the starter/motor  24  on its rotor shaft  50 . A servo-actuator  26  housed together with the clutch  22  can activate the clutch  22  to a closed and open position. The servo-actuator  26  can electronically control the engagement and disengagement of the clutch  22  by applying or releasing pressure on the friction components. These mechanisms are well known in the art. 
   The clutch  22  in a closed position allows the engine  20  to connect to the powertrain  18 . This closed position can serve three HEV powertrain functions. First, it allows the engine  20  to spin the starter/motor  24  to generate power to charge and discharge a high-powered energy storage device such as a battery  28  (the battery  28  is electrically connected to the starter/motor  24 ). Second, it allows the starter/motor  24  to spin the engine  20  during engine  20  start-up. And third, it allows both the engine  20  and starter/motor  24  to drive the vehicle powertrain  18  simultaneously. In an open position, the engine  20  is disconnected from the vehicle powertrain  18 . The clutch  22  would be open if the engine  20  is not running. 
   As illustrated in  FIG. 1 , the powertrain also has: a forward clutch  30  connected to the starter/motor  24 ; an electronically controlled converterless transmission (ECLT)  32  connected to the forward clutch  30 ; a differential and half-shafts combination (“differential”)  34  connected to the ECLT  32 ; and at least one drive wheel  36  connected to the differential  34 . Any of the vehicle wheels can be connected to a mechanical braking system  42  activated by operator using a brake activation means such as a brake pedal  44  well known in the art. Also, this powertrain is for illustrative purposes only. Several other powertrain configurations are possible using the present invention. 
   Each component of the illustrated powertrain  18  can have a sensor and an associated controller. A vehicle system controller (VSC)  38  can receive sensor input and control the components accordingly in this HEV configuration by connecting to each component&#39;s controller. Alternatively, controllers can be physically combined in any combination or can stand as separate units. The VSC  38  illustrated in  FIG. 1  can communicate with the servo-actuator  26  and other components through a communication network such as a controller area network (CAN)  40 . Sensor inputs can be included for the starter/motor  24  speed, engine  20  speed, clutch  22  position, and the position of driver operated braking means and accelerator means. The sensor for the accelerator means can be an accelerator position sensor  46 . 
   The present invention is a strategy to control the servo-actuator  26  to open and close the clutch  22 . This clutch controller as illustrated is within VSC  38 . In this illustration, the controller can generate a position command (Clutch_Position_Cmd) to the servo-actuator  26  as an eight-bit integer that represents a scaled, fixed-point representation of the interval 0.0 to 1.0, divided into 256 equal steps of value {fraction (1/256)}. The servo-actuator  26  can interpret the Clutch_Position_Cmd according to Table 1 below. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Condition 
               Clutch State 
             
             
                 
                 
             
           
           
             
                 
               Clutch_Pos_Cmd &gt; 0.85 
               Disengaged 
             
             
                 
               0.15 &lt; Clutch_Pos_Cmd &lt; 0.85 
               Slipping 
             
             
                 
               Clutch_Pos_Cmd &lt; 0.15 
               Engaged 
             
             
                 
                 
             
           
        
       
     
   
   For example, the VSC  38  can command only the starter/motor  24 , to provide motive force to the powertrain  18 . This command can include turning off the engine  20  and disconnecting the clutch  22 . The clutch  22  can be completely disengaged by generating a Clutch_Position_Cmd&gt;0.85. Any position value between 0.5 and 1.0 will result in activating the servo-actuator  26  to completely disengage the clutch  22 . Similarly, if the VSC  38  commands the engine  20  to connect to the powertrain  18 , the controller of the present invention can generate a Clutch_Position_Cmd&lt;0.15. Any position value between 0 and 0.15 will result in activating the servo-actuator  26  to completely engage the clutch  22 . 
   During clutch  22  transition from an engaged to disengaged state (and from disengaged to engaged) there is a period of decreasing (and increasing) clutch  22  engagement. This clutch  22  “slipping” state is a nonlinear relationship between the value of Clutch_Position_Cmd and the degree of clutch  22  engagement. For example, more slip is commanded as the eight-bit position value approaches 0.85 (i.e., less torque transmitted through the clutch  22 ). Similarly, less slip can be commanded as the position approaches 0.15 (i.e., more torque is transmitted through the clutch  22 ) and the closer the clutch is to being fully engaged. 
   The clutch  22  controller of the present invention controls clutch  22  slip during engagement and disengagement to provide a smooth transition, transparent to the driver in terms of noise, vibration and harshness (NVH) and performance feel. This smooth transition is important since an hybrid electric vehicle (HEV) can frequently transition between the various HEV operating modes such as: engine  20  only, starter/motor  24  only, engine  20  with starter/motor  24  boost, charging, and regenerative braking. 
   The present invention is a disconnect clutch control (Disconnect_Clutch_Control) and can have a top level structure of three main strategies: (1) Determ_Engine_Run_Cmd, (2) Determ_Filtered_Speed_Error, and (3) Generate_Clutch_Position_Cmd. 
   (1) Determ_Engine_Run_Cmd 
   One of the two outputs of the Disconnect_Clutch_Control can be an engine run command (Engine_Run_Cmd), where engine fueling is commanded to start (=1) or stop (=0). The other output is a Clutch_Position_Cmd. The Engine_Run_Cmd is a modified version of a VSC  38  signal Fuel_Engine_Request and can be set high whenever the engine  20  needs to be turned on to provide motive power or charge the battery  28 . Traditionally, once the VSC  38  determines the engine  20  needs to be started, it sets Fuel_Engine_Request high (=1) to commence engine  20  fueling. Nevertheless, if the clutch  22  is not yet engaged and the engine  20  is not rotating at sufficient speed, fueling must be prohibited. Therefore, the Determ_Engine_Run_Cmd delays the engine  20  fueling until the starter/motor  24  in combination with clutch  22  engagement has brought the engine  20  up to or beyond its “idle speed,” which in this embodiment can be 750 rpm. Only then is Fuel_Engine_Cmdset high and engine  20  fueling begins (See steps  82 ,  86 ,  90  and  92 ). 
   A sample code representation of the above description and the contents of FIG.  3 , Determ_Engine_Run_Cmd, is: IF (Clutch_Pos_Actual&lt;0.85) AND (Eng_Spd_GT_ 750 =1) AND (Fuel_Engine_Request=1), THEN (Engine_Run_Cmd=1) ELSE (Engine_Run_Cmd=0) END. 
   Here: 
   Clutch_Pos_Actual&lt;0.85: Clutch is slipping. 
   Eng_Spd_GT_ 750 =1: Engine speed is greater than 750 rpm. 
   Fuel_Engine_Request=1: The VSC has decided that the ICE needs to be running. 
   Engine_Run_Cmd=1: Begin fueling the ICE. 
   Engine_Run_Cmd=0: Do not fuel the ICE. 
   (2) Determ_Filtered_Speed_Error 
   This procedure determines the Speed_Error (rpm) between the starter/motor  24  speed and the engine  20  speed as a measure of clutch  22  slip (step  72  below). A very small gain multiplies the speed error to scale it to a range of approximately ±1 for use in the remainder of the strategy. This Scaled_Speed_Error (see step  70  below) can be the input to a Digital_Lowpass_Filter. This filter, which is a standard digital filter known in the art, can be determined by the following difference equation (see step  72 ): 
   Filtered_Speed_Error (k)=TIME_CONSTANT* Scaled_Speed_Error(k)+(1−TIME_CONSTANT)*Filtered_Speed_Error (k−1) 
   The value “k” refers to the current determination time step and “k−1” the determination from the previous time step. TIME_CONSTANT is a number between 0.0 and 1.0. The closer it is to 0.0, the more heavily filtered, or smoothed, the output Filtered_Speed_Error (k) will be; conversely, the closer it is to 1.0, the less filtered it will be. Also, the heavier the filtering, the slower the clutch  22  will be allowed to be engaged; consequently, the choice of TIME_CONSTANT is the key to proper tuning of the strategy. In one embodiment the constant can be TIME_CONSTANT=0.03. Here, very heavy filtering is performed to feather the clutch  22  engagement, ensuring a seamless, imperceptible transition from one HEV driving mode to the next. 
   (3) Generate_Clutch_Positon_Cmd 
   The primary output of Disconnect_Clutch_Control of the present invention is the Clutch_Pos_Cmd, (see steps  78 ,  92 , and  99  below). This command can be sent over the CAN  40  to the clutch servo-actuator  26  to position the clutch  22  plates according to the command. The servo-actuator  26  has a sensor to determine the actual clutch  22  position, Clutch_Position_Actual, and sends it back to the VSC  38  to the Disconnect_Clutch_Control strategy where it is used to determine Determ_Engine_Run_Cmd as previously described. The Generate_Clutch_Position_Cmd contains Switching_Logic_Subsystem to determine Eng_Spd_GT_ 750  (Engine Speed&gt;750 rpm) and sends it to Determ_Engine_Run_Cmd, and Engine_Off_and_Brk. Braking_Logic, determined in another VSC  38  procedure (see step  62  below), is high (=1) when the braking device such as a brake pedal  44  is applied or if the accelerator pedal position sensor  46  detects the accelerator is NOT applied, for instance, during braking or coasting. Braking_Logic is low (=0) when the accelerator pedal is applied. Switching_Logic_Subsystem logically ANDs Braking_Logic with Eng_Spd_GT_ 750  to produce Engine_Off_and_Brk. For example, with the mechanical brake applied (or, neither brake and accelerator pedal are not applied) and the engine  20  speed is greater than 750 rpm, this signal is high (=1), setting Clutch_Position_Cmd=1.0 to engage the clutch  22  fully. If the accelerator is applied, e.g., the operator&#39;s foot is on the accelerator pedal, Engine_Off_and_Brk=0 and the switch will pass through the lower signal whose determination is described next. 
   There can be several ways to determine engagement and disengagement of the clutch  22 . Simply, if Crank_Engine_Cmd=1 or if Fuel_Engine_Request=1 (in other words, if the VSC  38  has decided to crank the engine  20  or, it is already cranked and is ready to be fueled) then Filtered_Speed_Error is passed through the switch and subtracted from 1 (the output of Crank_Engine_Cmd OR Fuel_Engine_Request). This operation is why it is necessary to scale Speed_Error to Scaled_Speed_Error in Determ_Filtered_Speed_Error. The scaling factor is chosen so that when the clutch  22  is asked to engage, Filtered_Speed_Error is at some value near 0.5. 
     FIG. 2  can illustrate one embodiment the present invention Generate_Clutch_Positon_Cmd logic.  FIG. 2  shows several variables as a function of time (5 seconds) including: Crank —Engine _Cmd  100 , Clutch_Step_Input  102 , Filtered_Speed_Error  104 , Scaled_Speed_Error  106 , Clutch_Position_Cmd  108 , and Clutch_Pos_Actual  110 . In the example of  FIG. 2 , the Filtered_Speed_Error  102  value is roughly 0.4 when Crank_Engine_Cmd goes high. Clutch_Step_Input  102 =1−Filtered_Speed_Error  104  is then around 0.6 resulting in Clutch_Positon_Cmd  108 =approximately 40 after passing through the linear interpolation table Clutch_Pos_Map (Table 2, and step  99  below). 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Clutch_Pos_Map 
             
           
        
         
             
                 
               Clutch_Step_Input 
               Clutch State 
             
             
                 
                 
             
           
        
         
             
                 
               −1.0 
               Disengaged 
             
             
                 
               −0.5 
               Disengaged 
             
             
                 
               0 
               Disengaged 
             
             
                 
               0.5 
               Slipping 
             
             
                 
               1.0 
               Engaged 
             
             
                 
                 
             
           
        
       
     
   
   This Clutch_Position_Cmd is sent to the clutch&#39;s servo-actuator  26  that compresses the clutch  22  plates to achieve this commanded position. The bottom trace of  FIG. 2  shows the Clutch_Pos_Actual from the sensor output of the clutch position sensor. The mechanical dynamics of the clutch mechanism produce the filtering effect between the control signal, Clutch_Position_Cmd, and the physically measured Clutch_Pos_Actual. 
   The effect of Digital_Lowpass Filter described above is evident in  FIG. 2 , Filtered_Speed_Error  104  and Scaled_Speed_Error  106 . If the value of TIME_CONSTANT described above was not sufficiently small to provide enough smoothing, Filtered_Speed_Error  104  would tend to be more like Scaled_Speed_Error  106  (which was filtered to obtain Filtered_Speed_Error  104 ) resulting in very oscillatory engagement and disengagement processes and, therefore, unsatisfactory performance. 
     FIG. 3  shows a 20 second simulation of one embodiment of the present invention including: Clutch_Pos_Actual  120 , Eng/Motor Speed rpm  122 , Eng_Cranking  124 , Engine_Run_Cmd  126 , and Eng_Off &amp; Braking  128 .  FIG. 3  shows that the clutch  22  begins to engage when the engine  20  begins cranking.  FIG. 3  also shows a 3 to 5 second clutch  22  engagement period. The clutch  22  goes through a short period of slipping until the engine  20  speed equals the starter/motor  24  speed. The clutch  22  is then fully engaged while the vehicle operator speeds away until just after 12 seconds. Just after 12 seconds, the vehicle operator releases the accelerator pedal and either begins braking or is coasting with neither brake nor accelerator depressed. The clutch  22  stays engaged through this coast down period and disengages just before the 18 second mark when the engine  20  speed has dropped below 750 rpm.  FIG. 4  expands the engagement phase of  FIG. 3  (3 to 5 seconds) and  FIG. 5  expands the disengagement phase of  FIG. 3  (16 to 18 seconds). 
   The possible control strategy for the controller of the present invention is illustrated in  FIGS. 6A-6C . It can be housed within the VSC  38 . Many other control strategies using the present invention are possible. This strategy can start and end with each drive cycle (i.e., between “key-on” and “key-off”). In  FIGS. 6A-6C , the illustrated embodiment starts at Step  60  and determines whether the vehicle controller outputs have been initialized (Outputs_Initialized). Here, the outputs need to be initialized, given a known value, the first time through the algorithm after startup to ensure that the outputs are not set to an unwanted state by the power-up sequence of the controller. If yes, the strategy proceeds to step  62 . If no, the strategy proceeds to step  64  and commands “Initialize_Outputs” including: Clutch_Position_Cmd=Disengaged and Fuel_Engine_Cmd=False. The strategy proceeds next to step  66  and commands Outputs_Initialized=True and proceeds to step  62 . Once initialized in the first pass through the algorithm, subsequent output values are determined by the algorithm. As described above, the Clutch_Position_Cmd, for this step could be an eight-bit integer&gt;0.85. 
   At step  62  the strategy is commanded to read various vehicle inputs such as other VSC  38  commands and inputs various vehicle sensor outputs. In the illustration presented in  FIGS. 6A-C , the following examples are included: Crank_Engine_Cmd, Engine_Speed, Motor_Speed, Braking_Logic, Clutch_Position_Actual, Fuel_Engine Request. These examples represent various inputs that would be necessary to smoothly transition a clutch  22  between engaged and disengaged states. Crank_Engine_Cmd alerts the strategy whether the engine  20  has been commanded by the VSC  38  to start. Engine_Speed can originate from an engine  20  speed sensor well known in the art. Similarly, Motor_Speed can originate from a starter/motor  24  speed sensor known in the art. The difference in Engine_Speed and Motor_Speed can be used to determine actual clutch  22  slippage (see below). If a mechanical braking means such as a brake pedal  44  is depressed and a vehicle accelerator means such as an accelerator pedal is NOT depressed, then Braking_Logic=True. Otherwise, Braking_Logic=False. Accelerator pedal position is detected by the accelerator position sensor  46 . The Clutch_Position_Actual is the actual position of the clutch  22  in terms of engagement and disengagement sensed by a clutch  22  position sensor. The Fuel_Engine_Request is a VSC  38  command the controller of the present invention can use to indicate whether the engine  20  is running. 
   Once the inputs are read in step  62 , the strategy next proceeds to step  68  and determines Speed_Error. Speed_Error is the difference between the starter/motor  24  speed and engine  20  speed. 
   Next, the strategy proceeds to step  70  to determine Scaled_Speed_Error. The Scaled_Speed_Error multiples the Speed_Error determined in step  68  by Speed_Gain as described above. 
   Next the strategy proceeds to step  72  to determined Filtered_Speed_Error. The Filtered_Speed_Error as described above is:
 
(Time Constant) Scaled_Speed_Error)(k)+(1 Time Constant)*Filtered_Speed_Error) (k 1)
 
   Next, the strategy proceeds to step  74  and determines whether the VSC  38  has requested fuel to the engine  20 . If yes, the strategy proceeds to step  80 . If no, the strategy proceeds to step  76  and determines whether the VSC  38  has commanded the Crank_Engine_Cmd. If yes, the strategy proceeds to step  80 . If no, the strategy proceeds to step  78  and commands the clutch to disengage (i.e., Clutch_Position_Cmd=Disengaged), then proceeds to step  80 . 
   At step  80 , the strategy determines whether the Clutch_Position_Cmd is commanding the clutch  22  to slip. If no, the Fuel_Engine_Cmd is commanded false at step  82  and the strategy returns to the beginning. If yes, the strategy proceeds to step  84  and determines if the engine speed is greater than a predetermined start speed (as suggested above, a start speed could be under 750 RPM). If no at step  84 , the strategy commands the Fuel_Engine_Cmd=False and proceeds to step  94 . 
   If yes at step  84 , the strategy determines if the Braking_Logic=true (as described above) at step  88 . If no, the strategy proceeds to step  90  and commands Fuel_Engine_Cmd=True, then proceeds to step  94 . 
   If yes at step  88 , the strategy commands the clutch  22  to engage (Clutch_Positon_Cmd=Engaged) and the stop fuel to the engine  20  (Fuel_Engine_Cmd=False). The strategy next returns to the beginning. 
   At step  94 , the strategy determines Clutch_Step_Input as a value (Temp) of 1 the Filtered_Speed_Error (from step  72 ) and proceeds to step  95 . At step  95 , the strategy determines whether “Temp” from step  94  is less than 1. If yes, the strategy proceeds to step  96  and sets the Filtered_Speed_Error to 1 in step  96  and proceeds to step  99 . 
   If no at step  95 , the strategy proceeds to step  97  and determines whether “Temp” is &gt;−1. If no, the strategy proceeds to step  99 . If yes, the strategy proceeds to step  98  and sets the Filtered_Speed_Error to 1, then proceeds to step  99 . 
   At step  99 , the procedure performs a linear interpolation to smoothly transition the engagement of the clutch  22 . 
   To summarize, step  96  and step  98  are used to limit Temp to +1 or 1 if the calculation in  94  results in a value of Temp greater than +1 or less than 1. When Temp is between 1 and 1, the algorithm will proceed from step  94  to step  95  to step  97  and to step  99 . Command values can have only positive values between 0 and 1, whereas Clutch_Step_Input takes on values between 1 and 1. 
   The above-described embodiments of the invention are provided purely for purposes of example. Many other variations, modifications, and applications of the invention may be made.