Abstract:
A method of controlling engine idle speed in a motor vehicle having an internal combustion engine and multi-ratio transmission when launched from a neutral idle rest condition. At neutral idle, the transmission is controlled such that the input and output members are decoupled. During vehicle launch from neutral idle operation, two different strategies may be employed to control engine idle speed. One preferred method is to determine a feedforward or predicted torque converter load as a function of time. Another preferred method is to determine the feedforward or predicted torque converter load by comparing a maximum engine brake torque and the torque capacity of the forward clutch at any given time. Irrespective of the particular method used, other engine torque loads are added to the feedforward or predicted torque converter load to determine the engine output torque required to control engine idle speed and provide a smooth between transmission operating modes throughout the launch period.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to a method of controlling engine idle speed in an automotive vehicle. More particularly, this invention relates to a method of controlling engine idle speed when the transmission&#39;s forward clutch is engaged upon releasing the service brakes during launch from neutral idle operation. 
     BACKGROUND OF THE INVENTION 
     Neutral idle operation of a vehicle can be initiated when a vehicle is brought to a stand still position with the engine still running, as when a vehicle is stopped at a traffic light. In such situations, the transmission can be disengaged, i.e., neutral idle operation, which is beneficial to decrease overall vehicle fuel consumption by unloading the engine. 
     More technically speaking, neutral idle operation of a vehicle is generally characterized by (i) the vehicle being at rest, (ii) the service brakes applied, (iii) the gear select lever in a forward range, and (iv) all combinations of torque transmitting clutches that establish a speed ratio from the input to the output member of the transmission being disengaged. As a result, during neutral idle operation, the transmission input shaft rotates freely at a substantially synchronous speed with the engine output shaft. 
     When a vehicle begins to move after being held stationary, it is said that the vehicle is launching from neutral idle operation. One method of launching from neutral idle operation is for the vehicle operator to merely cease applying the service brakes and allow the vehicle to creep forward. The inventors herein have recognized that this type of slow launch from neutral idle operation sometimes causes the vehicle to exhibit undesirable noise, vibration, and harshness (NVH) during the launch. The undesirable NVH is primarily a result of the engine speed changing during the launch from neutral idle to accommodate and react to the increased engine load of the transmission upon re-engagement. Specifically, as the transmission&#39;s clutch pressure increases or decreases, the torque load on the vehicle&#39;s torque converter turbine, and therefore also on the impeller, also increases or decreases. As a result, the transmission&#39;s forward clutch begins to slowly engage to transfer the engine torque through the transmission, and the vehicle begins to slowly creep ahead. However, because conventional methods of engine speed control regulate the engine speed based on an assumption that the transmission is either in a “Drive” or “Neutral” mode of operation (as opposed to a partial mode in between “Drive” and “Neutral”), the engine speed control system tends to respond unevenly throughout the launch period. This is because the engine speed control system is reacting to the slowly increasing load on the engine. 
     Until recently, the NVH that results from the changing engine speed when the transmission engages has been relatively acceptable, primarily because vehicle operators have come to expect a certain amount of NVH when they physically move the vehicle&#39;s gear shifter from “Neutral” (or “Park”) to a “Drive” position. However, because neutral idle operation is typically initiated by the vehicle itself, without the vehicle operator physically moving the gear shifter, the NVH that results from conventional engine speed control methods is more noticeable and undesirable. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved method of controlling engine idle speed during launch from neutral idle operation. The present invention is particularly useful for minimizing undesirable noise, vibration, and harshness during a relatively slow launch from neutral idle operation, when the NVH is more noticeable to the vehicle operator. The present invention controls engine idle speed during launch by determining a “feedforward engine torque” required to maintain a constant engine idle speed when launching from neutral idle operation. The feedforward engine torque is an estimated engine torque that is predicted to be required at a time in the future during the launch. The feedforward engine torque is calculated by the vehicle&#39;s microprocessor based on an event that occurred in the drivetrain, such as the operator releasing the service brakes to begin engagement of the forward clutch. When the vehicle operator releases the service brakes, it is assumed that the vehicle is launching from neutral idle operation. The vehicle&#39;s microprocessor then controls the engine speed based upon the calculated feedforward engine torque. As a result of the present invention, the increased torque demands from the re-engagement of the transmission are anticipated, and the engine speed can be controlled proactively instead of reactively, as in the prior art. The proactive engine speed control minimizes the NVH common in prior art systems. 
     The feedforward engine torque required to maintain a constant engine idle speed is determined by one of two preferred methods. In the first preferred method, the commanded or measured clutch pressure is used to estimate the torque capacity of the forward clutch at any given time throughout the launch period using a mathematical predictive model. Using this preferred method, the torque capacity of the forward clutch is estimated at various times based upon certain operating parameters, such as clutch pressure. In addition, a maximum torque converter load is determined based on the engine speed and other transmission variables, such as transmission fluid temperature, clutch design, and the like. The torque converter load when the clutch is fully engaged (turbine speed equal to zero) is imposed as an upper limit on the estimation of the maximum torque converter load. Then, a feedforward or predicted torque converter load is determined by comparing the maximum torque converter load and the torque capacity of the forward clutch. 
     In the second preferred method, the feedforward torque converter load is determined by a predetermined mathematical function. Specifically, the feedforward torque converter load at any given time can be determined purely as a function of time based on the engine speed, the turbine speed, and other transmission variables, such as transmission fluid temperature, clutch design, and the like. Using this second preferred method, a predicted turbine speed can be estimated based on its current rate of change. Either of the preferred methods will result in a reduction of NVH compared to prior art methods. 
    
    
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a planetary gear transmission having a forward clutch located in the torque flow path between the hydrokinetic torque converter turbine and the input elements of the gearing. 
     FIG. 2 is a chart that shows a clutch and brake engagement and release pattern for the transmission shown in FIG.  1 . 
     FIG. 3 is a schematic representation of the control system including the electronic microprocessor for controlling clutch engagement in a closed-loop fashion. 
     FIG. 4 is a graph that shows output torque and clutch pressure changes as well as the engine speed and turbine speed changes during a neutral idle condition and during a subsequent engagement of the forward clutch for a typical transmission of the kind shown in the prior art references. 
     FIG. 5 is flow diagram of controlling engine idle speed during neutral idle operation according to a first method of the invention. 
     FIG. 6 is flow diagram of controlling engine idle speed during neutral idle operation according to a second method of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in FIG. 1, a hydrokinetic torque converter, shown generally at  10 , includes a bladed impeller  12  connected drivably to a vehicle engine crankshaft  14 . A bladed turbine  16  is connected to drive sprocket  18  of a chain transfer drive. A bladed stator  20  is located between the toroidal flow exit section of the turbine of the turbine flow entrance section of the impeller and acts in known fashion to change the direction of the toroidal fluid flow, thus making possible a torque multiplication in the torque converter  10 . 
     During steady-state operation in higher gear ratios, a friction bypass clutch  22  may be engaged to drivably connect the impeller  12  and the turbine  16 , thus effectively removing the hydrokinetic torque flow path from the driveline. Stator  20  is anchored against rotation in a direction opposite to the direction of rotation of the impeller by an overrunning brake  24 , which is grounded to stator sleeve shaft  26 . 
     A pair of simple planetary gear units  28  and  30  is rotatably mounted about the axis of output shaft  32  that is arranged in spaced parallel disposition with respect. to the engine crankshaft axis. Unit  28  includes ring gear  34 , sun gear  36 , carrier  38  and planet pinions  40  that are journalled on carrier  38  in meshing engagement with ring gear  34  and sun gear  36 . Gear unit  30  comprises ring gear  42 , sun gear  44 , carrier  46  and planet pinions  48 , which are journalled on carrier  46  in meshing engagement with sun gear  44  and ring gear  42 . 
     Carrier  46  forms a torque output element for the gearing and is drivably connected to output member  48 , which is connected to final drive sun gear  50  of final drive planetary gear unit  52 . 
     Final drive gear unit  52  includes, in addition to sun gear  50 , a ring gear  54 , a carrier  56  and planet pinions  58  journalled on carrier  56  in meshing engagement with sun gear  50  and ring gear  54 . Carrier  56  acts as a torque output element of the gear unit  52  and is connected to ring gear  60  and differential gear unit  62 . 
     A compound carrier  64  forms a part of the gear unit  62  that rotatably journals a first pair of pinions  66 , which mesh with a ring gear  60  and with a second set of planetary pinions  68 , the latter meshing with sun gear  70 . Sun gear  70  in turn is drivably connected to output shaft  32 . 
     Carrier  64  is drivably connected to a companion torque output shaft  72 . Shaft  32  is connected to one traction wheel of the vehicle, and the opposite traction wheel of the vehicle is connected to output shaft  72 . The connections between the traction wheels and the respective output shafts are achieved by universal coupling and half shaft assemblies in a known fashion. 
     A third simple planetary gear unit  74  is located between the pair of gear units previously described and the hydrokinetic torque converter. It comprises a ring gear  76 , a sun gear  78 , a carrier  80  and planet pinions  82  journalled on the carrier  80  in meshing engagement with ring gear  76  and sun gear  78 . Carrier  80  is connected to torque transfer sleeve shaft  84 , which is drivably connected to ring gear  34  of gear unit  28  and to ring gear  42  of gear unit  30 . An overrunning brake  86  that has an outer race  88  grounded to the transmission housing as shown at  90  is adapted to anchor sun gear  44  during operation in each of the first four overdriving ratios, thus providing a torque reaction point for the gear system. Ring gear  54  is permanently anchored to the housing as shown at  92 , thus permitting the final drive gear unit  52  to multiply the torque delivered through the gear units  74 ,  28  and  30  in each of the driving ratios. 
     A friction brake band  94  surrounds brake drum  96  which, in turn, is connected to sun gear  44 . The brake band  94  is applied to anchor the sun gear  44  during hill braking operation and during reverse-drive operation. 
     A disc brake shown generally at  98  is adapted to anchor the carrier  38  against the transmission housing during operation in the lowest ratio and in reverse drive. Sun gear  36  is a torque input-element flow transmission. During operation in reverse drive, sun gear  36  is connected to driven sprocket  100  by means of reverse clutch  102 , the latter acting as a driving connection between the driven sprocket  100  and brake drum  104 . Sun gear  36  is connected directly to the brake drum  104 . Driven sprocket  100  is connected to driving sprocket  18  through a torque transfer drive chain  106 . During forward drive operation, drive sprocket  100  is connected to sun gear  78  by forward drive clutch  106 . The forward drive clutch  106  is engaged during operation in the first three forward-driving ratios. 
     A direct-drive clutch  108  connects ring gear  76  with the driven sprocket  100  during operation in the third and fourth forward driving ratios as well as during the fifth driving ratio. When direct drive clutch  108  and the forward clutch  106  are engaged simultaneously, ring gear  76  is connected to sun gear  78  so that the elements of the gear unit  74  rotate in unison with a one-to-one speed ratio. 
     To effect a fifth forward-driving ratio, friction clutch  109  is applied, thus establishing a driving connection between sleeve shaft  84  and sun gear  44  of gear unit  30  to lock sun gear  44  to ring gear  42  so that the speed ratio developed by gear unit  30  is unity. 
     The neutral idle feature of the invention is achieved by controlling engagement and release of forward clutch  106 . When the vehicle is at a standstill and the engine is idling, the engine  10  will tend to drive the turbine because of the hydrokinetic torque multiplication effect of the converter  10 . Thus, a driving torque will be delivered to the traction wheels through the gearing, even when the engine is idling. 
     In prior art designs, it is necessary to maintain the accelerator pedal at a sufficiently advanced position so that the engine will idle at a speed that will avoid undue engine harshness. It further is necessary for the vehicle operator to maintain his foot on the vehicle brake to avoid creeping of the vehicle with the engine idling. By disengaging the clutch  106  to establish a neutral idle condition, the torque flow path to the traction wheel is interrupted when the engine is idling with the vehicle at a standstill. 
     FIG. 2 shows a chart that indicates the clutches and the brakes that are applied and released to establish each of the five forward-driving ratios as well as the reverse ratio. The sun gear  36  is anchored by a second and fourth ratio brake band  110 . That brake band is applied also during fifth ratio operation so that sun gear  36  may act as a reaction point as the ring gear  34  is overdriven and as torque is delivered to the gear unit  28  through the carrier  38  and through the direct-drive clutch  108 . In FIG. 2, the forward-drive clutch  106  is designated as clutch FWD, the direct-drive clutch  108  is designated as clutch DIR, the reverse disc brake  98  is referred to as the LO/REV brake, the fifth ratio clutch  109  is identified as 5CL clutch, and brake band  110  is identified as 2/4 band. 
     First ratio drive is achieved by engaging brake band  94 , which anchors sun gear  44 . Also, disc brake  98  is applied, and forward clutch  106  is applied. Thus, sun gear  78  is connected to the driven sprocket at  100 , and the underdriven motion imparted to the carrier  80  is transferred to the ring gear  42  of gear unit  30 . In FIG. 2, brake band  94  is referred to as the HB and REV band. The reverse clutch  102  is identified in FIG. 2 as the REV clutch. 
     A schematic representation of a microprocessor control system, shown generally at  200 , is shown in FIG.  3 . The engine is generally designated by reference numeral  228 . Operating variables for the engine, such as manifold pressure and coolant temperature and engine speed, are measured by analog sensors and distributed to an electronic microprocessor  230 . The signal passage for manifold pressure is shown at  232 . The engine coolant temperature signal is distributed to the processor  230  through signal line  234 . The engine speed signal is distributed to the processor  230  through line  236 . 
     Other variables that are measured and distributed to the processor are a signal indicating the range selection or transmission manual valve position. This signal is distributed through signal passage  238 . Turbine speed also is measured, and that value is distributed to the processor through signal line  240 . The torque output shaft speed for the transmission is distributed to the processor through signal line  242 . A bypass clutch pressure signal is distributed to the processor through signal line  244 , but that signal is irrelevant to the present invention. Transmission oil temperature for the engine is measured, and the signal representing that value is distributed to the processor through signal line  246 . A brake signal is distributed to the processor through signal line  248 . The presence of a signal at line  248  will indicate whether the vehicle brakes are applied or released by the vehicle operator. 
     The processor  230  will receive the information developed by the sensors and condition it so that it may be used in digital form by the central processor unit. The central processor unit identified at  250  processes the information delivered to the processor  230  in a manner that will be described subsequently using algorithms that are stored in memory  252 . The output signals from the processor  230  are delivered to a valve body  254  through signal line  256 . The output data includes shift signals delivered to the shift solenoids that control the ratio changes. The operation of the valve body  254  and the solenoid signals are described in commonly-assigned U.S. Pat. No. 5,272,630, herein incorporated by reference. The output signal developed by the valve body  254  delivered through signal line  258  controls the operation of the clutches and brakes of the transmission illustrated in FIG.  1 . 
     For purposes of describing the benefits of the present invention, a comparison to prior art neutral idle characteristics will first be made with reference to FIG. 4, which shows the prior art neutral idle clutch characteristics for a transmission having an open loop-type converter. This type of transmission is further described in commonly-assigned U.S. Pat. No. 5,272,630. In FIG. 4, time is plotted on the abscissa; and output shaft torque, clutch fluid pressure, engine speed and turbine speed are plotted on the ordinate. The forward clutch pressure, the engine speed, the turbine speed and the output shaft torque assume initially the values shown in region A of FIG.  4 . 
     It is seen from FIG. 4 that the turbine speed is zero since the vehicle is at rest. The difference between engine speed and turbine speed represents the slip that exists when the vehicle comes to rest and before the neutral idle mode begins. At time B, the neutral idle mode is initiated, which results in an exhaust of pressure from the forward clutch. This results in a decay of the forward clutch pressure over a short period of time, as indicated by the curve C in FIG.  4 . The output shaft torque decays, as shown by curve D, as the forward clutch pressure is relieved. 
     As the forward clutch looses capacity following initiation of a neutral start mode, the turbine speed will increase, as shown at E, until it reaches the normal turbine speed for engine idle, which may be 600 rpm as shown at F in FIG.  4 . The engine speed at that time in a typical vehicle installation may be about 800 rpm as shown at G. 
     FIG. 4 illustrates at point H what happens, according to the prior art, when the operator terminates the neutral idle mode by advancing the accelerator pedal. An immediate increase in the forward clutch pressure then will occur until a transition pressure indicated at I is reached. It is during this interval that the clutch servo cylinder is filling and the clutch servo piston is stroking. Because the engine throttle is advanced, the engine speed will respond to the advancing throttle and will increase as shown by the ramp J in FIG.  4 . The engine speed continues to increase until the clutch servo is fully stroked. At that time, the engine speed will have reached a peak value shown at K. 
     When the piston for the forward clutch servo is stroked and the forward clutch gains capacity, the output shaft torque will sharply rise, as indicated by the steep slope curved portion L, until it reaches a peak value shown at M. The achievement of the peak value M is coincident generally with the peak engine speed, the latter immediately decreasing in value at a fast rate, as shown at N. The decreasing engine speed is accompanied by a substantial inertia torque that contributes to the achievement of the peak value M for the output shaft torque. The clutch pressure will continue to increase following the stroking of the clutch servo piston and progressively increase at a rapid rate, as shown by the curve O, until a final clutch pressure value is reached, as shown at P. The output shaft torque will be subjected to torque fluctuations, as demonstrated by the oscillating torque values Q following clutch engagement. The control strategy of the present invention that avoids these undesirable features of the prior art will now be explained with reference to FIG.  5 . 
     Referring now to FIG. 5, the first preferred routine executed by microprocessor  230  for controlling engine idle speed during launch from neutral idle operation will now be described. The vehicle begins in the neutral idle operating condition (step  500 ). As described above, the neutral idle operating condition results in an exhaust of pressure from the forward clutch at time B in FIG.  4 . This results in a decay of the forward clutch pressure over a short period of time, as indicated by the curve C in FIG.  4 . The output shaft torque decays, as shown by curve D, as the forward clutch pressure is relieved. As the forward clutch loses capacity following initiation of a neutral start mode, the turbine speed will increase, as shown at E, until it reaches the normal turbine speed for engine idle, which may be 600 rpm as shown at F in FIG.  4 . The engine speed at that time in a typical vehicle installation may be about 800 rpm as shown at G. 
     At some point after being maintained in neutral idle operation for a period of time, the vehicle operator releases the service brakes (step  502 ). Though the present invention can be used in connection with a variety of types of launches from neutral idle, the present invention is particularly useful in situations where the vehicle operator does not immediately depress the accelerator pedal to terminate neutral idle operation, as shown at H in FIG.  4 . Rather, the vehicle operator merely allows the vehicle to creep forward, which results in a relatively slow engagement of the transmission. 
     According to the present invention, the engine idle speed during launch can be controlled by predicting a feedforward engine torque and then controlling the engine speed based upon that prediction. The feedforward engine torque is that predicted engine torque that is expected to be necessary to accommodate the increased torque required by the re-engaging transmission at a point in time in the near future. As used herein, the term “load” is a generic term that represents the amount of torque that the transmission exerts on the engine at a given time. 
     According to the first preferred embodiment of the invention, the predicted required engine torque is limited to the minimum of an estimated torque converter load when the forward clutch is fully engaged (i.e., turbine speed equal to zero) and the predicted torque required by the re-engaging transmission. This is done to allow the forward clutch to engage at a desired rate while preventing excessive clutch slippage and engine flare. Therefore, the maximum torque converter load is calculated (step  504 ) according to the following expression: 
     
       
         Max 13  Converter —   Tq _Load= Fn _Conv_Load ( Ne, Nt , other transmission variables), 
       
     
     where, 
     Ne is the engine speed, 
     Nt is the turbine speed and is assumed equal to zero, and 
     Fn 13  Conv_Load converts engine speed and turbine speed to engine brake torque independent of whether the forward clutch is engaged. Fn_Conv_Load is a function of engine speed, turbine speed and other transmission variables, such as transmission fluid temperature, temperature of the friction plates, slip across the clutch, clutch design, and the like. Fn_Conv_Load is easily determined by one skilled in the art from relationships known to the skilled practitioner, namely the torque ratio function, Fn_Conv, and a relationship that relates engine speed to the impeller torque. The relationship between engine speed and impeller torque, otherwise known as the K-factor, can be expressed as:          K        (       Turbine                 Speed       Engine                 Speed       )       =       Engine                 Speed         Impeller                 Torque                                
     The numerical values for the K-factor can be determined empirically by one skilled in the art. The above equation can be rearranged as follows:          Impeller                 Torque     =       (       Engine                 Speed       K        (       Turbine                 Speed       Engine                 Speed       )         )     2                            
     The above equation exemplifies that the impeller torque can be determined based on engine speed and turbine speed. It will be appreciated that the determination of Fn_Conv_Load may be further refined by one of skill in the art by taking into account other transmission variable, such as transmission fluid temperature, temperature of the friction plates, slip across the clutch, clutch design, and the like. 
     Then, a predicted future turbine speed, Nt_Pred, is calculated based on the current turbine speed and a rate of change of the turbine speed (step  506 ), according to the following expression: 
     
       
           Nt _Pred=max( Nt+Nt _Rate_Of_Change*Time_Into_Future,0) 
       
     
     Next, as shown in step  508 , the torque capacity of the forward clutch at any given time throughout the launch period is calculated using the following equation: 
     
       
         Forward_Clutch_Torque_Capacity= Fn _Cap (clutch pressure, other transmission variables) 
       
     
     where, 
     Fn_Cap converts either a commanded or measured forward clutch pressure into a torque capacity. Fn_Cap is a function of clutch pressure and other transmission variables, such as transmission fluid temperature, temperature of the friction plates, slip across the clutch, clutch design, and the like. Fn_Cap includes a conversion factor from torque measured at the forward clutch to torque measured at the turbine shaft and can be expressed as: 
     
       
           Fn _Cap= m *Clutch_Pressure− b   
       
     
     where, 
     m and b are constants that are determined empirically by one skilled in the art. 
     Then, a feedforward or predicted torque converter load is determined (step  510 ) by the following expression: 
     
       
         Pred_Load_Torque=Forward_Clutch_Torque_Capacity/ Fn _Conv( Nt _Pred/ Ne ) 
       
     
     where, 
     Fn_Conv is the torque multiplication or ratio of the torque converter. Fn_Conv can be determined by one skilled in the art by plotting the torque ratio (turbine torque/engine torque) as a function of the speed ratio (turbine speed/engine speed). The numerical values for turbine speed and torque, and engine speed and torque can be determined empirically by one skilled in the art. 
     Next, the feedforward or predicted torque converter load is limited to the lesser of the maximum torque converter load and the predicted torque converter load (step  512 ). In terms of a mathematical expression, the predicted torque converter load is expressed as follows: 
     
       
         Conv —   Tq _Load_Predicted=min(Max_Converter —   Tq _Load,Pred_Load_Torque) 
       
     
     Then, the actual engine torque for controlling engine idle speed is determined by adding any other engine torque loads to the feedforward or predicted torque converter load (step  514 ). Examples of other engine torque loads include, but are not limited to, engine accessories, such as air conditioning, pumps, and the like. Finally, the vehicle microprocessor determines whether the vehicle&#39;s launch from neutral idle is complete. If so, the algorithm ends (step  516 ). If not, then steps  502 - 514  are repeated. 
     Referring now to FIG. 6, a second preferred method of the invention will now be described. As in the first preferred method, the vehicle begins in the neutral idle operating condition (step  600 ). At some point, the vehicle operator releases the service brakes (step  602 ) and allows the vehicle to creep forward, thereby causing the transmission to re-engage relatively slowly. 
     Then, as in the first preferred embodiment, a feedforward turbine speed, Nt_Pred, is predicted at a time in the future based on a current turbine speed and a rate of change of the turbine speed (step  604 ). The same expression as in the first preferred embodiment is used here: 
     
       
           Nt _Pred=max( Nt+Nt _Rate_Of_Change*Time_Into_Future,0) 
       
     
     Unlike the first preferred embodiment though, the second preferred method calculates the feedforward or predicted torque converter load in a continuous loop using the following expression (step  606 ): 
     
       
         Conv —   Tq _Load_Predicted= Fn _Conv_Load( Ne, Nt _Pred, Other Transmission Variables) 
       
     
     where, 
     Ne is the engine speed, 
     Nt_Pred=max(Nt+Nt_Rate_Of_Change*Time_Into_Future,0), and 
     Fn_Conv_Load is identical to the mathematical function having the same name described in connection with the first preferred embodiment. The Fn_Conv_Load function converts engine speed and turbine speed to engine brake torque independent of whether the forward clutch is engaged and is a function of engine speed, turbine speed and other transmission variables, such as transmission fluid temperature, clutch design, and the like. 
     After the feedforward or predicted torque converter load is calculated, the actual engine torque required to control engine idle speed is determined by adding any other engine torque loads to the feedforward or predicted torque converter load (step  608 ). Examples of other engine torque loads include, but are not limited to, engine accessories, such as air conditioning, pumps, and the like. Then, the microprocessor  230  determines whether the launch from neutral idle is complete. If so, the algorithm ends (step  610 ). If not, then steps  602 - 608  are repeated. 
     Unlike the first preferred embodiment of the invention, in the second method of the invention, the torque capacity of the forward clutch is not predicted and converted to an impeller torque as in steps  508  and  510  of the first method of the invention. Rather, the feedforward or predicted torque converter load is determined in a continuous loop. 
     Irrespective of which of the preferred methods are used, the present invention maintains a constant engine idle speed during launch and provides a smooth transition between transmission operating modes. This is accomplished by predicting the feedforward torque converter load to control engine torque throughout the launch period, unlike conventional methods. Then, the engine speed can be controlled by adjusting certain engine operating parameters, such as fuel injection amounts, air intake amounts, and other parameters known to those of skill in the art. 
     Preferred embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. For example, the teachings of this invention apply when a different clutch other than the forward clutch or identified as the forward clutch is allowed to slip during neutral idle operation. More particularly, without limiting the generality of the foregoing, parameters from the reverse clutch of the vehicle may be used, instead of the forward clutch, to control the engine output a torque. Therefore, the following claims should be studied to determine the true scope and content of the invention.