Abstract:
An interactive engine and automatic transmission control system is provided wherein spark and air control are used to control engine speed to maintain a small positive torque on the transmission before and after coast down shifts. More particularly, a transmission controller is electrically coupled to an engine controller in a motor vehicle such that information can be passed therebetween. Such information includes a start of shift signal, a phase of the shift signal (i.e., clutch release phase, speed change phase, or clutch application phase), and a shift complete signal. In addition, the transmission controller identifies the type of shift that is occurring (i.e., fourth gear to third, third to second, or second to first), transmission oil temperature and the acceleration (braking) rate of the vehicle to the engine controller. During the three phases of a coast down shift, air flow control is used to supply an appropriate amount of air to the engine so that closed loop spark control can be used to adjust the speed of the engine. By maintaining the engine speed slightly above the turbine speed just before and just after a coast down shift, very little torque is transmitted through the transmission during the shift. As such, the coast down shift is practically imperceivable to the driver.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to transmission control systems and, more particularly, to an interactive engine and automatic transmission control system for improving vehicle drivability during coast down gear shifts and accelerations. 
     2. Discussion 
     When an automotive vehicle changes speeds from a cruising rate to a stop or near stop condition, the transmission shifts from a high forward gear down to a low forward gear. This event is known in the art as a coast down shift. In terms of shift quality and driver expectation, the points where the shifts occur are critical. 
     Shift points are chosen so that the torque on the transmission either remains entirely positive or entirely negative before, during, and after the shift. Because of the wide range of engine speeds that can exist during a coast down event, the shift points are confined to very narrow ranges. Shifting anywhere outside of these ranges has the potential to cause a torque reversal of the drive train which is detectable to the driver as a bump or knock. 
     Also, when braking moderately from a high speed, a driver expects the deceleration rate of the vehicle to remain constant. However, if a down shift occurs at a relatively high speed, it is possible for the turbine speed to increase substantially above the engine speed. The difference in speed between the engine and turbine will produce a negative torque which will tend to decelerate the vehicle. This torque will add to the driver&#39;s braking and cause an objectionable change in the deceleration rate of the vehicle. 
     For the above reasons, coast down shift points have been traditionally confined to low speed ranges. However, in terms of vehicle performance, low shift speeds can be undesirable. For example, when turning a corner a vehicle should optimally be in a down shifted gear just after entering the corner so that a better engine response and acceleration is provided out of the corner when the driver steps back into the throttle. If a vehicle is in a high gear when exiting the corner, time must be taken to perform a down shift and the desired vehicle response is delayed. 
     In addition, after braking to a near stop, a driver may tip back into the throttle to accelerate. This often happens when braking for a stop light and then accelerating when the light turns green before a complete stop is achieved. By placing the transmission in the appropriate gear prior to the driver pressing the accelerator pedal, vehicle performance is improved. Unfortunately, to date no effective control system has been provided for adequately down shifting the transmission. 
     In view of the foregoing, it would be desirable to provide a transmission controller which interacts with the engine controller to increase engine speed above turbine speed during a coast down shift with spark and air flow control so as to improve drivability. 
     SUMMARY OF THE INVENTION 
     The above and other objects are provided by an interactive engine and automatic transmission control system wherein spark and air control is used to control engine speed under high speed conditions to maintain a small positive torque on the transmission before and after coast down shifts. More particularly, a transmission controller is electrically coupled to an engine controller in a motor vehicle such that information can be passed therebetween. Such information includes a start of shift signal, a phase of the shift signal (i.e., clutch release phase, speed change phase, or clutch application phase), and a shift complete signal. In addition, the transmission controller identifies the type of shift that is occurring (i.e., fourth gear to third, third to second, or second to first), transmission oil temperature and the acceleration (braking) rate of the vehicle to the engine controller. During the three phases of a coast down shift, air flow control is used to supply an appropriate amount of air to the engine so that closed loop spark control can be used to adjust the speed of the engine. The amount of air supplied during a coast down shift is dependant on the acceleration rate of the vehicle and transmission oil temperature. Spark control is used in conjunction with the air flow control to change the speed of the engine to a level just above or at that of the turbine. By maintaining the engine speed slightly above the turbine speed just before and just after a coast down shift, very little torque is transmitted to the input of the transmission during the shift. As such, very little torque is transmitted through the transmission to the output shaft. Thus, the coast down shift is practically imperceivable to the driver. 
     As a further feature of the present invention, the capability of performing coast down shifts at higher speed enables shift points to be selected based on a particular vehicle driver&#39;s driving habits. For example, a person who is aggressive at braking and accelerating may have coast down shift points selected at a higher speed than a person who is less aggressive. As such, the aggressive driver is provided with good vehicle performance (acceleration and deceleration) while the less aggressive driver is provided with a smooth and quiet drive (e.g., less busy shift schedule). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to appreciate the manner in which the advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings only depict preferred embodiments of the present invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of a transmission having its coast down shifts controlled by the interactive control methodology of the present invention; 
     FIG. 2 is a schematic illustration of an idle air control device used in conjunction with transmission of FIG. 1; 
     FIG. 3 is a schematic illustration depicting interactive communication between a transmission controller and an engine controller in accordance with the teachings of the present invention; 
     FIG. 4 is a flowchart illustrating the transmission controller based interactive control activation logic of the present invention; 
     FIG. 5 is a flowchart depicting the clutch release phase logic of the interactive control methodology of the present invention; 
     FIG. 6 is a flowchart illustrating the clutch speed change logic of the interactive control methodology of the present invention; 
     FIG. 7 is a flowchart illustrating the clutch application phase logic of the interactive control methodology of the present invention; and 
     FIG. 8 is a flowchart illustrating the engine controller based down shift logic of the interactive control methodology of to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed towards a method of controlling coast down shifts in an automotive vehicle. More particularly, the present invention provides a method of increasing engine speed with spark control to a level at or slightly above that of the turbine speed just before and just after a coast down shift. By doing so, large levels of torque are prevented from being transmitted through the transmission during a coast down shift which improves drivability. 
     Turning now to the drawing figures, FIG. 1 illustrates a transmission  10  suitable for use in conjunction with the present invention. A torque converter clutch  12  and torque converter  14  connect an engine  16  to the transmission  10 . The torque converter  14  includes an impeller connected to the engine  16 , an overruning clutch connected through ground to a stator, and a gear system connected to a turbine with the input to the torque converter clutch  12 . An overrunning clutch  18  is coupled to the torque converter  14  to limit the transfer of torque to only one direction. A turbine speed sensor  20  is disposed downstream of the torque converter  14  and is operable for measuring the speed of the turbine. 
     The transmission  10  includes an under drive clutch  22 , an overdrive clutch  24 , and a reverse clutch  26 . The transmission  10  also includes a “ 24 ” clutch  28  and a reverse clutch  30 . An output speed sensor  32  is disposed downstream of the transmission  10  to measure a speed of the transmission output shaft. Depending upon the selective engagement of the above identified clutches, a rear planetary gear set  34  and a forward planetary gear set  36  effectuate different torque ratios through the transmission  10 . As such, ratio or output to input speed is controlled. 
     For example, in park and neutral only the low reverse clutch  30  is engaged and the torque ratio through the transmission  10  is zero. In reverse, the reverse clutch  26  and low reverse clutch  30  are engaged and the torque ratio through the transmission  10  is −2.214. In fourth gear, the overdrive clutch  24  and “ 24 ” clutch  28  are engaged and a torque ratio of 0.689 is established through the transmission  10 . In third gear, the under drive clutch  22  and overdrive clutch  24  are engaged and a torque ratio of 1.0 is established through the transmission  10 . In second gear the under drive clutch  22  and “ 24 ” clutch  28  are engaged in a torque ratio of 1.573 is established through the transmission  10 . In first gear, the under drive clutch  22  and low reverse clutch  30  are engaged and a torque ratio of 2.842 is established. 
     The transmission  10  is unique in the way that it interacts with the engine  16 . That is, the application of all forward gearing clutches in the transmission  10  are directly computer controlled through the use of three-way high flow solenoids (not illustrated). By pulse width modulating the solenoids, direct element pressure control is achieved. Also, there are no overrunning clutches or bands (uni-directional devices) in the gearing of the transmission  10 . Therefore, by applying pressure to a clutching element, either positive or negative torque can be transmitted to the output shaft. 
     When making a coast down shift, the active clutches in the transmission  10  experience many phases. Initially, the shift starts by lowering the pressure on the release clutch (which may be any of the clutches identified above) while pressure is increased on the apply clutch. This is called the “release phase” of the shift. Subsequently, the pressure on the release clutch drops to a low enough value so that it begins to slip. When the amount of slip is great enough, this is the start of the “speed change phase” of the shift. 
     During the speed change phase, the release clutch pressure is controlled to produce a desired acceleration rate of the turbine. For example, increasing the pressure on the clutch tends to pull the speed of the turbine toward the higher gear ratio speed and therefore slows the rate of change of turbine speed. The flow of hydraulic fluid into the apply clutch is also controlled during this phase so that the apply clutch begins to apply pressure when the speed of the turbine is at or above the lower gear ratio speed. 
     After the turbine reaches the lower gear ratio speed, the pressure on the apply clutch is increased at a slow rate until it has enough torque capacity to hold the transmission in gear. At the same time, the release clutch is controlled to be in a fully off state. This part of the shift is called the “apply phase”. Once the transmission controller (see FIG. 3) detects that the transmission  10  is in gear and that the apply clutch has a high enough capacity to hold the gear ratio, the shift is ended and the pressure in the apply clutch is raised to its maximum capacity. 
     Turning now to FIG. 2, the engine  16  includes an air control device  38  which enables the engine controller (see FIG. 3) to adjust the size of an air passage leading to the intake manifold of the engine  16 . Such air control devices  38  are commonly used for engine idle control or as electronic throttle devices. The air control device  38  is disposed along an alternate air passage  40  which bypasses a main air passage  42  formed in the throttle body  44 . The volume of air passing through the main air passage  42  is controlled by the vehicle operator depressing or releasing the accelerator pedal to move the throttle valve  46 . 
     The air control device  38  has a fast response and is capable of linearly varying air flow. In this way, the amount of air mass delivered to the engine  16  is adjusted in real time which enables a target engine speed to be obtained. Restricting the air flow also reduces the pressure in the manifold. As a result of a smaller manifold pressure, the air density drops and less air mass enters into the engine cylinders. This causes a drop in the amount of combustible mass that produces energy. The net result is a drop in engine output torque and speed. By opening the alternate air passage  40 , the opposite is accomplished and the speed and torque of the engine  16  are increased. 
     Although not illustrated, the engine  16  also includes conventional spark plugs and combustion chambers. As such, spark control also changes the speed and torque of the engine  16 . Igniting the spark plug when the piston is in its top-most position (top dead center) is considered zero spark advance. Igniting the spark plug before the piston reaches top dead center is called advancing the spark. The torque output of the engine  16  increases as spark is advanced to a mean best torque point. If the spark is advanced further, the torque begins to drop off. By advancing and retarding the spark between top dead center and the main best torque point, the output torque of the engine  16  is changed. 
     As compared to air control, spark control has the advantage of quicker responsiveness. For example, the torque output of the engine  16  can be changed within the next cylinder spark event with spark control while idle air control requires the rate of air volume in the intake manifold to change. However, at low engine speeds, spark advance does not have the large range of authority. For example, a 20% change in spark advance might only have a 5% effect on the level of engine output torque. As will be described in greater detail below, the present invention uses both spark advance and air control to achieve a quick response and a large range of authority over engine and engine torque particularly at slow engine speeds. 
     Turning now to FIG. 3, in order to coordinate spark control and air flow control with transmission coast down shifts, communication is established between the transmission controller  48  and the engine controller  50 . To accomplish this, two electronic communication paths are established therebetween. That is, an on board vehicle data bus  52  and a torque reduction line  54  are coupled between the transmission controller  48  and the engine controller  50 . The on board vehicle bus  52  enables the on board vehicle controllers  48  and  50  to communicate to one another and, preferably, allows at least 10 bytes of data to be sent in one message with a transmission time repeating rate of about 35 milliseconds or greater. 
     The torque reduction line  54  is preferably a dedicated single digital line that transmits either a high or low signal from the transmission controller  48  to the engine controller  50 . Messages are defined on the torque reduction line  54  by how long the signal is held in a certain state. For example, if the signal on the torque reduction line  54  is held low for greater than a pre-selected time, the engine controller recognizes the message as an “end of shift” or other pre-selected message. 
     To properly perform a downshift, the engine controller  50  receives a start of shift signal, a phase of the shift signal, and a shift complete signal from the transmission controller  48 . In addition, the engine controller  50  receives signals from the transmission controller  50  indicating the type of shift that is occurring (i.e., fourth gear to third, third to second, or second to first), transmission oil temperature, and an acceleration (braking) rate. It should be noted that alternate methods of communication are not available. 
     Turning now to FIG. 4, a flowchart of the transmission controller based interactive control activation logic is illustrated. This logic determines when control of coast down shifts are to be handled by the interactive control methodology as opposed to conventional control. The methodology starts in bubble  100  and falls through to decision block  102 . In decision block  102  the methodology determines if the communication paths between the transmission controller and engine controller are operating properly. If not, the methodology advances to block  104  and returns transmission control to the normal (i.e., non-integrated) shift logic. However, if the communication between the engine controller and transmission controller is operating properly at decision block  102 , the methodology continues to decision block  106 . 
     In decision block  106  the methodology determines if the engine speed control logic is operating properly. That is, the methodology determines if proper control over spark timing and air flow volume exists. If proper control does not exist, the methodology advances from decision block  106  to block  104  and returns shift control to conventional logic. However, if the integrity of the engine speed control logic is sound, the methodology advances from decision block  106  to decision block  108 . 
     In decision block  108 , the methodology determines if the position of the throttle is greater than that of a coasting position. If the throttle position is beyond a coast position, the engine is not operating in a coast down situation and therefore the interactive control logic of the present invention is not needed. As such, the methodology advances from decision block  108  to block  104  to return shift control to conventional shift logic. However, if the throttle is less than or equal to a coasting position at decision block  108 , a coast down condition may exist. As such, the methodology advances from decision block  108  to decision block  110 . 
     In decision block  110 , the methodology determines if the vehicle acceleration rate is less than zero (i.e., if the vehicle is decelerated). If the vehicle acceleration rate is not less than zero, the vehicle is not in a coast down condition. Therefore, the interactive shift control logic of the present invention is not necessary. As such, if the vehicle acceleration is greater than or equal to zero at decision block  110 , the methodology advances to block  104  and returns control to conventional logic. However, if the vehicle acceleration rate is less than zero at decision block  110 , a coast down condition exists and the methodology advances to block  112 . 
     In block  112 , the methodology calculates a time until the next shift point. This time is determined by dividing the vehicle&#39;s speed less the transmission shift speed by the negative of the vehicle&#39;s acceleration rate: 
     
       
         Time to shift=(vehicle speed−shift speed)/−(vehicle acceleration) 
       
     
     It should be appreciated that the transmission shift speed is a function of the transmission&#39;s current gear. After calculating the time until the next shift at block  112 , the methodology continues to decision block  114 . In decision block  114 , the methodology determines if the time until the next shift is less than a pre-selected threshold value. This threshold value corresponds to the time needed for the engine to respond to the control changes. If the time until the next shift event is greater than or equal to the threshold value, the methodology advances to bubble  116  and exits the subroutine pending a subsequent execution thereof. 
     However, if the time until the next shift event is less than the threshold value, the methodology advances to block  118 . In block  118 , the methodology activates the interactive shift logic in the transmission controller. From block  118 , the methodology advances through connector  120  to the release phase logic portion of the interactive engine and transmission control methodology which is illustrated in FIG.  5 . 
     Referring now to FIG. 5, the release phase logic begins by falling through connector  120  to block  122 . In block  122 , the methodology activates the release phase logic of the present invention. Accordingly, the methodology advances to block  124 . 
     In block  124 , the methodology calculates a target engine speed (RPM) value. The target engine RPM value is determined by adding a known tolerance value to the turbine speed (RPM): 
     
       
         Target engine RPM=turbine RPM+tolerance 
       
     
     It should be appreciated that the value of the tolerance is a function of the type of upcoming shift. After calculating the target engine RPM value at block  124 , the methodology continues to block  126 . 
     In block  126 , the methodology sends a type of shift signal, phase of shift signal, target engine speed signal, and vehicle acceleration rate signal from the transmission controller to the engine controller. As will be described in greater detail below, the engine controller uses this information to select the proper amount of spark advance and air flow retardation to increase the speed of the engine to be equal to or slightly above that of the turbine speed to effectuate a smooth coast down shift. It should also be appreciated that the type of shift signal is a function of the transmissions current gear. Further, the signals delivered may be in any form such as, for example, a flag. 
     After sending the type of shift signal, shift phase signal, target engine speed signal, and vehicle acceleration signal to the engine controller at block  126 , the methodology continues to block  128 . In block  128 , the methodology calculates the amount of in-gear slip. As described above, as the release clutch pressure is lowered, a slip is established within the active clutch of the transmission. When the amount of slip is great enough, the transmission is deemed to be in a speed change phase. When this occurs, additional coast down shift logic is employed. 
     The amount of in-gear slip is calculated by adding the turbine RPM to the negative of the transmission output speed multiplied by the current gear ratio speed: 
     
       
         In-gear slip=−(transmission output speed*current gear ratio)+turbine RPM. 
       
     
     After calculating the amount of in gear slip in block  128 , the methodology continues to decision block  130 . 
     In decision block  130 , the methodology determines if certain abort conditions exist. For example, such abort conditions may include an amount of gear slip which is greater than a fail safe value. If the abort conditions exist at decision block  130 , the methodology advances to bubble  132  and exists the subroutine pending a subsequent execution thereof. However, if the abort conditions do not exist at decision block  130 , the methodology advances to decision block  134 . 
     In decision block  134 , the methodology determines if the amount of in-gear slip calculated at block  128  is greater than a known threshold value. The known threshold value corresponds to the amount of slip required to indicate that the speed change phase of the shift has started. If the amount of in-gear slip is greater than the known threshold value at decision block  134 , the methodology advances to connector  136  for application of the speed change phase logic portion of the interactive control methodology. However, if the amount of in-gear slip is less than or equal to the threshold value, the methodology continues to block  138 . 
     In block  138 , the methodology calculates a release phase time limit which serves as a default for indicating the start of the speed change phase and therefore application of the speed change phase logic. The release phase time limit is a function of transmission oil temperature and clutch hydraulic fluid volume. After calculating the release phase time limit at block  138 , the methodology continues to decision block  140 . 
     In decision block  140 , the methodology determines if the release phase time limit calculated at block  138  has expired. If the time limit has expired, the release phase of the shift is deemed complete and the methodology advances to connector  142  for implementation of the speed change phase logic. However, if the release phase time limit has not expired at decision block  140 , the methodology continues to decision block  144 . 
     In decision block  144 , the methodology determines if the time since the last engine message was sent is greater than a known threshold value. The known threshold value corresponds to the maximum rate of communication (i.e., 35 milliseconds) which indicates that a new target engine RPM should be calculated. Thus, if the time since the last engine message was sent is greater than the threshold, the methodology advances from decision block  144  to block  124  and recalculates a new target engine RPM. However, if the time since the last engine message was sent is less than or equal to the threshold value, no new target engine RPM is needed. Therefore, the methodology advances from decision block  144  to block  128  to re-calculate the amount of in-gear slip. 
     Referring now to FIG. 6, the speed change phase logic of the interactive engine and transmission control methodology of the present invention is illustrated. The speed change phase logic begins in connector  146  which follows from connectors  136  and  142  in FIG.  5 . From the connector  146 , the methodology falls through to block  148 . In block  148 , the methodology activates the speed change logic in the transmission controller. Accordingly, the methodology advances from block  148  to block  150 . 
     In block  150 , the methodology calculates a new target engine speed. The new target engine speed equals the last target engine RPM plus the time span between messages to the engine controller as multiplied by the acceleration rate of the vehicle: 
     
       
         New target engine speed=last target engine speed+(message interval*acceleration rate) 
       
     
     It should be appreciated that the acceleration rate is equivalent to a known calibration that is present in the current shift control. After calculating the new target engine speed at block  150 , the methodology continues to block  152 . 
     In block  152 , the transmission controller sends a shift-type signal, a shift phase signal, a target engine speed signal, and a vehicle acceleration signal to the engine controller. As stated above, the engine controller uses these signals to select the proper spark advance and air flow retardation for increasing the engine speed to be at or slightly above the turbine speed so that the current coast down shift is smoothed. After sending the type of shift signal, phase of shift signal, target engine speed signal, and vehicle acceleration rate signal to the engine controller at block  152 , the methodology continues to decision block  154 . 
     In decision block  154 , the methodology determines if the turbine speed is greater than the gear ratio speed for the target gear (i.e., the gear being shifted into) less a pre-selected tolerance value. As noted above, when the turbine speed approaches the gear ratio speed of the target gear, the speed change phase of the shift is ending and the apply phase of the shift is beginning. As such, if the turbine RPM is greater than the target gear ratio speed less the tolerance value, the methodology advances from decision block  154  to connector  156  for implementation of the apply phase logic. However, if the turbine RPM is less than or equal to the target gear ratio speed less the tolerance, the methodology continues from decision block  154  to decision block  158 . It should be appreciated that the value of the tolerance is a function of the amount of slip that can be tolerated without causing a bump feeling when the apply clutch is engaged. 
     In decision block  158 , the methodology determines if certain abort conditions exist such as the turbine RPM being less than the target gear ratio speed by more than a fail safe value. If the abort conditions exist in decision block  158 , the methodology advances to bubble  160  and exist the subroutine pending a subsequent execution thereof. However, if the abort conditions do not exist at decision block  158 , the methodology continues to block  162 . 
     In block  162 , the methodology calculates the difference between the target engine speed and the turbine engine speed. This difference is used subsequently to re-determine target engine speed. After calculating the difference between the target engine speed and turbine engine speed at block  162 , the methodology advances to decision block  164 . 
     In decision block  164 , the methodology determines if the time since the last engine message was sent is greater than a known threshold value. The threshold value corresponds to a time indicating that a new target engine speed should be calculated. As such, if the time since the last engine message was calculated is greater than the threshold value, the methodology advances to block  150  for calculating a new target engine speed. However, if the time since the last engine message was sent is less than or equal to the threshold value, the methodology advances to decision block  154  for eventual advancement to connector  156  and implementation of the apply phase logic illustrated in FIG.  7 . 
     Turning now to FIG. 7, the apply phase logic portion of the interactive engine and transmission control methodology of the present invention is illustrated. The apply phase logic begins in connector  156  and falls through to block  166 . In block  166 , the methodology activates the apply phase logic in the transmission controller. As such, from block  166  the methodology continues to decision block  168 . 
     In decision block  168 , the methodology determines if the turbine speed plus a known threshold value is greater than the transmission output speed multiplied by the target gear ratio: 
     
       
         (Turbine RPM+threshold)&gt;transmission output speed*target gear ratio? 
       
     
     According to the relationship of the turbine speed to the transmission output speed and target gear ratio, one of two methods of calculating a new target engine speed is employed. Thus, if the turbine RPM plus threshold value is greater than the transmission output speed times the target gear ratio value at decision block  168 , the methodology advances to block  170 . However, if the turbine RPM plus threshold value is less than or equal to the transmission output speed times target gear ratio value at decision block  168 , the methodology advances to block  172 . 
     In block  170 , the methodology calculates a new target engine speed according to a first method. In this case, the target engine speed equals the target gear ratio multiplied by the transmission output speed, plus the last target engine speed minus the turbine speed: 
     
       
         Target engine speed=target gear ratio*transmission output speed+last (target engine speed−turbine speed) 
       
     
     After calculating the target engine speed at block  170 , the methodology continues to block  174 . 
     In block  172 , the methodology calculates the target engine speed according to a second method. In this case, the target engine speed equals the last target engine speed plus a time span between messages to the engine controller multiplied by the acceleration rate of the vehicle: 
     
       
         Target engine speed=last target engine speed+(message interval*acceleration rate) 
       
     
     After calculating the new target engine speed at block  172 , the methodology continues to block  174 . 
     In block  174 , the transmission controller sends a type of shift signal, phase of shift signal, target engine speed signal, and vehicle acceleration rate signal to the engine controller. As stated above, the engine controller uses these signals to determine the appropriate spark advance and air flow retardation for increasing engine speed during the shift. After sending the type of shift signal, phase of shift signal, target engine speed signal, and vehicle acceleration rate signal to the engine controller block  174 , the methodology advances to decision block  300 . 
     In decision block  300 , the methodology determines if certain abort conditions exist. For example, the methodology may determine if the calculated target engine speed is outside of a preselected range. If the abort condition exists at decision block  300 , the methodology advances to bubble  302  and exits the subroutine pending a subsequent execution thereof. However, if the abort conditions do not exist at decision block  300 , the methodology continues to decision block  176 . 
     In decision block  176 , the methodology determines if the current type of shift is from second gear to first gear. If the current shift involves a second gear to first gear transition, the engine target speed must be maintained above an idle engine speed. As such, if the current shift is from second gear to first gear, the methodology advances from decision block  176  to decision block  178 . However, if the current shift is not from second gear to first gear, the methodology advances to decision block  180 . 
     In decision block  178 , the methodology determines if the engine target speed is greater than the engine idle speed. If the target engine speed is less than or equal to the engine idle speed, the methodology advances to decision block  180 . However, if the engine target speed is greater than the engine idle speed, the methodology advances to block  182 . 
     In decision block  180 , the methodology determines if the shift is complete. As noted above, the shift is complete when the active clutch has a high enough capacity to hold the transmission in gear. Thus, if the shift is deemed complete at decision block  180 , the methodology advances to block  184  where the pressure in the clutch is raised to its maximum capacity and the subroutine is exited pending a subsequent execution thereof. However, if the shift is not complete at decision block  180 , the methodology advances to decision block  182 . 
     In decision block  182 , the methodology determines if a time since the last engine message was sent is greater than a known threshold value. The known threshold value corresponds to the maximum rate of communication which indicates that a new target engine RPM should be calculated. Thus, if the time since the last engine message was sent is greater than the threshold, the methodology advances from decision block  182  to block  168  to recompare the turbine speed to the transmission output speed and target gear ratio. However, if the time since the last engine message was sent is less than or equal to the threshold value, no new target engine RPM is needed. Therefore, the methodology continues from decision block  182  to decision block  300  to reverify the abort conditions. 
     Turning now to FIG. 8, the engine controller side of the interactive engine and transmission control methodology of the present invention is illustrated. The engine controller methodology starts in bubble  200  and falls through to decision block  202 . In decision block  202 , the methodology determines if a first coast down shift message for an upcoming shift has been received from the transmission controller. If no upcoming shift message has been received, the methodology advances to bubble  203  and exits the subroutine pending a subsequent execution thereof. However, if the first coast down shift message for an upcoming shift has been received by the engine controller from the transmission controller at decision block  202 , the methodology advances to decision block  204 . 
     In decision block  204 , the methodology determines whether an end of shift message has been received from the transmission controller. If the end of shift message has been received, the methodology advances to block  206  to begin an end of shift phase logic portion of the methodology. However, if the end of shift message has not yet been received from the transmission controller at decision block  204 , the methodology continues to decision block  208 . 
     In decision block  208 , the methodology determines if a fail safe timer has expired since the last message from the transmission controller was received. If the fail safe timer has expired, the methodology advances from decision block  208  to block  206  to begin the end of the shift phase logic of the methodology. However, if the fail safe timer has not yet expired since the last message was received from the transmission controller, the methodology advances from decision block  208  to decision block  210 . 
     In decision block  210 , the methodology determines if the vehicle throttle has opened. The throttle may open during a coast down shift, for example, when the vehicle operator applies pressure to the acceleration pedal during a coast down. If the throttle is open at decision block  210 , the methodology advances to block  206  to begin the end of shift phase logic of the methodology. However, if the throttle is not open at decision block  210 , the methodology continues to block  212 . 
     In block  212 , the vehicle acceleration rate signal and the target engine speed signal are received from the transmission controller and are saved. From block  212 , the methodology continues to block  214  where the methodology receives the type of shift signal and phase of shift signal from the transmissions controller. From block  214 , the methodology continues to block  216  and sets the flags for the spark proportional-integral-derivative gains and bypass air flow changes based on the type of shift signal identified from the transmission controller at block  214 . 
     From block  216 , the methodology continues to decision block  218 . In decision block  218 , the methodology determines if the phase of shift signal received at block  214  indicates that the transmission is in a release phase of the shift. If the phase of shift signal does not indicate that the transmission is in a release phase, the methodology advances to decision block  220 . However, if the phase of shift signal indicates that the transmission is in a release phase, the methodology advances from decision block  218  to block  222 . 
     In block  222 , the methodology begins to implement engine control logic particularly for the release phase of the shift. As such, in block  222 , the methodology freezes the base air flow at the value it had at the time of the start of the shift. After freezing the base air flow in block  222 , the methodology continues to block  224  where the bypass air control mode is set to a coast down shift mode. From block  224 , the methodology advances to block  226 . 
     In block  226 , the methodology sets the coast down air flow based on the type of shift signal received at block  214  (i.e., fourth gear to third, third to second, or second to first) and as a function of the transmission oil temperature and vehicle acceleration rate. As such, the total airflow is determined as follows: 
     
       
         Total airflow =base airflow+coast down airflow+load air-flow 
       
     
     After setting the coast down airflow in block  226 , the methodology advances to block  228  and enables proportional-integral-derivative spark control based on the target engine speed received at block  212 . During release phase spark control, the integral term is forced to zero. Thus, at block  228 , the methodology also forces the integral term to zero and limits the spark negative ramp to a calibratible value. 
     After enabling spark control at block  228 , the methodology continues to block  230  where the target engine speed is used to advance the spark such that the actual engine speed approaches the target engine speed. From block  230 , the methodology advances to block  232  where the methodology disables the vehicles air compressor. After disabling the A/C unit in block  232 , the methodology continues to bubble  234  where it exits the subroutine pending a subsequent execution thereof. 
     Referring again to decision block  220 , the methodology again checks the phase of shift signal received at block  214 . If the phase of shift signal indicates that the transmission is in a speed change phase, the methodology continues to block  236 . However, if the phase of shift signal does not indicate that the transmission is in a speed change phase, the methodology advances to block  238 . 
     In block  236 , the methodology continues proportional-integral-derivative spark control based on the target engine speed received from the transmission controller at block  212 . During the speed change phase, the integral term of the proportional-integral-derivative calculation is forced to zero. Thus, in block  236  the methodology also forces the integral term to zero. 
     After continuing spark control in block  236 , the methodology advances to block  240 . In block  240 , the methodology modifies the coast down air flow by a calibratible amount. This calibratible amount corresponds to a level necessary to maintain an engine speed slightly above the turbine speed during the apply phase. After modifying the coast down air flow by a calibratible amount in block  240 , the methodology advances to bubble  234  and exits the subroutine pending a subsequent execution thereof. 
     Referring again to block  238 , since the phase of shift signal received at block  214  did not indicate that the transmission was in a release phase at decision block  218  and did not indicate that it was in a speed change phase at decision block  220 , the phase of shift signal must indicate that the transmission is in an apply phase of the shift. As such, the methodology advances from block  238  to block  242 . In block  242 , the methodology continues proportional-integral-derivative spark control based on the target engine speed received at block  212 . During the apply phase of the shift, the methodology allows the integral term calculation to be performed. After controlling the spark at block  242 , the methodology continues to bubble  234  and exits the subroutine pending a subsequent execution thereof. 
     Referring again to block  206 , the end of shift phase portion of the engine controller methodology will now be described. After beginning the end of shift phase logic in block  206 , the methodology continues to block  244 . In block  244 , the methodology disables proportional-integral-derivative spark control and advances to decision block  246 . 
     In decision block  246 , the methodology determines if the spark advance setting is at the base calculated value. The base calculated value corresponds to desired emissions, efficiency, and smooth operation. If the spark setting is at the base calculated value at decision block  246 , the methodology advances to block  248  where the methodology returns spark control to normal control logic. However, if the spark advance setting is not at the base calculated value at decision block  246 , the methodology advances to block  250  where the methodology ramps out the spark setting by a calibratible amount. From blocks  248  and  250 , the methodology continues to decision block  252 . 
     In decision block  252 , the methodology determines if the air compressor delay is complete. If the delay is complete, the methodology advances to block  254  and enables the air compressor unit of the vehicle. However, if the air compressor delay is not complete at decision block  252 , the methodology bypasses block  254  and continues to bubble  234  where it exits the subroutine pending a subsequent execution thereof. Similarly, after enabling the air compressor at block  254 , the methodology continues to bubble  234  and exits the subroutine. 
     Thus, the present invention provides interactive control of coast down shifts with the transmission controller and engine controller of a motor vehicle. According to the present invention, a target engine speed is calculated during a shift based on various engine parameters. As such, when a coast down shift occurs, the engine speed is controlled through spark control and air flow control to a level at or slightly above that of the turbine speed so that the shift occurs smoothly thereby improving drivability. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.