Abstract:
A method and system for controlling engine speed during a vehicle launch (from a rest condition to a moving condition), such vehicle having the engine is coupled to an automatic shifting manual transmission. The method comprises determining a spark timing offset based on one or more of engine speed, a time rate of engine speed, relative air charge, engine coolant temperature, accelerator pedal position, and a time rate of change of accelerator pedal position. A new spark timing is determined as the base spark timing minus the spark timing offset. By adjusting spark timing based primarily on a time rate of change of engine speed, engine speed fluctuations arising during a clutch engagement associated with vehicle launch are largely attenuated.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a method and a system for controlling engine speed and more particularly to a method for controlling engine speed when a vehicle having such engine transitions from a rest condition to a moving condition (i.e., a vehicle launch).  
           [0003]    2. Background of the Invention  
           [0004]    As is known in the art, with a vehicle equipped with a manual transmission, during a typical, driver-actuated gearshift event, the driver matches the torque during the phases of the shift by adjusting the accelerator pedal position. The pedal position actuates either mechanically or electronically, the intake throttle valve.  
           [0005]    As is also known in the art, with an Automatic Shift Manual (ASM) transmission the traditional manual gearshift lever is replaced with operator hand-operated up-shift and down-shift paddles which are part of a driver interface. The ASM transmission uses a sophisticated electromechanical control system to eliminate clutch pedal control by the operator of the vehicle completely. More particularly, with an ASM transmission, the driver makes gear selections with the easy-to-operate electronic paddles while the vehicle&#39;s control system executes the driver&#39;s decision. During such execution, the system co-ordinates all gear-change events including engine torque ramp-down and ramp-up. An ASM transmission is an automatic manual gearbox because the mechanical linkages, which would commonly be controlled by the operator of the vehicle in a manual transmission, are supplanted by high-speed electrohydraulic actuators.  
           [0006]    There are two operator selectable modes by which an ASM transmission can operate: (1) An Operator Select -ASM (OS-ASM) mode; and (2) an Automatic Select-ASM (AS-ASM). The particular mode is selected by the operator by pressing one of a pair of operator hand-operated buttons which are also part of the driver interface. In the OS-ASM mode the operator depresses the appropriate upshift or downshift paddle to indicate a desire for a gear shift; in the AS-ASM mode the demand for a gearshift is not under the operator&#39;s control but rather under control of the system itself. For example, in the AS-ASM mode, the demand for a gearshift is produced by an engine control unit and is computed as a function of driver demanded torque and engine operating conditions.  
           [0007]    In the prior art, the ASM gearshift event is performed analogously to that of a manual transmission except that the engine control unit aboard the vehicle controls the intake throttle valve position and spark timing. A gearshift as a function of time, according to the prior art, is shown in FIG. 1 for a vehicle launch. A launch is when a resting vehicle is caused to drive away. At the left hand side of FIG. 1, the clutch is fully open. Shortly thereafter, a launch is requested and the clutch is caused to partially close. As a consequence of the clutch being partially closed, engine speed drops. Clutch pressure is controlled based on engine speed, as well as driver demanded torque. So, when engine speed starts to drop, the clutch is caused to retract to a fully open position. As a consequence of the clutch fully opening, engine speed flares. This continues in an unstable fashion causing the engine speed to deviate wider and wider. This vast fluctuation in clutch position and engine speed leads to an undesirable bucking of the vehicle. Typically, the driver intervenes, as was the case for that shown in FIG. 1, by adjusting the accelerator pedal position. If the driver depresses the accelerator pedal, the launch becomes a heavier launch and the condition can be exited. In the example shown in FIG. 1, engine speed fluctuations are mostly damped and the clutch is allowed to fully close by the end of the time shown in FIG. 1. In some situations, which require the most precise and slow vehicle maneuvers, such as positioning a car on a transporter for delivery, the operator of the vehicle desires a very slow vehicle speed. Thus, the driver intervenes by backing off the accelerator pedal and the clutch is returned to a fully open position. When the driver reapplies pressure to the accelerator pedal to attain a slightly higher vehicle speed, the bucking phenomenon recurs.  
           [0008]    In a transition from the vehicle being at rest to moving, (i.e., a vehicle launch), it has been found that it is difficult to provide a reliably smooth launch, particularly when the operator is requesting a gradual launch, such as might be requested to undergo a parking lot maneuver.  
         SUMMARY OF INVENTION  
         [0009]    In accordance with the present invention, a method and system are provided for controlling spark timing for spark plugs disposed in cylinders of an internal combustion engine. The method includes determining a base spark timing for such spark plugs. A time rate of change dNe/dt of engine speed is determined. The base timing is modified in accordance with the determined dNe/dt.  
           [0010]    In accordance with another feature of the invention, a method and system are provided for use in controlling engine torque of an internal combustion engine, such torque being controlled by a spark timing of spark plugs disposed in engine cylinders. A base spark timing for such spark plugs is determined in an engine control unit electronically coupled to the spark plugs. A time rate of change dNe/dt of engine speed is determined. A spark timing offset from the base timing is determined for the spark plugs. The spark timing offset is a function of the determined dNe/dt. A new spark timing is determined for the spark plugs, such new spark timing being a function of the base spark timing and said offset spark timing. The engine control unit commands said new spark timing to the spark plugs.  
           [0011]    According to another aspect of the invention, the spark timing offset is a function also of one or more of engine speed, engine coolant temperature, accelerator pedal position, a time rate of change of accelerator pedal position, a relative air charge, and engine speed divided by vehicle speed.  
           [0012]    According to an aspect of the invention, the offset spark timing is set to zero when the time rate of change of engine timing is negative.  
           [0013]    In one embodiment, a method and system are provided for controlling spark timing for spark plugs disposed in cylinders of an internal combustion engine of a vehicle. The method includes determining a launch requested by an operator of the vehicle. A determination is made of a base spark timing for such spark plugs. A time rate of change dNe/dt of engine speed is determined during the vehicle launch. The base timing is modified during the launch in accordance with the determined dNe/dt.  
           [0014]    In one embodiment, the base spark timing is a spark advance which provides the best torque (MBT spark timing, discussed in more detail below). Alternatively, the base spark timing is retarded from MBT spark timing thereby providing a torque reserve.  
           [0015]    The inventors of the present invention have found that the unstable vehicle launch can be prevented by a method for controlling torque in an internal combustion engine which is coupled to an automatic shifting manual transmission and installed in the vehicle. The torque is controlled by adjusting a spark timing of spark plugs disposed in engine cylinders. The method includes determining engine speed, and adjusting spark timing based on a derivative of engine speed.  
           [0016]    More particularly, the inventors of the present invention have recognized the problem underlying the phenomena is that the point at which the clutch plates first touch, the kiss point, i.e., when there is first some torque delivery between the clutch plate on the engine side to the clutch plate on the transmission side, is not known. One solution would be to very slowly engage the clutch so that the kiss point could be detected. However, this is an unsatisfactory clutch engagement which leads to excessive clutch slippage resulting to excessive clutch wear and overheating. An advantage of the invention is that spark timing affects engine torque rapidly. Therefore, when an initial portion of an engine overspeed condition is detected, spark is adjusted immediately and it affects engine torque in the next cycle, thereby thwarting engine overspeed. Because of the fast actuation provided by adjusting spark timing, an unstable control during vehicle launch is prevented. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:  
         [0018]    [0018]FIG. 1 is a timeline of a launch with an automatic shifting manual transmission coupled to an internal combustion engine according to the prior art.  
         [0019]    [0019]FIG. 2 is a schematic diagram of an internal combustion engine equipped with an ASM transmission, according to an aspect of the present invention;  
         [0020]    [0020]FIG. 2A is a schematic diagram of part of a vehicle dashboard showing driver interface components;  
         [0021]    [0021]FIG. 2B is a schematic diagram of a two-speed manual transmission in which the clutch is open and the transmission is in neutral;  
         [0022]    [0022]FIG. 2C is a schematic diagram of a two-speed manual transmission in which the clutch is closed and the transmission is a first gear;  
         [0023]    [0023]FIG. 3 is a graph of engine torque as a function of spark advance for a given engine operating condition;  
         [0024]    [0024]FIG. 4 is a flowchart of a launch strategy according to the present invention;  
         [0025]    [0025]FIG. 4A is a flowchart showing the determination of values of engine parameters;  
         [0026]    [0026]FIG. 5 is a flowchart of a launch strategy according to the present invention;  
         [0027]    [0027]FIG. 5A is a flowchart showing the determination of values of engine parameters; and  
         [0028]    [0028]FIG. 6 is a timeline of a launch with an automatic shifting manual transmission coupled to an internal combustion engine according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    Referring to FIG. 2, an engine  10  is shown coupled to an automatic shift manual (ASM) transmission  18 . The ASM transmission  18  is hydraulically actuated. The hydraulic fluid reservoir  1  is connected by hydraulic lines to an electrically-actuated hydraulic pump  2  and shift actuator  3 . Shift actuator  3  is connected by hydraulic lines to clutch actuator  5  (i.e., a clutch) and a pressure accumulator  4 . Hydraulic pump  2  is coupled to transmission control unit (TCU)  20  via a pump relay  16 . TCU  20  receives input from clutch position sensor  6 , input shaft speed sensor  7 , two gear position sensors  8 , output shaft speed sensor  9 , pressure sensor  12 , driver interface  14 , and crank interrupt relay  15 . Transmission control unit  20  is coupled to an engine control unit (ECU)  40  by a computer area network (CAN) connection, or other protocol capable of transferring data between the two control units, e.g., hardwired or wireless. TCU  20  controls four solenoid valves  11  which direct high pressure fluid to move the shift lever rods (not shown) along the H pattern to change gears.  
         [0030]    Referring now to FIG. 2A, driver interface  14  includes operator hand-operated shift paddles  206  and  208  and mode select buttons  202 . Driver interface  14  is electronically coupled to TCU  20 , as shown in FIG. 2. Shift paddles  206  and  208  are operated by the driver to indicate a desire for an upshift or a downshift, respectively. In one embodiment, one of mode buttons  202  is used by the operator to indicate AS-ASM or OS-ASM mode. The other mode button  202  is used to indicate a shift style desired by the operator: normal or aggressive. Alternatively, mode buttons  202  is a combination of push buttons, toggle switches, rotary switches, or any other switch.  
         [0031]    Also, shown in FIG. 2 is an accelerator pedal  44  coupled to pedal position sensor  38 . The driver of the vehicle actuates accelerator pedal  44  to indicate the driver request for torque. A signal indicative of position of accelerator pedal  44  is communicated to ECU  40  by pedal position sensor  38 . Also shown in FIG. 2 is a vehicle speed sensor  60  which receives signals from a plate  62  coupled to an axle (not shown) of the vehicle. Plate  62  has four teeth which cause a signal to be produced when they come into proximity with sensor  60 . By measuring the time in between pulses and knowing the wheel diameter, vehicle speed is determined. The vehicle speed sensing system shown in FIG. 2 is by way of example. Alternatively, other methods can be employed.  
         [0032]    Referring now to FIG. 2A, in the OS-ASM, mode the shift paddles  206 ,  208  are actuated to indicate both the type of shift, i.e., up or down, and when a gear shift is desired. In a second operating mode, AS-ASM, the TCU  20  requests a shift based on operating condition and communicates that request with ECU  40 . Alternatively, a request for a gearshift to an AS-ASM transmission could be provided by other modules within the vehicle.  
         [0033]    Referring again to FIG. 2, engine  10  has a lower end  34 , a cylinder head  22 , and a block  32 . Within the block are cylinders  30  in which pistons  28  reciprocate. Fuel injectors  24  and spark plugs  26  are disposed in cylinder head  22 . This fueling configuration is known as direct fuel injection. The present invention applies to other fuel delivery methods including, but not limited to, port fuel injection, in which the injectors are disposed in the intake ports outside the cylinders, carburetion, central fuel injection, in which injectors are disposed in the intake system upstream of where the intake splits to feed the cylinders, and combinations thereof. Engine  10  is supplied air through intake  47 , which has throttle valve  45 , which can be rotated to adjust the flow of air into engine  10 .  
         [0034]    Referring again to FIG. 2A, a portion of a dashboard  200  is shown. The steering wheel  204  is connected to a steering column (not shown), which comes through dashboard  200 . Shift paddles  206  and  208  are depressed by the operator of the vehicle to indicate a desire for an upshift or a downshift, respectively. For example, depressing paddle  206  indicates a desire for an upshift from the current gear to one gear higher; depressing paddle  206  twice indicates a desire for an upshift from the current gear to two gears higher. Paddles  206  and  208  are shown in FIG. 1A attached to steering wheel  204  such that when steering wheel  204  is rotated, paddles  206  and  208  also rotate. Alternatively, the paddles  204 ,  206  can be attached to the steering column but adjacent to the outside rim of steering wheel  204 . In this configuration, the paddles do not rotate with steering wheel  204 . Regardless of configuration, paddles  206  and  208  are electronically coupled to TCU  20 . Buttons  202  are on dashboard  200 . By manipulating buttons  202 , the operator indicates type of operating mode, OS-ASM or AS-ASM. In one embodiment, the driver can also indicate driving style desired: normal or aggressive, which refers to control of the transmission, which is not part of the present invention and not discussed further. Buttons  202  can be: toggle, rotating, push button, or other known types.  
         [0035]    Referring now to FIG. 2B, a clutch, including plates  152  and  154 , and a two-speed transmission is shown. Typically, manual transmissions have four to six gears. The gear set shown in FIG. 2B is merely an example and not intended to be limiting. Clutch plate  152  is fixed to shaft  150 , which couples to the engine. Thus, the clutch plate  152  rotates at engine rotational speed at all times. In FIG. 2B, clutch plates  152  and  154  are apart; thus, the clutch is disengaged or open. In this situation, engine  10  is decoupled from the transmission. Clutch plate  154  is fixed to gear  164 . Gear  164  meshes with gear  166 , which is fixed to layshaft  162 . Layshaft  162  also contains and is affixed to gears  168  and  170 . Gears  168  and  170  mesh with gears  158  and  156 , respectively. Shaft  172  is a spline shaft that is coupled to the driving wheels via a differential and driveshaft (not shown). Shaft  172  is not attached to gears  156  and  158 . Instead, gears  156  and  158  have bearings (not shown) in between shaft  172  and each of gears  156  and  158  to allow  156  and  158  to rotate independently of shaft  172  and each other. Collar  160  is connected, through the splines, to shaft  172 , thus spinning with shaft  172 . The teeth on collar  160 , called dog teeth, can be fit into corresponding holes on the sides of gears  156  and  158 . In FIG. 2B, the collar is in a center position, decoupled from both gears  156  and  158 . Thus, the transmission is in neutral. To select a gear, collar  160  is caused to move toward gear  156 , a lower gear, or toward gear  158 , a higher gear. Making a change from gear  156  to gear  158  is called an upshift and vice versa is a downshift. The lever, or other mechanism, by which collar  160  is caused to couple to a gear is not shown.  
         [0036]    Referring to FIG. 2C, clutch plates  152  and  154  are shown in proximity to each other. By a force applied to force clutch plates  152  and  154  together, the two to rotate together due to friction. The position shown in FIG. 2C is an engaged, or closed, clutch. Also shown in FIG. 2C is collar  160  with dog teeth coupled to gear  156 . In the configuration of FIG. 2C, shaft  150 , plates  152  and  154 , and gear  164  all rotate at engine speed. Layshaft  162 , gears  166 ,  168 , and  170  rotate at engine speed times the gear ratio between gears  164  and  166 . Gear  158  rotates at the rotational rate of gear  168  times the gear ratio between gears  168  and  158 . However, gear  158  is not coupled to layshaft  172  and has no effect on driving speed. Similarly, gear  156  rotates at the rotational rate of gear  170  times the gear ratio between gears  170  and  156 . Because collar  160  is coupled to gear  156  via the dog teeth, collar  160  and gear  156  rotate at the same speed. Collar  160 , being splined to shaft  172 , causes shaft  172  to rotate at this same speed, also. In this way, the rotational speed between shaft  150  and shaft  172  is based on gears  164 ,  166 ,  170 , and  158 . If collar  160  were, instead, coupled to gear  158 , the relative rotational speed of shafts  150  and  12  is based on gears  164 ,  166 ,  168 , and  158 .  
         [0037]    Referring again to FIG. 2, ECU  40  is provided to control engine  10 . ECU  40  has a microprocessor  50 , called a central processing unit (CPU), in communication with memory management unit (MMU)  48 . MMU  48  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  50 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  58 , random-access memory (RAM)  56 , and keep-alive memory (KAM)  54 , for example. KAM  54  may be used to store various operating variables while CPU  50  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  50  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  50  communicates with various sensors and actuators via an input/output (I/O) interface  52 . Example items actuated under control of CPU  50 , through I/O interface  52 , are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve position, spark plug timing, exhaust gas recirculation valve position, and others. Driver display  36 , which displays engine rpm, current gear and others to the operator, receives data via I/O interface  52 . Sensors  42  communicating input through I/O interface  52  preferably include sensors indicating piston position, engine rotational speed, vehicle speed, coolant temperature, barometric pressure, exhaust gas recirculation valve position, intake manifold pressure, accelerator pedal position  38 , throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, air flow, and others. Some ECU  40  architectures do not contain MMU  48 . If no MMU  48  is employed, CPU  50  manages data and connects directly to ROM  58 , RAM  56 , and KAM  54 . Of course, the present invention could utilize more than one CPU  50  to provide engine control and ECU  40  may contain multiple ROM  58 , RAM  56 , and KAM  54  coupled to MMU  48  or CPU  50  depending upon the particular application. In FIG. 2, ECU  40  and TCU  20  are separate units. However, the functionality of the two could be combined in a single control unit without departing from the spirit of the present invention.  
         [0038]    Spark timing is used, in the present invention, to control engine torque during a vehicle launch. The relationship between engine torque and spark advance is shown in FIG. 3. For a given air flow rate, fuel delivery rate, and engine speed, the relationship between engine torque and spark advance is shown as curve  90  in FIG. 3. There is a spark advance timing  92 , known as minimum spark advance for best torque (MBT) by those skilled in the art, which provides the highest engine torque for the given operating condition. If spark timing is either advanced or retarded from MBT, engine torque reduces. The fuel efficiency (not plotted in FIG. 3) is at a maximum at MBT. Thus, for fuel efficiency reasons it is desirable to operate the engine at MBT spark timing.  
         [0039]    Continuing to refer to FIG. 3, although MBT spark timing  92  provides the maximum fuel efficiency and maximum torque, it is a less desirable operating point from a control standpoint. Because MBT spark advance  92  provides the maximum torque for the given operating condition, by adjusting spark advance alone, torque can only be reduced. To control engine torque, it is desirable to have the capability to both increase and decrease engine torque. This can be accomplished by operating at a spark timing, which is retarded from MBT. (Although it appears that one could also choose to operate advanced of MBT, for exhaust emission reasons and others, it is more suitable to retard engine timing.) To determine the amount to retard the timing, a desired torque reserve is chosen. Torque reserve can be determined as an absolute number or a percentage of torque at MBT spark timing. Such a torque reserve is shown in FIG. 3. By intersecting the torque reserve with curve  90 , point  94  is found, which is the spark timing with torque reserve. When operating at point  94 , engine torque can be increased, by advancing spark timing toward MBT, and decreased, by retarding spark timing further from MBT.  
         [0040]    Referring to FIG. 4, a strategy according to the present invention is shown. In step  210 , the strategy is initiated when the vehicle is at rest. In step  212 , a determination is made whether a launch is requested. A launch is determined based on the operator depressing paddle  206  and based the position of accelerator pedal  44 , i.e., the driver indicating a desire for the vehicle to accelerate. When the operator has depressed paddle  206 , the transmission waits until the operator depresses the accelerator pedal  44  before a clutch engagement is begun. A negative response from step  212  causes looping through  212 , until a positive response in step  212 , which causes control to pass to step  214 . In step  214 , it is determined what kind of launch type. The strategy of the present invention applies to light launches, such as might be encountered in a parking lot maneuver. A heavy launch is, for example, an acceleration from a red light. The determination of light or heavy launch is determined by at least one of: the pedal position, θ, and a time rate of change of pedal position, d θ/dt. If it is determined that a heavy launch is requested, control passes to step  216  in which an alternate strategy is used that is not part of the present invention. If the strategy is light, control passes to step  218  in which it is determined whether the rate of change in engine speed is positive or negative. If the engine speed is in control, engine speed remains at the desired speed. But, when engine speed is not in control, engine speed ramps up and down. Because the base spark advance, SA b , is MBT, adjusting spark timing cannot be used to increase engine torque, only decrease engine torque. So, if the engine speed is ramping down, no measure is taken in the engine. Instead, a measure is taken in the clutch. Step  218  is looped repeatedly until the slope of a time rate of change of engine speed (dNe/dt)is positive. At this point, control passes to step  220  in which a spark timing offset is determined. In step  222 , a new spark timing is computed as the base spark timing minus the spark timing offset computed in step  220 . Control passes to step  224  in which it is determined whether the clutch is fully engaged. If so, the routine of the present invention is ended in step  226 . If not, control passes back to step  220 , in which a new offset spark timing is determined.  
         [0041]    Thus, the method determines a new spark timing for the spark plugs as a function of the base spark timing and said offset spark timing. This new spark timing is sent by the ECU  40  to the spark plugs.  
         [0042]    The steps shown in FIG. 4 depend on knowing the values of a number of engine parameters. Referring now to FIG. 4A, the values of various engine parameters are determined based on signals from sensors: engine speed (Ne), vehicle speed (V), engine speed divided by engine speed (Ne/V), relative air charge (ac_rel), accelerator pedal position (θ), and engine coolant temperature (ECT) are determined, as well as time derivatives: dNe/dt and d θ/dt. Time derivatives are known to be noisy signals; thus, in step  232 , these are filtered. Base spark timing (SA b ) is determined in the ECU  40  based on such variables as Ne, ac_rel, ECT, and others. In the strategy of FIG. 4, SA b  is nominally the MBT spark advance. For fuel economy purposes, it is desirable to operate close to or at MBT spark advance.  
         [0043]    The relative air charge, ac_rel, is the amount of air trapped in the cylinder divided by the amount of air that could be trapped in the cylinder at standard conditions. In one embodiment, the amount of air trapped in the cylinder is based on the conditions in the engine intake, i.e., intake pressure and temperature. In an alternative embodiment, the amount of air trapped in the cylinder is determined from a mass airflow sensor (not shown in FIG. 1). Based on the mass air flowrate to the engine and the rate of intake strokes (proportional to engine speed), the mass inducted into the cylinder is computed. In either case, the trapped charge is normalized by the amount of air which could be inducted at standard pressure and temperature.  
         [0044]    Referring now to FIG. 4A, in steps  230  and  232 , various engine parameters are determined, which are used in steps  212 ,  214 ,  218 ,  220 , and  222  of FIG. 4. Engine speed (Ne), vehicle speed (V), engine speed divided by engine speed (Ne/V), relative air charge (ac_rel), accelerator pedal position (θ), and engine coolant temperature (ECT) are determined based on sensor signals. Time derivatives are also found in step  230 : dNe/dt and d θ/dt. Time derivatives are known to be noisy signals; thus, in step  232 , these are filtered. Base spark timing (SA b ) is determined in the ECU  40  based on such variables as Ne, ac_rel, ECT, and others. In this case, SA b  is nominally the MBT spark advance. For fuel economy purposes, it is desirable to operate close to or at MBT spark advance.  
         [0045]    Referring now to FIG. 5, an alternate strategy is shown. As in the prior strategy shown in FIG. 4, the strategy begins, here in step  240 , with the vehicle at rest and it is determined in step  242  whether a launch has been requested. Again, a desire for launch is indicated by the operator by depressing paddle  206  and depressing the accelerator pedal  44 . If a launch is detected in step  242 , control passes to step  244  in which it is determined whether it is a heavy launch or a light launch. If it is a heavy launch, control passes to step  246  in which an alternate strategy, not part of the present invention, is used. If a light launch is detected, control passes to step  250  in which a spark timing offset, SA offset  is computed. Control passes to step  252  in which a new spark timing, SA new  is found as the difference between SA b—   tr  and SA offset . The new spark timing is commanded to the spark plugs. Control passes to step  254  in which it is determined whether the clutch is fully engaged. If a positive result, the routine is ended in step  256 . If a negative result from step  254 , control passes back to step  250  in which spark offset is determined again.  
         [0046]    In step  260 , the spark timing, SA b—   tr , is determined; whereas, in step  230 , of FIG. 4, SA b  is found. As mentioned above in regards to FIG. 4, SA b  is substantially the MBT spark timing. SA b—   tr  is a spark advance which is retarded from MBT, thereby capable of providing a torque reserve, as shown in FIG. 3. By operating the spark timing at SA b—   tr , engine torque can be increased or decreased by advanced or retarding spark timing, respectively. Thus, the method determines a desired amount of torque reserve and computes a predetermined amount of spark retardation to provide said desired amount of torque reserve.  
         [0047]    Referring now to FIG. 5A, in steps  260  and  262 , value of engine parameters are determined, which are used in steps  242 ,  244 ,  250 , and  252  of FIG. 5. Engine speed (Ne), vehicle speed (V), engine speed divided by engine speed (Ne/V), relative air charge (ac_rel), accelerator pedal position (θ), and engine coolant temperature (ECT) are determined based on sensor signals. Time derivatives are also found in step  250 : dNe/dt and d θ/dt. Time derivatives are known to be noisy signals; thus, in step  252 , these are filtered. Base spark timing (SA b—   tr ) is determined in the ECU  40  based on such variables as Ne, ac_rel, ECT, and others.  
         [0048]    In step  260  of FIG. 5A, the spark timing, SA b—   tr , is determined; whereas, in step  230 , of FIG. 4A, SA b  is found. As mentioned above in regards to FIG. 4A, SA b  is substantially the MBT spark timing. SA b—   tr  is a spark advance which is retarded from MBT spark timing, thereby capable of providing a torque reserve, as shown in FIG. 3. By using a spark timing at SA b—   tr  in the strategy of FIG. 5, engine torque can be increased or decreased by advanced or retarding spark timing, respectively, back to step  250  in which a new spark timing offset is computed.  
         [0049]    Referring now to FIG. 6, a launch according to the present invention is shown. At the left hand side, the clutch is fully open. Shortly thereafter, the clutch is caused to close partially. By adjusting spark timing throughout the clutch engagement process, it can be seen that the clutch position and engine speed, although not perfectly constant, are much improved over the prior art (FIG. 1). As a consequence of the smooth clutch engagement, the vehicle does not buck. Instead, engine speed smoothly increases as desired by the operator.  
         [0050]    While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.