Abstract:
Systems and methods for improving emissions and drivability of a hybrid vehicle that includes a motor/generator and an engine are presented. The systems and methods may allow vehicle emissions regulations to be met while at the same time providing driveline torque that matches driver demand torque so that vehicle drivability may be maintained or improved.

Description:
FIELD 
     The present description relates to methods and a system for starting an engine of a hybrid vehicle. The methods may be particularly useful for hybrid vehicles that include a driveline integrated starter/generator. 
     BACKGROUND AND SUMMARY 
     Hybrid vehicles may be required to meet emissions regulations for hydrocarbons, carbon monoxide, and oxides of nitrogen. One way to meet emissions regulations is to couple a three-way catalyst to an engine of the hybrid vehicle so that engine emissions are oxidized and reduced to more desirable gases. However, even with a three-way catalyst, a hybrid vehicle may not meet emissions regulations because the three-way catalyst may have to reach a light-off temperature (e.g., a temperature where catalyst efficiency reaches a threshold efficiency) before engine exhaust gases may be processed. One way to shorten an amount of time a catalyst takes to reach light-off temperature is to retard engine spark timing away from minimum spark advance for best torque (MBT). By retarding spark timing, exhaust gases may transfer additional heat to the engine&#39;s exhaust system and its components. Nevertheless, retarding engine spark timing may be insufficient to heat a catalyst to a light off temperature soon enough to meet emissions regulations. Therefore, it would be desirable to provide a way to reach catalyst light off temperature sooner. 
     The inventors herein have recognized the above-mentioned disadvantages and have developed a method, comprising: operating an engine with a substantially constant air mass and spark timing in response to catalyst temperature less than a threshold; varying engine torque as engine speed varies while operating the engine with the substantially constant air mass; and providing driver demand torque via engine torque and motor torque while operating the engine with the substantially constant air mass. 
     By operating an engine with a substantially constant air mass flowing through the engine, substantially constant spark retard, and varying engine torque as engine speed varies, it may be possible to provide the technical result of quickly heating a catalyst while producing a desired driver demand torque. In particular, the engine air mass may be selected to provide a desired rate of thermal energy from the engine to a catalyst so that the catalyst lights off within a desired time even in the presence of varying vehicle speed and driver demand torque. A motor coupled to the engine may augment or lower engine torque to provide a driver demand torque at a torque converter impeller as engine speed changes during vehicle acceleration and deceleration. In this way, flow of air through an engine may be held substantially constant even as engine speed changes so that a catalyst lights off in a repeatable fashion as a vehicle accelerates or decelerates. 
     The present description may provide several advantages. In particular, the approach may improve vehicle emissions. Further, the approach may improve vehicle drivability during engine starting. Additionally, the approach may allow more accurate air-fuel ratio control while engine emissions components are being heated to operating temperature. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, where: 
         FIG. 1  is a schematic diagram of an engine; 
         FIG. 2  shows an example vehicle driveline configuration; 
         FIG. 3  shows an example hybrid vehicle operating sequence; and 
         FIG. 4  shows an example method for operating a hybrid vehicle driveline. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to improving hybrid vehicle emissions after an engine is started. The hybrid vehicle may include an engine as is shown in  FIG. 1 . Further, the engine may be included in a driveline of the hybrid vehicle as is shown in  FIG. 2 . Engine emissions may be reduced via heating a catalyst by operating an engine and driveline integrated starter/generator (DISG) as shown in the sequence of  FIG. 3 . The engine and DISG may be operated according to the method of  FIG. 4  in the system of  FIGS. 1 and 2  to provide the operating sequence shown in  FIG. 3 . 
     Referring to  FIG. 1 , internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in  FIG. 1 , is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Flywheel  97  and ring gear  99  are coupled to crankshaft  40 . Starter  96  (e.g., low voltage (operated with less than 30 volts) electric machine) includes pinion shaft  98  and pinion gear  95 . Pinion shaft  98  may selectively advance pinion gear  95  to engage ring gear  99 . Starter  96  may be directly mounted to the front of the engine or the rear of the engine. In some examples, starter  96  may selectively supply torque to crankshaft  40  via a belt or chain. In one example, starter  96  is in a base state when not engaged to the engine crankshaft. Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Each intake and exhaust valve may be operated by an intake cam  51  and an exhaust cam  53 . The position of intake cam  51  may be determined by intake cam sensor  55 . The position of exhaust cam  53  may be determined by exhaust cam sensor  57 . 
     Fuel injector  66  is shown positioned to inject fuel directly into cylinder  30 , which is known to those skilled in the art as direct injection. Alternatively, fuel may be injected to an intake port, which is known to those skilled in the art as port injection. Fuel injector  66  delivers liquid fuel in proportion to the pulse width from controller  12 . Fuel is delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     In addition, intake manifold  44  is shown communicating with turbocharger compressor  162 . Shaft  161  mechanically couples turbocharger turbine  164  to turbocharger compressor  162 . Optional electronic throttle  62  adjusts a position of throttle plate  64  to control air flow from air intake  42  to compressor  162  and intake manifold  44 . In one example, a high pressure, dual stage, fuel system may be used to generate higher fuel pressures. In some examples, throttle  62  and throttle plate  64  may be positioned between intake valve  52  and intake manifold  44  such that throttle  62  is a port throttle. 
     Distributorless ignition system  88  provides an ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . 
     Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. 
     Controller  12  is shown in  FIG. 1  as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106  (e.g., non-transitory memory), random access memory  108 , keep alive memory  110 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  134  coupled to an accelerator pedal  130  for sensing force applied by foot  132 ; a position sensor  154  coupled to brake pedal  150  for sensing force applied by foot  152 , a measurement of engine manifold pressure (MAP) from pressure sensor  122  coupled to intake manifold  44 ; an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position; a measurement of air mass entering the engine from sensor  120 ; and a measurement of throttle position from sensor  58 . Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
     In some examples, the engine may be coupled to an electric motor/battery system in a hybrid vehicle as shown in  FIG. 2 . Further, in some examples, other engine configurations may be employed, for example a diesel engine. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  54  closes and intake valve  52  opens. Air is introduced into combustion chamber  30  via intake manifold  44 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  52  and exhaust valve  54  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  54  opens to release the combusted air-fuel mixture to exhaust manifold  48  and the piston returns to TDC. Note that the above is shown merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
       FIG. 2  is a block diagram of a vehicle  225  including a driveline  200 . The driveline of  FIG. 2  includes engine  10  shown in  FIG. 1 . Driveline  200  may be powered by engine  10 . Engine  10  may be started with an engine starting system shown in  FIG. 1  or via driveline integrated starter/generator (DISG)  240 . DISG  240  (e.g., high voltage (operated with greater than 30 volts) electrical machine) may also be referred to as an electric machine, motor, and/or generator. Further, torque of engine  10  may be adjusted via torque actuator  204 , such as a fuel injector, throttle, etc. 
     An engine output torque may be transmitted to an input side of driveline disconnect clutch  236  through dual mass flywheel  215 . Disconnect clutch  236  may be electrically or hydraulically actuated. The downstream side of disconnect clutch  236  is shown mechanically coupled to DISG input shaft  237 . 
     DISG  240  may be operated to provide torque to driveline  200  or to convert driveline torque into electrical energy to be stored in electric energy storage device  275 . DISG  240  has a higher output torque capacity than starter  96  shown in  FIG. 1 . Further, DISG  240  directly drives driveline  200  or is directly driven by driveline  200 . There are no belts, gears, or chains to couple DISG  240  to driveline  200 . Rather, DISG  240  rotates at the same rate as driveline  200 . Electrical energy storage device  275  (e.g., high voltage battery or power source) may be a battery, capacitor, or inductor. The downstream side of DISG  240  is mechanically coupled to the impeller  285  of torque converter  206  via shaft  241 . The upstream side of the DISG  240  is mechanically coupled to the disconnect clutch  236 . 
     Torque converter  206  includes a turbine  286  to output torque to input shaft  270 . Input shaft  270  mechanically couples torque converter  206  to automatic transmission  208 . Torque converter  206  also includes a torque converter bypass lock-up clutch  212  (TCC). Torque is directly transferred from impeller  285  to turbine  286  when TCC is locked. TCC is electrically operated by controller  12 . Alternatively, TCC may be hydraulically locked. In one example, the torque converter may be referred to as a component of the transmission. 
     When torque converter lock-up clutch  212  is fully disengaged, torque converter  206  transmits engine torque to automatic transmission  208  via fluid transfer between the torque converter turbine  286  and torque converter impeller  285 , thereby enabling torque multiplication. In contrast, when torque converter lock-up clutch  212  is fully engaged, the engine output torque is directly transferred via the torque converter clutch to an input shaft (not shown) of transmission  208 . Alternatively, the torque converter lock-up clutch  212  may be partially engaged, thereby enabling the amount of torque directly relayed to the transmission to be adjusted. The controller  12  may be configured to adjust the amount of torque transmitted by torque converter  212  by adjusting the torque converter lock-up clutch in response to various engine operating conditions, or based on a driver-based engine operation request. 
     Automatic transmission  208  includes gear clutches (e.g., gears 1-6)  211  and forward clutch  210 . The gear clutches  211  (e.g., 1-10) and the forward clutch  210  may be selectively engaged to propel a vehicle. Torque output from the automatic transmission  208  may in turn be relayed to wheels  216  to propel the vehicle via output shaft  260 . Specifically, automatic transmission  208  may transfer an input driving torque at the input shaft  270  responsive to a vehicle traveling condition before transmitting an output driving torque to the wheels  216 . 
     Further, a frictional force may be applied to wheels  216  by engaging wheel brakes  218 . In one example, wheel brakes  218  may be engaged in response to the driver pressing his foot on a brake pedal (not shown). In other examples, controller  12  or a controller linked to controller  12  may apply engage wheel brakes. In the same way, a frictional force may be reduced to wheels  216  by disengaging wheel brakes  218  in response to the driver releasing his foot from a brake pedal. Further, vehicle brakes may apply a frictional force to wheels  216  via controller  12  as part of an automated engine stopping procedure. 
     Controller  12  may be configured to receive inputs from engine  10 , as shown in more detail in  FIG. 1 , and accordingly control a torque output of the engine and/or operation of the torque converter, transmission, DISG, clutches, and/or brakes. As one example, an engine torque output may be controlled by adjusting a combination of spark timing, fuel pulse width, fuel pulse timing, and/or air charge, by controlling throttle opening and/or valve timing, valve lift and boost for turbo- or super-charged engines. In the case of a diesel engine, controller  12  may control the engine torque output by controlling a combination of fuel pulse width, fuel pulse timing, and air charge. In all cases, engine control may be performed on a cylinder-by-cylinder basis to control the engine torque output. Controller  12  may also control torque output and electrical energy production from DISG by adjusting current flowing to and from field and/or armature windings of DISG as is known in the art. 
     When idle-stop conditions are satisfied, controller  12  may initiate engine shutdown by shutting off fuel and spark to the engine. However, the engine may continue to rotate in some examples. Further, to maintain an amount of torsion in the transmission, the controller  12  may ground rotating elements of transmission  208  to a case  259  of the transmission and thereby to the frame of the vehicle. When engine restart conditions are satisfied, and/or a vehicle operator wants to launch the vehicle, controller  12  may reactivate engine  10  by craning engine  10  and resuming cylinder combustion. 
     Referring now to  FIG. 3 , an example hybrid vehicle operating sequence is shown. The sequence of  FIG. 3  may be provided by the system of  FIGS. 1 and 2  executing the method of  FIG. 4  stored as instructions in non-transitory memory. The vertical lines at T 1 -T 5  represent particular time of interest during the sequence. 
     The first plot from the top of  FIG. 3  is a plot of vehicle speed versus time. The Y axis represents vehicle speed and vehicle speed increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. 
     The second plot from the top of  FIG. 3  is a plot of active transmission gear versus time. The Y axis represents active transmission gear and the active transmission gears are indicated along the Y axis. The X axis represents time and time increases from the left to right side of the figure. 
     The third plot from the top of  FIG. 3  is a plot of engine and DISG speed versus time. The Y axis represents engine and DISG speed and engine and DISG speed increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. The engine and the DISG are coupled together during the sequence via the driveline disconnect clutch. 
     The fourth plot from the top of  FIG. 3  is a plot of driver demand torque versus time. The Y axis represents driver demand torque and driver demand torque increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. 
     The fifth plot from the top of  FIG. 3  is a plot of engine air mass or mass of air flowing through the engine versus time. The Y axis represents engine air mass and engine air mass increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. 
     The sixth plot from the top of  FIG. 3  is a plot of DISG torque versus time. The Y axis represents DISG torque and DISG torque increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. Horizontal line  302  represents a maximum DISG torque at DISG speeds below a DISG speed where the DISG changes from having a constant maximum torque output to having a constant maximum power output. 
     The seventh plot from the top of  FIG. 3  is a plot of engine torque versus time. The Y axis represents engine torque and engine torque increases in the direction of the Y axis arrow. The X axis represents time and time increases from the left to right side of the figure. 
     At time T 0 , a driver inputs a driver demand torque after a cold engine start and vehicle speed begins to increase. The engine air mass or air flowing through the engine is at a predetermined constant level. The DISG torque begins to increase in response to the driver demand torque and the engine torque begins to decrease so that DISG torque plus engine torque meets the driver demand torque at a torque converter impeller that is downstream of the DISG. The engine speed increases since the DISG and engine are coupled and because the combined DISG and engine torque is increasing in response to the driver demand torque. The transmission is in first gear and the vehicle speed begins to increase in response to the driver demand torque. 
     At time T 1 , the transmission shifts into second gear. The transmission shifts in response to the driver demand torque and vehicle speed. The vehicle speed continues to increase and the engine speed and DISG speed decrease in response to shifting into a higher gear. The driver demand torque is slowly being reduced in response to a driver operating the accelerator pedal, and the engine air mass remains constant even though engine speed is reduced. Engine air mass may be held constant when engine speed is reduced by opening the engine&#39;s throttle and/or advancing intake valve timing. Opening the engine throttle and/or advancing intake valve timing increases engine torque. The DISG torque is reduced in response to the increase in engine torque. 
     Between time T 1  and time T 2 , the engine&#39;s throttle is closed (not shown) to maintain constant engine air flow as engine speed and DISG speed increase. Closing the engine&#39;s throttle reduces intake manifold pressure so that engine cylinders produce less torque for each combustion event. Consequently, engine torque decreases in response to engine speed increasing and maintaining constant engine air flow. 
     At time T 2 , the transmission shifts from second gear to third gear in response to vehicle speed and driver demand torque. The engine and DISG speed are reduced in response to the transmission entering third gear. The engine air mass remains constant and the engine torque increases in response to the decrease in engine speed to maintain the constant engine air mass. The engine torque is increased via opening the engine&#39;s throttle or advancing intake valve opening timing. The DISG torque is reduced in response to increasing engine torque. The engine torque plus the DISG torque provides the desired driver demand torque at the vehicle&#39;s torque converter impeller. 
     At time T 3 , the vehicle speed has reached a higher level and the driver reduces the driver demand torque via partially releasing the accelerator pedal. The engine torque increases to maintain engine air flow and DISG torque is decreased in response to the decreased driver demand torque and the increased engine torque. The engine speed and DISG speed are reduced in response to the reduced driver demand torque. The transmission remains in third gear and the vehicle speed begins to decrease. 
     Between time T 3  and time T 4 , the driver demand torque remains low and the engine speed and DISG speed decrease in response to the low driver demand torque. The engine torque increases a slight amount to maintain the engine air amount and the DISG torque decreases in response to the increase in engine torque. The vehicle speed continued to slow. 
     At time T 4 , the driver increases the driver demand torque via applying the accelerator pedal. The transmission remains in third gear and the engine and DISG speed begin to increase in response to the combined DISG torque and engine torque providing the driver demand torque. The engine torque decreases as the engine speed increases to maintain the constant engine air flow. The DISG torque increases with the increasing driver demand torque and decreasing engine torque. 
     At time T 5 , the DISG torque reaches torque limit  302 . Torque limit  302  may be a maximum engine torque at the present DISG speed. The maximum DISG torque is a function of DISG speed. DISG torque is maintained at the maximum DISG torque and engine torque is increased so that the DISG torque plus engine torque provides the driver demand torque at the vehicle&#39;s torque converter impeller. The engine air flow is increased to increase engine torque after the DISG is at its maximum torque. Thus, if the DISG provides its maximum torque and additional torque is needed to meet driver demand torque, the engine air mass may be increased to meet the driver demand torque. In this way, the engine air flow may be held at a constant flow until driver demand torque exceeds maximum DISG torque plus engine torque when the engine is operated with the predetermined constant air mass. 
     Referring now to  FIG. 4 , a method for operating a hybrid vehicle driveline is shown. The method of  FIG. 4  may be included in the system of  FIGS. 1 and 2  as executable instructions stored in non-transitory memory. Additionally, the method of  FIG. 4  may provide the operating sequence shown in  FIG. 3 . 
     At  402 , method  400  judges if the engine is being cold started. Alternatively, or in addition, method  400  may judge if the engine is operating within predetermined conditions after a cold start, or if the engine is operating within predetermined conditions after a warm engine start. The predetermined conditions after cold and/or warm start may be that a catalyst temperature is less than a first threshold temperature and/or that engine temperature is less than a second threshold temperature. The engine may be considered to be cold started when engine and/or exhaust component temperature is less than a threshold temperature (e.g., 20° C.) and before the engine has been operating for a predetermined amount of time or before the engine has reached a threshold temperature. If method  400  judges that the engine is being cold started or if the engine is operating within predetermined conditions after a start, the answer is yes and method  400  proceeds to  404 . Otherwise, the answer is no and method  400  proceeds to  450 . 
     At  450 , method  400  adjusts the engine air mass in response to the driver demand torque and spark is adjusted to knock limited or MBT spark timing. For example, if driver demand torque increases, the engine air amount increases. If driver demand torque decreases, the engine air amount decreases. Additionally, the engine air-fuel ratio averages a near stoichiometric air-fuel ratio. Method  400  proceeds to exit after engine air-fuel ratio is adjusted. 
     At  404 , method  400  determines engine speed. In one example, engine speed is determined via measuring time between engine positions via an engine position sensor. Further, method  400  determines driver demand torque at  404 . In one example, driver demand torque may be based on accelerator pedal position and vehicle speed. Specifically, vehicle speed and accelerator pedal position are used to index a table containing empirically determined driver demand torques. The table outputs the driver demand torque based on the accelerator pedal position and vehicle speed. Method  400  proceeds to  406  after engine speed is determined. 
     At  406 , method  400  determines desired engine air mass or the desired amount of air to flow through the engine. In one example, the desire engine air mass is empirically determined and stored in a table or function that is indexed based on engine temperature and/or catalyst temperature. Additionally, the table or function may be indexed via time since engine stop. The table may contain desired engine air mass amounts that allow a catalyst in the engine exhaust system to reach a desired temperature within a threshold amount of time. The desired engine air mass may be a substantially constant value (e.g., varying less than 10%) from the time since engine speed reaches a threshold speed after engine stop until a catalyst reaches a desired temperature or until driver demand exceeds a threshold torque, including all time between. Further, in some examples, the substantially constant air mass may be based on engine temperature or catalyst temperature during engine starting. For example, the engine air mass may be a greater value for lower catalyst and engine temperatures, though the engine air mass remains constant from a time the engine reaches a threshold speed after engine stop until predetermined conditions are achieved (e.g., the catalyst or engine reach a threshold temperature). For example, if engine temperature is 20° C. during a first start, the engine air flow may be X Kg/sec. However, if engine temperature is 15° C. during a second start, the engine air flow may be Y Kg/sec, where Y is greater than X. The respective X and Y air masses may flow through the engine from the time since engine speed reaches a threshold speed after engine stop until a catalyst reaches a desired temperature or until driver demand exceeds a threshold torque. The desired engine air mass is output from the table and method  400  proceeds to  408 . 
     At  408 , method  400  determines a desired spark retard from minimum spark advance timing for best engine torque (MBT). In one example, the spark retard from MBT is empirically determined and stored in a table or function that may be indexed based on time since engine stop and/or engine or catalyst temperature. The table or function outputs a spark retard and method  400  proceeds to  410 . In one example, the spark retard from MBT spark timing may be substantially constant (e.g., changing by less than 5 crankshaft angle degrees) from the time since engine stop until a catalyst reaches a desired temperature or until driver demand exceeds a threshold torque. 
     At  410 , method  400  determines desired engine torque to provide the desire engine air mass determined at  406 . In one example, the desired engine air flow determined at  406  is multiplied by a fuel to air ratio to determine a fuel flow rate. The fuel flow rate may be used to index a table or function that outputs engine torque based on fuel flow rate and engine speed. The table or function outputs empirically determined engine torque values corresponding to the engine torque produced at the present engine speed when engine fuel flow is based on the desired air flow and fuel to air ratio. Method  400  proceeds to  412  after the desired engine torque is determined. 
     At  412 , method  400  determines the desired DISG or motor torque. In one example, the desired motor torque is determined via the following equation:
 
 T   MOT   =T   DD   −T   DES   _   ENG  
 
where T MOT  is the desired motor torque, T DD  is the driver demand torque, and where T DES   _   ENG  is the desired engine torque determined at  410 . Method  400  proceeds to  414  after desired motor torque is determined.
 
     At  414 , method  400  judges if motor torque (e.g., T MOT ) is less than maximum motor torque (e.g., T MOT   _   MAX ). If so, the answer is yes and method  400  proceeds to  416 . Otherwise, the answer is no and method  400  proceeds to  418 . 
     At  416 , method  400  determines the motor and engine torque commands. In particular, the motor torque command is T MOT   _   CMD =T MOT , or the motor torque command is the motor torque determined at  412 . The engine torque command is T ENG   _   CMD =T DES   _   ENG , or the engine torque command is the desired engine torque determined at  410 . Method  400  proceeds to exit after the engine and motor commands are determined. 
     Additionally at  416 , method  400  provides for operating the engine with a substantially constant air mass (e.g., air mass that changes by less than 10%) as a transmission shifts gears. Further, method  400  may upshift a transmission from a lower gear to a higher gear in response to speed of the motor being within a threshold speed of a speed where the motor transitions from providing a constant maximum torque to providing a constant maximum power. By upshifting the transmission, the maximum DISG torque may be held at a higher value than if the DISG speed were to continue increasing. Consequently, the engine may be held with a constant air mass flowing through engine even as the vehicle speed increases. Thus, method  400  may limit DISG speed to a speed less than a speed where the DISG transitions from providing a constant maximum torque to providing a constant maximum power to provide a greater maximum DISG torque. 
     During conditions where engine torque is greater than driver demand torque, the DISG may be transitioned from a motor mode (e.g., providing positive torque to the driveline) to a generator mode (e.g., providing negative torque to the driveline) while the engine operates at the substantially constant air mass. 
     At  418 , method  400  determines the motor and engine torque commands. In particular, the motor torque command is T MOT   _   CMD =T MOT   _   MAX , or the motor torque command is the maximum motor torque at the present motor speed. The engine torque command is T ENG   _   CMD =T DD −T MOT   _   MAX , or the engine torque command is the driver demand torque determined at  404  minus the maximum motor torque at the present motor speed. The engine torque is adjusted via adjusting throttle position, intake valve closing timing, and/or fuel injection. Motor torque is adjusted by adjusting an amount of current supplied to the motor. Further, if the motor torque command is negative, the motor is operated as a generator to absorb engine torque. Thus, at  418  the engine torque command increases with driver demand torque such that the engine air flow increases from the substantially constant air amount in response to driver demand torque being greater than maximum engine torque while the engine operates with the substantially constant air amount and maximum DISG torque at a present DISG speed. Method  400  proceeds to exit after the engine and motor commands are determined. 
     The engine torque may be adjusted via adjusting the amount of fuel injected and the engine throttle position or intake valve closing timing. In one example, as desired engine torque is adjusted to provide the desired engine air mass as engine speed changes, the throttle or intake valve timing may be adjusted to provide a desired intake manifold pressure that corresponds to the desired engine air-flow rate at the present engine speed. In particular, engine intake manifold pressure may be adjusted to provide the desired engine air mass via adjusting the engine throttle or intake valve closing time based on the following speed/density equation: 
             P   =       R   ·   T   ·   Me   ·   2         η   v     ·     N   e               
where Me is the desired engine air flow, R is a gas constant, T is air temperature, N e  is engine speed, P is manifold pressure, and η v  is engine volumetric efficiency. The intake manifold pressure may be closed loop control based on intake manifold pressure. For example, if intake manifold pressure is greater than desired based on intake manifold pressure feedback from a pressure sensor, the throttle may be closed further.
 
     Thus, the method of  FIG. 4  provides for a method, comprising: operating an engine with a substantially constant air mass and spark timing in response to catalyst temperature less than a threshold; varying engine torque as engine speed varies while operating the engine at the substantially constant air mass; and providing driver demand torque via engine torque and motor torque while operating the engine at the substantially constant air mass. The method includes where the spark timing is retarded from minimum spark timing for best engine torque. The method includes where engine torque is adjusted via adjusting a position of a throttle. 
     In some examples, the method includes where engine torque is further adjusted via adjusting an amount of fuel injected to the engine. The method also includes where engine torque is adjusted via adjusting a position of an intake cam or timing of an intake valve. The method includes where the engine is operated with the substantially constant air mass as a transmission shifts gears. The method also includes where the substantially constant air mass is varied in response to engine or catalyst temperature during engine starting. The method further comprises upshifting a transmission gear in response to speed of the motor being within a threshold speed of a speed where the motor transitions from providing a constant maximum torque to providing a constant maximum power. 
     The method of  FIG. 4  also provides for: varying engine torque as engine speed varies while operating an engine at a substantially constant air mass in response to a temperature being less than a threshold and driver demand torque being less than a maximum engine torque plus a maximum driveline integrated starter/generator (DISG) torque, where the maximum engine torque is produced while the engine operates at the substantially constant air mass, and where the maximum (DISG) torque is at a present DISG speed; and providing driver demand torque via engine torque and DISG torque while operating the engine at the substantially constant air mass. In some examples, the method includes where the engine is operated with a substantially constant spark timing when the engine is operated with the substantially constant air mass. The method also includes where the temperature is a catalyst temperature or an engine temperature. The method further comprises increasing engine air amount from the substantially constant air amount in response to driver demand torque being greater than maximum engine torque while the engine operates with the substantially constant air amount and maximum DISG torque at a present DISG speed. The method further comprises upshifting a transmission gear in response to speed of the DISG being within a threshold speed of a speed where the DISG transitions from a constant maximum torque to a constant maximum power. The method includes where the substantially constant air mass is adjusted in response to a temperature at engine start. The method further comprises limiting DISG speed to a speed less than a speed where the DISG transitions from providing a constant maximum torque to providing a constant maximum power. 
     In some examples, the method of  FIG. 4  provides for a method, comprising: varying engine torque as engine speed varies while operating the engine at a substantially constant air mass; transitioning a driveline integrated starter/generator (DISG) from a motor mode to a generator mode in response engine torque exceeding driver demand torque while the engine operates at the substantially constant air mass; and providing driver demand torque via engine torque and motor torque while operating the engine at the substantially constant air mass. The method further comprises increasing engine air amount from the substantially constant air amount in response to driver demand torque being greater than maximum engine torque while the engine operates with the substantially constant air amount and maximum DISG torque at a present DISG speed. The method further comprises upshifting a transmission gear in response to speed of the DISG being within a threshold speed of a speed where the DISG transitions from a constant maximum torque to a constant maximum power. The method further comprises limiting DISG speed to a speed less than a speed where the DISG transitions from providing a constant maximum torque to providing a constant maximum power. The method includes where the engine is operated with a substantially constant spark timing when the engine is operated with the substantially constant air mass. 
     As will be appreciated by one of ordinary skill in the art, the methods described in  FIG. 4  may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the objects, features, and advantages described herein, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations, methods, and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, 13, 14, 15, V6, V8, V10, and V12 engines operating in natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage.