Patent Publication Number: US-6209526-B1

Title: Direct injection engine system

Description:
FIELD OF THE INVENTION 
     This invention relates to mode transition control of a direct injection spark ignition engine. 
     BACKGROUND OF THE INVENTION 
     In direct injection spark ignition engines, the engine operates with stratified air/fuel operation in which the combustion chamber contains stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. 
     The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during the intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry. 
     Direct injection engines are also coupled to conventional three-way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx. 
     The stratified mode of operation is typically utilized when the engine is operating in light to medium loads. The homogeneous mode of operation is typically used from medium to heavy load operating conditions. In certain conditions, it is necessary to transition from one engine mode of operation to the other. During these mode transitions, it is desired to deliver the requested engine output torque to provide good drive feel. However, in some circumstances, the range of acceptable lean air/fuel ratios of stratified operation do not overlap with the acceptable air/fuel ratios of homogeneous operation. Therefore, during the mode transition, a torque shock occurs because of the step change in engine air/fuel ratio. 
     One method of preventing the engine torque disturbance during mode transition is to change the injection mode one cylinder at a time according to the required amount of fuel to be injected. This reduces a large torque disturbance to several smaller torque disturbances. Such a method is described in U.S. Pat. No. 5,170,759. 
     The inventors herein have recognized a disadvantage with the above approach. Even though the large torque jump during mode transition is avoided, there are still several smaller torque jumps experienced. In other words, a single, large torque disturbance is substituted with multiple smaller torque disturbances which are still noticeable by the vehicle driver. 
     SUMMARY OF THE INVENTION 
     An object of the invention herein is to control an engine during mode transitions to provide a smooth torque output. 
     The above object is achieved and disadvantages of prior approaches overcome by a method for controlling an engine during a cylinder air/fuel ratio change from a first cylinder air/fuel ratio to a second cylinder air/fuel ratio. The method comprises the steps of changing the cylinder air/fuel ratio, and in response to said step of changing said cylinder air/fuel ratio, changing a number of cylinders carrying out combustion in the engine so that abrupt changes in an engine torque of the engine will be avoided during the cylinder air/fuel ratio change. 
     By using cylinder deactivation as an additional degree of freedom, abrupt changes in average engine torque can be avoided even during air/fuel ratio changes. Thus, during a mode transition where engine air/fuel ratio is constrained to change due to, for example, combustion limits, abrupt changes in average engine torque can be avoided by changing a number of active cylinders. 
     An advantage of the above aspect of the invention is that abrupt changes in engine output torque can be avoided during mode transitions. 
     Another advantage of the above aspect of the invention is that the range of stratified operation can be extended since the range of available transitions is increased. 
     Yet another advantage of the above aspect of the invention is that emissions can be reduced since the engine can be operated farther from air/fuel ratio combustion limits. 
     In another aspect of the invention, the above object is achieved by a control method for a spark ignited engine having multiple combustion chambers capable of operating in a stratified mode where fuel is injected during a compression stroke of the engine, wherein the engine is further capable of operating in a homogeneous mode of operation where fuel is injected during an intake stroke of the engine. The method comprises transitioning from said stratified mode to said homogeneous mode based on an operating condition; in response to said transition, disabling a number of cylinders by discontinuing fuel injection into disabled cylinders; and in response to said transition, changing an injection timing from compression stroke injection to intake stroke injection and changing a fuel injection amount to a remaining set of activated cylinders. 
     In yet another aspect of the invention, the above object is achieved by a control method for a spark ignited engine having multiple combustion chambers capable of operating in a stratified mode where fuel is injected during a compression stroke of the engine, wherein the engine is further capable of operating in a homogeneous mode of operation where fuel is injected during an intake stroke of the engine. The method comprises transitioning from said homogeneous mode to said stratified mode based on an operating condition; in response to said transition, enabling a number of cylinders by starting fuel injection into disabled cylinders; and in response to said transition, changing an injection timing from intake stroke injection to compression stroke injection and changing a fuel injection amount to an already enabled set of activated cylinders. 
     Therefore, when transitioning, for example, from stratified to homogenous mode, where cylinder air/fuel ratio discontinuities are unavoidable, abrupt changes in average engine torque can be avoided by compensating for increased torque in individual cylinders by deactivating fuel injection into a predetermined number of cylinders. Similarly, when transitioning from homogenous to stratified mode, abrupt changes in average engine torque can be avoided by compensating for decreased power in individual cylinders by reactivating fuel injection into a predetermined number of cylinders. 
     An advantage of the above aspect of the invention is that abrupt changes in engine output torque can be avoided during mode transitions. 
     Another advantage of the above aspect of the invention is that the range of stratified operation can be extended since the range of available transitions is increased. 
     Yet another advantage of the above aspect of the invention is that emissions can be reduced since the engine can be operated farther from air/fuel ratio combustion limits. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the invention claimed herein will be more readily understood by reading an example of an embodiment in which the invention is used to advantage with reference to the following drawings wherein: 
     FIG. 1 is a block diagram of an embodiment in which the invention is used to advantage; and 
     FIGS. 2-3 are high level flowcharts which perform a portion of operation of the embodiment shown in FIG.  1 . 
    
    
     DESCRIPTION OF AN EXAMPLE OF OPERATION 
     Direct injection spark ignited internal combustion engine  10 , comprising a plurality of combustion chambers, is controlled by electronic engine controller  12 . Combustion chamber  30  of engine  10  is shown in FIG. 1 including combustion chamber walls  32  with piston  36  positioned therein and connected to crankshaft  40 . In this particular example piston  36  includes a recess or bowl (not shown) to help in forming stratified charges of air and fuel. Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valves  52   a  and  52   b  (not shown), and exhaust valves  54   a  and  54   b  (not shown). Fuel injector  66  is shown directly coupled to combustion chamber  30  for delivering liquid fuel directly therein in proportion to the pulse width of signal fpw received from controller  12  via conventional electronic driver  68 . Fuel is delivered to fuel injector  66  by a conventional high pressure fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail. 
     Intake manifold  44  is shown communicating with throttle body  58  via throttle plate  62 . In this particular example, throttle plate  62  is coupled to electric motor  94  so that the position of throttle plate  62  is controlled by controller  12  via electric motor  94 . This configuration is commonly referred to as electronic throttle control (ETC) which is also utilized during idle speed control. In an alternative embodiment (not shown), which is well known to those skilled in the art, a bypass air passageway is arranged in parallel with throttle late  62  to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway. 
     Exhaust gas oxygen sensor  76  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . In this particular example, sensor  76  provides signal EGO to controller  12  which converts signal EGO into two-state signal EGOS. A high voltage state of signal EGOS indicates exhaust gases are rich of stoichiometry and a low voltage state of signal EGOS indicates exhaust gases are lean of stoichiometry. Signal EGOS is used to advantage during feedback air/fuel control in a conventional manner to maintain average air/fuel at stoichiometry during the stoichiometric homogeneous mode of operation. 
     Conventional distributorless ignition system  88  provides ignition spark to combustion chamber  30  via spark plug  92  in response to spark advance signal SA from controller  12 . 
     Controller  12  causes combustion chamber  30  to operate in either a homogeneous air/fuel mode or a stratified air/fuel mode by controlling injection timing. In the stratified mode, controller  12  activates fuel injector  66  during the engine compression stroke so that fuel is sprayed directly into the bowl of piston  36 . Stratified air/fuel layers are thereby formed. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. During the homogeneous mode, controller  12  activates fuel injector  66  during the intake stroke so that a substantially homogeneous air/fuel mixture is formed when ignition power is supplied to spark plug  92  by ignition system  88 . Controller  12  controls the amount of fuel delivered by fuel injector  66  so that the homogeneous air/fuel mixture in chamber  30  can be selected to be at stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. The stratified air/fuel mixture will always be at a value lean of stoichiometry, the exact air/fuel being a function of the amount of fuel delivered to combustion chamber  30 . 
     Nitrogen oxide (NOx) absorbent or trap  72  is shown positioned downstream of catalytic converter  70 . NOx trap  72  absorbs NOx when engine  10  is operating lean of stoichiometry. The absorbed NOx is subsequently reacted with HC and catalyzed during a NOx purge cycle when controller  12  causes engine  10  to operate in either a rich homogeneous mode or a stoichiometric homogeneous mode. 
     Controller  12  is shown in FIG. 1 as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as read only memory chip  106  in this particular example, 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: measurement of inducted mass air flow (MAF) from mass air flow sensor  100  coupled to throttle body  58 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  40 ; and throttle position TP from throttle position sensor  120 ; and absolute Manifold Pressure Signal MAP from sensor  122 . Engine speed signal RPM is generated by controller  12  from signal PIP in a conventional manner and manifold pressure signal MAP provides an indication of engine load. 
     In this particular example, temperature Tcat of catalytic converter  70  and temperature Ttrap of NOx trap  72  are inferred from engine operation as disclosed in U.S. Pat. No. 5,414,994 the specification of which is incorporated herein by reference. In an alternate embodiment, temperature Tcat is provided by temperature sensor  124  and temperature Ttrap is provided by temperature sensor  126 . 
     Referring now to FIG. 2 a routine for performing mode transitions from stratified to homogeneous, and from homogeneous to stratified is described. First, a determination is made in step  210  if a mode transition is required. A mode transition is required based on engine operating conditions. For example, as the required engine torque is gradually decreased from a large value to a small value, a transition from homogeneous mode of operation to stratified mode may be required. Conversely, if the required engine torque is gradually changed from a low value to a high value, transition from stratified mode to homogeneous mode may be required. Also, fuel economy requirements or emission device conditions may dictate mode transitions. For example, when trap  72  is reaches a certain capacity of stored NOx, a transition may be required from stratified mode to homogeneous mode so that a rich or stoichiometric air/fuel ratio can be combusted, thereby allowing purging of trap  72 . Further, this transition is required to be smooth so that driver comfort is not affected. 
     Continuing with FIG. 2, if the answer to step  210  is yes, a determination is made in step  212  if the engine is currently operating in the stratified mode. If the answer to step  212  is yes, a new determination is made in step  214  if cylinder deactivation is required. The determination in step  214  is made using engine mapping data described by the following equation: 
     
       
         min spark   T   i (spark, a/f   max   homogenous )&gt;max spark   T   i (spark, a/f   min   stratified )  
       
     
     where the equation determines if the minimum indicated engine torque (T i ) over available ignition timings (spark) for homogenous operation at the maximum lean homogenous air/fuel ratio (a/f max   homogenous ) is greater than the maximum indicated engine torque over available ignition timings for stratified operation at the minimum lean stratified air/fuel ratio (a/f max   homogenous ) at the current operating conditions define by, for example, engine speed (RPM), fresh air flow, exhaust gas recirculation amount, and any other variables known to those skilled in the art to affect engine indicated torque. In other words, if this condition is true, then a torque disturbance will occur when transitioning from stratified to homogenous mode if only ignition timing is used to compensate for the mode change. Also, these torque calculations are scaled for the current number of active cylinders (Ncyl). Thus, one of ordinary skill in the art will recognize the general applicability of the present invention to any number of cylinders including when some cylinders may be deactivated. 
     When this condition holds, cylinder deactivation is used. If cylinder deactivation is not used, a mode transition can be carried out using other control variables such as spark timing in step  216 . Otherwise, in step  218 , the number of cylinders to deactivate is determined. The number of cylinders to deactivate is based on the size of the torque gap and several other factors, as described later herein with particular reference to FIG.  3 . Next, in step  220 , the determined number of cylinders are deactivated while at the same time the operating mode of the remaining cylinders is changed. In addition, during the mode change, the air/fuel ratio is also abruptly changed. However, according to the present invention, the average output torque of the engine avoids abrupt changes. 
     Continuing with FIG. 2, if the answer to step  212  is no, then the engine is currently operating in the homogeneous mode and a transition from the homogeneous mode to the stratified mode is required. In step  230 , a determination is made if cylinder reactivation is required based on the following equation. 
     
       
         min spark   T   i (spark, a/f   max   homogenous )&lt;max spark   T   i (spark, a/f   min   stratified )  
       
     
     If the answer to step  230  is no, the routine continues to step  216  previously described herein. Otherwise, in step  232 , the number of cylinders to enable is determined as described later herein with particular reference to FIG.  3 . Then, in step  234 , the determined cylinders are enabled and the engine operating mode is simultaneously changed from the homogeneous mode to the stratified mode. In addition, during this transition, the air/fuel ratio in the cylinders is jumped. 
     Referring now to FIG. 3, a routine for determining the number of cylinders to enable/disable is described. In step  312  a determination is made if the engine is currently operating in the stratified mode. If the answer to step  312  is yes, the number of cylinders to deactivate (M) is determined in step  314  from the following equations, where:          Δ                 T     =         min   spark            T   i          (     spark   ,     a   /     f   max   homogeneous         )         -     T   requested               M   =     roundup              [         Δ                 T         min   spark            T   i          (     spark   ,     a   /     f   max   homogeneous         )                N   cyl       ]                     
     Again, these torque calculations are scaled for the current number of active cylinders (Ncyl). Thus, one of ordinary skill in the art will recognize the general applicability of the present invention to any number of cylinders including when some cylinders may already be deactivated. 
     If the answer to step  312  is no, the number of cylinders to activate (P) is determined in step  316  from the following equation, where:          Δ                   T   ′       =       T   requested     -       max   spark            T   i          (     spark   ,     a   /     f   min   stratified         )                   P   =     roundup              [         Δ                   T   ′           max   spark            T   i          (     spark   ,     a   /     f   min   stratified         )                N   cyl       ]                     
     Thus, according to the present invention, during the mode transition, where the air/fuel ratio is discontinuously jumped due to engine performance constraints, abrupt changes in engine torque are avoided by either activating, or deactivating, cylinders concurrently with the air/fuel ratio jump. 
     Also, the “round up” function is used since the number of cylinders to activate or deactivate to compensate the engine torque during cylinder air/fuel ratio changes may not be exactly equal to an integer value. Therefore, the remaining torque difference can be compensated using methods known to those skilled in the art, such as ignition timing. 
     In an alternative embodiment, the torque difference can be calculated according to the formula:                Δ                 T     =                  Δ                   T   ′       =              min   spark            T   i          (     spark   ,     a   /     f   max   homogeneous         )         -                     =                         min   spark            T   i          (     spark   ,     a   /     f   max   homogeneous         )         -                                    max   spark            T   i          (     spark   ,     a   /     f   min   stratified         )              .                         
     In another an alternative embodiment, where cylinder air/fuel ratio is abruptly changed for reasons other than mode transitions, the method described above with particular reference to FIGS. 2-3 can be applied. In particular, increases or decreases in engine torque can be used for selecting either enabling, or disabling cylinders. For example, when an increase in engine torque will be produced (current engine torque is less than the future engine torque, equivalent to switching from stratified to homogenous mode), cylinder deactivation can be used. Similarly, when a decrease in engine torque will be produced (current engine torque is greater than the future engine torque, equivalent to switching from homogenous to stratified mode), cylinder activation can be used. The corresponding current and future engine torques and air/fuel ratios can be substituted into the above equations. 
     While the invention has been shown and described in its preferred embodiments, it will be clear to those skilled in the arts to which it pertains that many changes and modifications may be made thereto without departing from the scope of the invention. Accordingly, it is intended that the scope of the invention be limited only by the following claims.