Patent Publication Number: US-6671613-B2

Title: Cylinder flow calculation system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/769,800 entitled “Method and system for engine air-charge estimation”, filed on Jan. 25, 2001, the entire subject matter thereof is being incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a system and a method for controlling an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     In order to efficiently operate an internal combustion engine, it is important to achieve good control of the air-fuel ratio. This can be accomplished by determining the cylinder flow and adjusting the amount of fuel to be injected accordingly to achieve a desired air-fuel ratio. Therefore, it is important to obtain an accurate estimate of the cylinder flow. One method is described in a pending U.S. application Ser. No. 09/769,800 owned by the assignee of the present invention and incorporated herein by reference, which teaches an estimation algorithm for determining engine cylinder flow using both an airflow sensor (MAF) and an intake manifold pressure (MAP) sensor. This MAP-MAF estimation algorithm uses the information on the time rate of change of the intake manifold pressure signal to correctly estimate cylinder flow during transients, and precisely matches the MAF sensor measurement at steady state. 
     However, under some circumstances the MAF sensor reading may become less accurate, thus negatively affecting the overall accuracy of the cylinder flow estimate. For example, in systems where a hot wire-type MAF sensor is used, the sensor does not reach operating temperature immediately upon start-up of the engine. Therefore, it is possible for the MAF sensor reading to not be accurate for the first 30-60 seconds of engine operation. Additionally, at high throttle angles, pulsation and backflow may affect the accuracy of the MAF sensor reading. Therefore, under the circumstances where MAF sensor reading accuracy is reduced, other methods of estimating cylinder flow that are not dependent on the MAF sensor reading are required. One such system is described in U.S. Pat. No. 4,644,474 owned by the assignee of the present invention, wherein engine operating conditions are monitored to determine when to switch between the MAF sensor reading and the estimate of the airflow based on the speed-density equation. 
     While this system provides satisfactory results, the inventors herein have recognized that an improved performance can be achieved. Specifically, since there is always some difference between an estimated and an actual reading, or between two different types of estimates, switching between them may cause abrupt fluctuations in the air-fuel ratio and engine torque, thus degrading vehicle drivability, fuel economy, and emission control. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a method for accurately estimating cylinder flow under all operating conditions while eliminating any fluctuations that may result due to switching between different types of estimates. 
     In accordance with the present invention, a method and system for estimating cylinder flow in an internal combustion engine include: calculating a first cylinder flow estimate based on a first algorithm; providing an indication of an operating condition; in response to said indication, calculating a second cylinder flow estimate based on a second algorithm; and adjusting said second cylinder flow estimate based on said first cylinder flow estimate for a predetermined period of time thereby providing a smooth transition between said first estimate and said second estimate. 
     An advantage of the present invention is that a more accurate method of estimating cylinder flow is achieved during all operating conditions, therefore resulting in improved air-fuel ratio control, and thus improved fuel economy, emission control and vehicle drivability. 
     Another advantage of the present invention is that it results in a smooth transition between the two types of estimates, and therefore eliminates abrupt torque fluctuations and improves driver satisfaction. 
     The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and 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 Description of Preferred Embodiment, with reference to the drawings, wherein: 
     FIG. 1 is a block diagram of an internal combustion engine illustrating various components related to the present invention. 
     FIG. 2 is a block diagram of an example of an embodiment in which the invention is used to advantage. 
     FIG. 3 is a graphic description of an example of a transition between the two types of flow estimates according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     As will be appreciated by those of ordinary skill in the art, the present invention is independent of the particular underlying engine technology and configuration. As such, the present invention may be used in a variety of types of internal combustion engines, such as conventional engines in addition to direct injection stratified charge (DISC) or direct injection spark ignition engines (DISI). 
     A block diagram illustrating an engine control system and method for a representative internal combustion engine according to the present invention is shown in FIG.  1 . Preferably, such an engine includes a plurality of combustion chambers only one of which is shown, and is controlled by electronic engine controller  12 . Combustion chamber  30  of engine  10  includes combustion chamber walls  32  with piston  36  positioned therein and connected to crankshaft  40 . In addition, the 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). A 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 the 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, the throttle plate  62  is coupled to electric motor  94  such that the position of the 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 plate  62  to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway. 
     Exhaust gas sensor  76  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . In this particular example, sensor  76  is a universal exhaust gas oxygen (UEGO) sensor, also known as a proportional oxygen sensor. The UEGO sensor generates a signal whose magnitude is proportional to the oxygen level (and the air-fuel ratio) in the exhaust gases. This signal is provided to controller  12 , which converts it into a relative air-fuel ratio. 
     Advantageously, signal UEGO is used during feedback air-fuel ratio control in to maintain average air-fuel ratio at a desired air-fuel ratio as described later herein. In an alternative embodiment, sensor  76  can provide signal EGO, exhaust gas oxygen (not shown), which indicates whether exhaust air-fuel ratio is lean or rich of stoichiometry. In another alternate embodiment, the sensor  76  may comprise one of a carbon monoxide (CO) sensor, a hydrocarbon (HC) sensor, and a NOx sensor that generates a signal whose magnitude is related to the level of CO, HC, NOx, respectively, in the exhaust gases. 
     Those skilled in the art will recognize that any of the above exhaust gas sensors may be viewed as an air-fuel ratio sensor that generates a signal whose magnitude is indicative of the air-fuel ratio measured in exhaust gases. 
     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 ratio mode or a stratified air-fuel ratio 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 stratum closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. 
     In 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 ratio mixture in chamber  30  can be selected to be substantially at (or near) stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. Operation substantially at (or near) stoichiometry refers to conventional closed loop oscillatory control about stoichiometry. The stratified air-fuel ratio mixture will always be at a value lean of stoichiometry, the exact air-fuel ratio being a function of the amount of fuel delivered to combustion chamber  30 . An additional split mode of operation wherein additional fuel is injected during the exhaust stroke while operating in the stratified mode is available. An additional split mode of operation wherein additional fuel is injected during the intake stroke while operating in the stratified mode is also available, where a combined homogeneous and split mode is available. 
     Lean NOx trap  72  is shown positioned downstream of catalytic converter  70 . Both devices store exhaust gas components, such as NOx, when engine  10  is operating lean of stoichiometry. These are subsequently reacted with HC, CO and other reductant and are catalyzed during a purge cycle when controller  12  causes engine  10  to operate in either a rich mode or a near stoichiometric mode. 
     Controller  12  is shown in FIG. 1 as a conventional microcomputer including but not limited to: 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  giving an indication of engine speed (RPM); 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. 
     Fuel system  130  is coupled to intake manifold  44  via tube  132 . Fuel vapors (not shown) generated in fuel system  130  pass through tube  132  and are controlled via purge valve  134 . Purge valve  134  receives control signal PRG from controller  12 . 
     Exhaust sensor  140  is a NOx/UEGO sensor located downstream of the LNT. It produces two output signals. First output signal (SIGNAL 1 ) and second output signal (SIGNAL 2 ) are both received by controller  12 . Exhaust sensor  140  can be a sensor known to those skilled in the art that is capable of indicating both exhaust air-fuel ratio and nitrogen oxide concentration. 
     The diagram in FIG. 2 generally represents operation of one embodiment of a system or method according to the present invention. As will be appreciated by one of ordinary skill in the art, the diagram 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 of the invention, 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. 
     Referring now to FIG. 2, a routine is described for selecting between a MAF-independent and a MAF-dependent cylinder flow estimate based on operating conditions, and for facilitating a smooth transition between the two types of estimates via the “switchover coordinator”. 
     First in step  100 , a determination is made whether the mass airflow (MAF) sensor is operational. For example, MAF sensor may not be operational and therefore not provide accurate readings when its temperature is below a predetermined temperature, such as at engine startup. Under these circumstances, time since engine start can be monitored and compared to a predetermined constant to make the decision in step  100 . Alternatively, throttle position sensor signal may be monitored in step  100  to determine whether the MAF sensor is operational or not, since MAF sensor accuracy decreases at high throttle angles due to air pulsation and backflow. 
     If the answer to step  100  is NO, indicating that the MAF sensor is not operational, the routine proceeds to step  800  wherein a “transition completed” flag is set to 0. The routine then proceeds to step  900  wherein cylinder flow is estimated without relying on the information supplied by the MAF sensor. For example, cylinder flow can be estimated using the speed-density equation:          W   cyl     =       η   vk            n   e     2          V   d          P   RT                       
     where η vk  is a volumetric efficiency estimated from a nominal map as a function of engine speed and valve timing, V d  is the engine displacement volume (a predetermined constant), P is the intake manifold pressure measured by the MAP sensor, T is the intake manifold temperature either measured by a senor or estimated, R is a gas constant (difference of specific heats), n e  is the engine speed in revolutions per second. The routine then returns to step  100 . 
     If the answer to step  100  is YES, the routine proceeds to step  200  wherein a determination is made whether the transition between the two types of cylinder flow estimates is completed. If the answer to step  200  is YES, the routine proceeds to step  300 , wherein cylinder flow is estimated using MAF sensor information:          W   cyl     =         η   vk            n   e     2          V   d          P   RT       +       (     ɛ   -     γ                 P       )            V   im     RT                         
     where η vk  is a volumetric efficiency estimated from a nominal map as a function of engine speed and valve timing, V d  is the engine displacement volume (a predetermined constant), P is the intake manifold pressure measured by the MAP sensor, T is the intake manifold temperature measured by a senor or estimated, R is a gas constant (difference of specific heats), n e  is the engine speed in revolutions per second, V im  is the intake manifold volume, γ is the estimator gain, and ε is the estimator state. The estimator state is updated in accordance with the following equation:          ɛ        (     t   +   Δ     )       =       ɛ        (   t   )       +     Δ        (       -     γɛ        (   t   )         -         γη   vk          (   t   )                n   e          (   t   )       2          V   d            P        (   t   )         V   im         +     γ          RT        (   t   )         V   im              W   th          (   t   )         +       γ   2          P        (   t   )         +     γ          RT        (   t   )         V   im              W   egr          (   t   )           )                         
     where W th  is the mass flow rate through the throttle as measured by the MAF sensor, and Δ is the sampling period, and W egr  is an estimate of an amount of recirculated exhaust gas inducted into the intake manifold. The routine then exits. 
     If the answer to step  200  is NO, indicating that even though MAF sensor is operational, the transition between the MAF-independent and MAF-dependent estimates is still in process, the routine proceeds to step  400  wherein the “switchover coordinator” algorithm is employed to achieve a smooth transition between the two different estimates according to the following equation: 
     
       
           y ( t +Δ)= y ( t )+Δ·(−γ 1 ( y ( t )− x ( t ))−γ 2 ( y ( t )− z ( t ))−γ 3 sign( y ( t )− z ( t ))) 
       
     
     where γ 1 , γ 2  and γ 3  are nonnegative gains, x(t) is a first type of cylinder flow estimate and z(t) is a second type of estimate. The initial time t=0 coincides with the start of the transition between the two types of estimates. 
     The routine then proceeds to step  500  wherein a determination is made whether the switchover condition has been satisfied. The switchover condition is satisfied when the difference between the two types of estimates is less than a small predetermined value, e. For example, the condition that may be satisfied at the time instant t when: 
     
       
         ( z ( t )− y ( t ))·( z ( t− 1)− y ( t −1))&lt; e   
       
     
     If the answer to step  500  is YES, which means that y(t) has crossed z(t), the routine proceeds to step  600  wherein a the “transition completed” flag is set to 1, and the routine ends. If the answer to step  500  is NO, indicating that the transition is not completed yet, the routine proceeds to step  700  wherein the “switchover coordinator” gains are updated according to the following equation: 
     
       
         γ 1 ( t +Δ)=γ 1 ( t )−Δ·α·γ 1   
       
     
     
       
         γ 2 ( t +Δ)=γ 2 ( t )−Δ·α·(γ 2 −γ 20 ) 
       
     
     
       
         γ 3 ( t +Δ)=γ 3 ( t )−Δ·α·(γ 3 −γ 30 ) 
       
     
     where the constants and initial conditions are set so that they satisfy 
     
       
         γ 1 (0)&gt;0 
       
     
     
       
         γ 20 &gt;0, γ 2 (0)=0 
       
     
     
       
         γ 30 &gt;0, γ 3 (0)=0 
       
     
     The routine then cycles back to step  200 . 
     Referring now to FIG. 3, a graphical depiction of an example of how the “switchover coordinator” is employed to achieve a smooth transition between the two different estimation methods is presented. X(t) is a MAF-independent cylinder flow estimate plotted as a function of time, z(t) is a MAF-dependent estimate of the flow as a function of time, and y(t) is the output of the “switchover coordinator”. Time t 0  corresponds to step  400  of the above-described FIG. 2, wherein the MAF sensor becomes operational and the transition between the two types of estimates begins. Time t 1  corresponds to step  600  of FIG. 2, wherein the switchover condition is satisfied when the output of the “switchover coordinator”, y(t) crosses the MAP-MAF flow estimate z(t). Therefore, any control strategy that requires an estimate of cylinder flow (such as the air-fuel ratio control strategy, or an engine torque control strategy) can use the estimate depicted by the curve x(t) prior to time t 0 , the estimate depicted by z(t) after time t 1 , and the output of the “switchover coordinator” y(t) during the time period between t 0  and t 1 . In this way, abrupt fluctuations in the air-fuel ratio or engine torque that may occur due to switching between the two types of estimates can be avoided. 
     Alternatively, the “switchover coordinator” can be used to smoothly transition between the cylinder flow estimate based on the estimated throttle flow and the one based on the throttle flow as measured by the MAF sensor. For example, the cylinder flow equation described above in step  300 , FIG. 2, can be used:          W   cyl     =         η   vk            n   e     2          V   d          P   RT       +       (     ɛ   -     γ                 P       )            V   im     RT                         
     where ε is updated in accordance with the following equation:          ɛ        (     t   +   Δ     )       =       ɛ        (   t   )       +     Δ        (       -     γɛ        (   t   )         -         γη   vk          (   t   )                n   e          (   t   )       2          V   d            P        (   t   )         V   im         +     γ          RT        (   t   )         V   im              W   th          (   t   )         +       γ   2          P        (   t   )         +     γ          RT        (   t   )         V   im              W   egr          (   t   )           )                         
     and W th  is either the mass flow rate through the throttle as measured by the MAF sensor (when the MAF sensor is operational) or estimated via the orifice equation:          W   th     =           C   d          A   thr          P   b           RT   b            θ                     
     where C d  is the orifice discharge coefficient, A thr  is the throttle valve area which is a function of the throttle position, T b  is the temperature upstream of the throttle (measured or estimated), R is a gas constant (difference of specific heats), P b  is the ambient pressure before the throttle, and θ is a function of the ratio of the intake manifold pressure Pi and the ambient pressure before the throttle, P b  defined by the following equations:              (       P   i       P   b       )       1   /   r       ·         2     (     r   -   1     )       ·     [     1   -       (       P   i       P   b       )         (     r   -   1     )     /   r         ]           ;       (       P   i       P   b       )     &gt;     P     crit        
                     θ   =         (     2     (     r   +   1     )       )         (     r   +   1     )     /     (     r   -   1     )             ;       (       P   i       P   b       )     ≤     P   crit                       
     where P crit  is the critical pressure ratio of 0.5283, and r is a ratio of specific heats. 
     Therefore, it is possible to obtain an accurate estimate of cylinder flow at all operating conditions by using a MAF-independent estimate when MAF sensor is not operational (such as at engine start-up of at high throttle angles) and using a “switchover coordinator” to smoothly transition to a MAF-dependent cylinder flow estimate when the MAF sensor is operational. Using the “switchover coordinator” avoids abrupt jumps in cylinder flow estimates and thus eliminates resulting air-fuel ratio and torque fluctuations. 
     This concludes the description of the invention. 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 invention. Accordingly, it is intended that the scope of the invention be defined by the following claims: