Patent Publication Number: US-2004045526-A1

Title: Method and system for controlling partial pressure of air in an intake manifold of an engine

Description:
TECHNICAL FIELD  
       [0001] This invention relates generally to methods and systems for controlling the partial pressure of air in an intake manifold of an engine.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0002] As is known in the art, the mass of air, or cylinder air charge, inducted into each cylinder of an internal combustion engine must be known as precisely as possible in order to match the air mass with an appropriate mass of metered fuel. Placing sensors at the intake port of each cylinder is technically very difficult and expensive. Instead, a sensor is typically located either inside the intake manifold or at the throttle opening into the intake manifold. A physics model is then used to estimate the air mass propagation through the intake manifold into each cylinder.  
       [0003] Two types of the above-described sensors are typically employed in internal combustion engines. One type is a manifold absolute pressure (MAP) sensor. An estimation algorithm treats the manifold pressure as an input to the system and uses mapped engine data and engine speed to estimate air flow into the engine cylinders. The other type of sensor is a relatively expensive mass air flow (MAF) sensor used to directly measure mass air flow at the throttle body. For the MAF based system, fresh air from the throttle is directly measured. EGR gas content is left out of the cylinder port air charge estimation. Other air flows not from the throttle (via vacuum lines from the brakes, canister purge system, etc.) are not accounted for by the MAF measurement and must be accounted for by other means.  
       [0004] The MAP sensor measures the absolute pressure in the intake manifold and thus incorporates the air flow from all sources. Difficulties arise, however, when gases other than air are introduced into the intake manifold. For the MAP based system (often referred to as a speed density system), gases other than air, such as the deliberately introduced exhaust gas (referred to as EGR or exhaust gas recirculation), increase the manifold pressure. These gases should not be matched by fuel. However, the MAP sensor cannot distinguish between fresh air and EGR. Thus, EGR mass in the intake manifold must be measured or estimated.  
       [0005] More particularly, control of the partial pressure of air has to be achieved under uncertainties in the EGR flow. These uncertainties are due to the soot deposits in the EGR valve conduit and the fact that the exhaust pressure and temperature are not measured. Additionally, air is present in the EGR flow during lean operation and this air needs to be accounted for in the partial pressure of air estimate.  
       [0006] In accordance with the present invention, a method is provided for controlling partial pressure of air in an intake manifold of an engine. The engine has an intake throttle device for controlling a flow of air to the intake manifold. An EGR valve is provided for controlling a flow of exhaust gas from the engine to the intake manifold downstream of the intake throttle. The engine has at least one cylinder fed a flow comprising air passing through the throttle to the intake manifold and exhaust products passing through the EGR valve to the intake manifold. Both the air through the throttle and the exhaust gas products in the intake manifold are passed as a combined flow to the intake manifold and then to the at least one cylinder. The method includes: specifying a dynamic reference model for the desired partial pressure of the air as a function of time; and controlling the flow through the intake throttle device in accordance with an estimated EGR flow obtained by a dynamic observer and an estimate of partial air fraction in the exhaust gas products.  
       [0007] In one embodiment, the partial air fraction is estimated in accordance with intake to exhaust delay and fuel injection to exhaust delay.  
       [0008] In one embodiment the dynamic observer does not require information of engine exhaust temperature, engine exhaust pressure, or EGR valve position.  
       [0009] According to the present invention, there is provided a method controlling partial air pressure in an intake manifold of an engine. The engine has an intake throttle device for controlling a flow of air to the intake manifold. An EGR valve is provided for controlling a flow of exhaust gas from the engine to the intake manifold downstream of the intake throttle. The engine has at least one cylinder fed a flow comprising air passing through the throttle to the intake manifold and exhaust products passing through the EGR valve to the intake manifold. Both the air through the throttle and the exhaust gas products in the intake manifold are passed as a combined flow to the intake manifold and then to the at least one cylinder. The method includes: calculating the desired partial pressure of air dynamically, as a function of time in accordance with a reference model, estimating the flow of exhaust gas products passing through the EGR valve to the intake manifold from engine operating parameters; estimating the air fraction in the estimated flow of exhaust gas products passing through the EGR valve to the intake manifold; determining the partial pressure of air in the intake manifold from such estimate of the flow of exhaust gas products and such estimate of the air fraction; and, adjusting the intake throttle device in accordance with a difference between a desired partial pressure of the air in the intake manifold and the determined partial pressure of air in the intake manifold.  
       [0010] In a preferred embodiment of the invention, the estimate of the flow of gas products passing through the EGR valve comprises providing such estimate in accordance with an open loop estimator.  
       [0011] In accordance with the present invention, a method is provided for controlling partial air pressure in an intake manifold of an engine. The method includes estimating partial air pressure in intake manifold based on open loop observer and estimated partial air fraction in the flow of exhaust to the intake manifold.  
       [0012] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
    
     DESCRIPTION OF DRAWINGS  
     [0013] The single figure is a schematic diagram of an engine system having control of partial air pressure at an intake manifold thereof. 
    
    
     DETAILED DESCRIPTION  
     [0014] Referring now to FIG. 1, a gasoline engine system  10  is shown to include an engine block  16  is shown having for example, four cylinders  18 . Each of the combustion chambers  18  includes here for example direct-injection fuel injectors  20 . The duty cycle of the fuel injectors  20  is determined by the engine control unit (ECU)  24  and transmitted along signal line  22 .  
     [0015] The engine system  10  has an intake throttle device  49  for controlling a flow of air to an intake manifold  26 . An EGR valve  12  is provided for controlling a flow of a portion of exhaust gas (shown by arrow  31 ) passing from the engine to the intake manifold  26  downstream of the intake throttle  49 . The portion of the exhaust gas passing through the EGR valve  12  is indicated by arrow  32 . The cylinders  18  are thus fed a flow comprising air passing through the intake throttle device  49  to the intake manifold  26  and the portion of the exhaust gas products passing through the EGR valve  12  to the intake manifold  26 . Both the air through the throttle  49  and the exhaust gas products in the intake manifold are passed as a combined flow to the cylinders  18 , such combined flow being indicated by the arrow  35 .  
     [0016] The EGR system is provided to reduce the level of NOx emissions. The EGR system comprises the EGR valve  12  disposed in a conduit  33  connecting the exhaust manifold  28  to the intake manifold  26 . This allows a portion of the exhaust gases to be circulated from the exhaust manifold  28  to the intake manifold  26  as described above. It is noted that the flow of exhaust gas though the EGR valve  12  is a function of the pressure across such valve  12  in addition to the electrical signal provided to the valve on line  46  from the ECU  24 . Here, there is no pressure sensor at the input to the EGR valve  34  (i.e., in the exhaust manifold  28 ). The electrical signal on line  46  is produced by the ECU  24  from relationships stored a priori in the ECU  24  in accordance with a computer program stored in a memory  25  in the ECU  24 .  
     [0017] All of the engine systems, including the EGR valve  12 , fuel injectors  20 , intake throttle device  49  are controlled by the ECU  24 . For example, signal  46  from the ECU  24  regulates the EGR valve  12  position and a signal on line  47  controls the position of the intake throttle device  49 .  
     [0018] In the ECU  24 , the command signal  47  to the intake throttle device  49  will be described in detail below. Suffice it to say here, however, that the signal on line  47  for the intake throttle device  49  is produced by the ECU  24  to provide a desired partial air pressure in the intake manifold  26 . Additional sensory inputs are also received by the ECU  24  via lines  62  along with: engine intake manifold temperature, TEMP, as measured by temperature sensor  50  which produces a signal on line  52 ; mass air flow, MAF, to the intake throttle device  49  as measured by flow sensor  59  which produces a signal on line  61 ; mass air pressure (MAP) as measured by intake manifold pressure sensor  58  which produces a signal on line  52 ; and engine speed which is fed to the ECU  24  as the signal, n e , etc. Additional operator inputs  68  are received along signal  70  such as the accelerator pedal position.  
     [0019] As will be described in more detail below, a set of control instructions or code are stored in a memory  25  in the ECU  24 . Execution of the stored code by the ECU  24  results in a method being performed which estimates the flow of exhaust gas products passing through the EGR valve  12  to the intake manifold  26  from engine operating parameters; estimates the air fraction in the estimated flow of exhaust gas products passing through the EGR valve  12  to the intake manifold; determines the partial pressure of air in the intake manifold  26  from such estimate of the flow of exhaust gas products and such estimate of the air fraction; and, adjusts the intake throttle device  49  position in accordance with a difference between a desired partial pressure of the air in the intake manifold and the determined partial pressure of air in the intake manifold. The estimate of the flow of gas products passing through the EGR valve  12  provides such estimate in accordance with an open loop estimator to be described.  
     [0020] It should first be noted that the following notation is used herein, reference being made to FIG. 1:  
     [0021] {circumflex over ( )} denotes an estimated value of an engine operating parameter;  
     [0022] t k  is the time of a sample of the parameter;  
     [0023] dT is the period between samples of the parameter;  
     [0024] T, is the temperature measured in the engine intake manifold (i.e., the signal TEMP on line  60 );  
     [0025] W th  is the mass air flow measured through the engine intake throttle (i.e., the signal MAF on line  61 );  
     [0026] W cyl  is the total flow into a cylinder of the engine;  
     [0027] W cyl,air  is the partial flow of air into a cylinder of the engine (estimated in a manner to be described below);  
     [0028] W egr  is the exhaust gas recirculation (EGR) flow (estimated in a manner to be described below);  
     [0029] χ is the air fraction in the exhaust gas of the engine (estimated in a manner to be described below);  
     [0030] V IM  is a priori measured intake manifold volume;  
     [0031] V d  is a priori measured cylinder displacement;  
     [0032] P air  is the partial pressure of air measured in the intake manifold (i.e., the MAP signal on line  52 );  
     [0033] P air,d  is the desired partial pressure in the intake manifold  26 ;  
     [0034] P=p air +P bg  (where p air  is the total pressure in the intake manifold and p bg  is the partial pressure of burnt gas in the intake manifold);  
     [0035] n e  is measured engine speed in revolutions per second;  
     [0036] η v  is engine volumetric efficiency;  
     [0037] Δ fi  is fuel injection to exhaust delay  
     [0038] Δ io  is intake to exhaust delay  
     [0039] R is the gas constant  
     [0040] To maintain good engine performance it is desirable to have a well-controlled partial pressure of air response in the intake manifold  26 . The desired response for the partial pressure of air is here defined by a reference model, here a first order system:  
       p   air ( t   k+1 )= p   air ( t   k )+ dT (−λ·( p   air ( t   k )−p air,d ))  (1)  
     [0041] where;  
     [0042] p air  is the actual partial pressure in the intake manifold  26 ; and  
     [0043] p air,d  is the desired behavior of partial pressure of air in the reference model, as presented above in equation (1).  
     [0044] In order to achieve this desired partial pressure, p air,d,  the intake throttle  49  is adjusted to provide the following air flow through such throttle  49 :  
                 W     th   ,   d            (     t   k     )       =       -         λ        (           p   ^     air          (     t   k     )       -     p     air   ,   d         )            V   IM         R                     T   ^          (     t   k     )             +         W   ^       cyl   ,   air            (     t   k     )       -         χ   ^          (     t   k     )       ·         W   ^     egr          (     t   k     )                   (   2   )                       
 
     [0045] In order to determine W th,d (tk) from equation (2), while the following parameters are known: V IM , R, T and, as will be described, λ, an estimate of W cyl,air , χ, W egr . and p air  are determined as described below.  
     [0046] Given this desired flow rate of air through the throttle, W th,d  we backtrack the desired throttle device  49  position to provide this flow and set the throttle device  49  to that position via the signal on line  47 .  
     [0047] The parameter λ is adjusted to ensure the desired shape of the engine torque response. A larger λ provides faster torque response; however, very large λ s should not be used since fast torque response may cause driveline oscillations and other driveability problems and also may cause the estimators employed throughout perform poorly (i.e., not be able to track or catch up with a fast engine behavior).  
     [0048] The estimates of W cyl,air , χ, W egr . and p air  are determined as follows:  
     [0049] (1) Determine throttle flow, W th , by estimates thereof or by measuring the mass air flow (MAF) with MAF sensor  59 , as indicated in FIG. 1  
     [0050] (2) Estimate or measure the intake manifold pressure p(t) with a MAP sensor  50  as indicated in FIG. 1;  
     [0051] (3) Estimate cylinder flow W cyl (t k ) at the present sampling time instant, t k , in accordance with:  
             W   ^     cyl          (     t   k     )       =         η   v          (     t   k     )                n   e          (     t   k     )       2          V   d            p        (     t   k     )           T   ^          (     t   k     )                         
 
     [0052] where:  
     [0053] η n (t k )=η n (η e (t k ), p(t k )) is volumetric efficiency obtained from a look-up table or a regression equation  
     [0054] (4) Determine the estimate of the EGR flow in accordance with the following:  
     [0055] It is first noted from FIG. 1 that the amount of air flow in the exhaust gas returned to the intake manifold through the EGR valve is χ·W egr , where, as noted above, W egr  is the total exhaust gas recirculation flow and χ is air fraction in the exhaust gas. Here, estimates are made of the air fraction in the exhaust gases by making estimates of χ and W egr  in accordance with:  
           χ   ^          (     t   k     )       =       max        {             W   ^       cyl   ,   air            (       t   k     -     Δ   io       )       -         W   f          (       t   k     -     Δ   fi       )           (     A   /   F     )     s         ,   0     }               W   ^     cyl          (       t   k     -     Δ   io       )       +       W   f          (       t   k     -     Δ   fi       )                         
 
     [0056] where:  
     [0057] (A/F)s is the stoichiometric air-to-fuel ratio (approx. 14.64), and  
               W   ^     egr          (     t   k     )       =         V   IM       R                     T   ^          (     t   k     )                (       ɛ        (     t   k     )       -     γ   ·     p        (     t   k     )           )         ,                 
 
     [0058] respectively,  
     [0059] where ε is an open loop estimator state which is updated in accordance with:  
         ɛ        (     t     k   +   1       )       =       ɛ        (     t   k     )       +     dT        (         -   γ                     ɛ        (     t   k     )         -     γ          R                     T   ^          (     t   k     )           V   IM            (         W   th          (     t   k     )       -         W   ^     cyl          (     t   k     )         )       +       γ   2          p        (     t   k     )           )                       
 
     [0060] The estimate of the partial pressure of air in the intake manifold is given by:  
             p   ^     air          (     t     k   +   1       )       =           p   ^     air          (     t   k     )       +     dT        (         R                     T   ^          (     t   k     )           V   IM            (         W   th          (     t   k     )       +           W   ^     egr          (     t   k     )              χ   ^          (     t   k     )         -         W   ^       cyl   ,   air            (     t   k     )         )       )                       
 
     [0061] Having determined the desired throttle flow, W th,d  as function of p air,d  in accordance with equation (2), the intake throttle device  49  position signal on line  47  is set as a function of p(t k )/p amb  and W th,d  to match W th,d  
           α   th     =       A     -   1       (         W     th   ,   d       /       f   th     (       p        (     t   k     )         p   amb       )       /       p   amb         T   amb           )       ,                 
 
     [0062] where A is the throttle position to throttle effective flow area (geometric flow area times the discharge coefficient) map, A −1  is its inverse, α th  is the throttle position, T amb  is the ambient temperature, p amb  is ambient pressure and  
           f   th          (   x   )       =     {                 γ   0.5          (     2     γ   +   1       )           γ   +   1       2        (     γ   -   1     )           ,     x   ≤   0.5     ,                   x     1   γ              {         2      γ       γ   -   1            [     1   -     x       γ   -   1     γ         ]       }       1   2         ,     x   ≥   0.5                             
 
     [0063] where γ=1.4  
     [0064] The EGR flow is controlled with EGR valve  12  via the signal on line  46  while the fuel and spark are adjusted as desired with the remainder of the control system.  
     [0065] In summary, the method described above combines an estimator for the EGR flow that uses intake manifold pressure and throttle flow measurements, with an open-loop estimator for the air fraction in the exhaust gas. This EGR flow estimator provides a robust way of estimating the EGR flow in presence of significant uncertainties in the EGR valve conduit. The controller for the electronic intake throttle device is then developed to enforce the desired response of the partial pressure of air estimate.  
     [0066] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.