Abstract:
A model-based estimation of mass airflow is provided which provides an accurate estimation of mass airflow without introducing undesirable time delays characteristic of filtered mass airflow signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/794,580 filed on Apr. 24, 2006 which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to internal combustion engines. More particularly, the invention is concerned with accurately estimating mass airflow to the engine. 
     BACKGROUND OF THE INVENTION 
     The combustion process of homogeneous charge compression ignition (HCCI) engines depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. Hence, the control inputs to the engine, for example, fuel injection mass and timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion. Generally, for best fuel economy, an HCCI engine operates un-throttled and with a lean air-fuel mixture. Further, in an HCCI engine using an exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by advancing the exhaust valve close timing from nominal. The opening timing of the intake valve is retarded from nominal to a later time preferably symmetrical to the exhaust valve closing timing about top-dead-center (TDC) intake. Both the cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from the previous cycle can be retained with earlier closing of the exhaust valve which leaves less room for the incoming fresh air mass. The net effects are higher cylinder charge temperature and lower cylinder oxygen concentration. The negative valve overlap (NVO), defined as the crank-angle period where both intake and exhaust valves are simultaneously closed around TDC intake, is indicative of the trapped amount of hot residuals. 
     Robust HCCI combustion has been demonstrated using a variable valve actuation system such as a fully flexible valve actuation (FFVA) system (e.g. electrically variable, hydraulically variable or electro-hydraulically variable valves) or a simplified mechanical two-step valve lift system with a dual cam phasing system. In particular, optimal combustion phasing can be maintained by adjusting both intake and exhaust valve profiles in conjunction with engine control inputs such as injection mass and timing, spark timing, throttle and EGR valve positions. Furthermore, air-fuel ratio control is critical for maintaining robust HCCI combustion especially during transients. 
     In conventional gasoline spark-ignition engines, airflow is controlled by the throttle, and the fuel is metered proportional to the measured mass airflow at the throttle body using a MAF sensor. The noise level (i.e. high frequency components) of the MAF signal is low as long as the intake manifold absolute pressure (MAP) is far below the ambient pressure (i.e. throttled engine operation). However, during minimally throttled operation, noise levels can be substantial due to significant coupling of intake dynamics of the cylinders with the intake manifold and MAF sensor. During HCCI engine operations, the throttle is usually kept wide-open to minimize pumping losses, and the airflow is controlled by the exhaust and intake valve profiles (i.e. combinations of lift, duration and phase). Therefore, engines operating in an HCCI mode are also affected by MAF signals which can be substantially noisy. Similarly, in diesel engines, which operate without air throttling, MAF signals can similarly be substantially noisy. Although the high-frequency components in the MAF measurement can be reduced using a low pass filter, a filtered signal produces an undesirable time delay in the MAF measurement. Adapting fuel injection command using a filtered, and hence time delayed, MAF measurement can cause significant air-fuel ratio deviations during engine transient operations resulting in undesirable combustion results including, for example, partial burn, misfires, excessive emissions, combustion phase shifts, etc. 
     SUMMARY OF THE INVENTION 
     In the present invention, model-based estimation and control methodology based on MAF measurement is developed to accurately estimate mass airflow without introducing time delay for robust transient operations. 
     A method for unfiltered intake airflow determination in a substantially unthrottled internal combustion engine includes modeling intake airflow using a low-order differential equation. The low-order differential equation includes an estimated airflow term and a desired airflow term, wherein the actual airflow follows the desired airflow as described by the low-order differential equation. The low-order differential equation is tuned in accordance with adaptive parameters operative on the estimated airflow term and the desired airflow term. The tuning minimizes error between the estimated airflow term and the actual airflow. 
     An apparatus for unfiltered intake airflow determination in an internal combustion engine includes airflow control means for controlling airflow to engine cylinders without any substantial airflow throttling and an airflow sensor measuring substantially unthrottled airflow. Further included is a closed-loop airflow controller for controlling the airflow control means based on a desired airflow and the measured airflow from the airflow sensor. The controlled airflow follows the desired airflow in such a way that can be described by low-order dynamics. Finally included is an adaptive airflow estimator for providing an undelayed estimate of airflow based on the desired airflow and adaptive parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of invention may take physical form in certain parts and arrangement of parts, the preferred embodiment of which will be described in detail and illustrated in the accompanying drawings which form a part hereof and wherein: 
         FIG. 1  schematically illustrates an HCCI engine and control system; 
         FIG. 2  illustrates measured mass airflow response to a desired mass airflow signal; 
         FIG. 3  illustrates measured, filtered and modeled mass airflow in accordance with the present invention; and, 
         FIGS. 4A-4D  illustrate various data graphs corresponding to an HCCI engine operated in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described with respect to a HCCI engine. However, the invention is fully applicable to other engine types, including conventionally throttled spark-ignited engines, diesel cycle engines, or any variety of engines employing measured mass airflow. 
     Referring now to  FIG. 1 , illustrated is a block diagram showing an engine  12  capable of operating with homogeneous charge compression ignition (HCCI) and a combustion control system  14  and method for controlling combustion in the engine. 
     The engine  12  may include various features or devices, including power producing combustion chambers  13  connected with an intake air system  17  and an exhaust system  19 , intake  21  and exhaust  23  valves with some form of variable valve actuation system  15  operative to control intake flow to and exhaust flow from the combustion chambers, an external exhaust recirculation system  25  including an EGR valve  27  connected between the intake and exhaust systems, and fuel injection and spark ignition systems (not separately illustrated) for supplying fuel to and igniting or assisting ignition of combustible mixtures in the combustion chambers. 
     The engine  12  is designed to operate on fuel injected gasoline or similar blends, unthrottled with HCCI combustion over an extended range of engine speeds and loads, which may include engine starting where possible. However spark ignition and throttle controlled operation may be utilized with conventional or modified control methods under conditions not conducive to HCCI operation and to obtain maximum engine power. Applicable fueling strategies may including direct cylinder injection, port fuel injection or throttle body fuel injection. Widely available grades of gasoline and light ethanol blends thereof are preferred fuels; however, alternative liquid and gaseous fuels such as higher ethanol blends (e.g. E80, E85), neat ethanol (E99), neat methanol (M100), natural gas, hydrogen, biogas, various reformates, syngases etc. may also be used in the implementation of the present invention. 
     The described control system  14  and method are of particular benefit to unthrottled operation of the engine wherein time delays, introduced for example by signal filtering, of a MAF signal are undesirable. The combustion control system  14  includes one or more computers or controllers adapted to carry out a repetitive series of steps or functions in a method of combustion control according to the invention. The main controllers include a feedforward controller and a feedback controller. 
     In the present application of the invention, it is assumed that an HCCI engine is operating with exhaust recompression strategy with one of electro-hydraulic, hydraulic, or electric cam phaser, and that mass air flow (MAF) measurement is available with a MAF sensor. The present invention comprises an airflow control using NVO via a variable valve actuation system, and an adaptive airflow model based on the MAF measurement. The overall control structure is shown represented by control system  14  of  FIG. 1 . 
     Airflow to the engine is measured by a MAF sensor  30  located at the throttle body, and a feedback controller  61  adjusts NVO to achieve desired airflow based on the MAF measurement. The feedback controller is designed such that response of actual airflow to the desired airflow can be approximated by low-order dynamics (e.g. first or second order). Then, closed-loop response of airflow can be modeled using a low-order differential equation. 
     An example is shown in  FIG. 2  when the feedback controller  61  is designed such that closed-loop dynamics of airflow can be approximated by a 1 st  order differential equation as follows: 
     
       
         
           
             
               
                 
                   
                     x 
                     . 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           τ 
                         
                       
                       ⁢ 
                       x 
                     
                     + 
                     
                       
                         1 
                         τ 
                       
                       ⁢ 
                       r 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where x is the airflow measured by a sensor, r is the desired airflow, and τ is the time constant of the closed-loop system. To estimate the airflow into the engine, a 1 st  order adaptive airflow model  63  is introduced as follows: 
     
       
         
           
             
               
                 
                   
                     
                       x 
                       . 
                     
                     e 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           
                             τ 
                             e 
                           
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           1 
                           + 
                           α 
                         
                         ) 
                       
                       ⁢ 
                       
                         x 
                         e 
                       
                     
                     + 
                     
                       
                         1 
                         
                           τ 
                           e 
                         
                       
                       ⁢ 
                       β 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       r 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where x e  is the estimated airflow, τ e  is the estimated time constant of the closed-loop system, α and β are control parameters employed by an adaptive controller so that the difference between response of the model and that of actual airflow is minimized. Since from the first order behavior of the airflow under control, the error between the actual and the estimated model airflow is given by the following relationship which relies, in part, upon a desired airflow term: 
     
       
         
           
             
               
                 
                   
                     e 
                     . 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           τ 
                         
                       
                       ⁢ 
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                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             
                               τ 
                               e 
                             
                           
                           + 
                           
                             
                               1 
                               
                                 τ 
                                 e 
                               
                             
                             ⁢ 
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                           - 
                           
                             1 
                             τ 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         x 
                         e 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             
                               1 
                               
                                 τ 
                                 e 
                               
                             
                             ⁢ 
                             β 
                           
                           - 
                           
                             1 
                             τ 
                           
                         
                         ) 
                       
                       ⁢ 
                       r 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where e=x e −x. Adaptation laws for α and β can be derived using, for example, a Lyapunov function as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     = 
                     
                       
                         
                           
                             1 
                             2 
                           
                           ⁢ 
                           
                             ⅇ 
                             2 
                           
                         
                         + 
                         
                           
                             
                               τ 
                               e 
                             
                             
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               γ 
                             
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     τ 
                                     e 
                                   
                                 
                                 + 
                                 
                                   
                                     1 
                                     
                                       τ 
                                       e 
                                     
                                   
                                   ⁢ 
                                   α 
                                 
                                 - 
                                 
                                   1 
                                   τ 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                         + 
                         
                           
                             
                               τ 
                               e 
                             
                             
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               γ 
                             
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   
                                     1 
                                     
                                       τ 
                                       e 
                                     
                                   
                                   ⁢ 
                                   β 
                                 
                                 - 
                                 
                                   1 
                                   τ 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                       &gt; 
                       0 
                     
                   
                   , 
                   
                     γ 
                     &gt; 
                     0 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Finally, it can be shown that the following adaptation law guarantees 
                 V   .     =         -     1   τ       ⁢     ⅇ   2       ≤   0       ,         
and that e→0 as τ→∞ while α and β are bounded:
 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             
                               ⅆ 
                               α 
                             
                             
                               ⅆ 
                               t 
                             
                           
                           = 
                           
                             γ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               x 
                               e 
                             
                             ⁢ 
                             e 
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               ⅆ 
                               β 
                             
                             
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                           = 
                           
                             
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             re 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
       FIG. 3  shows MAF sensor output from a multi-cylinder HCCI engine operated at constant engine speed of 2000 RPM, with 95 kPa of MAP. In addition, both filtered and adaptive model estimated signals are presented in the figure. 
     It can be seen from the  FIG. 3  that MAF sensor signal (measured) contains high-frequency components which requires heavy filtering to smooth. Filtering (dashed line), however, introduces a time delay of about 1 sec. The estimated MAF signal from the adaptive model (solid line) show a negligible time delay. With the estimated airflow from the adaptive model, desired air-fuel ratio can be controlled with correct fuel injection command. 
     A method in accordance with an embodiment has been tested with a multi-cylinder HCCI engine, and the result is shown in  FIGS. 4A-4D . The fueling rate was scheduled based on desired air-fuel ratio and estimated airflow using the present invention. The engine was operating with 95 kPa of MAP, with exhaust recompression valve strategy at constant engine speed of 2000 RPM. The desired MAF was changed from 6.5 to 8.5 g/s, with roughly 2 g/s 2  of change rate. The desired air-fuel ratio was set to be constant at 16:1, and the fueling rate was determined by the estimated airflow from the adaptive model and the desired air-fuel ratio. The  FIG. 4C  shows that peak-to-peak air-fuel ratio excursion was below 1 during load transients. Also, the combustion phasing, defined as the crank angle position of 50% fuel burned (CA50), is also shown in the figure.  FIG. 4D  illustrates satisfactory combustion phasing control during transients with the present invention. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.