Abstract:
The amount of fuel to be injected in each cylinder of a multi-cylinder spark ignition internal combustion engine may be determined with enhanced precision if the fuel injection durations are determined as a function of the sensed mass air flow in all the cylinders of the engine, instead of considering only the air flow in the same cylinder. This finding has led to the realization of a more efficient method of controlling a multi-cylinder spark ignition internal combustion engine and a feedforward control system.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to internal combustion engines, and, more particularly, to a method and associated control system for determining the width (duration) of the fuel injection pulse in each cylinder of a multi-cylinder internal combustion engine.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the feedforward part of an SI (Spark Ignition) engine Air/Fuel control system, the in-cylinder mass air flow rate must be accurately estimated in order to determine the amount of fuel to be injected. Generally, this evaluation is performed either with a dedicated physical sensor (MAF sensor) or more often through an indirect evaluation.  
         [0003]     In order to meet the stricter and stricter emission regulations, automobile gasoline engines are equipped with a three-way catalytic converter (TWC). A precise control of the air-fuel ratio (A/F) to maintain it as close as possible to the stoichiometric value is necessary to achieve a high efficiency of the TWC converter in the conversion of the toxic exhaust gases (CO, NOx, HC) into less harmful products (CO 2 , H 2 O, N 2 ). Typically, in a spark-ignition engine, this control is performed through a so-called lambda sensor. The lambda sensor generates a signal representative of the value of the ratio  
       λ   =       Air   /   Fuel       Air   /     Fuel   stoichiometric             
 
 from the amount of oxygen detected in the exhaust gas mixture. If λ&lt;1 the mixture is rich of fuel, while if λ&gt;1 the mixture is lean of fuel. 
 
         [0004]     In order to keep the air/fuel ratio (AFR) as close as possible to unity, the lambda sensor is introduced in the conduit of exhaust gases for monitoring the amount of oxygen present in the exhaust gas mixture. The signal generated by the lambda sensor is input to the controller of the engine that adjusts the injection times and thus the fuel injected during each cycle for reaching the condition λ=1.  
         [0005]     Traditional Air/Fuel control systems include a feed-forward part, in which the amount of fuel to be injected is calculated on the basis of the in-cylinder mass air flow, and a feedback part that uses the signal of the oxygen sensor (lambda sensor) in the exhaust gas stream, to ensure that the Air/Fuel remain as close as possible to the stoichiometric value [1].  
         [0006]      FIG. 1  shows a block diagram of a traditional Air/Fuel control system. Generally, the feedback part of the Air/Fuel control system is fully active only in steady-state conditions. Moreover, the lambda sensor signal is made available only after this sensor has reached a certain operating temperature. In transients and under cold start conditions, the feedback control is disabled, thus the feedforward part of Air/Fuel control becomes particularly important.  
         [0007]     As mentioned above, air flow estimation is often the basis for calculating the amount of injected fuel in the feedforward part of Air/Fuel control system.  
         [0008]     A conventional technique [1] for estimating a cylinder intake air flow in a SI (Spark Ignition) engine involves the so-called “speed-density” equation:  
           m   .     ap     =       η   ⁡     (       p   m     ,   N     )       ·         V   d     ·   N   ·     p   a         120   ·   R   ·     T   m               
 
 where {dot over (m)} ap  is the inlet mass air flow rate, V d  is the engine displacement and N is the engine speed; T m  and p m  are the average manifold temperature and pressure and η is the volumetric efficiency of the engine. This is a nonlinear function of engine speed (N) and manifold pressure (p m ), that may be experimentally mapped in correspondence with different engine working points. 
 
         [0009]     A standard method is to map the volumetric efficiency and compensate it for density variations in the intake manifold.  
         [0010]     One of the drawbacks in using the “speed-density” equation for the in-cylinder air flow estimation is the uncertainty in the volumetric efficiency. Generally, the volumetric efficiency is calculated in the calibration phase with the engine under steady state conditions. However variations in the volumetric efficiency due, for example, to engine aging and wear, combustion chamber deposit buildup etc., may introduce errors in the air flow estimation.  
         [0011]     The low-pass characteristic of commercial sensors (Manifold Absolute Pressure or MAP sensors) used for the determination of the manifold pressure p m , introduces a delay that, during fast transients, causes significant errors in the air flow determination.  
         [0012]     This problem is not solved by using a faster sensor because in this case the sensor detects also pressure fluctuations due to the valve and piston motion [2].  
         [0013]     In engines equipped with an EGR (Exhaust Gas Recirculation) valve, the MAP (Manifold Absolute Pressure) sensor cannot distinguish between fresh air (of known oxygen content) and inert exhaust gas in the intake manifold. Therefore, in this case the speed-density equation (1) cannot be used and the air charge estimation algorithm should provide a method for separating the contribution of recycled exhaust gas to the total pressure in the intake manifold [4].  
         [0014]     An alternative method for the air charge determination is to use a dedicated Mass Air Flow (MAF) physical sensor, located upstream the throttle, that directly measures the inlet mass air flow. The main advantages of a direct air flow measurement are [1]: utomatic compensation for engine aging and for all other factors that modify engine volumetric efficiency; improved idling stability; and lack of sensibility of the system to EGR (Exhaust Gas Recirculation) since only the fresh air flow is measured.  
         [0015]     Anyway, air flow measurement by means of a MAF sensor (which is generally a hot wire anemometer) accurately estimates the mass flow in the cylinder only in steady state because during transients the intake manifold filling/empting dynamics play a significant role [3], [5]. Moreover, for commercial automotive applications, the fact that a MAF sensor has a relatively high cost compared to the cost of MAP (Manifold Absolute Pressure) sensor used with the “speed density” evaluation approach, should be accounted for.  
       SUMMARY OF THE INVENTION  
       [0016]     Test carried out by the applicants have unexpectedly shown that the amount of fuel to be injected in each cylinder of a multi-cylinder spark ignition internal combustion engine may be determined with enhanced precision if the fuel injection durations are determined as a function of the sensed mass air flow in all the cylinders of the engine, instead of considering only the air flow in the same cylinder.  
         [0017]     This surprising finding has led the applicants to devise a more efficient method of controlling a multi-cylinder spark ignition internal combustion engine and an innovative feedforward control system.  
         [0018]     The feedforward control system may be embodied in a feedforward-and-feedback control system of a multi-cylinder spark ignition engine, including also a lambda sensor, that effectively keeps the composition of the air/fuel ratio of the mixture that is injected into the combustion chamber of each cylinder at a pre-established value.  
         [0019]     Experimental tests carried out by the applicants demonstrated that the feedforward-and-feedback control system of this invention is outstandingly effective in controlling the engine such to keep the lambda value as close as possible to any pre-established reference value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Preferred embodiments of the invention will be described referring to the attached drawings, wherein:  
         [0021]      FIG. 1  shows a block diagram of the traditional air/fuel control system for an internal combustion engine as in the prior art;  
         [0022]      FIG. 2  is a block diagram of a feedforward injection control system devised by the same applicants; and  
         [0023]      FIG. 3  shows an injection control system of this invention for an internal combustion engine with N cylinders. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The amount of fuel to be injected in each cylinder of a spark ignition (SI) internal combustion engine having N cylinders is determined by a feedforward fuel injection control system as that surrounded by the broken line perimeter in  FIG. 3 .  
         [0025]     The block A IR -C YLINDER  generates signals MAF 1 , . . . , MAF N  representative of the Mass Air Flow aspired by each cylinder of the engine. This block may be easily realized by juxtaposing N mass air flow sensors.  
         [0026]     The block I NJECTION  C ONTROL  M APS  has as inputs the signals MAF 1 , . . . , MAF N  and a signal representing the speed of the engine, and generates as a function thereof a feedforward signal I FF1 , . . . , I FFN  for each cylinder.  
         [0027]     According to an innovative aspect of this invention, each feedforward signal is determined as a function of the speed of the engine and of all the Mass Air Flow values MAF 1 , . . . , MAF N  of all the cylinders. The feedforward signals I FF1 , . . . , I FFN  are generated in this case by pointing respective locations of a look-up table that is established during a test phase of the engine.  
         [0028]     Tests carried out by the applicants have demonstrated that generating each feedforward signal I FFi  for a certain cylinder as a function of all the mass air flow values detected or estimated for all the cylinders of the engine, enhances the apparent correctness of the composition of the air/fuel mixture that is injected into each cylinder of the engine.  
         [0029]     This unpredictable result may be explained by the fact that there is an apparent non-homogeneous air filling for the different cylinders of the engine. This phenomenon is induced by air backflow in the intake manifold and air turbulences. For this reason, even if each cylinder of the engine is maintained nominally to the stoichiometric condition, the global exhaust gas could not have the oxygen content needed to guarantee the maximum efficiency of the three-way catalytic converter. For this reason, it appears that the injected fuel amount for each cylinder of the engine should be dependent not only by the related mass air flow value but also by the mass air flow incoming into the other cylinders.  
         [0030]     According to a preferred embodiment of this invention, the amount of fuel to be injected in each cylinder of an internal combustion engine having N cylinders is determined with a feedforward-and-feedback fuel injection control system as depicted in  FIG. 3 .  
         [0031]     A lambda sensor, introduced in the outlet conduit of exhaust gases for monitoring the amount of oxygen in the exhaust gases, determines whether the lambda ratio is above or below unity from the amount of oxygen detected in the exhaust gas mixture. The lambda sensor provides a signal representative of the value of the ratio:  
       λ   =       Air   /   Fuel       Air   /     Fuel   stoichiometric             
 
         [0032]     If λ&lt;1 the mixture is rich of fuel, while if λ&gt;1 the mixture is lean of fuel.  
         [0033]     The feedback-and-feedforward control system comprises an array of controllers CONTROLLERB 1 , . . . , CONTROLLERB N  each input with a respective feedforward signal I FFi  and with an error signal Δ LAMBDA  representing the difference between the actual lambda ratio L AMBDA - VALUE  and a reference value L AMBDA - REF.  Each controller adjusts the injection duration I 1 , . . . , I N  of a respective cylinder and thus the amount of fuel that is injected during each cycle in the respective cylinder for eventually reach the condition L AMBDA - VALUE =L AMBDA - REF .  
         [0034]     The lambda sensor may be preferably a virtual lambda sensor of the type described in the cited prior European Patent application No. 05,425,121.0.  
         [0035]     According to a preferred embodiment of this invention, each controller CONTROLLERB i  is realized using a Fuzzy Inference System properly set in a preliminary calibration phase of the system, according to a common practice.  
         [0036]     Preferably, each mass air flow sensor is a soft computing mass air flow estimator, of the type disclosed in the European patent application No. 06,110,557.3 in the name of the same applicants and shown in  FIG. 2 . This estimator is capable of estimating both in a steady state and in transient conditions the in-cylinder mass air flow of a single-cylinder SI engine, basically using a combustion pressure signal of the cylinder. A learning machine, such as for example a MLP (Multi-Layer Perceptron) neural network, trained on the experimental data acquired in different operating conditions of a gasoline engine, may be used for realizing the inlet mass air flow estimator.  
         [0037]     A traditional combustion pressure piezoelectric transducer, or any other low-cost pressure sensor, may provide the required raw information. As disclosed in the cited European Patent application, the cylinder combustion pressure is correlated with the inlet mass air flow of the cylinder, thus a signal produced by a combustion pressure sensor is exploited for producing through a soft-computing processing that utilizes information on throttle opening, speed and angular position, a signal representative of the inlet mass air flow.  
       REFERENCES 1. Heywood, J. B.,—“Internal combustion engine fundamentals”—McGraw-Hill Book Co., 1988.  
       [0038]     2. Barbarisi, O., Di Gaeta, A., Glielmo, L., and Santini, S., “An Extended Kalman Observer for the In-Cylinder Air Mass Flow Estimation”, MECA02 International Workshop on Diagnostics in Automotive Engines and Vehicles, 2001.  
         [0039]     3. Grizzle, J. W., Cookyand, J. A., and Milam, W. P., “Improved Cylinder Air Charge Estimation for Transient Air Fuel Ratio Control”, Proceedings of American Control Conference, 1994.  
         [0040]     4. Jankovic, M., Magner, S. W., “Air Charge Estimation and Prediction in Spark Ignition Internal Combustion Engines”, Proceedings of the American Conference, San Diego, Calif., June 1999.  
         [0041]     5. Stotsky, I., Kolmanovsky, A., “Application of input estimation and control in automotive engines” Control Engineering Practice 10, pp. 1371-1383, 2002.