ENGINE CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINES

Abstract: An internal combustion engine has an intake air flowmeter which includes a first air flow rate computing means (6) for computing the flow rate of intake air from an output of a hot-wire type air flow­ meter in accordance with a first flow rate conversion function, a second air flow rate computing means (13) for computing the intake air flow rate from data con­ cerning a given opening degree of the throttle valve (4) and measured engine speed in accordance with a second flow rate conversion function, and a calibration device (8) for calibrating the first conversion function in accordance with the air flow rate computed by the second air flow rate computing means (13). When the extent of secular change due to contamination of the hot wire of the air flowmeter has exceeded a predeter­ mined limit, the calibration device (8) is put into effect to calibrate the first flow rate conversion function, so that the intake air flow rate can be measured with a high degree of accuracy without being influenced substantially by the secular change of the hot-wire type air flowmeter.

Description: BACKGROUND OF THE INVENTION The present invention relates to an engine control system for internal combustion engines having an intake air-flow sensor capable of measuring the flow rate of intake air, which is improved to eliminate any undesirable effect of a secular change of the sensor caused by contamination of the surface of a sensor element by contaminant particles such as dust and oil particles suspended in the intake air. Current demands for higher performance of internal combustion engines and a higher degree of purification of the exhaust gases from such engines require a higher degree of accuracy of the air-fuel ratio control of the intake mixture. This in turn has given rise to the demand for a higher measuring precision and durability of intake air-flow sensors. A hot-wire type air-flow sensor, for example, encounters a problem in that the measuring accuracy is impaired by contaminants such as dust particles sticking on the surface of the hot-wire sensor element. More specifically, this type of air-flow sensor employs a heat-generating resistor, i.e., a hot-wire, placed in the stream of flowing air such that the rate of heat radiation from the hot wire is changed in accordance with a change in the air-flow rate. The heat-generating resistor constitutes one of four sides of a bridge circuit. The air-flow rate therefore can be measured by detecting the voltage across the heat-generating resistor. The circuit is usually constructed such that the resistance value, i.e., the temperature, of the heat-generating resistor is maintained constant. The heat-generating resistor is made from a material the resistance of which has a large temperature dependency, e.g., of platinum, nickel and so forth, in the form of a wire, foil or a film, independently or in the form of a coil on a bobbin made from ceramics, glass, or a polyimide resin, or adhering to a substrate. The heat-generating resistor in any of such forms will be generally referred to as "hot wire" hereinafter. Dust and other contaminants suspended in the intake air may stick on the hot wire so as to cause a change in the coefficient of heat transfer from the hot wire to the air, resulting in a change in the cooling characteristics of the hot wire. As a consequence, the voltage across the hot wire as the sensor output, necessary for maintaining a constant resistance value of the hot wire, is undesirably changed even if the air-flow rate is maintained constant. The determination of the air-flow rate employs a curve representing the relationship between the sensor output and the air-flow rate. The output characteristic of the sensor, however, undergoes a secular change due to contaminant sticking on the hot wire as described above, so that the measuring accuracy of the air-flow sensor is progressively degraded with the result that the precision of the air-fuel ratio control is impaired. Various methods have been proposed to overcome this problem. For instance, JP-A-54-76182 discloses a method in which a large electric current is supplied to the hot wire to raise its temperature to a level higher than the ordinary operation temperature and to burn the contaminants sticking on the surface of the hot wire thereby. On the other hand, JP-A-59-190624 discloses a method in which an obstacle is disposed upstream of the hot wire as viewed in the direction of the air-flow, so as to reduce the amount of contaminants sticking on the hot wire. However, the method which relies upon heating the hot wire up to a temperature higher than the ordinary operation temperature so as to burn the contaminants sticking on the surface of the hot wire is disadvantageous in that substances such as silicates contained in the dust particles are molten and vitrified to adhere to the hot wire surface more strongly to cause a change in the heat-radiation characteristic of the hot wire. On the other hand, the method which makes use of an obstacle upstream of the hot wire so as to reduce the amount of contaminants sticking on the hot wire encounters a problem in that a noise is uncurred in the sensor output because the stream of air is disturbed by the presence of the obstacle which is provided immediately upstream of the hot wire. Thus, these proposals for eliminating undesirable influences of a secular change of hot-wire type air-flow sensors cannot overcome the above-mentioned problems and, hence, are still unsatisfactory. JP-A-63-88237 relates to a system for correcting the fuel amount to be injected by correcting the calculated intake air amount if the intake air amount calculated from the throttle valve position, preferably the full-open position, and the engine speed differs from the intake air amount calculated on the basis of the output of the air-flow sensor by more than a predetermined amount. However, this document does not disclose how this correction has to be performed, and how the correction is stored or considered for the subsequent operation of the engine. US-A-4 264 961 discloses an apparatus for measuring the intake air-flow rate on the basis of the characteristic of the air-flow sensor which may be calibrated on the basis of the air-flow rate value measured by a separate air-flow sensor of differential pressure type. This known apparatus has the disadvantage that a separate air-flow sensor is required for the recalibration. SUMMARY OF THE INVENTION Accordingly, it is the object of the present invention to provide an engine control system for internal combustion engines in which the air-fuel ratio of the mixture is controlled with a high degree of accuracy, through a compensation for any secular change in the air-flow sensor, thereby overcoming the above-described problems of the prior art. The above object is achieved according to claim 1. The dependent claims relate to preferred embodiments. The engine control system of the present invention comprises an air-flow sensor mounted in the intake passage of an internal combustion engine generating a signal corresponding to the flow rate of the intake air supplied to the internal combustion engine, a throttle valve mounted downstream the air-flow sensor in the intake pasage, a fuel injection device mounted downstream the throttle valve, air-flow signal computing means receiving signals from the air-flow sensor and determining the intake air-flow signal according to a first air-flow signal conversion function based on the signal from the air-flow sensor, and air-flow signal determining means for determining a second air-flow signal in accordance with the engine speed and a predetermined throttle valve opening; it is characterized in that calibration means are provided for calibrating the first air-flow signal conversion function to determine a new first air-flow signal conversion function in accordance with the value of the intake air-flow rate determined by the air-flow signal determining means during driver-commanded idling with forcibly opened throttle valve, and for delivering the new first air-flow conversion function to the air-flow signal computing means. In accordance with one embodiment of this invention, the air-flow signal determining means determine the flow rate of the intake air by calculation from the speed of the engine in accordance with a second flow rate conversion function which defines the relationship between the speed and the flow rate of the intake air as obtained when the throttle valve is fixed at a predetermined opening. The second flow rate conversion function may comprise also the temperature of the ambient air and the pressure of the atmosphere as parameters in addition to the flow rate of the intake air and the engine speed so that the air-flow signal determining means compute the intake air-flow rate from the temperature of the ambient air, the pressure of the atmosphere, the engine speed, and the throttle valve position in accordance with the second flow rate conversion function. The means for opening the throttle valve for setting it at a predetermined opening may include a spring which operates to fully close the throttle valve during idling of the engine, a wire through which the throttle valve is connected to the accelerator pedal so that the opening of the throttle valve can be varied by means of the accelerator pedal, the connection wire being cut at an intermediate portion thereof, and a shape memory alloy connected between the cut ends of the wire, the shape memory alloy being controllable to selectively fully open the throttle valve during idling. The signal from the air-flow sensor is input to the air-flow signal computing means which compute the intake air-flow rate in accordance with the first air-flow signal conversion function. On the other hand, the air-flow signal determining means receive a signal representative of the engine speed under a condition where the throttle valve is fixed at a predetermined opening, and compute the intake air-flow rate in accordance with the second air-flow signal conversion function. The calibration means calibrate the first air-flow signal conversion function of the air-flow signal computing means on the basis of the intake air-flow rate determined by the air-flow signal determining means. When the second air-flow signal conversion function comprises the ambient air temperature and the atmospheric pressure as the computing parameters in addition to the engine speed, the accuracy of determination of the intake air-flow rate by the air-flow signal determining means is enhanced. The throttle valve is normally fully closed during idling of the engine. The shape memory alloy connected in the wire for operating the throttle valve acts to forcibly open and to fix the throttle valve at the full open position during idling, thereby enabling computation of the air-flow rate by the air-flow signal determining means. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustration of an intake system of an internal combustion engine, shown together with a block diagram of a fuel injection system, a hot-wire type air-flow sensor and a calibration device; Fig. 2 is a flow chart showing a process for performing a flow rate signal conversion and calibration of a characteristic curve; and Figs.3 and 4 are schematic illustrations of a throttle valve opening setting device used in the arrangement of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described with reference to Figs. 1 to 4. Fig. 1 schematically shows parts constituting an intake passage of an internal combustion engine, together with a hot-wire type air-flow sensor 5 and a calibration device. Ambient air is sucked into a suction chamber 3 of the engine through an air intake 1 via a passage 2. A throttle valve 4 for controlling the flow rate of the intake air is disposed between the intake 1 and the suction chamber 3. The engine comprises a hot-wire type air-flow sensor of the type disclosed, for example, in US patent application serial No. 207,525. This air-flow sensor 5 has a sensor element disposed in the passage 2 at a position immediately upstream of the throttle valve 4. Downstream of the throttle valve 4, a fuel injector 7 is disposed which may be of the type disclosed in US patent application serial No. 211,261. The flow rate of the intake air flowing through the passage 2 is usually determined in the form of a flow rate signal which is obtained by processing the output from the air-flow sensor 5 through air-flow signal computing means 6 with a memory which stores a characteristic curve g representing the relationship between the output signal and the flow rate signal. Next, a description will be given of the calibration device 8. This device 8 is adapted to be put into operation manuelly or automatically during idling of the engine or cruising of an automobile carrying the engine, when the extent of the secular change of the hot-wire type air-flow sensor has exceeded a predetermined value, e.g., when the total travel distance of the automobile has exceeded a predetermined value, or when the difference between the intake air-flow rate determined from the throttle valve opening ϑ₀ and the engine speed Nj and the flow rate measured by the air-flow sensor 5 has exceeded a predetermined limit. When the calibration device 8 is put into operation, the opening degree of the throttle valve 4 is set at ϑ₀ by a signal from a valve opening setting device 9. Then, the fuel injection rate is set to Gj by a signal given from the fuel injection rate setting device 10 while the throttle opening ϑ₀ is maintained. As a result, the output from an engine speed sensor 11 is set constant at Nj. The engine speed Nj is input to air-flow signal determining means 13 which also receive signals T₀ and P₀ representing the temperature and the pressure of the ambient air derived from a temperature/pressure sensor 12 installed separately from the air-flow sensor 5. The means 13 have a memory which stores data concerning the air-flow rate M at the throttle valve opening ϑ₀ with parameters of a standard ambient air temperature T s and a standard atmospheric pressure P s . Upon receipt of the signals representing the engine speed Nj and the temperature and pressure of the ambient air, therefore, the air-flow signal determining means 13 compute the air-flow rate signal Mj which corresponds to the set values of the throttle valve opening ϑ₀ and the engine speed Nj. The air-flow rate signal Mj representing the thus determined air-flow rate is sent to a characteristic curve calibration circuit 14 which also receives an output signal e j from the air-flow sensor 5. Using these signals, the calibration circuit 14 forms a pair of calibration data. This operation is repeated n times while varying values of the fuel injection rate G j so that the relationship between the air-flow rate signal Mj and the sensor output e j is determined, so that a recalibration of the characteristic curve g is conducted to determine a new characteristic curve. The calibration device 8 stops its operation after delivering the new characteristic curve to the air-flow signal computing means 6. In the following, a description will be given of the processes for the flow rate conversion and calibration of the characteristic curve, with reference to the flow chart shown in Fig. 2. The flow shown in Fig. 2 is executed by the software of a microcomputer which constitutes the air-flow signal determining means 13 and the characteristic curve calibration circuit 14 of Fig. 1. In Step 21, a function f = f(N) representing the relationship between the engine speed and the air-flow rate is computed by least squares method, from previously input data concerning the engine speed Nj and the air-flow rate f j (j = 1, 2, ..., M) at the standard ambient air temperature and pressure T s and P s . In Step 22, j is set as j = 0, and data necessary for the present air-flow rate computation are input in Step 23. In Step 24, the air flow rate signal Mj at the engine speed Nj under the standard atmospheric condition is determined in accordance with the formula Mj = f(Nj), and, in Step 25, the air-flow rate signal Mj at air temperature T₀ is determined by correcting the air-flow rate signal Mj computed in Step 24. In Step 26, the air flow rate signal Mj and the sensor output signal e j are stored as a set of data. In Step 27, the preceding steps 23 to 26 are repeated. The number of repetition corresponds to the number j max of the data of the engine speed. Subsequently, using these data (Mj, e j ) (j = 1, 2, ..., jmax), the relationship between the air-flow rate M and the sensor output signal e, represented by M = g(e), is determined by means of the least squares method, whereby a new characteristic curve correctly representing the relationship between the sensor ouput and the intake air-flow rate can be obtained. Because of this calibration, the sensor output e is located on the new characteristic curve of the intake air-flow rate and the fuel injector 7 is controlled in accordance with the thus determined intake air-flow rate so as to provide the desired air-fuel ratio of the mixture. The above-described calibration can be accomplished in quite a short time. Alternatively, the calibration may be conducted with reference to a data map which stores data of the air-flow rate signal Mj in relation to the throttle valve opening ϑ₀ and the engine speed N j . When the requirements for the measuring accuracy are not so strict, the computation of the air-flow rate signal may be executed on the basis of the assumption that the atmospheric pressure P₀ is constant. In such a case, the term of the pressure P₀ is omitted from the function f for determining the air-flow rate, so that the formula can be simplified advantageously. The function f may be set in the factory at the time of assembly of the engine or, alternatively, determined on the basis of the output signal from the air-flow sensor 5 in the running-in period of the engine. Fig. 3 illustrates a device for setting the throttle valve opening at ϑ₀ (full opening). In general, when the engine is idling, the throttle valve 4 is kept in fully closed position by the force of a spring 21, because no tension is applied to the wire 20. However, when the calibration device 8 is put into operation, the shape memory alloy 22 constituting a portion of the wire 20 contracts due to heat generated as a result of supply of electrical power, so as to tense the wire 20, whereby the throttle valve 4 is opened against the force of the spring 21 and is fixed by a stopper 23 at the valve opening ϑ₀ (full opening). Reference numeral 24 denotes the accelerator pedal. Fig. 4 shows an arrangement in which the throttle valve opening is controlled by an electric motor 31, comprising a reduction gear 32. The throttle valve opening can be set freely by suitably setting the angle of rotation of the rotor of the electric motor. The calibration of a hot-wire type air-flow sensor would be possible by the use of a separate hot-wire type air-flow sensor intended exclusively for calibration, or a Pitot-tube type flowmeter. These measures, however, are not recommended because such a system is complicated due to the provision of two air-flow sensors. In contrast thereto, in the present invention, the calibration can advantageously be enabled by a simple modification of parts which are originally installed on the engine. As has been described, the intake air-flow rate is determined from the engine speed while setting the throttle valve at a constant opening during idling of the engine. Simultaneously, the air-flow rate is measured with the air-flow sensor. The measuring characteristic of the air-flow sensor is then calibrated on the basis of the intake air-flow rate determined from the engine speed. It is therefore possible to avoid any deterioration in the measuring accuracy of the air-flow sensor, and therefore, to precisely control the air-fuel ratio of the mixture to be fed to the engine. The accuracy of the calibration can be enhanced by adopting, in addition to the speed of the engine, the temperature and the pressure of the ambient air as parameters. The use of a shape memory alloy as a part of the member for actuating the throttle valve provides represents simple means for setting the throttle valve to a predetermined opening during idling of the engine.

Claims: Engine control system comprising an air-flow sensor (5) mounted in the intake passage (2) of an internal combustion engine, a throttle valve (4) mounted downstream the air-flow sensor (5) in the intake passage (2), a fuel injection device (7) mounted downstream the throttle valve (4), air-flow signal computing means (6) receiving signals (e) from the air-flow sensor (5) and determining the intake air-flow signal (M) according to a first air-flow signal conversion function (M=g(e)) based on the signal from the air-flow sensor (5), and air-flow signal determining means (13) for determining a second air-flow signal (Mj) in accordance with the engine speed (Nj) and a predetermined throttle valve opening (ϑ₀), characterized in that calibration means (14) are provided for calibrating the first air-flow signal conversion function (M=g(e) ) to determine a new first air-flow signal conversion function (M=g(e) ) in accordance with the value of the intake air flow rate (Mj) determined by the air-flow signal determining means (13) during driver-commanded idling with forcibly opened throtlee valve, and for delivering the new first air-flow conversion function to the air-flow signal computing means (6). The system according to claim 1, characterized in that the air-flow signal determining means (13) compute the intake air-flow rate (Mj) in accordance with a second air-flow signal conversion function (Mj=f(Nj, ϑ₀)). The system according to claim 1, characterized in that the air-flow signal determining means (13) reads the intake air flow rate (Mj) from a data map in dependence of the throttle valve opening (ϑ₀) and the engine speed (Nj). The system according to one of claims 1 to 3, characterized in that the air-flow signal determining means (13) determine the intake air-flow rate (Mj) with use of the temperature (T₀) and the pressure (P₀) of the ambient air as parameters. The system according to one of claims 1 to 4, characterized by means for setting the throttle valve (4) at a predetermined opening (ϑ₀), comprising a spring (21) for closing the throttle valve (4) during the idling state of the internal combustion engine, a wire (20) connecting the throttle valve (4) with an accelerator pedal (24) so that the opening (ϑ₀) of the throttle valve (4) can be changed by the accelerator pedal (24), the wire (20) being cut at an intermediate portion thereof, and a shape memory alloy (22) being connected between the cut ends of the wire (20), the shape memory alloy (22) being controllable to selectively fully open the throttle valve (4) during the idling state of the internal combustion engine. The system according to one of claims 1 to 5, characterized in that the calibration means (14) are correcting the first air-flow signal conversion function (M=g(e) ) when the difference between the value of the intake air-flow signal (M) determined by the air-flow signal computing means (6) and the air-flow signal (Mj) determined by the air-flow signal determining means (13) exceeds a predetermined value.

Inventors: KOBAYASHI, JUNICHI, MIZUMORI, TAKASHI
Applicants: HITACHI, LTD.
Application year: 1989
Main classification: F02D41
IPC codes: F02D41