Air-fuel ratio control device for internal combustion engine

Abstract: An air-fuel ratio control device for an internal combustion engine controls an air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio by the use of a factor determined in accordance with inlet pipe pressure and engine speed. A throttle opening degree sensor is provided to detect a degree of throttle opening, and on the basis of the degree of throttle opening, the factor determined in accordance with the inlet pipe pressure and the engine speed is corrected in a high load range of the engine. Since the output of the throttle opening degree sensor in the high load range of the engine is more accurate than the output of a pressure sensor for detecting the inlet pipe pressure, the air-fuel ratio can be controlled accurately in the high load range by correcting the factor in accordance with the degree of throttle opening.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine of the lean-burn control type, wherein the air-fuel ratio is controlled to a lean target air-fuel ratio rather than to a stoichiometric air-fuel ratio; in other words, a type wherein a lean mixture is used. Description of the Related Art: Generally, a basic fuel injection time is determined on the basis of engine speed and inlet pipe pressure or intake air quantity, which is then further corrected in accordance with engine cooling water temperature, intake air temperature, and so on, to determine an execution fuel injection time, on the basis of which fuel injection is performed. In addition, a lean-burn control system is known in which the air-fuel ratio is controlled in the lean air-fuel ratio range rather than to a stoichiometric air-fuel ratio. Since the peak value of NOx generation is normally found somewhat on the lean side, deviating from the stoichiometric air-fuel ratio, the air-fuel ratio in the lean-burn control system is controlled to the lean side beyond a level corresponding to the peak of NOx generation for the purpose of reducing NOx generation and to improve fuel consumption. Japanese Patent Application Laid-Open No. 62-199943 discloses a system in which lean-burn control is performed by determining a lean correction factor on the basis of inlet pipe pressure and engine speed, and by multiplying the basic fuel injection time by the lean correction factor. A pressure sensor for detecting inlet pipe pressure is accurate in low and medium load ranges where a degree of opening of a throttle valve is small; however, in a high load range, the change of output of the sensor is small in comparison to the change of opening of the throttle valve. That is, the resolving power of the sensor becomes degraded. Particularly, while a vehicle is running at high altitudes (high-altitude atmospheric pressure PA is lower than low altitude atmospheric pressure PAo), the output of the pressure sensor in the high load range (where inlet pipe pressure PM is substantially equal to the atmospheric pressure PA) changes little and not in proportion to the change of opening of the throttle valve. That is, an air quantity being sucked into a combustion chamber of the engine cannot be detected accurately in the high load range by the pressure sensor. Therefore, an adequate lean correction factor cannot be obtained in the high load range, with the result that lean burn control can not accurately be performed. A like a problem also arises when the lean correction factor is determined using an air-flow meter for detecting the intake air quantity instead of the inlet pipe pressure. Furthermore, from document EP-A 164 558, a control device for controlling an air-fuel ratio and a spark timing of an internal combustion engine is known, in which a lean air-fuel ratio is realized and the ignition timing is controlled in relation to the change of the air-fuel ratio. From a predetermined map, a fundamental fuel injection value FI and a correction factor KLEAN are first calculated according to values of engine speed Ne and intake air pressure Pm. Depending on the position of a throttle valve opening detection switch LS, when the engine is not in idle state, the correction factor KLEAN is further modified to predetermined specific air-fuel ratios according to the available throttle valve opening ranges below or above a predetermined throttle valve opening threshold given by predetermined positions of the switch LS. When the throttle valve opening degree exceeds the threshold defined by the switch LS, the correction factor KLEAN remains based on the intake air pressure and is compared to a lean limit value and set to a predetermined maximum lean limit air-fuel ratio when the actual air-fuel ratio is determined to be above the lean limit value, whereas when the actual air-fuel ratio is determined to be below the lean limit value, the correction factor KLEAN is not modified and, thus, not dependent on the degree of the throttle valve opening. Moreover, document EP-A 163 955 discloses an apparatus for controlling the ignition timing of an internal combustion engine in accordance with changes in the engine operating conditions. In this apparatus, a basic injection signal pulse width TP is determined by the use of a predetermined map in accordance with the values of the engine speed NE and the absolute pressure PM. An injection signal pulse width TAU is calculated in accordance with the basic injection pulse signal TP, and a lean correction coefficient KLEAN is used for changing the desired air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. A first value of KLEAN based on engine speed NE and the absolute pressure PM is modified dependent on the throttle valve opening degree given by the position of switches LS and VL. A final air-fuel ratio is calculated on the basis of the obtained correction factor KLEAN which serves as a condition for the selection of an ignition timing. However, when the throttle valve opening degree exceeds the first threshold defined by the switch LS, the correction factor KLEAN is not determined according to the actual throttle valve opening degree, but rather set to several specific values corresponding to other engine operating conditions or being determined by a comparison with predetermined maximum values, and when the throttle valve opening degree exceeds the second threshold defined by the switch VL, the correction factor KLEAN is merely set to a predetermined value corresponding to the stoichiometric air-fuel ratio. SUMMARY OF THE INVENTION It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine which can accurately perform lean-burn control in the high load range as well as in the low and medium load ranges. To achieve the foregoing object, the air-fuel ratio control device for an internal combustion engine according to the present invention comprises means for detecting one of an inlet pipe pressure and an intake air quantity, means for detecting an engine speed, means for detecting a degree of throttle opening, means for calculating a basic fuel injection time on the basis of the engine speed and the one of the inlet pipe pressure and the intake air quantity, means for calculating a correction factor on the basis of the engine speed and the one of the inlet pipe pressure and the intake air quantity that is used for controlling the air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio, means for controlling the air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio on the basis of the basic fuel injection time and the correction factor when the degree of throttle opening is besides substantially full-open, and means for correcting the correction factor on the basis of at least the degree of throttle opening when the degree of the throttle opening exceeds a given value in a high load range of the engine when the air-fuel ratio is controlled to the lean side, wherein the given value of the degree of throttle opening is set so as to increase as the engine speed increases. The means for calculating a basic fuel injection time, the means for calculating a correction factor and the means for controlling the air-fuel ratio are included in a control means C. According to the present invention, when the detection value of the throttle opening degree detection sensor exceeds a given but with the engine speed increasing level indicating the high load range, the correction factor determined on the basis of engine speed and either inlet pipe pressure or intake air quantity is corrected in accordance with a correction value determined in accordance with at least the degree of throttle opening. Since the degree of throttle opening is detected accurately in the high load range, and inadequate correction factor based on the inlet pipe pressure can be corrected and changed to an adequate correction factor in the high load range, whereby an accurate lean-burn control can he performed in the high load range as well as in low and medium load ranges. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a block diagram explaining the present invention; Fig. 1B is a schematic diagram of an internal combustion engine to which the present invention is applied; Fig. 2 is a block diagram showing in greater detail a control device shown in Fig. 1B; Fig. 3 is a control flow chart showing a fuel injection time calculation routine including lean-burn control; Fig. 4 is a characteristic graph showing a lean-burn control factor calculated in relation to inlet pipe pressure and throttle opening; Fig. 5 is a distribution characteristic graph showing a correction factor in relation to engine speed and inlet pipe pressure; and Fig. 6 is a distribution characteristic graph showing a correction factor in relation to engine speed and throttle opening. DESCRIPTION OF THE PREFERRED EMBODIMENT An internal combustion engine equipped with a control device according to the present invention will now be described in detail with reference to the drawings. Fig. 1B schematically shows an internal combustion engine. An intake air temperature sensor 14 for detecting an intake air temperature is provided in the vicinity of an air cleaner 10. Downstream, a throttle valve 12 is provided whose opening is controlled by an accelerator pedal. Attached to the throttle valve 12 is a throttle opening degree sensor 16 for delivering a signal proportional to the degree of opening of the throttle valve 12. One end of a pipe 15 is connected downstream from the throttle opening degree sensor 16 to an inlet pipe so as to communicate with the inlet pipe. Attached to the other end of the pipe 15 is a semiconductor pressure sensor 13 which detects the absolute pressure of the inlet pipe or, in other words, inlet pipe pressure. Downstream from the throttle valve 12 is a surge tank 18 which communicates with combustion chambers formed in an engine body through an intake manifold 20. A fuel injection valve 22 for each cylinder projects into the intake manifold 20. The combustion chambers formed in the engine body communicate with a catalyst unit 25 filled with catalytic converter rhodium through an exhaust manifold 24. Attached to the exhaust manifold 24 is an O₂ sensor 26 which detects the density of residual oxygen in exhaust gas and delivers a signal whose polarity is inverted at the point of a stoichiometric air-fuel ratio. Attached to an engine block of the engine body is a water temperature sensor 28 for detecting an engine cooling water temperature, which projects through the engine block into a water jacket. Each cylinder of the engine body is provided with a spark plug 46, which projects through a cylinder head into the combustion chamber, and which is connected via a distributor 48 and an ignitor 50 to a control circuit 52. Provided inside the distributor 48 is a rotational angle sensor 54 which comprises a signal rotor secured to a distributor shaft and a pickup secured to a distributor housing. The rotational angle sensor 54 outputs an engine speed signal to the control circuit 52 in the form of a pulse train with one pulse being generated for example, every 30 degrees, of CA (crank angle). The control circuit 52 includes a microcomputer. Specifically, as shown in Fig. 2, the control circuit 52 comprises a RAM 56, a ROM 58, an MPU 60, an input/output port 62, an input port 64, output ports 68 and 70, and a bus 72 including a data bus, a control bus, etc. The input/output port 62 is connected to an analog-to-digital converter (A-D converter) 74 and a multiplexer 76. The multiplexer 76 is respectively connected through a buffer 75 to the inlet pipe pressure sensor 13, through a buffer 78 with the water temperature sensor 28, through a buffer 80 with the throttle opening degree sensor 16, and through a buffer 821 with the intake air temperature sensor 14. The MPU 60 controls the A-D converter 74 and the multiplexer 76 via the input/output port 62, successively converts the outputs of the pressure sensor 13, water temperature sensor 28, intake air temperature sensor 14, and throttle opening degree sensor 16 from analog to digital, and stores them in digital form in the RAM 56. The O₂ sensor 26 is connected through a comparator 84 and a buffer 86 to the input port 64. The rotation angle sensor 54 is connected through a waveform shaping circuit 88 to the input port 64. The output port 68 is connected through a drive circuit 92 to the ignitor 50. The output port 70 is connected through a drive circuit 94 provided with a down counter to the fuel injection valve 22. In the drawings, 96 designates a clock, and 98 a timer. Previously stored in the ROM 58 are a control routine program, a basic ignition timing table, a basic fuel injection time table, and the like. Basic fuel injection time TP is calculated using the basic fuel injection time table and on the basis of the inlet pipe pressure defined by the output of the inlet pipe pressure sensor 13 and the engine speed defined by the output of the rotational angle sensor 54 as will be described later. This basic fuel injection time TP is corrected on the basis of the outputs of the intake air temperature sensor 14, the O₂ sensor 26, and the water temperature sensor 28, whereby an execution fuel injection time TAU is obtained. Similarly to the calculation of the basic fuel injection time TP, a basic ignition timing A BASE is calculated using the basic ignition timing table and on the basis of the outputs of the inlet pipe pressure sensor 13 and the rotational angle sensor 54, and corrected on the basis of the outputs of the intake air temperature sensor 14, the water temperature sensor 28, and the like, whereby an execution ignition timing SA is obtained. A control routine of the embodiment will now be described with reference to the flow chart (Fig. 3). The calculation and execution routines for the execution ignition timing SA are identical with those used in controlling a conventional electronically-controlled internal combustion engine and, thus, will not be described. In step 100, engine speed NE, inlet pipe pressure PM, and throttle opening TA are read. In step 102, a correction factor KAFB is read from an NE-PM characteristic map as shown in Fig. 5 on the basis of the inlet pipe pressure. In step 104, a correction factor KTAAF is read from an NE-TA characteristic map as shown in Fig. 6 on the basis of the degree of throttle opening. In step 106, the value of the factor KAFB read in step 102 is multiplied by the value of the factor KTAAF read in step 104, whereby a lean control factor KAF is obtained as below: KAF = KAFB · KTAAF (l) As shown in Fig. 6, the correction factor KTAAF based on the degree of throttle opening is equal to one (l) when the degree of throttle opening TA is smaller than a given value, and the lean correction factor KAF of the expression (l) is influenced only by the correction factor KAFB based on the inlet pipe pressure. When the degree of throttle opening exceeds a given value, the correction factor KTAAF based on the degree of throttle opening becomes smaller than one (l); therefore, the lean control factor KAF is influenced by both the correction factor KAFB based on the inlet pipe pressure and the correction factor KTAAF based on the degree of throttle opening. Accordingly, in a range where the degree of throttle opening is larger than a given value, the lean control factor decreases as the degree of the throttle opening increases even if the inlet pipe pressure PM and the engine speed NE show no change. As shown in Fig. 6, the degree of throttle opening corresponding to the correction factor KTAAF being smaller than one (l) increases as the engine speed NE increases. Further, at "wide open throttle (WOT)" or a degree of throttle opening TA2 near "full load", the correction factor KTAAF is zero (0), so that the lean control factor KAF also becomes zero (0); therefore, as will be understood from expressions (2) and (3) as described later, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. In step 108, an execution air-fuel ratio correction factor KAFS is calculated in accordance with the following expression: KAFS = (l - KAF) (2) In step 110, the basic fuel injection time TP is calculated on the basis of the inlet pipe pressure PM and the engine speed NE. The basic fuel injection time TP is corrected on the basis of the engine cooling water temperature (the output of the water temperature sensor 28), the intake air temperature (the output of the intake air temperature sensor 14), and the like, whereby the execution fuel injection time TAU is obtained. In this embodiment, lean-burn control is performed using the air-fuel ratio correction factor KAFS. That is, the execution fuel injection time TAU is calculated in accordance with the following expression: TAU = (A · TP) · KAFS + B (3) where A and B are correction factors determined in accordance with the engine cooling water temperature, the intake air temperature, and the like. After the execution fuel injection time TAU is calculated, the fuel injection execution routine controls the fuel injection valve 22 on the basis of the execution fuel injection time TAU, whereby fuel injection is performed. The characteristic of the lean control factor KAF calculated in accordance with the expression (l), which is dependent on a load change, will now be described with reference to Fig. 4, where high altitude running and low altitude running of the engine are considered. When the degree of throttle opening TA becomes equal to a given opening TA1, the inlet pipe pressure becomes such that the pressure during low altitude running (for example, the atmospheric pressure PAo) is higher than the pressure during high altitude running (for example, the atmospheric pressure PA). During high altitude running, the lean control factor KAF reaches a peak value when TA equals TA1. When the degree of throttle opening TA exceeds a given value TA1, the correction factor KTAAF based on the degree of throttle opening TA is influenced, so that the lean control factor KAF decreases gradually from its value before being influenced by the high altitude running mode during high altitude running, or from its value before being influenced of the low altitude running mode during low altitude running, and becomes zero (0) when TA equals TA2. In this way, in the high load range, the setting of the lean control factor KAF by the correction factor KAFB based on the inlet pipe pressure PM is not switched to setting the lean control factor KAF by the correction factor KTAAF based on the degree of throttle opening TA. Instead, in the high load range, the correction factor KAFB based on the inlet pipe pressure PM is influenced by the correction factor KTAAF based on the degree of throttle opening TA. Therefore, the target air-fuel ratio can be varied smoothly irrespective of whether the altitude is high or low. As described above, according to the present embodiment, the lean-burn control process in the high load range is influenced by the correction factor KTAAF based on the degree of throttle opening TA. Therefore, accurate lean-burn control can be performed in all load ranges, thereby resulting in improved driveability, driving force output, fuel consumption, etc. It should be noted that the intake air quantity may be used in place of the inlet pipe pressure, and the correction factor KTAAF may be determined in accordance with only the degree of throttle opening TA.

Claims: An air-fuel ratio control device for an internal combustion engine, comprising means (13) for detecting one of an inlet pipe pressure (PM) and an intake air quantity, means (54) for detecting an engine speed (NE), means (16) for detecting a degree of throttle opening (TA), means (C, 52) for calculating a basic fuel injection time (TP) on the basis of the engine speed (NE) and the one of the inlet pipe pressure (PM) and the intake air quantity, means (C, 52) for calculating a correction factor (KAFB) which is used for controlling the air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio on the basis of the engine speed (NE) and the one of the inlet pipe pressure (PM) and the intake air quantity, means (C, 52) for controlling the air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio on the basis of the basic fuel injection time (TP) and the correction factor (KAFB) when the degree of throttle opening (TA) is besides substantially full-open, and means (E, 52) for correcting the correction factor (KAFB) on the basis of at least the degree of throttle opening (TA) when the degree of the throttle opening exceeds a given value in a high load range of the engine when the air-fuel ratio is controlled to the lean side, wherein the given value of the degree of throttle opening (TA) is set so as to increase as the engine speed (NE) increases. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the correcting means (E, 52) corrects the correction factor (KAFB) such that the air-fuel ratio approaches the stoichiometric air-fuel ratio as the degree of throttle opening (TA) increases. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the correcting means (E, 52) corrects the correction factor (KAFB) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 2, wherein the correcting means (E, 52) corrects the correction factor (KAFB) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the correcting means (E, 52) corrects the correction factor (KAFB) on the basis of the degree of throttle opening (TA) and the engine speed (NE). An air-fuel ratio control device for an internal combustion engine according to claim 5, wherein the correcting means (E, 52) corrects the correction factor (KAFB) such that the air-fuel ratio approaches the stoichiometric air-fuel ratio as the degree of throttle opening (TA) increases. An air-fuel ratio control device for an internal combustion engine according to claim 5, wherein the correcting means (E, 52) corrects the correction factor (KAFB) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full open. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the correction factor correcting means (E, 52) includes an additional correction factor calculating means (E, 52) for calculating an additional correction factor (KTAAF) on the basis of at least the degree of throttle opening (TA) that is used in correcting the correction factor (KAFB) only when the degree of throttle opening (TA) exceeds a given value , and the air-fuel ratio controlling means (C, 52) includes control means for correcting the basic fuel injection time (TP) in accordance with the correction factor (KAFB) and the additional correction factor (KTAAF) and for controlling the air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio in accordance with the thus corrected basic fuel injection time (TP) when the degree of throttle opening (TA) is besides substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that the air-fuel ratio approaches the stoichiometric air-fuel ratio as the degree of throttle opening (TA) increases. An air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 9, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) on the basis of the degree of throttle opening (TA) and the engine speed (NE). An air-fuel ratio control device for an internal combustion engine according to claim 12, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that the air-fuel ratio approaches the stoichiometric air-fuel ratio as the degree of throttle opening (TA) increases. An air-fuel ratio control device for an internal combustion engine according to claim 12, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that the air-fuel ratio becomes identical with the stoichiometric air-fuel ratio when the degree of throttle opening (TA) becomes substantially full-open. An air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the given value of the degree of throttle opening (TA) is set so as to increase as the engine speed (NE) increases. An air-fuel ratio control device for an internal combustion engine according to claim 8, wherein the control means (52) controls the air-fuel ratio in accordance with A * TP(1 - KAFB * KTAAF) + B , where A and B are constants, TP is the basic fuel injection time, KAFB is the correction factor, KTAAF is the additional correction factor. An air-fuel ratio control device for an internal combustion engine according to claim 16, wherein the second correction factor calculating means (E, 52) calculates the additional correction factor (KTAAF) such that in a range where the degree of throttle opening (TA) exceeds a given value, the second correction factor (KTAAF) gradually decreases from a value close to and smaller than one (1) to zero (0) as the degree of throttle opening (TA) increases. An air-fuel ratio control device for an internal combustion engine according to claim 17, wherein the degree of throttle opening (TA) at which the second correction factor (KTAAF) becomes smaller than one (1) increases as the engine speed (NE) increases.

Inventors: KAWAI TAIYO, NAKAGAWA NARIHISA, KAWAI, TAIYO, NAKAGAWA, NARIHISA
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1990
Main classification: F02D41
IPC codes: F02D41