EP0296323B1

EP0296323B1 - Engine control method

Info

Publication number: EP0296323B1
Application number: EP88106047A
Authority: EP
Inventors: Mineo Kashiwaya; Kiyomi Morita; Masahide Sakamoto
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1982-11-24
Filing date: 1983-11-23
Publication date: 1991-03-20
Anticipated expiration: 2003-11-23
Also published as: EP0296323B2; EP0296323A3; EP0296323A2

Description

ENGINE CONTROL METHOD

CROSS REFERENCE OF RELATED

APPLICATIONS

This application relates to the subject-matter of EP-A-110312.
The present invention relates to an engine control method for a car employing a microcomputer, and particularly to an engine control method in which the engine revolution can be controlled stably and/or smoothly in idle running.
Recently, general control for an engine is performed by using a microcomputer for the purpose of improvement in engine control performance.
Various functions are required for the engine control depending on the kind or type/use of car, and, therefore, in the engine control system utilizing a microcomputer, a general purpose software, that is a software in which correction, modification or addition can be effected onto the various control functions depending on the kind/use of car, is required in view of improvement in cost and/or in controllability.
Conventionally, in idle running, that is, in the ON state of an idling switch, the ON duty factor of a bypass valve is determined on the basis of the sum of a value determined in accordance with the cooling water of the engine and a value representing the quantity of feedback of the number of engine revolution for controlling the number of engine revolution to be a reference number of engine revolution for idle running. In idle running, however, it is difficult to control the number of engine revolution to be the reference number of engine revolution for idle running stably and/or smoothly, when the engine state is changed from normal running to idle one.
GB-A-2 053 508 discloses an intake air flow rate control system for an internal combustion engine having an open-loop control system carried out in unstable engine driving condition. In the open-loop control, the pulse signal applied to an airflow rate control valve means is determined both by open loop ratio and feedback ratio. The pulse duty of the pulse signal can also be varied corresponding to engine load condition or an engine starting condition.
GB-A-2 073 451 discloses an idling speed control system for an internal combustion machine. The control operation is thereby based on a cooling watertem- perature of the engine in either of open-loop control and feedback control mode. In cases of shifting from an open-loop control to a closed-loop (feedback) control, at first a large amount of intake air is supplied to an engine and then the amount of intake air is gradually decreased, thereafter shifting to the closed-loop control.
DE-A-3 138 058 discloses that when a target engine speed is changed to a new target speed, a duty factor is not controlled so as to trace the new target speed but a target speed is gradually changed toward the new target speed and the duty factor is changed so as to trace the gradually changing target speed.
In contrast to the above cited publications the technical concept of the present invention resides in that the feedback control of the duty factor is not carried out immediately after the change of an idle switch from an OFF state to an ON state but carried out when the engine speed reached to a speed which is slightly higher than a desired engine speed.
An object of the present invention is to provide an engine control method in which the bypass valve ON duty factor is controlled so that the number of engine revolution can be smoothly changed when the idling switch is turned ON from the OFF state, that is when the engine state is changed from normal running to idle one.
This object is achieved according to the present invention by an engine control system which is characterized by the features recited in the claim.
The above and otherobjects, features and advantages of the present invention will be more clear from the following description with reference to the accompanying drawings, in which :

Fig. 1 is a schematic diagram of the control device for the whole of the engine system;
Fig. 2 is a block diagram generally illustrating the engine control system;
Fig. 3 (A)-(F) is a time chart from OFF to ON of the idling switch ;
Fig. 4 (A)-(D) is a time chart of the number of engine revolution and the ISC duty factor when the engine brake is actuated ;
Fig. 5 (A) is a time chart of the reduction in the number of engine revolution ;
Fig. 5 (B) is a time chart of the ISC duty factor ;
Fig. 6 is a flowchart of the ISC duty factor control in the loaded state.

Referring to the drawings the present invention will be described hereunder.
In Fig. 1 a control apparatus for the whole of an engine system is illustrated. In Fig. 1, suction air is supplied to a cylinder 8 through an air cleaner 2, a throttle chamber 4, and a suction pipe 6. A gas burnt in the cylinder 8 is discharged from the cylinder 8 to the atmosphere through an exhaust pipe 10. An injector 12 for injecting fuel is provided in the throttle chamber 4. The fuel injected from the injector 12 is atomized in an air path of the throttle chamber 4 and mixed with the suction air to form a fuel-air mixture which is in turn supplied to a combustion chamber of the cylinder 8 through the suction pipe 6 when a suction valve 20 is opened.
Throttle valves 14 and 16 are provided in the vicinity of the output of the injector 12. The throttle valve 14 is arranged so as to mechanically interlocked with an accelerator pedal (not shown) so as to be driven by the driver. The throttle valve 16 is arranged to be driven by a diaphragm 18 such that it becomes its fully close state in a range where the air flow rate is small, and as the air flow rate increases the negative pressure applied to the diaphragm 18 also increases so that the throttle valve 16 begins to open, thereby suppressing the increase of suction resistance.
An air path 22 is provided at the upper stream of the throttle valves 14 and 16 of the throttle chamber 4 and an electrical heater 24 constituting a thermal air flow rate meter is provided in the air path 22 so as to derive from the heater 24 an electric signal which changes in accordance with the airflow velocity which is determined by the relation between the air flow velocity and the amount of heat transmission of the heater 24. Being provided in the air path 22, the heater 24 is protected from the high temperature gas generated in the period of back fire of the cylinder 8 as well as from the pollution by dust or the like in the suction air. The outlet of the air path 22 is opened in the vicinity of the narrowest portion of the venturi and the inlet of the same is opened at the upper stream of the venturi.
Throttle opening sensors (not shown in Fig. 1 but generally represented by a throttle opening sensor 116 in Fig. 2) are respectively provided in the throttle valves 14 and 16 for detecting the opening thereof and the detection signals from these throttle opening sensors, that is the sensor 116, are taken into a multiplexer 120 of a first analog-to-digital converter as shown in Fig. 2.
The fuel to be supplied to the injector 12 is first supplied to a fuel pressure regulator 38 from a fuel tank 30 through a fuel pump 32, a fuel damper 34, and a filter 36. Pressurized fuel is supplied from the fuel pressure regulator 38 to the injector 12 through a pipe 40 on one hand and fuel is returned on the other hand from the fuel pressure regulator 38 to the fuel tank 30 through a return pipe 42 so as to maintain constantthe difference between the pressure in the suction pipe 6 into which fuel is injected from the injector 12 and the pressure of the fuel supplied to the injector 12.
The fuel-air mixture sucked through the suction valve 20 is compressed by a piston 50, burnt by a spark produced by an ignition plug 52, and the combustion is converted into kinetic energy. The cylinder 8 is cooled by cooling water 54, the temperature of the cooling water is measured by a water temperature sensor 56, and the measured value is utilized as an engine temperature. A high voltage is applied from an ignition coil 58 to the ignition plug 52 in agreement with the ignition timing.
A crank angle sensor (not shown) for producing a reference angle signal at a regular interval of predetermined crank angles (for example 180 degrees) and a position signal at a regular interval of a predetermined unit crank angle (for example 0.5 degrees) in accordance with the rotation of engine, is provided on a not-shown crank shaft.
The output of the crank angle sensor, the output 56A of the water temperature sensor 56, and the electrical signal from the heater 24 are inputted into a control circuit 64 constituted by a microcomputer or the like so that the injector 12 and the ignition coil 58 are driven by the output of this control circuit 64.
In the engine system controlled by the arrangement as described above, a bypass 26 bypassing the throttle valve 16 to communicate with the suction pipe 6 is provided and a bypass valve 62 is provided in the bypass 26. A control signal is inputted to a drive section of the bypass valve 62 from the control circuit 64 to control the opening of the bypass valve 62.
That is, the opening of the bypass valve 62 is controlled by a pulse current such that the cross-sectional area of the bypass 26 is changed by the amount of lift of valve which is in turn controlled by a drive system driven by the output of the control circuit 64. That is, the control circuit 64 produces an open/close period signal for controlling the drive system so that the drive system responds to this open/close period signal to apply a control signal for controlling the amount of lift of the bypass valve 62 to the drive section of the bypass valve 62.
Fig. 2 is a diagram showing the whole configuration of the control system which is constituted by a central processing unit (hereinafter abbreviated as CPU) 102, a read only memory (hereinafter abbreviated as a ROM) 104, a random access memory (hereinafter abbreviated as RAM) 106, and an input/output (hereinafter abbreviated as I/O) circuit 108. The CPU 102 operates input data from the 1/0 circuit 108 in accordance with various programs stored in the ROM 104 and returns the result of operation to the I/O circuit 108. Temporary data storage necessary for such an operation is performed by using the RAM 106. Exchange of various data among the CPU 102, the ROM 104, the RAM 106, and the 1/0 circuit 108 is performed through a bus line 110 constituted by a data bus, a control bus, and an address bus.
The I/O circuit 108 includes input means such as the above-mentioned first analog-to-digital converter (hereinafter abbreviated as ADC1), a second analog-to-digital converter (hereinafter abbreviated as ADC2), an angular signal processing circuit 126, and a discrete I/O circuit (hereinafter abbreviated as DIO) for inputting/outputting one bit information.
In the ADC1, the respective output signals of a battery voltage sensor (hereinafter abbreviated as VBS) 132, the above-mentioned cooling water temperature sensor (hereinafter abbreviated as TWS) 56, an atmosphere temperature sensor (hereinafter abbreviated as TAS) 112, a regulation voltage generator (hereinafter abbreviated as VRS) 114, the above-mentioned throttle opening sensor (hereinafter referred to as OTHS) 116, and a λ sensor 118 (hereinafter abbreviated as λS) 118 are applied to the above-mentioned multiplexer 120 (hereinafter abbreviated as MPX) 120 which selects one of the respective input signals and inputs the selected signal to an analog-to-digital converter circuit (hereinafter abbreviated as ADC) 122. The digital value of the output of the ADC 122 is stored in a register (hereinafter abbreviated as REG) 124.
An output signal of an air flow rate sensor (hereinafter abbreviated as AFS) 24 is inputted to the ADC2 in which the signal is AID converted in an ADC 128 and set in a REG 130.
An angle sensor (hereinafter abbreviated as ANGS) 146 produces a reference signal representing a reference crank angle (hereinafter abbreviated as REF), for example as a signal generated at an interval of 180 degrees of crank angle, and a position signal representing a small crank angle (hereinafter abbreviated as POS), for example 1 (one) degree. The REF and POS are applied to the angular signal processing circuit 126 to be waveform-shaped therein.
The respective output signals of an idle switch 148 (hereinafter abbreviated as IDLE-SW) 148, a top gear switch (hereinafter abbreviated as TOP-SW) 150, and a starter switch 152 (hereinafter abbreviated as START-SW) are inputted into the DIO.
Next, a circuit for outputting pulses in accordance with the result of operation of the CPU 102 and an object to be controlled will be described hereunder. An injector circuit (hereinafter abbreviated as INJC) 134 is provided for converting the digital value of the result of operation into a pulse output. Accordingly, a pulse having a pulse width corresponding to the amount of fuel injection is generated in the INJC 134 and applied to the injector 12 through an AND gate 136.
An ignition pulse generating circuit (hereinafter abbreviated as IGNC) 138 includes a register (hereinafter referred to as ADV) for setting ignition timing and another register (hereinafter referred to as DWL) forsetting initiating timing of the primary current conduction of the ignition coil 58 and these data are set by the CPU 102.
The rate of opening of the bypass valve 62 is controlled by a pulse supplied thereto by a control circuit (hereinafter referred to as ISCC) 142 through an AND gate 144. The ISCC 142 has a register ISCD for setting a pulse width and another register ISCP for setting a repetitive pulse period.
The one-bit I/O signals are controlled by the circuit DIO. The 1/0 signals include the respective output signals of the IDLE-SW 148, the TOP-SW 150 and the START-SW 152 as input signals, and include a pulse signal for controlling the fuel pump 32 as an output signal. The DIO includes a register DDRfordetermin- ing whether a terminal be used as a data inputting one or a data outputting one, and another register DOUT for latching the output data.
A register (hereinafter referred to as MOD) 160 is provided for holding commands instructing various internal states of the I/O circuit 108 and arranged such that, for exmaple, all the AND gates 136, 140, 144, and 156 are turned on/off by setting a command into the MOD 160. The stoppage/start of the respective outputs of the INJC 134, IGNC 138, and ISCC 142 can be thus controlled by setting a command into the MOD 160.
An embodiment of the invention will be explained referring to Figs. 3 to 6, in which the bypass valve ON duty factor is controlled so that the number of engine revolution can be smoothly changed when the idling switch is turned ON from the OFF state, that is when the engine state is changed from normal running to idle one.
If the idling switch is turned ON from its OFF state at the time t₁ as shown in Fig. 3(A), the feedback control with respect to the bypass valve ON duty factor is started as shown in Fig. 3(B). That is, the ON duty fac- torforthe OFF state of the idlinge switch, i.e. the value (Ko + ISC_FB) which is the sum of the ON duty factor fixed component Ko and the ON duty factor feedback component ISC_FB corresponding to the difference AN between the actual value of the number of engine revolution N and the reference value of the number of engine revolution for idle running N_REF, is outputted as the ON duty factor at this time. That is, if the feedback component ISC_FB has a negative value (hereinafter, it is assumed that the value ISC_FB is negative in this embodiment), the value ISC_FB is decreased at regular or predetermined intervals of time by a feedback component changing value AD (negative value) which is determined by the above-mentioned difference value AN in the number of engine revolution and therefore the bypass valve ON duty factor gradually decreases after the time t₁. as shown in Fig. 3(B). In the case where the ON duty factor is determined to control the number of engine revolution to the reference number of engine revolution N_REF by feedback control, however, the number of engine revolution may be so reduced below the reference number of engine revolution N_REF (overshoot) as shown in Fig. 3(C) when it is reduced toward the reference number of engine revolution by the feedback control, with the possibility of occurrence of engine stoppage. If the number of engine revolution N comes below the reference number of engine revolution for idle running N_REF (at the time t₃), the difference AN = N - N_REF becomes negative and therefore the changing value AD becomes positive so that the feedback component ISC_FB increases gradually. To cope with this problem, a method has been proposed conventionally, in which the feedback control is started upon the turning ON of the idling switch at the time t₁ as shown in Fig. 3(D) so as to decrease the ON duty factor step by step by the ON duty factor changing value AD to thereby reduce the number of engine revolution N toward the reference number of engine revolution N_REF, and when the number of engine revolution N has reached a given value which is the sum (N_REF+ ΔNo) of the reference numberof engine revolution N_REF and a predetermined fixed vaiue ANo (for example, 400 r.p.m.) at the time t₂, the ON duty factor feedback control is stopped, that is the decreasing of the value ISC_FB is stopped, so that the bypass valve ON duty factor is caused to come back to the fixed component Ko and the feedback control is effected again to thereby converge the number of engine revolution to the desired value, as shown by the broken curve in Fig. 3(C). In this method, however, the number of engine revolution may overshoot to downward exceed the desired value N_REF as shown by the broken curve in Fig. 3(C) even if the ON duty factor is increased at the time where the number of engine revolution has reached the value which is the sum of the desired value N_REF and the predetermined value ANo.
In the embodiment according to the present invention, therefore, the feedback control is not immediately effected upon the turning ON of the idling switch at the time t₁ but started when the number of engine revolution has reduced to the value which is larger than the reference or desired value N_REF by a predetermined value ANo (for example, 400 r.p.m.), as shown in Fig. 3(E) and (F). Although it takes a longer time for the number of engine revolution to reach the value of the sum of the desired number of engine revolution N_REF and the fixed value ANo in comparison with the case of Fig. 3(D) (actually, the period of time is so short that it is difficult to clearly find the difference by measurement), the number of engine revolution can be quickly converged, after the initiation of the feedback control, to the desired reference value in comparison with the conventional case without overshooting.
In such a method in which the feedback control is started from the time where the number of engine revolution has reached the value which is larger than the desired reference value N_REF by the fixed value ANo, assume that after normal running, the accelerator is released at the third, fourth, or top gear position to effect engine braking to gently decrease the number of engine revolution and the clutch is turned OFF before the number of engine revolution has reached a sufficiently low value at which knocking may occur. Then, the number of engine revolution may largely fall down at the time t₂ as shown in Fig. 4(A) because the engine load becomes light at that time. That is, if the idling switch is turned ON and the engine braking is effected at time t₁ where the number of engine revolution has reached the value N₁ (N₁= N_REF+ ANo), the decrease of the number of engine revolution becomes gentle so that rate of reduction of the value AN = (N_REF) becomes smaller to maintain the changing value AD of the feedback component ISC_FB large, whereby the ON duty factor (Ko + ISC_FB) decreases rapidly as shown in Fig. 4(B) to rapidly reduce the number of engine revolution toward the desired value. In such a case where the changing value AD of the feedback component ISC_FB has reached a large value, if the clutch is turned OFF at the time t₂ where the number of engine revolution is N₂ so as to provide non-braking condition, the engine load decreases rapidly and the number of engine revolution falls down rapidly to overshoot or downward exceed the reference value N_REF. If the number of engine revolution becomes lower than the reference value N_REF, the reduction of the number of engine revolution cannot be sufficiently recovered although the changing value AD becomes positive to act to increase the value ISC_FB. To cope with this problem, the rate of change of the number of engine revolution upon the initiation of the feedback control, i.e. immediately after the idling switch has been turned ON from its OFF state, is obtained so that when the rate of change is smaller than a predetermined value, the gain of feedback control, i.e. the feedback changing value AD, is made small to increase the rate of change of the ON duty factor (Ko + ISC_FB) to effect the feedback control gently as shown in Fig. 4(C). Thus, the number of engine revolution can be converged to the desired value rapidly without overshooting, as shown in Fig. 4(D).
Further, the feedback control is started at the time where the number of engine revolution is larger than the desired value N_REF by ANo and therefore the rate of reduction of the number of engine revolution may be large if there exists a load such as airconditioner at the time when the feedback control is started.
Accordingly, if the feedback control is started at the time t₁ at which the number of engine revolution has become a value N₁ which is larger than the desired value N_REF by the value ΔNo so as to perform ordinary feedback control as shown by the curve P₁ in Fig. 5(B), the number of engine revolution decreases suddenly so as to downward exceed the desired value N_REF as shown by solid line in Fig. 5(A). To cope with this problem, when the rate of reduction of the number of engine revolution is larger than a predetermined value at the time at which the number of engine revolution becomes the above-mentioned value N₁. the ON duty factor increment ISCD is obtained in accordance with the rate of reduction as shown in Fig. 5(B) and the increment ISCD is added to the ON duty factor (Ko + ISC_FB) so as to perform the feedback control by using this sum (Ko + ISC_FB + ISCD) as the ON duty factor which is decreased step by step at regular or predetermined intervals of time (for example, every 160 msec) by the ON duty factor changing value AD determined on the basis of the difference AN between the present number of engine revolution N and the desired or reference number of engine revolution N_REF. In this manner, the number of engine revolution gradually decreases after the time t₁ and smoothly reaches the desired or reference value N_REF without downwardly exceeding the desired value. The ON duty factor increment ISCD is maintained constant while the rate of reduction of the number of engine revolution is substantially constant, and increased or decreased in accordance the value of the rate of reduction of the number of engine revolution when the rate of reduction increases or decreases respectively.
Referring to the flowchart shown in Fig. 33, the embodiment in which the bypass valve ON duty factor after the turning-ON of the idling switch is controlled as shown in Figs. 3 to 5 will be described hereunder. It is assumed that the processing flow of Fig. 33 is executed every 160 msec and that the feedback component ISC_FB has a negative value in this processing flow as shown in Figs. 3 to 5.
In the step 1201, first, the number of engine revolution is read and be stored as N_NEW in a predetermined area of the RAM and the previously read value is shifted as N_OLD to another area in the RAM. Next, judgement is made as to whether the ON duty factor increment ISCD is zero or not in the step 1202. If the result of judgement proves that the increment ISCD is not zero, a predetermined ON duty factor value Ad is subtracted from the ON duty factor increment ISCD and the resulted value is stored in a predetermined area of the RAM in the step 1203, and the processing is shifted to the step 1204. When the result of judgement proves that the increment ISCD is zero in the step 2102, on the contrary, the processing is shifted to the step 1204. In the step 1204, judgement is made as to whether the idling switch is in the ON state or not. If the result of judgement in this step proves that the idling switch is in the OFF state, a flag 1 is set to "1" in the step 1205 and a flag 2 is reset to "0" in the step 1206. The flag 1 is for indicating the OFF state of the idling switch and the flag 2 is for executing the control to minimize the changing value AD for the ON duty factor feedback component ISC_FB. When the result of judgement proves that the idling switch is in the OFF state, it is considered that system is to be subjected to open loop control and the ON dutyfactorfixed component Ko is map-retrieved on the basis of the cooling water temperature in the step 1207 so as to be set into the register ISCC 142.
If the result of judgement in the step 1204 proves that the idling switch is ON, the reference number of engine revolution for idle running N_REF is computed on the basis of the cooling water temperature and stored in a predetermined area of the RAM in the step 1208. In the step 1209, next judgement is made as to whether "1" is set in the flag 1 or not. If the result of judgement proves that "1" is not set to the flag 1, it is considered that the idling switch has been left in the ON state and the processing is shifted to the step 1214. If the result of judgement in the step 1209 proves that "1" is set in the flag 1, it is considered that the state of the idling switch has been changed from its OFF state to ON and judgement is made in the step 1210 as to whether the number of engine revolution N_NEW taken-in in the step 1201 is not smaller than the value obtained by adding the value ANo to the reference number of engine revolution N_REF for idle running. If the result of judgement in this step 1210 proves that the value N_NEW is equal to or larger than the sum of the value N_REF and the value ANo, it is considered that the ON duty factor is not yet to be subjected to the number-of-engine-revolution feedback control but to the open loop control and the processing is shifted to the step 1224. In the step 1224, the ON duty factor fixed component Ko is map-retrieved on the basis of the cooling water temperature and set into the register ISCC 142. Thus, open loop control is effected after the turning ON of the idling switch and before the time t₁. If the result of judgement in the step 1210 proves that the value N_NEW is smaller than the sum of the value N_REF and the value ANo, on the contrary, it is considered that the number-of-engine-revolution feedback control for the ON duty factor is to be effected and the flag 1 is reset in the step 1211. In the step 1212, then, the rate of change of the number of engine revolution (AN = N_OLD- N_NEW) is obtained from the respective values of the number of engine revolution N_NEW and N_OLD taken-in in the step 1201 and judgement is made as to whether this An is smaller than a predetermined value Δn_o or not. If the result of judgement in the step 1212 proves that the rate of reduction of the number of engine revolution (An ⁼ N_OLD- N_NEW) is equal to or larger than the predetermined value An_o, the processing is shifted to the step 1214. If the value Δn is smaller than the predetermined value An_o, "1" is set in the flag 2 in the step 1213. That is, the changing value AD for the feedback component ISC_FB is set to a minimum value when the rate of reduction of the number of engine revolution An is smaller than the predetermined value Δn_o at the time t₁ as shown in Fig. 4, and "1" is set to the flag 1 to indicate such control.
Upon the resetting of the flag 1 in the step 1211, the processing is shifted from the step 1209 to the step 1214 after the time t₁. In the step 1214, next, the ON duty factor increment ISCD_a is computed from the rate of reduction of the number of engine revolution An = N_OLD - N_NEW and stored into the RAM.
The increment ISCD, is set such that it is larger as the rate of reduction An is larger and set to zero when it is smaller than the predetermined value Δn₁, i.e. (An, < Ana). That is, as shown in Fig. 5, when the rate of reduction of the number of engine revolution An is equal to or larger than the predetermined value An₁ after the time t₁, the increment ISCD in accordance with the rate of reduction An is add to the ON duty factor to prevent the sudden reduction in the engine speed.
Next, judgement is made as to whether the increment ISCD obtained in the step 1214 is larger than the increment ISCD_o obtained in the step 1203. If the result of judgement proves that the increment ISCD_o is not larger than the increment ISCD, that is when the rate of reduction of the number of engine revolution An is smaller than the previous value of the same, the processing is shifted to the step 1217, and the increment ISCD which has been decreased by Ad obtained in the step 1203 is used in the ON duty factor computing in the later step 1223. In this manner, as shown in Fig. 5(B) after the time t₂, the increment ISCD is decreased by Ad step by step at regular or predetermined intervals of time as the rate of reduction of engine speed becomes smaller so that the reference number of engine revolution N_REF can be reached smoothly.
If the result of judgement proves that the increment ISCD_o is larger than the increment ISCD, that is when the rate of reduction of the number of engine revolution An is substantially equal to or larger than the previous value of the same, on the contrary, the increment ISCD_o obtained in the step 1214 is made to be the increment ISCD which is used in the ON duty factor computing operation in the step 1223. This is because, for example in Fig. 5, when the rate of reduction of the number of engine revolution becomes larger after the ON duty factor feedback control has been started and the ON duty factor has been increased by the increment ISCD after the time t₁, the increment ISCD is renewed to a larger value determined corresponding to the rate of reduction of the number of engine revolution An to thereby prevent the engine speed from suddenly falling down.
In the step 1217, next, the reference number of engine revolution N_REF obtained in the step 1208 is compared with the number of engine revolution N_NEW taken-in in the step 1201 to judge whether the former is not smaller than the latter. If the result of judgement in this step 1201 proves that N_REF is smaller than N_NEW, the flag 2 is reset in the step 1218. That is, it is considered that the control to minimize the changing value AD for the ON duty factor feedback component ISC_FB has been completed.
In the step 1219, then, judgement is made as to whether "1" is set in the flag 2 or not, so that when the flag 2 is set to "1 ", the changing value AD for the feedback component ISC_FB is set to a minimum value in the step 1220, while if the result of judgement in the step 1219 proves that the flag 2 is not set to "1", the changing value AD for the feedback component ISC_FB is obtained in accordance with the value AN = N_REF-N_NEW in the step 1221. The changing value AD is set such that it is larger as the value AN = N_REF- N_NEW becomes larger.
In the step 1222, next, the new feedback component ISC_FB(NEW) is obtained from the previous feedback component ISC_FB(OLD) (this value is assumed to be negative, here) and the changing value AD obtained in the step 1221. That is, the value (ISC_FB(OLD) - AD) is made ISC_FB(NEW).
In the step 1223, then, ON duty factor is obtained from the value of increment ISCD determined in the steps 1215 and 1216 and the feedback component ISC_FB(NEW) obtained in the step 1222. That is the value Ko + ISC_FB(NEW)+ ISCD is computed and set in the ISCC 142.
Thus, as shown in Fig. 4, if the number of engine revolution decreases gradually when the engine brake is actuated at the third, the fourth, or the top gear position under the condition that the number of engine revolution feedback control has been started at the time t₁, the flag 2 is set to "1" in the steps 1212 and 1213 and the change value AD for the feedback component ISC_FB is minimized, as shown in the steps 1217 to 1220, to thereby prevent the number of engine revolution from falling down suddenly. It is noted that the ON duty factor increment ISCD is zero in this case.
As shown in Fig. 5, if the engine load is large and the rate of reduction of the number of engine revolution is larger than the predetermined value An₁ under the condition that the number of engine revolution feedback control has been started at the time t₁, the ON duty increment ISCD is obtained in the step 1214 on the basis of An, the larger one between this value ISCD and the value of difference obtained by subtracting the predetermined value Ad from the previous increment obtained in the step 1203 is obtained in the step 1215 and 1216, and the thus obtained value is added to the fixed and feedback components of the ON duty factor in the step 1223. In this manner, the ON duty factor is made larger to prevent the number of engine revolution from falling down when the rate of reduction of the number of engine revolution is large.
Although the description as to the embodiment shown in Figs. 3 to 5 is made on the assumption that the feedback component ISC_FB of the ON duty factor is negative, the present invention can be applied to the case where the feedback component ISC_FB takes a positive value. In this case the feedback control is effected from the beginning because the number of engine revolution N is always smaller than the sum N_REF+ ANo. Further, the changing value Ad in the step 1203 and the changing value AD in the step 1221 are assumed to be negative, and the changing value ISCD for the ON duty is also assumed to be negative.

Claims

1. In an engine control system comprising :

(A) a plurality of sensors including a throttle opening sensor (148) for detecting the closed state of a throttle valve (16),

(B) the throttle valve (16) provided in an intake path for controlling a flow rate of intake air being introduced into the engine,

(C) a bypass (26) provided in parallel with the throttle valve,

(D) a bypass valve (62) for controlling a flow rate of auxiliary intake air being introduced into the engine,

(E) central processing means (64) for computing a value of duty factor of the bypass valve (62) in accordance with a real engine speed in order to keep the engine speed at a predetermined value, and

(F) a pulse generating circuit (142) for generating pulses in accordance with the value of duty factor computed by the central processing means (64) ; an engine control method, characterized in that said method comprising the steps of :

(G) judging whether the engine speed has decreased or not to a predetermined engine speed (ANo + N_REF) which is higher than a desired engine speed (N_REF) in an engine idle operation, when said throttle valve (16) has been closed, and

(H) in a case that it is judged that the engine speed has decreased to said predetermined engine speed, the duty cycle of said bypass valve being controlled in accordance with the actual engine speed and in a case that it is judged that the engine speed is greater than said predetermined engine speed, the duty cycle of said bypass valve being controlled in an open loop manner.