Abstract:
A feedback control system has a comparing section to compare idling engine speed with a wide dead zone and a narrow dead zone. The wide dead zone has a predetermined width with respect to a desired idle speed. The narrow dead zone width is narrower than the wide dead zone width. When the idling speed is out of the wide dead zone, the system selects the narrow dead zone for rapid convergence of the idling speed. When the idling speed is within the narrow dead zone, the system selects the wide dead zone to prevent hunting.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for controlling idle speed of an automotive engine through a feedback control operation for causing the actual idle speed to converge to a desired idle speed. 
     In an idle speed control system, a bypass having an idle speed control valve (ISCV) is provided around a throttle valve of the engine. The idle speed control valve is operated to control the amount of intake air for causing the actual idle speed to converge to the desired idle speed by a feedback control system. 
     In the feedback control system, a dead zone is provided with respect to the desired idle speed in order to prevent the opening degree of the ISCV from hunting when the idle speed approximates the desired idle speed. When the engine speed is within the dead zone, the feedback operation is stopped so that the opening degree of the ISCV is held constant Hence, the convergence of the idle speed is improved. 
     Japanese Patent Application Laid-Open 59-226250 discloses such an idle control system where the feedback operation is carried out only in a transient state, such as the time when an air-conditioner is started and stopped, which causes fluctuation of the idle speed. 
     In the feedback control operation, a dead zone is provided. 
     In a no load state, the engine speed is controlled by an open-loop control. Accordingly, there are disadvantages in that the idle speed can fluctuate without converging to a desired speed. 
     Furthermore, the width of the dead zone is obtained by adding a predetermined value to the average of the differences of peak engine speeds at the time of fluctuation. Since the width of the dead zone changes with the fluctuation of the engine speed, it is difficult for the engine speed to accurately converge to the desired idle speed. More particularly, the width of the dead zone for the desired idle speed (for example 800 rpm) is set at a value (for example ±50 rpm) which is slightly larger than the fluctuating width of the engine speed. Thus, an undesirable feedback operation caused by the fluctuation of the engine speed is avoided. However, since the dead zone is wide (750-850 rpm), the range of the controlled engine idling speed becomes large. To the contrary, if the dead zone is reduced, the ISCV is operated even if a slight fluctuation of the engine speed occurs. As a result, the idling speed fluctuates. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an idle speed control system for an automotive engine where the engine speed is accurately converged to a desired idle speed while preventing the hunting of the control system. 
     According to the present invention, there is provided a system for controlling idling speed of an engine, the system having an engine speed sensor for detecting engine speed, an idle speed control valve for controlling air flow quantity for said engine during idling, an actuator for operating said idle speed control valve so as to control said idling speed, and a control unit for comparing said engine speed with a desired idle speed for controlling said idling speed through a feedback operation, wherein the control unit comprises: dead zone width providing means for providing a first width and a second width which is wider than said first width; and comparator means comprising: means for defining a first dead zone with said first width with respect to said desired idle speed, and further for defining a second dead zone with said second width with respect to said desired idle speed, and each dead zone having an upper limit value and a lower limit value respectively; selecting means for selecting either of said first width and said second width in accordance with said engine speed; and first comparing means for comparing said engine speed with said upper limit value and lower limit value of the dead zone corresponding to the selected width for producing a control signal to actuate said actuator for decreasing said idling speed when said engine speed is larger than said upper limit value of said dead zone corresponding to the selected width, and for producing a control signal to actuate said actuator for increasing said idling speed when said engine speed is smaller than said lower limit value of said dead zone corresponding to the selected width so as to accurately converge said idling speed to said desired idle speed and prevent hunting of said idling speed. 
    
    
     The other objects and features of this invention will be apparently understood from the following description with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic illustration showing a system of the present invention and a block diagram of a control unit used in the system; 
     FIG. 2 is a graph showing dead zones in a feedback control system; 
     FIG. 3 is a flowchart showing the operation of the system of the present invention; and 
     FIG. 4 is a timechart showing variations of the engine speed and ISCV opening degree in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an automotive engine E has an intake pipe 1 and a throttle body 1a connected to the intake pipe 1 and provided with a throttle valve 2. A solenoid-operated idle speed control valve (ISCV) 3 is provided in a bypass 4. A solenoid 3a of the ISCV 3 is applied with a control signal from a control unit 10 in a control system of the present invention. 
     The control system has an engine speed sensor 5 and an idle switch 6 which is closed when the throttle valve 2 is closed. Engine speed Ne from the engine speed sensor 5 and the output signal of the idle switch 6 are fed to idle determining means 11 which produces an idle signal when the idle switch 6 is closed while the engine speed is smaller than a predetermined reference speed. A desired idle speed No is provided in a desired idle speed providing means 12. 
     The control unit further has a comparator means 13 and a dead zone width providing means 14. The engine speed Ne is fed to the dead zone width providing means 14 for determining a width ΔN of a dead zone. As shown in FIG. 2, a narrow dead zone width ΔN1, for example, 50 rpm (±25 rpm) and a wide dead zone width ΔN2, for example 100 rpm (±50 rpm), are provided in the dead zone width providing means 14. 
     The dead zone widths ΔN1, and ΔN2, the engine speed Ne and the desired idle speed No are fed to the comparator means 13. In the comparator means, the engine speed Ne is compared with the desired idle speed in three comparing zones in conjunction with the narrow and wide dead zone widths ΔN1 and ΔN2. More particularly, the comparator means has first comparing means and second comparing means. The second comparing means determines one of the following three zones, in which the engine speed falls. 
     
         Ne&gt;No+ΔN2/2                                          (1) 
    
     
         No+ΔN1/2≧Ne≧No-ΔN1/2             (2) 
    
     
         Ne&lt;No-ΔN2/2                                          (3) 
    
     In other words, in the zone (1), the engine speed is larger than an upper limit of a wide reference zone which is determined by the wide dead zone width ΔN2 and the desired idle speed No. In the zone (3), the engine speed is smaller than a lower limit of the wide reference zone. In the zone (2), the engine speed is within a narrow reference zone which is determined by the narrow dead zone width ΔN1 and the desired idle speed No. When the engine speed Ne is in the zone (1), where the engine speed is too large for idling, or in the zone (3) where the engine speed is too small, the narrow dead zone width ΔN1 is selected as the dead zone width ΔN in order to cause the engine speed Ne to precisely converge to the desired idle speed No. When the engine speed Ne is in the zone (2), the wide dead zone width ΔN2 is selected, thereby preventing hunting of the engine speed. 
     In the first comparing means, the engine speed Ne is compared with the upper limit and the lower limit of the wide reference zone. When the engine speed exceeds the upper limit (Ne&gt;No+ΔN2/2), an ISCV opening degree decreasing signal is fed to ISCV operating quantity providing means 15. If the engine speed is smaller than the lower limit (Ne&lt;No-ΔN2/2), an ISCV opening degree increasing signal is fed to the ISCV operating quantity providing means 15. The ISCV operating quantity providing means 15 provides a predetermined ISCV operating quantity or a quantity based on the difference between the actual engine speed Ne and the desired idle speed No when the ISCV opening degree changing signal is applied. A driver 16 which is fed with the operating quantity applies pulses, the duty ratio of which corresponds to the operating quantity, to the solenoid 3a of the ISCV 3. 
     The operation of the control system of the present invention is described hereinafter with reference to the flowchart and the timechart shown in FIGS. 3 and 4, respectively. 
     Referring to FIG. 3, at a step S100, the engine speed Ne is read out, and at a step S101, it is determined whether the engine is idling or not in accordance with the output signal of the idle switch 6. When the engine is not idling, the program goes to a step S102, where the opening degree of the ISCV 3 is maintained at a predetermined degree. 
     On the other hand, in the idle state, the desired idle speed No is provided at a step S103. At steps S104 and S105, the engine speed Ne is checked. When the engine speed Ne is excessively larger or smaller than the desired idle speed No, that is, in zone (1) (Ne&gt;ΔNo+ΔN2/2) or zone (3) (Ne&lt;No-ΔN2/2), the program goes to a step S106. At the step S106, the narrow dead zone width ΔN1 is chosen as the dead zone width ΔN. At a step S107, the engine speed Ne is compared with the upper engine speed Limit No+ΔN/2 of the narrow reference zone. When the engine speed Ne is larger than the upper limit, the program goes to a step S109 where the opening degree of the ISCV 3 is reduced. When the engine speed Ne is smaller than the upper limit, it is determined at a step S108 whether the engine speed Ne is lower than the lower limit No-ΔN/2. When the engine speed is smaller than the lower limit, the program goes to a step S110 where the opening degree of the ISCV 3 is increased. If the engine speed is between the limits No+ΔN/2 and No-ΔN/2, the opening degree of the ISCV 3 is maintained. 
     Referring to FIG. 4, before a time t1 the engine speed Ne is in the zone (1), so that the dead zone width ΔN1 is selected The program goes from the step S107 to the step S109, thereby decreasing the opening degree of the ISCV 3. As a result, the quantity of intake air supplied to the engine E is decreased so as to reduce the engine speed Ne accordingly. The opening degree of the ISCV 3 is gradually decreased until the engine speed Ne becomes smaller than the upper limit No+ΔN1/2 of the narrow dead zone. Thereafter, the opening degree of the ISCV 3 is maintained so that the average engine speed approximates the desired idle speed No. 
     As a consequence, the engine speed Ne is in neither of the zones (1) or (3) so that the program proceeds from the step S105 to a step S111 where it is determined whether the engine speed Ne is in the zone (2) (No+ΔN1/2≧Ne≧No-Δ N1/2). When the engine speed is in the zone (2), the wide dead zone width ΔN2 is selected as the dead zone width ΔN at a step S112. Thereafter, the program proceeds to the step S107. Thus, even though the engine speed Ne largely fluctuates over the narrow dead zone at a time t2 in FIG. 4, since the engine speed Ne is still smaller than the upper limit No+ΔN2/2 of the wide dead zone, the ISCV 3 is kept unchanged, hence preventing hunting. 
     When the engine speed Ne becomes smaller than the lower limit No-ΔN2/2, at a time t3 in FIG. 4, the program proceeds from the step S105 to the step S106 so that the narrow width ΔN1 is again selected. The program goes from the step S108 to the step S110 where the opening degree of the ISCV 3 is increased. The quantity of the intake air increases accordingly to raise the engine speed Ne. When the engine speed Ne increases to No-ΔN1/2, the opening degree of the ISCV 3 is no longer increased. Thus, the average engine speed converges to the desired idle speed No without causing hunting. 
     The present invention may be so modified to correct the desired idle speed at a cold engine state. 
     In accordance with the present invention, the narrow dead zone and the wide dead zone are provided. The feedback operation is performed by using the narrow dead zone when the engine speed is extremely large or small. Thus, the engine speed accurately converges to the desired idle speed. Once the engine speed approximates the desired idle speed, the wide dead zone is used so as to effectively prevent hunting. Selection of the narrow dead zone or the wide dead zone is determined in accordance with the comparison of the actual engine speed with the upper and lower limits so that the control operation becomes simple. 
     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.