System for controlling the idle speed of an automotive engine
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for controlling the idle speed of an automotive engine. 
     A conventional idle speed control system with a microcomputer is adapted to control the idle speed to a desired idle speed stored in a look-up table in accordance with the temperature of the coolant of the engine. The desired idle speed is set at a relatively high value in order to achieve fast idle warm up. However, if a motor vehicle is started at such a high idle speed during the warm up, it is difficult to slowly and moderately start the motor vehicle. Furthermore high idle speed causes loud exhaust noises. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a system which controls the idle speed of an automotive engine at a high idle speed upon starting the engine, and at a low idle speed when a driver depresses the accelerator pedal of the vehicle at high idle speed, whereby high idle speed can be changed to a low idle speed engine as desired. 
     The above and other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view showing a system according to the present invention; 
     FIG. 2 shows look-up tables used for controlling the idle speed of an automotive engine; and 
     FIGS. 3a and 3b are flowcharts showing the operation of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an electronic control unit (computer) 1 comprises a CPU 2, ROM 3, RAM 4, input interface 6, output interface 7 and BUS 5. An automotive engine 12 is provided with a fuel injection system with fuel injectors 23 and has various sensors, such as an idle switch 11 producing an idle signal when a throttle valve 10 provided in an intake passage 24 for the engine is at an idle position, a mass air flow meter 9 downstream of an air cleaner 8, a coolant temperature sensor 13, an engine speed sensor 14, a throttle position sensor 19, and a neutral switch 16 provided on a transmission 15 which produces a neutral signal when a gear select lever 15a is at neutral position. The engine further has a starter switch 18 for a starter 17. An air bypass 21 is provided around the throttle valve 10. An idle speed control valve 20, for example a solenoid operated valve, is provided for controlling the air flow passing through the bypass 21. 
     Output signals of the mass air flow meter 9, idle switch 11, throttle position sensor 19, coolant temperature sensor 13, engine speed sensor 14, neutral switch 16, and starter switch 18 are applied to the input interface 6. Idle operation is detected by the signal of the idle switch 11 or the neutral switch 16. The ROM 3 stores two kinds of look-up tables one of which is for high idle speed before depression of an accelerator pedal 25 (FIG. 1) which is operatively connected to the throttle valve 10 and the other is for low idle speed after depression of the accelerator pedal. These idle speeds change in dependency on coolant temperature. FIG. 2 shows a look-up table (NE) for desired high idle speed and a look-up table (NR) for desired low idle speed. 
     In order to control idle speed, the control unit determines whether the accelerator pedal is depressed and produces an output signal. The output signal is applied to an actuator 22 (a solenoid) of the idle speed control valve 20 so as to control the idle speed in accordance with conditions of the engine. 
     The operation of the system is described hereinafter with reference to FIGS. 3a and 3b. When the engine is first energized by closure of an ignition switch (not shown), the computer program is initiated at a point 28 and then proceeds to a step 30. At step 30, if the engine is not started at the first program, namely the engine speed does not reach a predetermined speed, an accelerator pedal depression flag is set at step 32 and a control value for operating the actuator of the control valve 20 is read out from another look-up table (not shown) for starting the engine at a step 33. 
     If the engine is started and the starter switch 18 is OFF, the program proceeds to step 34 from step 31. When the idle switch 11 is ON the program goes to step 34a and when the neutral switch 16 is also ON and when the pedal depression flag is set, the desired high idle speed is read out from the table NE at a step 35. The computer operates to determine the control signal for the actuator at step 36 so as to control the actual idle speed detected by the engine speed sensor 14 to the desired high idle speed by feeding back the engine speed. The control signal is applied to the actuator 22 of the control valve 20 thereby controlling the idle speed to the desired high idle speed. 
     When the idle switch 11 or the neutral switch 16 is OFF, which means that the vehicle is being started or has been started, the pedal depression flag is reset at step 37. Accordingly, the program proceeds to step 38. On the other hand, if the pedal depression flag is reset, at step 40 the program goes to step 39. At the step 39, the idle speed for the low idle state is read out from the table NR. Accordingly, the desired low idle speed is set to a lower speed than the high idle speed. On the other hand at the step 38, the feedback control of idle speed is cut off. 
     In accordance with the present invention, the engine is fast quickly warmed up by fast (high) idle before depression of the accelerator pedal, and high idle speed can be changed to low idle speed by depression of the accelerator pedal. 
     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.