Idling speed controlling system for an internal combustion engine

An idling speed control system for an internal combustion engine of an automotive vehicle having intake air flow quantity control means located in an auxiliary passage beside an intake air passage, the control means actuated in response to a pulse signal with a calculated duty ratio, in which either feedback control or open-loop control is selectively carried out;

• in the mode of feedback control, the control operation being determined on a basis of actual engine speed and a reference engine speed so that a deviation of the actual engine speed from the reference engine speed is reduced substantially toward zero and in the mode of open-loop control, the control operation being determined on a basis of a cooling water temperature of the engine, the system comprising:
• (a) a first means for determining the reference engine speed with respect to the cooling water temperature of the engine;
• (b) a second means for discriminating engine operating conditions to determine when to perfrom feedback control and when to perform open-loop control;
• (c) a third means for determining a basic duty ratio of a pulse signal outputted from the system on a basis of cooling water temperature, the basic duty ratio of the pulse signal being a basic control ratio of open-loop control and feedback control operations;
• (d) a fourth means for determining a first correction value for combining with the basic duty ratio obtained from said third means on a basis of an actual engine speed and reference engine speed, said fourth means being operative after said second means determines to perform feedback control operation;
• (e) a fifth means for determining a second correction value for combining with the basic duty ratio obtained from said third means on a basis of an actual engine speed and a reference engine speed;
• (f) a sixth means for decreasing the second correction value gradually;
• and
• (g) a seventh means for additively combining the duty ratio obtained from said third, fourth and fifth means and outputting a pulse signal of a constant frequency and amplitude having the duty ratio obtained from said third, fourth and fifth means into the intake air quantity control means according to the control operation mode, whereby the output engine speed gradually nears the
• reference engine speed without overshooting so that the reference engine speed can be set lower and engine hunting and stalling can be prevented.