Engine control system for maintaining idle speed

A control method for an engine having an ignition system responsive to an ignition signal, a bypass throttle valve responsive to a throttle position signal for engine idle speed control, and an exhaust coupled to a catalytic converter, comprising the steps of:

• generating the throttle position signal with a duty cycle related to a desired quantity of air to be inducted through the bypass throttle valve;
• setting said desired air quantity signal to an initial air quantity value based on predetermined conditions to approach a desired engine idle speed;
• adjusting said desired air quantity signal in response to a difference between actual engine idle speed and said desired engine idle speed;
• retarding the ignition signal during cold engine operation by a preselected amount to increase exhaust gas temperature for rapid warmup of the converter;
• and
• correcting said initial air quantity value as a function of said preselected retard amount during said retarding step to compensate for loss of engine idle speed which said ignition timing retarding step would otherwise cause.

A control system for an engine having an exhaust coupled to a catalytic converter, comprising:

• an ignition system responsive to an ignition signal;
• a bypass throttle valve for engine idle speed control responsive to a throttle position signal having a duty cycle related to a desired quantity of air to be inducted through the bypass throttle valve;
• open loop means for setting said desired air quantity signal to an initial air quantity value based on predetermined conditions to approach a desired engine idle speed;
• feedback means for adjusting said desired air quantity signal in response to a difference between actual engine idle speed and said desired engine idle speed;
• said ignition system retarding the ignition signal during cold engine operation by a preselected amount to increase exhaust gas temperature for rapid warmup of the converter;
• and
• correction means for correcting said initial air quantity value as a function of said preselected retard amount during said retarding step to compensate for loss of engine idle speed which said ignition timing retarding step would otherwise cause.