1. AbstractControl of internal combustion engines depends on reduced idle speed and stabilized idling to improve fuel consumption.. Using a valve (ISC/V) to adjust air intake at idling, various idle speed control techniques have been proposed to improve response to variations in the target idle speed due to disturbance and to enhance speed control stability.Many conventional idle speed control systems utilize P and I control. They cannot compensate for the phase difference caused by the intake air volume between the throttle and intake valves. This leads to poor response to variations in the target idle speed and unsatisfactory control accuracy, that is, problems in preventing engine stalling and maintaining idle speed stability.To solve these problems, an idle speed control system based on fuzzy control(1)* theory is proposed. However, it is impossible to determine the control constant (membership function) theoretically. This must be determined on an experimental basis and it is very labor-intensive.This paper describes a control system with the following features: (1) As idle speed control using engine torque signals offers an effective response to disturbance, a model was prepared that provides the desired control characteristics of both engine speed and torque. (2) A feedback control system with rapid following to model output was built. (3) Phase difference caused by intake air volume is compensated. The model following idle speed control system with these features was installed on an engine control unit and its evaluated performance was far superior to conventional control systems.