Idle speed controller for internal combustion engines

Claim 1

An idle speed controller for an internal combustion engine having a throttle valve, comprising an idle positioner for setting the idle position of said throttle valve, a diaphragm actuator for biasing said idle positioner in the directions of opening and closing said throttle valve, a rotational speed sensor for detecting a signal which represents engine speed, and an operational control means which operates said diaphragm actuator depending upon rotational information received from said rotational speed sensor, said operational control means comprising: an intake vacuum take-out port; a vacuum modulator, which modulates intake vacuum taken from said port and supplies a modulated vacuum to said diaphragm actuator, comprising a housing, a solenoid mounted in said housing, an armature-flapper valve adapted to be actuated by said solenoid, a first nozzle adapted to be controlled by said flapper valve to selectively communicate the internal space of said housing to the atmosphere, a second nozzle adapted to be controlled by said flapper valve to selectively communicate the internal space of said housing to said manifold vacuum take-out port, and a conduit means which communicates the internal space of said housing to said diaphragm actuator; and an electric control circuit for controlling said vacuum modulator in accordance with said rotational speed information, which generates an electric pulse ouput for engergizing said solenoid, and which comprises a wave-shaping circuit which processes said rotational speed information to generate a trigger signal having frequency proportional to the rotational speed of the engine, a saw-wave generator for generating saw-waves each being triggered by said trigger signal, a comparison voltage generating circuit for generating a comparison voltage, a pressure switch for detecting intake vacuum incorporated in said comparison voltage generating circuit so as to substantially lower said comparison voltage when intake vacuum has increased beyond a predetermined value, and a difference amplifier which compares said saw-waves and said comparison voltage and generates said pulse output having a duty ratio which is the ratio of a first duration wherein said saw-wave is higher than said comparison voltage to a second duration wherein said saw-wave is lower than said comparison voltage.