Computers and Control Systems: Description and Operation

Fig. 1 Electronically controlled Hitachi carburetor:

The Electronically Controlled Carburetor System,Fig. 1 maintains the air/fuel ratio at nearly ideal conditions at all times, thereby controlling emissions while providing satisfactory driveability. The main components of this system are an oxygen sensor, electronic control module, duty solenoid valve, thermosensor, engine speed sensing circuit, and two vacuum switches.
The oxygen sensor is a concentration cell that generates a voltage according to ratio of oxygen between ambient air and exhaust gasses, thereby maintaining proper air/fuel ratio. The oxygen sensor will generate up to one volt in the absense of oxygen (rich mixture) or as little as zero volts when there is a lot of oxygen (lean mixture). It is installed on exhaust manifold and connected to a tube with a protective cap.
The electronic control module operates the duty solenoid valves, which control ambient air flow into carburetor, in response to signals from oxygen sensor
. The two duty solenoid valves, control ambient air flow to carburetor according to voltage supplied by electronic control module and provide greater vehicle acceleration at higher engine RPM.
The vacuum switch (I), thermosensor, and engine speed sensing circuit assure better driveability by preventing application of feedback.
The vacuum switch (II), controls operation of duty solenoid valves.