An Electronically Controlled Carburetor (ECC) is used on late models in conjunction with a three-way catalyst, EGR system, air injection system, ignition control system and (in some cases) a high altitude kit.
The three-way catalyst reduces HC, CO and NOx in exhaust gases and permits simultaneous oxidation. The concentration of exhaust gas pollutants varies with the air/fuel mixture (ratio).
The air/fuel ratio needs to be controlled to a value within a very narrow range to purify the exhaust gas components. The ECC system is employed to control the air/fuel ratio. Improper air/fuel ratios such as a rich mixture can damage the catalytic convertor which can also lead to dangerous emission levels.
The system includes: an oxygen sensor, and Electronic Control Module (ECM), a duty solenoid and a carburetor. The components provide feedback to control the air/fuel ratio during operation by supplying a measured amount of air into the carburetor air bleeders. Vacuum switches, a thermosensor and an engine speed sensing circuit are used to avoid feedback during certain driving conditions.
The oxygen sensor is installed on the exhaust manifold and provides information to the ECM on the exhaust gases from a rich or lean air/fuel ratio.
The oxygen sensor is a kind of concentration cell that generates electromotive force according to the ratio of oxygen concentration in the air to that in the exhaust gases, and has a characteristic that the electromotive force is charged drastically with respect to the stoichiometric air fuel ratio. The force is larger on the rich side (smaller air-fuel ratio) and smaller on the lean side (larger air-fuel ratio) of the mixture.
Upon receiving information from the O2sensor the ECM signals the duty solenoid to allow either more or less air into the carburetor to maintain the correct air/fuel mixture.
The Duty Solenoid(s) is (are) controlled by the ECM. At a given signal, the Duty Solenoid will admit more or less air to the carburetor, maintaining the proper air/fuel ratio.
NOTE: On late models equipped with a Hitachi carburetor, two duty solenoids are installed on the exhaust manifold. Models equipped with a C-W carburetor have the duty solenoid mounted on the carburetor.
Vacuum switches, thermosensors (and speed sensing circuits) signal the ECM to help determine the air/fuel ratio.