Oxygen Sensor: Description and Operation

Oxygen (Lambda) sensor feedback is used to provide more precise control of air/fuel mixtures. This system operates by measuring oxygen content in exhaust gasses, as the amount of oxygen remaining in the exhaust gasses is directly proportional to the air/fuel ratio of mixtures entering the engine. A sensor mounted in the exhaust stream measures oxygen remaining in unburned exhaust gasses and produces a variable voltage signal depending upon the oxygen content of the gasses.

Voltage signals from the oxygen sensor are transmitted to an Electronic Control Module (ECM). The ECM computes the proper amount of fuel necessary to maintain ideal air/fuel mixtures based on throttle position, engine temperature, engine speed and vehicle speed, compares air/fuel mixture ratios measured by the oxygen sensor with the ideal stored in its memory, then controls operation of the frequency valve to provide the fuel necessary to maintain the ideal mixtures.

The frequency valve resembles an electrically controlled fuel injector that has been modified to accept fuel lines at the inlet and outlet. The valve is installed inline between the lower control plunger chamber and the fuel return, and modulates pressure in the lower plunger chamber depending upon signals from the ECM. Modulating pressure in the lower plunger chamber alters the pressure differential between the control plunger inlet and outlet, allowing the quantity of fuel supplied to the injectors to be controlled independent of control plunger position.

The ECM controls lower plunger chamber pressure during normal operation by cycling the frequency valve on and off a fixed number of times per second. When the valve is energized the fuel return passage is closed, pressure in the lower chamber rises and forces the equalization regulator diaphragms upward, and less fuel is supplied to the injectors. When the frequency valve is off the fuel return passage is open, pressure applied to the equalization valve lower chambers is reduced, and more fuel is supplied to the injectors. The ECM controls lower plunger chamber pressure, and thereby the amount of fuel supplied to the injectors by varying the amount of time that the frequency valve is energized during each cycle (OCR ratio). If the valve is energized for a longer period than it is de-energized, less fuel is supplied to the injectors. However, if the valve is de-energized for a longer time per cycle than it is energized, more fuel is delivered to the injectors and air/fuel mixtures are enriched. The ECM constantly varies the OCR ratio operation of the frequency valve during normal operation in order to maintain the ideal air/fuel mixture ratio.

During cold engine operation (coolant temperature below 65°F), full throttle operation with engine warm, and on deceleration, the ECM maintains the OCR ratio at a fixed value in order to provide compensation for these conditions. When the engine is cold, the frequency valve is energized longer during each cycle, providing necessary mixture enrichment. During wide open throttle operation with engine warm, the frequency valve is energized for a shorter period of time to provide a high speed ``lean-out'' condition necessary to provide proper engine operation and effective emission control. During deceleration, the fixed OCR ratio provides proper air/fuel mixtures to compensate for high vacuum conditions and prevent engine backfire.