A closed loop integral control system for the air/fuel management of an internal combustion engine is disclosed. An oxygen sensor positioned in the exhaust gas of the internal combustion engine is biased with a constant current source to provide a signal indicative of the oxygen content of the exhaust gas over a significant range of air/fuel ratios. The signal waveform from the sensor is compared to a threshold value of a comparator to produce level changes in the comparator output depending on whether the output of the sensor is above or below the threshold. An integrator, receiving these level changes as commands to increase or decrease the fuel pulse widths, controls the air/fuel ratio of the engine in a limit cycle around a scheduled value. By changing the current bias on the sensor and thus modifying the unbiased waveform of the sensor to intercept the threshold value at various points different average air/fuel ratios are obtainable from the system. According to another feature of the invention, cruise detection circuitry determines when the engine is in a stable non- accelerating/decelerating mode and enables the current source to bias the sensor to produce a relatively lean air/fuel ratio from the system for an economical optimum cruising operation.
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