Page 185 - Understanding Automotive Electronics

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5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.17
Simplified Waveforms in a Closed-Loop Fuel Control System

The air/fuel ratio in a The electronic fuel controller changes the mixture by changing the
closed-loop system is duration of the actuating signal to each fuel injector. Increasing this duration
always increasing or causes more fuel to be delivered, thereby causing the mixture to become more
decreasing in the vicinity rich. Correspondingly, decreasing this duration causes the mixture to become
of stoichiometry. This is more lean. Figure 5.17b shows the fuel injector signal duration.
in response to the EGO In Figure 5.17a the EGO sensor output voltage is at the higher of two levels
sensor’s output, which over several time intervals, including 0 to 1 and 1.7 to 2.2. This high voltage
indicates a rich or lean indicates that the mixture is rich. The controller causes the pulse duration (Figure
fuel mixture. 5.17b) to decrease during this interval. At time 1 sec the EGO sensor voltage
switches low, indicating a lean mixture. At this point the controller begins
increasing the actuating time interval to tend toward a rich mixture. This
increasing actuator interval continues until the EGO sensor switches high, causing

172 UNDERSTANDING AUTOMOTIVE ELECTRONICS