Page 184 - Understanding Automotive Electronics
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                                         THE BASICS OF ELECTRONIC ENGINE CONTROL                           5




                                         2 that this type of control is provided by a limit-cycle controller (e.g., a typical
                                         furnace controller). The important parameters for this type of control include
                                         the amplitude and frequency of excursion away from the desired stoichiometric
                                         set point. Fortunately, the three-way catalytic converter’s characteristics are such
                                         that only the time-average air/fuel ratio determines its performance. The
                                         variation in air/fuel ratio during the limit-cycle operation is so rapid that it has
                                         no effect on engine performance or emissions, provided that the average air/fuel
                                         ratio remains at stoichiometry.
                                         Exhaust Gas Oxygen Concentration
                    The EGO sensor is used    The EGO sensor, which provides feedback, will be explained in Chapter
                    to determine the air/fuel   6. In essence, the EGO generates an output signal that depends on the amount
                    ratio.               of oxygen in the exhaust. This oxygen level, in turn, depends on the air/fuel
                                         ratio entering the engine. The amount of oxygen is relatively low for rich
                                         mixtures and relatively high for lean mixtures. In terms of equivalence ratio (λ),
                                         recall that λ = 1 corresponds to stoichiometry, λ > 1 corresponds to a lean
                                         mixture with an air/fuel ratio greater than stoichiometry, and λ < 1 corresponds
                                         to a rich mixture with an air/fuel ratio less than stoichiometry. (The EGO
                                         sensor is sometimes called a lambda sensor.)
                                              Lambda is used in the block diagram of Figure 5.16 to represent the
                                         equivalence ratio at the intake manifold. The exhaust gas oxygen concentration
                                         determines the EGO output voltage (V ). The EGO output voltage abruptly
                                                                          o
                                         switches between the lean and the rich levels as the air/fuel ratio crosses
                                         stoichiometry. The EGO sensor output voltage is at its higher of two levels for a
                                         rich mixture and at its lower level for a lean mixture.
                    In a closed-loop system,   The operation of the control system of Figure 5.16 using EGO output
                    the time delay between   voltage is complicated somewhat because of the delay from the time that λ
                    sensing a deviation and   changes at the input until V  changes at the exhaust. This time delay, t , is in the
                                                                o                                  D
                    performing an action to   range of 0.1 to 0.2 second, depending on engine speed. It is the time that it
                    correct for the deviation   takes the output of the system to respond to a change at the input. The electrical
                    must be compensated   signal from the EGO sensor voltage going into the controller produces a
                    for in system design.  controller output of V , which energizes the fuel metering actuator.
                                                            F
                                         Closed-Loop Operation
                                              Reduced to its essential features, the engine control system operates as a
                                         limit-cycle controller in which the air/fuel ratio cycles up and down about the
                                         set point of stoichiometry, as shown in Figure 5.17. The air/fuel ratio is either
                                         increasing or decreasing; it is never constant. The increase or decrease is
                                         determined by the EGO sensor output voltage. Whenever the EGO output
                                         voltage level indicates a lean mixture, the controller causes the air/fuel ratio to
                                         decrease, that is, to change in the direction of a rich mixture. On the other
                                         hand, whenever the EGO sensor output voltage indicates a rich mixture, the
                                         controller changes the air/fuel ratio in the direction of a lean mixture.



                                         UNDERSTANDING AUTOMOTIVE ELECTRONICS                            171
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