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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