Page 186 - Understanding Automotive Electronics

2735 | CH 5 Page 173 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

the controller to decrease the fuel injector actuating interval. The process continues
this way, cycling back and forth between rich and lean around stoichiometry.
During any one of the intervals shown in Figure 5.17, the fuel injectors
may be activated several times. The engine controller continuously computes
the desired fuel injector actuating interval (as explained later) and maintains the
current value in memory. At the appropriate time in the intake cycle (see
Chapter 1), the controller reads the value of the fuel injector duration and
generates a pulse of the correct duration to activate the proper fuel injector.
Figure 5.17c illustrates the actuating signals for a single fuel injector. The
pulses correspond to the times at which this fuel injector is activated. The
duration of each pulse determines the quantity of fuel delivered during that
activation interval. This fuel injector is switched on repeatedly at the desired
time. The on duration is determined from the height of the desired actuator
duration of Figure 5.17b. Note that the first pulse corresponds to a relatively
low value. The second corresponds to a relatively high value, and the duration
of the on time shown in Figure 5.17c is correspondingly longer. The last pulse
shown happens to occur at an intermediate duration value and is depicted as
being of duration between the other two. The pulses depicted in Figure 5.17c
are somewhat exaggerated relative to an actual fuel control to illustrate the
principle of this type of control system.
One point that needs to be stressed at this juncture is that the air/fuel
ratio deviates from stoichiometry. However, the catalytic converter will
function as desired as long as the time-average air/fuel ratio is at stoichiometry.
The controller continuously computes the average of the EGO sensor voltage.
Ideally the air/fuel ratio should spend as much time rich of stoichiometry as it
does lean of stoichiometry. In the simplest case, the average EGO sensor voltage
should be halfway between the rich and the lean values:

V + V
Lean
Rich
avg.V EGO = ----------------------------
2
Whenever this condition is not met, the controller adapts its computation of
pulse duration (from EGO sensor voltage) to achieve the desired average
stoichiometric mixture. Chapter 7 explains this adaptive control in more detail.

Frequency and Deviation of the Fuel Controller
Recall from Chapter 2 that a limit cycle controls a system between two
limits and that it has an oscillatory behavior; that is, the control variable oscillates
about the set point or the desired value for the variable. The simplified fuel
controller operates in a limit-cycle mode and, as shown in Figure 5.17, the air/fuel
ratio oscillates about stoichiometry (i.e., average air/fuel ratio is 14.7). The two
end limits are determined by the rich and lean voltage levels of the EGO sensor,
by the controller, and by the characteristics of the fuel metering actuator. The time
necessary for the EGO sensor to sense a change in fuel metering is known as the
transport delay. As engine speed increases, the transport delay decreases.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 173