Page 248 - Understanding Automotive Electronics

2735 | CH 7 Page 235 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

These latter variables are determined from the output of the exhaust gas oxygen
(EGO) sensor as described in Chapter 6.
Whenever the EGO sensor indicates a rich mixture (i.e., EGO sensor
voltage is high), then the integral term is reduced by the controller for the next
cycle,

(
In + 1) = In() – 1

for a rich mixture.
Whenever the EGO sensor indicates a lean mixture (i.e., low output
voltage), the controller increments I(n) for the next cycle,

(
In + 1) = In() + 1
for a lean mixture. The integral part of C continues to increase or decrease in a
L
limit-cycle operation, as explained in Chapter 5 for continuous-time operation.
The computation of the closed-loop correction factor continues at a rate
determined within the controller. This rate is normally high enough to permit
rapid adjustment of the fuel injector pulse width during rapid throttle changes
at high engine speed. The period between successive computations is the
computation cycle described above.
In addition to the integral component of the closed-loop correction to
pulse duration is the proportional term. This term, P(n), is proportional to
the deviation of the average EGO sensor signal from its mid-range value
(corresponding to stoichiometry). The combined terms change with
computation cycle as depicted in Figure 7.6. In this figure the regions of lean
and rich (relative to stoichiometry) are depicted. During relatively lean
periods the closed-loop correction term increases for each computation cycle,
whereas during relatively rich intervals this term decreases.
Once the computation of the closed-loop correction factor is completed,
the value is stored in a specific memory location (RAM) in the controller. At
the appropriate time for fuel injector activation (during the intake stroke), the
instantaneous closed-loop correction factor is read from its location in RAM
and an actual pulse of the corrected duration is generated by the engine
control.

Acceleration Enrichment
The mixture is enriched During periods of heavy engine load such as during hard acceleration, fuel
to maximize torque dur- control is adjusted to provide an enriched air/fuel ratio to maximize engine
ing very heavy load (for torque and neglect fuel economy and emissions. This condition of enrichment
example, a wide open is permitted within the regulations of the EPA as it is only a temporary
throttle). condition. It is well recognized that hard acceleration is occasionally required
for maneuvering in certain situations and is, in fact, related at times to safety.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 235