Page 247 - Understanding Automotive Electronics

2735 | CH 7 Page 234 Tuesday, March 10, 1998 1:15 PM

7 DIGITAL ENGINE CONTROL SYSTEM

In any closed-loop control system a measurement of the output variables
is compared with the desired value for those variables. In the case of fuel
control, the variables being regulated are exhaust gas concentrations of HC,
CO, and NO , as explained in Chapter 5. Although direct measurement of
x
these exhaust gases is not feasible in production automobiles, it is sufficient
for fuel control purposes to measure the exhaust gas oxygen concentration.
Recall from Chapter 5 that these regulated gases can be optimally controlled
with a stoichiometric mixture. Recall further from Chapter 6 that the EGO
sensor is, in essence, a switching sensor that changes output voltage abruptly
as the input mixture crosses the stoichiometric mixture of 14.7.
The closed-loop mode can only be activated when the EGO (or
HEGO) sensor is sufficiently warmed. Recall from Chapter 6 that the output
voltage of the sensor is high (approximately 1 volt) when the exhaust oxygen
concentration is low (i.e., for a rich mixture relative to stoichiometry). The
EGO sensor voltage is low (approximately .1 volt) whenever the exhaust
oxygen concentration is high (i.e., for a mixture that is lean relative to
stoichiometry).
The time-average EGO sensor output voltage provides the feedback
signal for fuel control in the closed-loop mode. The instantaneous EGO sensor
voltage fluctuates rapidly from high to low values, but the average value is a
good indication of the mixture.
As explained earlier, fuel delivery is regulated by the engine control system
by controlling the pulse duration (T) for each fuel injector. The engine
controller continuously adjusts the pulse duration for varying operating
conditions and for operating parameters. A representative algorithm for fuel
injector pulse duration for a given injector during the nth computation cycle,
T(n), is given by

Tn() = T b n() × [ 1 + C L n()]

where
T (n) is the base pulse width as determined from measurements of mass
b
air flow rate and the desired air/fuel ratio
C (n) is the closed-loop correction factor
L
For open-loop operation, C (n) equals 0; for closed-loop operation, C is given by
L
L
C L n() = αIn() + βPn()

where
I(n) is the integral part of the closed-loop correction
P(n) is the proportional part of the closed-loop correction
α and β are constants

234 UNDERSTANDING AUTOMOTIVE ELECTRONICS