Page 91 - Automotive Engineering

CH AP TER 4 .1 Digital engine control systems

measure the EGO concentration. These regulated gases Whenever the EGO sensor indicates a lean mixture
can be optimally controlled with a stoichiometric mix- (i.e., low output voltage), the controller increments I(n)
ture. The EGO sensor is, in essence, a switching sensor for the next cycle,
that changes output voltage abruptly as the input mixture
crosses the stoichiometric mixture of 14.7. Iðn þ 1Þ¼ IðnÞþ 1
The closed-loop mode can only be activated when the
for a lean mixture. The integral part of C L continues
EGO (or HEGO) sensor is sufficiently warmed. That is, to increase or decrease in a limit-cycle operation.
the output voltage of the sensor is high (approximately The computation of the closed-loop correction factor
1 volt) when the exhaust oxygen concentration is low continues at a rate determined within the controller. This
(i.e., for a rich mixture relative to stoichiometry). The rate is normally high enough to permit rapid adjustment
EGO sensor voltage is low (approximately 0.1 volt) of the fuel injector pulse width during rapid throttle
whenever the exhaust oxygen concentration is high (i.e., changes at high engine speed. The period between suc-
for a mixture that is lean relative to stoichiometry). cessive computations is the computation cycle described
The time-average EGO sensor output voltage provides above.
the feedback signal for fuel control in the closed-loop In addition to the integral component of the closed-
mode. The instantaneous EGO sensor voltage fluctuates loop correction to pulse duration is the proportional
rapidly from high to low values, but the average value is term. This term, P(n), is proportional to the deviation of
a good indication of the mixture. the average EGO sensor signal from its mid-range value
As explained earlier, fuel delivery is regulated by the (corresponding to stoichiometry). The combined terms
engine control system by controlling the pulse duration change with computation cycle as depicted in Fig. 4.1-6.
(T ) for each fuel injector. The engine controller contin- In this figure the regions of lean and rich (relative to
uously adjusts the pulse duration for varying operating stoichiometry) are depicted. During relatively lean pe-
conditions and for operating parameters. A representa- riods the closed-loop correction term increases for each
tive algorithm for fuel injector pulse duration for computation cycle, whereas during relatively rich in-
a given injector during the nth computation cycle, T(n), is tervals this term decreases.
given by:
Once the computation of the closed-loop correction
factor is completed, the value is stored in a specific
TðnÞ¼ T ðnÞ ½1 þ C L ðnފ memory location (RAM) in the controller. At the
b
appropriate time for fuel injector activation (during the
where intake stroke), the instantaneous closed-loop correction
T b (n) is the base pulse width as determined from factor is read from its location in RAM and an actual
measurements of MAF rate and the desired air/fuel pulse of the corrected duration is generated by the engine
ratio control.
C L (n) is the closed-loop correction factor
For open-loop operation, C L (n) equals 0; for closed-loop 4.1.4.5 Acceleration enrichment
operation, C L is given by:
During periods of heavy engine load such as during hard
C L ðnÞ¼ alðnÞþ bPðnÞ acceleration, fuel control is adjusted to provide an
enriched air/fuel ratio to maximize engine torque and
where neglect fuel economy and emissions. This condition of
enrichment is permitted within the regulations of the
I(n) is the integral part of the closed-loop correction EPA as it is only a temporary condition. It is well rec-
P(n) is the proportional part of the closed-loop ognized that hard acceleration is occasionally required for
correction maneuvering in certain situations and is, in fact, related at
a and b are constants times to safety.
These latter variables are determined from the output The computer detects this condition by reading the
of the EGO sensor. throttle angle sensor voltage. High throttle angle corre-
Whenever the EGO sensor indicates a rich mixture sponds to heavy engine load and is an indication that
(i.e., EGO sensor voltage is high), then the integral term heavy acceleration is called for by the driver. In some
is reduced by the controller for the next cycle, vehicles a switch is provided to detect wide open
throttle. The fuel system controller responds by in-
Iðn þ 1Þ¼ IðnÞ 1 creasing the pulse duration of the fuel injector signal for
the duration of the heavy load. This enrichment enables
for a rich mixture. the engine to operate with a torque greater than that

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