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is. Sensors used: coolant tempera-
ture, throttle position, manifold
absolute pressure, mass air flow,
RPM.
• ...how much air is coming into the
engine. Sensors used: mass air flow
or a combination of manifold absolute
pressure, manifold air temperature,
RPM.
• ...how much fuel is being delivered. The
computer knows this by how long it
turns on the fuel injectors. (The
computer uses a "feedback control" or
"duty cycle" solenoid on electronic
controlled carburetors.)
• ...that everything is working the way it
should. Sensor used: exhaust gas
oxygen sensor.
Note: Not all engines use every sensor
listed above.
"Open Loop" operation
The coolant temperature sensor tells the
computer how warm the engine is.
Factory engineers know what the best
air/fuel mixture is for the engine at
various operating temperatures. (More
fuel is needed for a cold engine.) This
information is permanently programmed
into the computer. After the computer
knows the engine temperature, it
determines the amount of air coming in,
then it will look at its programming to find
out how much fuel to deliver and operate
the fuel injectors accordingly. (Engines
with feedback carburetors don't do any
of this. They use a "Variable Voltage
Choke." The computer controls the
amount of choke opening.)
This is an example of "Open Loop"
operation by the computer. The control
system performs an action (expecting a
certain result), but has no way of
verifying if the desired results were
achieved. In this case, the computer
pulses a fuel injector expecting a certain
amount of fuel to be delivered. (The
computer assumes everything in the fuel
system is operating as expected.) In
open loop operation, the computer has
no way of checking the actual amount of
fuel delivered. Thus, a faulty injector or
incorrect fuel pressure can change the
amount of fuel delivered and the
computer would not know it.
"Closed Loop" operation
The computer watches the coolant
temperature and throttle position sensors
to tell when the engine is all warmed up
and cruising. As before, the computer
determines the amount of air coming into
the engine, then delivers the amount of
fuel that should provide the optimum air/
fuel mixture. The big difference is that
this time the computer uses the oxygen
sensor to check how well it's doing and
readjust things, if needed, to make sure
the fuel delivery is correct.
This is an example of "Closed Loop"
operation. The control system performs
an action (expecting a certain result),
then checks the results and corrects its
actions (if necessary) until the desired
results are achieved.
The oxygen sensor only works when it is
very hot. Also, it can only monitor the
"hot engine" air/fuel mixture value and
send back a signal to the computer. The
sensor can not monitor the other air/fuel
mixture values used during engine
warm-up, so the computer must operate
"open loop" at that time.
As long as the engine and oxygen
sensor are hot, the computer can
operate "closed loop" for best economy
and least emissions. During the drive
conditions listed above, the computer
may have to ignore the sensor and run
"open loop," relying on internal program-
ming for fuel delivery instructions. During
idle, for example, the oxygen sensor may
cool down and stop sending a signal. A
different situation can occur during wide-
open-throttle acceleration. The computer
sometimes adds additional fuel (on
purpose) for temporary acceleration
power. The computer knows it is running
"rich" so it ignores the sensor signal until
the wide-open-throttle condition is over.
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