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Other output signals
Not all of the computer outgoing signals
go to actuators. Sometimes information
is sent to electronic modules, such as
ignition or trip computer.
How the computer controls Fuel
Delivery
Good driveability and emission perfor-
mance depends upon precise fuel
control. Early computer controlled
vehicles used electronically adjustable
carburetors, but fuel injectors were soon
used later on.
The job of the computer is to provide the
optimum mixture of air and fuel (air/fuel
ratio) to the engine for best performance
under all operating conditions.
The computer needs to know:
• ...what the engine operating condition
is.
Sensors used: coolant temperature,
throttle position, manifold absolute
pressure, mass air flow, RPM.
• ...how much air is coming into the
engine.
Sensors used: mass air flow, vane air
meter 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 solenoid to adjust
air/fuel mixture 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.
Cold engine warm-up condition
“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. (Computer-
ized engines with carburetors don’t do
any of this. They use a conventional
carburetor choke mechanism.)
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.
Hot engine cruise condition
“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 re-adjust 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
Summary of Contents for Code Scanner CP9025
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