Haltech F9A - Manual (Page 84)

83
Chapter 4
Closed Loop Control
By fitting an oxygen sensor to the exhaust system of an engine, the F9A is able to perform a
feedback correction to maintain a consistent air-fuel ratio around stoichiometric mix; i.e..
when exactly the correct amount of fuel is provided to consume all the oxygen of the air
drawn into the engine, without any unburnt fuel remaining after combustion. Using closed
loop fuel control adapts for small variations in fuel quality and day-to-day running, provides
better fuel economy and lower emissions.
Figure 4.1 Typical lambda sensor output.
An oxygen sensor (or lambda sensor) is
placed in the exhaust gas stream usually
after the collector but before the catalytic
converter. The O2 sensor possesses an
output voltage characteristic similar to that
in figure 15-1. When the exhaust gas is free
from oxygen (i.e.. mixture is rich), the
sensor reads around 1 volt. When there is
an excess of oxygen, the sensor reads
closer to 0 volts. Most oxygen sensors'
transfer curves change very suddenly
around stoichiometric mixtures.
The object in closed loop control is to measure the voltage of the oxygen sensor, determine
whether the engine is running lean or rich, and compensate accordingly by adjusting the
injection time. The ECU may overcorrect slightly, and then will pull the mixture back
towards the desired air-fuel ratio. This slight oscillation either side of stoichiometric mixture
aids the function of the catalytic converter.
In order to use the closed loop fuel control function, you must have the following:
- the F9A secondary harness, incorporating the oxygen sensor connector;
- an appropriate lambda sensor mounted in the exhaust stream;
- F9 programming software and cable.
Mounting and connection of the sensor is all that is required in installation. Check figure 4-2
for appropriate wiring for different oxygen sensors. Run the F9 programming software Online
and go to the Output Options Page. Ignition will need to be switched on. Select Closed Loop
Control, and adjust the following parameters as necessary.
NOTE: CLOSED LOOP CONTROL WILL NOT WORK FOR THE FIRST 2
MINUTES THE ECU IS ON, THIS ALLOWS SUFFICIENT TIME FOR THE
OXYGEN SENSOR TO WARM UP TO OPERATING TEMPERATURE.