Suzuki F6A Service Manual page 21

Air/fuel ratio feedback compensation
SUZUKI uses a rhodium catalytic converter to process C0, HC and NOx contents in the exhaust gas. It oxidize CO
and HC and reduces NOx simultaneously into non–toxic CO2 H2O Oz and N2 respectively; although only near the
optimum air/fuel ratio range. In other words, when the air/fuel ratio becomes leaner than its optimum value, more
NOx is generated and when it becomes richer, more CO and HC are generated. In order to process the exhaust gas
by making an effective use of the catalytic converter rhodium, it is necessary to keep the air/fuel ratio accurately to its
optimum value. However, as the air/fuel ratio range in which C0, HC and NOx are processed is small, it is impossible
to keep it to its optimum value constantly with the open loop control. To make it possible, the air/fuel ratio feedback
control (compensation) is executed. When the air/fuel ratio feedback compensation is executed, first, whether the
air/fuel ratio is richer or leaner than its optimum value is judged by using the signal from the Oz sensor and the fuel
injection is decreased when it is rich and increased when it is lean to keep it to the optimum air/fuel ratio. Such feed-
back compensation is not executed under following conditions to ensure optimum operation. (This is called "open
loop control".)
Conditions under which air/fuel ratio feedback control is not executed:
Cooling water temperature
During after start increase compensation
When O2 sensor is judged as inactive
During output increase compensation
During fuel cut
The electromotive voltage of the O2 sensor becomes high (about 1 V) when the air/fuel ratio is richer than its optimum
value, and becomes low (about V) when it is leaner. The ECM receivers this signal and compares it with the reference
value to determine whether it is richer or leaner. When it is richer, the ECM reduces the air/fuel ratio feedback compensa-
tion factor (air/fuel ratio compensation value) in steps to reduce the amount of injection (step reduction) and then at a
constant rate (skip reduction). When it is leaner, it is increased in steps first and then at a constant rate. (This is called
"closed loop control".)
There is a limit to the air/fuel ratio feedback compensation factor. It varies within the range between –0.20 to +0.20. Also,
when the ECM lean judgement continues about 10 seconds, the ECM judges the O2 sensor inactive and sets the air/fuel
ratio feedback compensation factor to "0". (This is called "open loop control".)
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