Haltech E8 - Manual (Page 96)

Haltech E11/E8 Instruction Manual
Figure 101
O2 Closed Loop Tuning
Narrow Band Sensors
By fitting an oxygen sensor to the exhaust system of an engine, the ECU is able to perform corrections
based on feedback from the O2 sensor to maintain a consistent air-fuel ratio around the stoichiometric
mixture. The stoichiometric mixture is when exact 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. Closed loop control does not compensate for poor tuning however, and only allows for
small mixture changes that are already close to the correct target mixture.
Figure 102 - Narrow Band O2 Sensor Output
A 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
the diagram above. 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.
The characteristic of most narrow band sensors only enables accurate measurement about the
stoichiometric mixture point. As a result, O2 closed loop is only useful under cruise and light load
conditions where a stoichiometric mixture is desirable. Most narrow band O2 closed loop systems actively
Page: 96 Copyright © Haltech 2008
Staging Bar
-100 -90 -80 -70 -60 -50 -40
Extra bar from
staged injection
map
-100 -90 -80 -70 -60 -50 -40
Staging Bar
With Staged Injection Map Without Staged Injection Map
Rich spot between
–70 and –60 load
points