The air/fuel mixture is expressed either as the ratio of air to fuel
vapor or as a lambda value. The lambda value is derived from the
stoichiometric air/fuel ratio, which is the chemically correct ratio of
air to fuel for complete combustion to take place. The
stoichiometric ratio is 14.7:1 when expressed as an air/fuel ratio, or 1
when expressed as a lambda value. A richer mixture will have a lower
air/fuel ratio and lower lambda value. e.g. an air/fuel ratio of 12.5:1
equals a lambda value of 0.85, and is a typical value for a naturally
aspirated engine under full load.
The ECU aims to keep the air/fuel ratio close to the stoichiometric
air/fuel ratio in order for the catalytic converter to work at maximum
efficiency. This air/fuel ratio also gives good fuel economy.
Under increased engine load the optimum air/fuel ratio is richer than the
stoichiometric air/fuel ratio in order to give maximum engine output and
prevent engine damage.
An oxygen sensor produces an electric voltage from the different levels
of oxygen present in the air and the exhaust gas. If the mixture is rich
then the exhaust gas will contain very little oxygen. The oxygen sensor
will therefore product a voltage output, which the ECU senses and
determines that the fuel mixture is rich. Conversely if the fuel mixture
is lean then the exhaust gas will contain higher levels of oxygen, which
produces a lower voltage output. The normal range of the oxygen sensor
output signal is about 0.2V to 1.2V It should be noted that most
stock oxygen sensors are designed to be particularly sensitive around the
stoichiometric air/fuel ratio.
In closed loop operation the ECU uses one or more oxygen sensors as a
feedback loop in order to adjust the fuel mixture. This gives the name ‘closed
loop’ from the closed feedback loop. The ECU won’t run in a closed
feedback loop all the time, so ‘open loop’ is used to describe the
operation of the ECU when the mixture is not being adjusted in this way
(usually when the engine is cold or when running under high load).
In closed loop operation the ECU uses the oxygen sensor to tell if the
fuel mixture is rich or lean. However, due to the characteristics of the
oxygen sensor it can’t tell exactly how rich or lean, it only knows that
the mixture is richer or leaner than optimum. The ECU will enrich
the mixture if the oxygen sensor shows that the mixture is lean, and lean
the mixture if it looks rich. The result of this is that the mixture
will swing back and forward around the stoichiometric point.
Short Term Adjustment
The ECU uses the short term adjustment to alter the injector duration,
and therefore the mixture, in order to make the oxygen sensor voltage
swing around 0.6V.
In the above picture where a short term mixture adjustment of
approximately +-5% is used to keep the oxygen sensor voltage swinging
about 0.6V The vertical lines on the graph are 1 second apart. The
above graph was measured at idle. At higher engine speeds and loads the
oxygen sensor voltage will pass 0.6V up to 20 times per second.
Long Term Adjustment
Over time the ECU will look at the average short term oxygen sensor
adjustment and determine if the engine is running rich or lean
overall. The ECU will alter the long term oxygen sensor adjustment
based on the average value of the short term oxygen sensor
adjustment. This has the effect of compensating for differences in
each individual engine and other factors such as environmental conditions
in order for the engine to run at the correct air/fuel ratios. There
is a limit on the amount of adjustment of approximately +-30%
In the above picture the short term oxygen sensor adjustment shows that
the ECU is on average leaning the mixture out by about 15%. Because
of this the long term adjustment value is slowly being reduced to lean out
It is best to disable closed loop operation while tuning.
Otherwise what commonly occurs is that the ECU will alter the mixture
using the long term adjustment while the car is idling between dyno runs,
which means that the mixture is not repeatable between dyno runs.
If changes are made to the engine which alter the amount of fuel that
is delivered (bigger injectors, increased fuel pressure or altering the
air temperature sensor voltage) the ECU will compensate the best it can
using the long term adjustment. Under high load when the ECU stops
running in closed loop the long term adjustment is not used so
increasing fuel delivery via these means in not recommended unless the ECU
is recalibrated or closed loop disabled.
At part load it is best if the ECU is tuned so that the mixture is
close to stoichiometric. This reduces the amount of time the ECU
will take to use the short term adjustment to alter the mixture to get the
oxygen sensor voltage to swing past 0.6V, and keeps the long term
adjustment from the zero position.
Pre-OBD I Engines
Early VTEC engines use two oxygen sensors arranged to read one cylinder
pair per oxygen sensor. The mixture for each cylinder pair is tuned
separately. It is important not to wire the sensors around the wrong
way, otherwise one cylinder pair will run lean, and the other pair
rich. It is also important not to wire one oxygen sensor into both
sensor inputs, otherwise the engine will run either very lean or very
OBD II Engines
OBD II engines use one oxygen sensor before the catalytic converter,
and one oxygen sensor after the catalytic converter. The function of
the second oxygen sensor is to determine if the catalytic converter is
functioning. It does this by looking at the difference between the
two oxygen sensors. If the catalytic converter is functioning
correctly there will be a reduction in the exhaust oxygen content as
carbon monoxide and carbon dioxide is catalyzed in the converter.