Mixture

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.

Stoichiometry

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.

Oxygen Sensors

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.

Closed Loop

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 the mixture.

Tuning Implications

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 still 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 rich.

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.