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The Stoichiometric air/fuel ratio for a petrol combustion engine is 14.7:1.
Stoichiometric is the term used for the correct ratio of air to fuel for complete combustion to take place.
A richer mixture will have a lower air/fuel i.e. 12.8:1.
A leaner mixture will have a higher air/fuel ratio i.e. 16.9:1
Under idling and cruising condition the ECU aims to keep the air/fuel ratio close to 14.7:1 in order for the CAT to work at its maximum efficiency. This air/fuel ratio also gives the best fuel economy.
Under increased engine load the optimum air/fuel ratio is richer than 14.7:1 in order to give maximum engine output and to prevent damage to the engine.
The Primary oxygen sensor (O2 Sensor) in the DC5 produces an electric voltage from the different levels of oxygen present in the exhaust gases.
If the mixture is rich then the exhaust gas will contain very little oxygen.
If the mixture is lean then the exhaust gas will contain a higher level of oxygen.
A voltage is created and sent to the ECU. The ECU determines whether the mixture is rich or lean.
Am OEM ECU operates in a mode called “Closed Loopâ€.
The ECU uses the oxygen sensor(s) as a feedback loop (hence the name “CLOSED†Loop) in order to adjust the fuel mixture to keep the air/fuel ratio as close to stoichiometric as possible.
The ECU doesn‘t run in a Closed Loop all the time, “Open Loop†is used to describe the operation of the ECU when the mixture is not being adjusted by the use of the O2 sensor and runs entirely on the map preset on the ECU.
The ECU operates in Open Loop when the engine is cold and when operating under high load (WOT).
In Closed Loop the ECU uses the oxygen sensor to tell if the fuel mixture is rich or lean. However, due to the characteristics of the O2 Sensor it can‘t tell exactly how rich or lean.
(A normal O2 Sensor is sometimes referred to as a Narrowband O2 Sensor, where as to tune a car a Wideband O2 Sensor is needed is order to tell the exact air/fuel ratio in the exhaust gases).
The ECU will enrich the mixture if the oxygen sensor shows that the mixture is lean, and lean the mixture if it looks rich. This results in the air/fuel ratio swinging over 14.7:1.
This constant adjustment of the ECU is called Short Term Adjustment.
If the car has been modified it will tend to run lean as the engine is breathing more efficiently, this will cause the Short Term Adjustment to add fuel all the time in order to stay at 14.7:1.
Say for example the Short Term Adjustment reads consistently at +10% then a further adjustment is made to adjust the entire map in order to get the Short Term Adjustment as close to 0%.
This further adjustment which affects the entire map is called Long Term Adjustment.
Over time the ECU will look at the average Short Term Adjustment and determines whether the engine is running rich or lean OVERALL. The ECU alters the Long Term Adjustment based on the average of the Short Term value.
Under high load (WOT) when the ECU stops running in Closed Loop and switches to Open Loop the long term adjustment is still used which can cause the car to over fuel or run lean.
OBD II engines use one O2 Sensor before the CAT and one after the CAT.
The function of the second O2 Sensor is to determine if the catalytic converter is working correctly. The ECU does this by looking at the difference between the two O2 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 CAT.
I‘ve tried to explain this the best I can but have no doubt missed something.
Stoichiometric is the term used for the correct ratio of air to fuel for complete combustion to take place.
A richer mixture will have a lower air/fuel i.e. 12.8:1.
A leaner mixture will have a higher air/fuel ratio i.e. 16.9:1
Under idling and cruising condition the ECU aims to keep the air/fuel ratio close to 14.7:1 in order for the CAT to work at its maximum efficiency. This air/fuel ratio also gives the best fuel economy.
Under increased engine load the optimum air/fuel ratio is richer than 14.7:1 in order to give maximum engine output and to prevent damage to the engine.
The Primary oxygen sensor (O2 Sensor) in the DC5 produces an electric voltage from the different levels of oxygen present in the exhaust gases.
If the mixture is rich then the exhaust gas will contain very little oxygen.
If the mixture is lean then the exhaust gas will contain a higher level of oxygen.
A voltage is created and sent to the ECU. The ECU determines whether the mixture is rich or lean.
Am OEM ECU operates in a mode called “Closed Loopâ€.
The ECU uses the oxygen sensor(s) as a feedback loop (hence the name “CLOSED†Loop) in order to adjust the fuel mixture to keep the air/fuel ratio as close to stoichiometric as possible.
The ECU doesn‘t run in a Closed Loop all the time, “Open Loop†is used to describe the operation of the ECU when the mixture is not being adjusted by the use of the O2 sensor and runs entirely on the map preset on the ECU.
The ECU operates in Open Loop when the engine is cold and when operating under high load (WOT).
In Closed Loop the ECU uses the oxygen sensor to tell if the fuel mixture is rich or lean. However, due to the characteristics of the O2 Sensor it can‘t tell exactly how rich or lean.
(A normal O2 Sensor is sometimes referred to as a Narrowband O2 Sensor, where as to tune a car a Wideband O2 Sensor is needed is order to tell the exact air/fuel ratio in the exhaust gases).
The ECU will enrich the mixture if the oxygen sensor shows that the mixture is lean, and lean the mixture if it looks rich. This results in the air/fuel ratio swinging over 14.7:1.
This constant adjustment of the ECU is called Short Term Adjustment.
If the car has been modified it will tend to run lean as the engine is breathing more efficiently, this will cause the Short Term Adjustment to add fuel all the time in order to stay at 14.7:1.
Say for example the Short Term Adjustment reads consistently at +10% then a further adjustment is made to adjust the entire map in order to get the Short Term Adjustment as close to 0%.
This further adjustment which affects the entire map is called Long Term Adjustment.
Over time the ECU will look at the average Short Term Adjustment and determines whether the engine is running rich or lean OVERALL. The ECU alters the Long Term Adjustment based on the average of the Short Term value.
Under high load (WOT) when the ECU stops running in Closed Loop and switches to Open Loop the long term adjustment is still used which can cause the car to over fuel or run lean.
OBD II engines use one O2 Sensor before the CAT and one after the CAT.
The function of the second O2 Sensor is to determine if the catalytic converter is working correctly. The ECU does this by looking at the difference between the two O2 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 CAT.
I‘ve tried to explain this the best I can but have no doubt missed something.