OK in relation to another Post regarding Tuning the AFR on a H22A I would like to elaborate on the subject. This covers just the Basics of how the ECU determines Air Fuel Ratios. If you want to get to the guts of the Subject do what I done and do a Trade Course on Engine Management Systems.
The BASIC AIM
Modern Engine Management Systems (Including the HONDA PGMFI System)are Tuned or Mapped to achieve the optimum AFR of 14.7:1 which is universally considered the best trade off between emissions, fuel economy and power production.
In any type of internal combustion engine, to achieve complete combustion the fuel needs to be burned at a ratio of 14.7 parts of air to 1 part fuel. This air to fuel ratio (AFR) is called Stoichiometric or Lambda = 1.
The CLOSED LOOP SYSTEM
When the engine reaches Normal Operating Temperature the ECU receives a signal from the Coolant Temperature Sensor allowing it to enter Closed Loop Mode. The ECU uses an Oxygen Sensor (Lambda Sensor) to determine how much Oxygen is left in the exhaust after combustion. The O2 Sensor converts this to a voltage signal so the ECU can understand what the AFR is.
The ECU will adjust the AFR so that the engine runs at the mapped optimum AFR of 14.7:1 in Closed Loop Mode. Actually, the ECU will alternate the mixture rich to lean and back so it can centre it around 14.7:1.
It does this by Increasing or Decreasing the Injector Pulse Width in Milliseconds(The time the Injector Pintle is open) which determines the actual volume of fuel that is injected during this time. This is also determined by the Fuel Pressure in the Fuel Injector Rail. Fuel Pressure is normally regulated between 220 to 250 Kpa by the Fuel Pressure Regulator.
The ECU doesn't just rely on the O2 Sensor to determine the AFR. It also references the Coolant Temperature Sensor, Throttle Position Sensor, Intake Air Temperature Sensor, Manifold Absolute Pressure Sensor, Crank Position Sensor, Engine Speed Sensor & the No 1 Cyl Position Sensor. In an Auto It also references the Auto Transmission Control Unit and its sensors.
The ECU constantly makes adjustments to the AFR using data from all the Input Sensors, this is called the Short Term Fuel Trim(STFT).
The ECU keeps an average of what adjustments it has made over time. That average is applied against the Long Term Fuel Trim (LTFT), this is known as 'Learning' .
During closed loop operation both the STFT and the LTFT will have an effect on the AFR.
SHORT TERM FUEL TRIM - LONG TERM FUEL TRIM
Lets say the ECU detects the Short Term AFR at roughly 5.0% RICH when we take the car for a drive with a Diagnostic Scanner. When we reset the Fuel Cells or Wipe the ECU Memory, the LTFT is at 0.0%.The ECU will detect, using the O2 sensors, that the Short Term AFR is 5.0% rich. The STFT will equal -5.0%. The ECU will lean out the fuel by 5%. This results in the AFR being brought back to 14.7:1.
At first the LTFT will be at 0.0% but over time the LTFT will slowly move negative. As that occurs the STFT will move back towards 0.0% because the LTFT is now also removing some of the extra fuel(Leaning Out). Therefore the correction needed by the STFT will be less. After driving for a while you would see something like this:
LTFT= 0.0% STFT= -5.0%
LTFT= -1.0% STFT= -4.0%
LTFT= -2.0% STFT= -3.0%
LTFT= -3.0% STFT= -2.0%
LTFT= -4.0% STFT= -1.0%
LTFT= -5.0% STFT= 0.0%
If the conditions stay the same the car has 'finished' Re-Learning. Because the STFT is now @ 0.0%, adjustments to the LTFT will stop until STFT moves away from 0.0% again. Actually the STFT will move all over the place if you watched it regularly while someone else was driving, which is why the ECU slowly adjusts the LTFT based on what the STFT tends to be centered around. The Re-Learning Procedure is important to understand because the LONG TERM TRIM is applied against your Wide Open Throttle (W.O.T) FUEL Maps as the ECU does not use the O2 Sensor at W.O.T.
OPEN LOOP MODE
When the Engine is Cold or at W.O.T, the ECU is running in "Open Loop Mode"(No O2 sensor feedback). During Open Loop Mode the ECU maps are programmed to intentionally run somewhat richer for a higher & smoother idle when cold (which operates like a Choke) or for more power at W.O.T.
MODS to ECU
If you Mod your engine to run richer all the time, say by putting in larger fuel injectors, adding a resistor to the Coolant Temp Sensor or increasing the Fuel Pressure by bypassing the Fuel Pressure Reg the ECU is going to know it is running rich based on the O2 sensor feedback in Closed Loop. It will lean out the extra fuel, at first with the STFT then later more and more with the LTFT to get the AFR back to 14.7:1 to reduce emissions. Now when you go WOT the ECU will stop using O2 sensors feedback but it will still remove however much fuel the LTFT tells it to. The net effect is that your mod to richen the mixture won't work after you have driven the car a little while. In fact it could lean it out depending upon how much fuel the LTFT removes resulting in reduced power. The short term power advantages are there but the long term effects will negate the cheap 'Mods'.
This can work in reverse to. If you lean out your mixture during Closed Loop the ECU will ADD fuel as needed to get back to a 14.7:1 AFR and this will be seen in the LTFT. It does this because running lean increases emissions which the ECU doesn't want to do. Now when you go WOT the LTFT will add fuel. A Pre-O2 Sensor Exhaust Leak can also trick the O2 Sensor into thinking there is more oxygen left after combustion that there actually is. The O2 Sensor will send a False signal to the ECU which will adjust the STFT in turn affecting the LTFT. It creates one big visious circle.
RICH & LEAN Mixtures.
A RICH Mixture is when your AFR has more FUEL than Air.
The AFR is less then 14.7:1 or Lambda <1. Running rich increases emissions, decreases heat(Short Term), affects fuel economy & INCREASES POWER(Short Term).
A LEAN Mixture is when the AFR has more AIR than fuel.
The AFR is greater than 14.7:1 or Lambda >1. Running lean increases emissions, increases heat, affects fuel economy and REDUCES POWER.
PROBLEMS
Now Running RICH for too long can cause detrimental long term problems like carbon deposits building up on the back of valves, spark plugs, pistons, O2 sensors and Cat Convertor.
When the Carbon builds up (Mushrooms) on the back of the valves it affects the Air/Fuel Flow into the combustion chamber. The Carbon soaks up the incoming fuel and also stops the valve from dissipating heat properly.
When it Builds up on the Piston Surface it creates Hot Spots and changes the Compression Ratio. This causes Pre-Ignition which leads to Power Loss, Burnt Valves & Melted Pistons etc.
When it builds up on the Spark Plugs the power of the spark decreases and affects the combustion process.
When it builds up on the O2 Sensor, it contaminates the O2 Sensor and makes it lazy. It will send an incorrect/slow signal back to the ECU which affects both the Long Term & Short Term Fuel Trim.
When it build up on the Cat Convertor it can cause it to melt down, which can block the Exhaust Pipe resulting in No Power on acceleration.
LIMP HOME MODE
The ECU is programmed to recognise minimum & maximum parameters from each Input Sensor. If any Input Sensor operates outside these parameters the ECU references Pre-Programmed Default Maps and goes into LIMP HOME MODE The ECU will Turn on the Check Engine Light to warn the Driver of the problem. The O2 Sensor is more or less the 'Last Line Of Defence'.
So it all comes down to this.
"If everything in the engine is operating normally and your AFR is 'perfect' at 14.7:1 all the time then Re-Learning will NOT occur and you will achieve Optimum Power, Emissions & Fuel economy"
The BASIC AIM
Modern Engine Management Systems (Including the HONDA PGMFI System)are Tuned or Mapped to achieve the optimum AFR of 14.7:1 which is universally considered the best trade off between emissions, fuel economy and power production.
In any type of internal combustion engine, to achieve complete combustion the fuel needs to be burned at a ratio of 14.7 parts of air to 1 part fuel. This air to fuel ratio (AFR) is called Stoichiometric or Lambda = 1.
The CLOSED LOOP SYSTEM
When the engine reaches Normal Operating Temperature the ECU receives a signal from the Coolant Temperature Sensor allowing it to enter Closed Loop Mode. The ECU uses an Oxygen Sensor (Lambda Sensor) to determine how much Oxygen is left in the exhaust after combustion. The O2 Sensor converts this to a voltage signal so the ECU can understand what the AFR is.
The ECU will adjust the AFR so that the engine runs at the mapped optimum AFR of 14.7:1 in Closed Loop Mode. Actually, the ECU will alternate the mixture rich to lean and back so it can centre it around 14.7:1.
It does this by Increasing or Decreasing the Injector Pulse Width in Milliseconds(The time the Injector Pintle is open) which determines the actual volume of fuel that is injected during this time. This is also determined by the Fuel Pressure in the Fuel Injector Rail. Fuel Pressure is normally regulated between 220 to 250 Kpa by the Fuel Pressure Regulator.
The ECU doesn't just rely on the O2 Sensor to determine the AFR. It also references the Coolant Temperature Sensor, Throttle Position Sensor, Intake Air Temperature Sensor, Manifold Absolute Pressure Sensor, Crank Position Sensor, Engine Speed Sensor & the No 1 Cyl Position Sensor. In an Auto It also references the Auto Transmission Control Unit and its sensors.
The ECU constantly makes adjustments to the AFR using data from all the Input Sensors, this is called the Short Term Fuel Trim(STFT).
The ECU keeps an average of what adjustments it has made over time. That average is applied against the Long Term Fuel Trim (LTFT), this is known as 'Learning' .
During closed loop operation both the STFT and the LTFT will have an effect on the AFR.
SHORT TERM FUEL TRIM - LONG TERM FUEL TRIM
Lets say the ECU detects the Short Term AFR at roughly 5.0% RICH when we take the car for a drive with a Diagnostic Scanner. When we reset the Fuel Cells or Wipe the ECU Memory, the LTFT is at 0.0%.The ECU will detect, using the O2 sensors, that the Short Term AFR is 5.0% rich. The STFT will equal -5.0%. The ECU will lean out the fuel by 5%. This results in the AFR being brought back to 14.7:1.
At first the LTFT will be at 0.0% but over time the LTFT will slowly move negative. As that occurs the STFT will move back towards 0.0% because the LTFT is now also removing some of the extra fuel(Leaning Out). Therefore the correction needed by the STFT will be less. After driving for a while you would see something like this:
LTFT= 0.0% STFT= -5.0%
LTFT= -1.0% STFT= -4.0%
LTFT= -2.0% STFT= -3.0%
LTFT= -3.0% STFT= -2.0%
LTFT= -4.0% STFT= -1.0%
LTFT= -5.0% STFT= 0.0%
If the conditions stay the same the car has 'finished' Re-Learning. Because the STFT is now @ 0.0%, adjustments to the LTFT will stop until STFT moves away from 0.0% again. Actually the STFT will move all over the place if you watched it regularly while someone else was driving, which is why the ECU slowly adjusts the LTFT based on what the STFT tends to be centered around. The Re-Learning Procedure is important to understand because the LONG TERM TRIM is applied against your Wide Open Throttle (W.O.T) FUEL Maps as the ECU does not use the O2 Sensor at W.O.T.
OPEN LOOP MODE
When the Engine is Cold or at W.O.T, the ECU is running in "Open Loop Mode"(No O2 sensor feedback). During Open Loop Mode the ECU maps are programmed to intentionally run somewhat richer for a higher & smoother idle when cold (which operates like a Choke) or for more power at W.O.T.
MODS to ECU
If you Mod your engine to run richer all the time, say by putting in larger fuel injectors, adding a resistor to the Coolant Temp Sensor or increasing the Fuel Pressure by bypassing the Fuel Pressure Reg the ECU is going to know it is running rich based on the O2 sensor feedback in Closed Loop. It will lean out the extra fuel, at first with the STFT then later more and more with the LTFT to get the AFR back to 14.7:1 to reduce emissions. Now when you go WOT the ECU will stop using O2 sensors feedback but it will still remove however much fuel the LTFT tells it to. The net effect is that your mod to richen the mixture won't work after you have driven the car a little while. In fact it could lean it out depending upon how much fuel the LTFT removes resulting in reduced power. The short term power advantages are there but the long term effects will negate the cheap 'Mods'.
This can work in reverse to. If you lean out your mixture during Closed Loop the ECU will ADD fuel as needed to get back to a 14.7:1 AFR and this will be seen in the LTFT. It does this because running lean increases emissions which the ECU doesn't want to do. Now when you go WOT the LTFT will add fuel. A Pre-O2 Sensor Exhaust Leak can also trick the O2 Sensor into thinking there is more oxygen left after combustion that there actually is. The O2 Sensor will send a False signal to the ECU which will adjust the STFT in turn affecting the LTFT. It creates one big visious circle.
RICH & LEAN Mixtures.
A RICH Mixture is when your AFR has more FUEL than Air.
The AFR is less then 14.7:1 or Lambda <1. Running rich increases emissions, decreases heat(Short Term), affects fuel economy & INCREASES POWER(Short Term).
A LEAN Mixture is when the AFR has more AIR than fuel.
The AFR is greater than 14.7:1 or Lambda >1. Running lean increases emissions, increases heat, affects fuel economy and REDUCES POWER.
PROBLEMS
Now Running RICH for too long can cause detrimental long term problems like carbon deposits building up on the back of valves, spark plugs, pistons, O2 sensors and Cat Convertor.
When the Carbon builds up (Mushrooms) on the back of the valves it affects the Air/Fuel Flow into the combustion chamber. The Carbon soaks up the incoming fuel and also stops the valve from dissipating heat properly.
When it Builds up on the Piston Surface it creates Hot Spots and changes the Compression Ratio. This causes Pre-Ignition which leads to Power Loss, Burnt Valves & Melted Pistons etc.
When it builds up on the Spark Plugs the power of the spark decreases and affects the combustion process.
When it builds up on the O2 Sensor, it contaminates the O2 Sensor and makes it lazy. It will send an incorrect/slow signal back to the ECU which affects both the Long Term & Short Term Fuel Trim.
When it build up on the Cat Convertor it can cause it to melt down, which can block the Exhaust Pipe resulting in No Power on acceleration.
LIMP HOME MODE
The ECU is programmed to recognise minimum & maximum parameters from each Input Sensor. If any Input Sensor operates outside these parameters the ECU references Pre-Programmed Default Maps and goes into LIMP HOME MODE The ECU will Turn on the Check Engine Light to warn the Driver of the problem. The O2 Sensor is more or less the 'Last Line Of Defence'.
So it all comes down to this.
"If everything in the engine is operating normally and your AFR is 'perfect' at 14.7:1 all the time then Re-Learning will NOT occur and you will achieve Optimum Power, Emissions & Fuel economy"
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