I'm too stupid to understand that chart without more help.... what bits am I supposed to understand is improving ?
bottom axis is ? revs ? or time and what happening is that as time ticks by things change ?
Ok. Here goes: "Lambda, fueling and AF-XIED for Dummies". (No pun intended
)
(This will be a long one. Please bear with me)
Later years emission regulation requires engines to limit pollution. It's a fairly complex picture, but let's focus on the subject that annoys us, the fueling of the engine, or rather, lack of fuel.
Background: Lambda vs AFR
In an ideal world, perfect combustion for an engine is when every drop of gasoline find it's oxygen molecule to bond with, so when the mixture is ignited, every drop of gasoline has exploded, with no excess unburnt gasoline.
To analyze the combustion, the manufacturer installs an O2 sensor. The O2 sensor reacts to the amount of vacant (not used) oxygen molecules. And when there are no remaining oxygen in the exhaust, this condition is defined to Lambda 1.0
Tuners use the Air to Fuel Ratio, AFR. For a gasoline engine, the Lambda 1.0 will be reached when the AFR is 14,7, i.e 14,7g of air to 1g of gasoline.
Then why mess with Lambda, rather than AFR? The O2 sensor is searching for excessive oxygen. Different type og gasoline (as in mixed with ethanol etc) may offer different AFR. However, regardless of what type of gasoline the engine is burning. Lambda 1 will ALWAYS be the condition where all the oxygen is used.
In order to not add to confusion, we will from now on stick to the most common gasolin, that will offer AFR 14,7 for Lambda 1.
The Lambda sensor used in the debate (Used in BMW boxers up to introduction of the 1250 shiftcam engine, which use a different sensor type.)
So we are debating the Narrow band (NB) O2 sensor.
Here is a picture of the output from a warm NB sensor. When warm, the sensor will emit a voltage, according to the chart below.
(Picture is borrowed from NightRider that produce the AF-XIED)
The scales of AFR is adjusted to reflect gasoline that produce Lambda 1 at AFR 14,7. and the voltage output from the sensor for the different AFR values.
The different curves shows output vs sensor temperature. However, have a look at the 0,5V line. All curves emits the same voltage at AFR 14,6 - 14,8.
The ECU uses the O2 sensor feedback to inform if it runs too lean or too rich, i. e the interesting part is if the voltage from the sensor is above or below 0,5V (AFR 14,7)
But even so, at 0,8V output, the AFR is down to 14,2-ish. From there on, the voltage reading vs AFR will be less accurate.
The ECU couldn't care less about what happens when voltage is above or below 0.5V. the ECU sees richer than or leaner than AFR 14,7, even if the voltage output slightly varies with AFR in a highly non-linear manner.
So, when the engine runs, the amount of fuel injected is calculated from tables inside the ECU (called maps, hence remapping meant changing the value in the tables). The tables offers information to how long (time in milliseconds ) the injectors are opened, depending on RPM, throttle position etc. There are actually several maps, but for simplicity lets just say there are preinstalled maps.
Now the engine is running, and fuel is delivered according to the maps. However, in order to fine tune the fueling, the O2 sensor offers feedback of the combustion. These small adjustments, called for by the O2 sensor, are called Short Term Trim, and it is basically a number that adds or subtracts to whatever time value for the opening of the injectors.
But keep in mind, the exhaust reflects the result after the combustion, i.e the O2 sensor will not tell ECU what to do. It tells the ECU how it did.
Let's say the airfilter is clogging up, restricting the airflow to the cylinder. Now, the pre-determened values from the maps will give the engine too much fuel, since the engine gets less air than expected. And the O2 sensor will tell the engine that the fueling is too rich, thus it needs to trim down on the amount of fuel. In a steady condition, this will work pretty well, but it will always need a couple of strokes of the piston in order to first analyze, and then inform if too lean or rich. And this goes on and on.
Her comes the Long term trim: At every condition (rpm vs throttle position), the Short term trim will be stored. It's tendency for richening or leaning is stored in small steps. But the point is, if a given condition consistently calls for a need of adjustment, this adjustment will be stored in the Long Term Trim table. And this table is part of the equation when the ECU collects data from different maps in order to determin the fueling. BMW calls the Long Term Trim the Adaptive map This is the map that gets zeroed out when ever BMW talks about restoring the adaptive values.
So much for reading the chart in the previous post.
So, what does the AF-XIED do, and how?
Take a look at the curve below.
The curve shows the voltage from a warm O2 sensor, and this is what the ECU reads. (Data from a -15 LC boxer)
Up to 10 seconds, the engine is running with a steady throttle. At 10 seconds and after, the engine is accelerated.
Pre-10, the voltage from O2 sensor flickers between 0,1V and up to 0,9. When voltage is < 0,5 V the ecu sees 'Lean' and it will add a small amount of fuel. Then it keeps adding until the O2 reads back >0,5V. Now it's too rich, so time to reduce the fuel. This is done by correcting the short term trim. Go back to previous post and see how the Short term trim table varies...
At time mark 10 sec, the throttle is opened up. Now we see that the voltage jumps up to a steady 0,9V. Refer the table og O2 sensor voltage output vs AFR. The green curve reflects the 1200F temp of the O2 sensor, a fairly common condition, i.e when adding throttle, the AFR is richened to 13,6.
(What is the big dela about 13,6?
Well, the world is not perfect, hence in order to utilize all the oxygen molecules, we need to throw in some extra fuel for good measure. Practical tests have shown that adding a bit more fuel will make sure all the O2 molecules get their share of gas, and more molecules with fuel + O2 mixed means a more powerful explosion during the combustion, i.e more power)
The point I'm trying to say is: Any tuning of the engine that only involves controlling the fuel to air mixture will offer very little top end power, as the ECU OEM fueling allready offers the added fuel at full throttle (Full throttle has not been the focus of emission control, even if that may change in the future).
Tuning involving changing AFR will benefit when throttle is less than fully open, as in the condition in the first ten seconds og the chart above. And generally, when throttle is opened up in the higher range (the limit varies depending of the type of engine. For the F800, i know the limit is 80%) the fueling is richer than AFR 14,7
Here is where the AF-XIED shines:
Have a look at the chart below. It shows what the AF-XIED at setting 8 does to the engine.
Red curve is the voltage from the O2 sensor and now being analyzed by the AF-XIED, the blue curve shows the voltage produced by the AF-XIED and transmitted to the engine.
The Voltage from the O2 sensor (red) is read by the AF-XIED, and stops there. This voltage is analyzed, and the AF-XIED creates the blue curve and transmits this to the ECU. The ECU has no knowledge about the AF-XIED and behaves like this is the feedback from the O2 sensor.
Remember: High voltage = Rich. Low Voltage = Lean.
If we amplify the red curve, we will notice that every time the red curve drops below a straight line, the AF-XIED signals a low voltage to the ECU (And Low voltage = Lean condition). The ECU gets a Lean condition signal and reacts to it by richening the fuel.
Now, we don't want the fuel to go excessive rich, so when the red curve rise to slightly above the straight line, (hence going rich), it's time for the ECU to be stopped from keep richening the fuel mix. And this is done by the AF-XIED rising the voltage emulating the O2 sensor. Now the voltage output follows the actual O2 voltage and reports this to the ECU. The ECU will react to this richer than 0,5 V condition by once again starting to lean the mix, and on and on....
Clever little bugger, isn't it
The OEM variation between lean and rich being so slow actually does not reflect Bosch ability for accurate control. An engine with CAT needs to feed the CAT with oxygen. The cat use oxygen to convert CO to CO2, and this oxygen is delivered to the CAT through the exhaust, that is controlled to produce periodes with excessive O2 in order to feed the CAT. Hence the slow reaction time that allows for periods with too much oxygen. Hence, it is preferred to set the AF-XIED to a value rich enough for a smooth running, and no more. That's why they recommend to go from a low value and slowly increase the setting, one step at a time. The point of allowing time between each change is to allow for the ECU to rebuild the Long Term Trim for each time the setting has been changed.
Also, remember the NB sensor inaccuracy to AFR vs voltage depending on the sensor voltage. The AF-XIED aims to maintain a steady voltage feedback from the O2 sensor.
The LC2 system reading in previous post is based on a different type of sensor. It reports back the AFR. Hence even if the AF-XIED keeps a steady voltage from the NB sensor, varying sensor temperature will mean a slightly variable AFR . Thus, the AFR curve in the previous post shows a curve that is less straight than the direct voltage reading from the OEM NB O2 sensor.
If the goal is perfect, then the NB sensor needs to be replaced. But, given how well it works, even if less than perfect, it works well enough to where rebuilding to more accuracy, the extra improvement will not justify the cost and extra complexity.
