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Old February 10th, 2013, 17:56   #1
tisho80
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Default ECU calculations

Hi.I wonder about specified and actual MAF readings.My question is what's the input(or inputs) to ECU which helps calculating the actual MAF(mass air flow).Is that only MAF sensor or MAF sensor in collaboration with other sensors like IAT,MAP......
And my second question is about the specified MAF.How ECU knows at every single moment what mass air flow rate the engine need?What are the inputs here?It depens at engine rpm or something else?
Thank you in advance for your reply.
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Old February 12th, 2013, 06:24   #2
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Specified MAF is going to depend on what the engine needs. So the ECU will be programmed by a lot of testing. This means, for each situation, given the relative humidity, barometric pressure, engine speed (rpm) and load (how much power the engine needs to make), there is a given amount of oxygen required in order to burn the fuel needed.

The smoke map will be used to keep the actual fuel injected from being greater than the actual oxygen available.

The oxygen available depends on how much air is ingested. And in modern cars of all types, a single point where the MAF is placed will be used to measure the oxygen flowing into the engine.

There will be other factors, and the MAF itself should take care of compensating for those factors (e.g. humidity, air temperature), so that the ECU receives the signal that says how much air mass is entering the engine. For the engine speed and load, the ECU knows how much air is required. The required figure is the specified air mass. The actual is how much the MAF says is entering the engine.

The specified figure will have some factors changing it, such as whether you are slowing down, just punched the go-pedal and are trying to pass, or if you just downshifted and are engine braking going down a hill.

But when it comes down to the basics, the specified air mass will be how much air is needed to burn the fuel for that situation.

Exactly what is going on in the ECU, that is, what else the ECU uses to decide how much air to specify, or how the factors alluded to above are used to determine the specified air mass, depends on a lot of information we of the unwashed masses are not allowed to see.
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Old February 12th, 2013, 07:52   #3
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In EDC15 the specified MAF is 100% about the EGR. Nothing more, nothing less
In newer models the MAF is used for other things, but it's still mostly about the EGR. Keep in mind, the airmass entering the intake manifold is not equal to the airflow at the maf.

manifold = maf + EGR

through experimentation on a test stand, The factory knows how much airflow the engine consumes without a maf. What you see in VCDS is the outside air being consumed WITH egr flow lowering the outside air requirement. So in other words, the maf reading is:

MAF = manifold - EGR

Maf is not equal to the total quantity of gas being consumed by the engine at any one time. It is equal to the total consumption of the engine minus the egr contribution to the consumption.

The EGR rate needed for emissions certification is found by experimentation under standard conditions on a test stand (engine dyno). Then they empirically measure the MAF airflow under the prescribed egr rates. a bunch of times, under a number of operating rpm and load conditions in this controlled laboratory environment, to come up with an "average" for each load and rpm. This data is processed into a basic setpoint map (basis map) inside the ecu. Then some environmental variables are changed in a controlled way, and the airflow changes measured. These environmental effects are programmed into other (complimentary) ecu maps which are used as modifying variables, to "educate" the setpoint under various environmental conditions. (essentially creating a dynamic, "virtual" setpoint map at runtime) The goal here is to keep emissions compliance in almost any conceivable environmental condition, since the cars will be driven in a variety of weather and altitude. Both the setpoints maps and duty cycle maps are dynamically generated this way at runtime, however...

Certain operating conditions have the ecu in closed loop mode (feedback, or regulating, or governor if you will) and others have the ecu in open loop mode (feedforward, or steering mode). Closed loop uses all the modifying factors and feeds the data in to a PID controller to actively regulate the EGR valve duty cycle based on the calculated desired flow at the maf, to achieve the desired egr rate. Open loop ignores the actual maf and simply follows the egr duty cycle basis map to control the egr without looking at what's actually happening at the MAF. there is another set of maps and setpoints to tell the ecu when to use each feedback mode for the particular parameter being controlled. (note: this schema applies to just about everything that's ecu controlled, such as boost, fuel, timing, cruise control, etc, only different in the variables being montiored/controlled and the inputs/outputs. this is how the factory (and good aftermarket tuning) tunes cars to operate reliably and consistently under all environmental and altitude conditions)

What you see in VCDS as the "specified" is the airflow value being used to control the EGR, however it's derived. You also see the egr valve duty cycle, though you won't be able to tell in vcds whether it's open or closed loop mode.

I get calls sometimes from guys who are worried to death that their MAF is reading higher or lower than specified. I usually get the call after their mechanics go to great lengths to try and get actual and specified to match. I end up having to explain why this is not a concern. The "specified" is only a crude estimate of what the MAF airflow should generally look like with the egr exactly like the engine on the test stand. There is a fairly wide tolerance band, something like 125 or 200 mg/r plus or minus, depending on the exact model of ecu. If you unplug the egr, you will see the actual maf and specified maf will not match at any time. My point here is, just keep in mind there's a wide tolerance allowed by the factory and it only relates to how much egr flow. I know VCDS shows actual/requested maf air flow, but please realize this maf flow actual/requested comparison is only intended to give the ecu some rough idea of how to adjust the egr valve duty cycle. Nothing more, nothing less. (yes, I know the maf actual MAF value is used for other things but I'm only talking about the actual/specified comparison here)

It's not about what the engine "needs", it's about the EGR, and that's all. I think a lot of mechanics are still in gasser-think mode and don't understand that diesels don't care about stoichiometric air to fuel ratios. In a gas car, if you run too lean, you will burn up the valves or worse. So the correct a/f ratio is very important in a gasser. In a diesel, too lean is "just right". So drive more, worry less.
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Last edited by KERMA; February 12th, 2013 at 11:54.
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Old February 12th, 2013, 12:12   #4
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So this would imply that the adaptation frequently mentioned as a way to 'turn off' EGR functions to bypass the ECU considerations and just keep it at a minimal EGR duty cycle. Do you agree?
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Old February 12th, 2013, 13:15   #5
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adaptation settings appear to offset the requested values (setpoint) by some amount

In other words, the calculated setpoint gets shifted by some offset that's associated with the adaptation number.

For example, when you set the EGR adaptation to 33768, you are telling the ecu to try and achieve a higher airflow as seen at the maf.

More maf requested = less egr

When people ask me about this I always tell them to leave it at stock 32768, because it gains you no extra performance and changing it can hurt your fuel economy.
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Last edited by KERMA; February 13th, 2013 at 12:47.
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