Why do engines with a MAF sensor still use an o2 sensor?

[ELS]

Can't you just calculate the required injector duty cycle depending on the MAF reading?

Also, wouldn't EGR mess up the reading of an O2 sensor?

[BLSTIC]

Your entire car is a collection of mass produced components that only get tested when you turn the key and drive it off the lot, it's necessary for all the minor variations.

Also wear and tear on all components. It's impossible tell if you've got a fuel pressure fault on a car without a rail pressure sensor, let alone model how dirty your maf sensor may be.

[07GTS]

the maf dosnt know what air is flowing thru it, the ecu maf table is applying a airflow value to the hz feedback its seeing, having the 02's is for closed loop control of the fueling so as mentioned it stays closest to the set maf table values throughout the yearly seasons, there is also the SD side that is needed as the maf isnt good with transients

[RDY4WAR]

The MAF measures flow, not mass. Whether you're in Miami or Denver, the MAF sees the same volume of air though the mass of air is very different. Air/fuel ratio is mass of air to mass of fuel, not volume. The O2 sensor gives a readout of the amount of oxygen in the exhaust gases which gets translated into an air/fuel ratio. With volume information from the MAF, it can determine the air density, and injector pulse width is adjusted to compensate. At least that's the case in closed loop. In open loop, the ECU delivers fuel based on preset fueling tables and basically ignores the MAF and O2 sensor. You usually only see open loop during cold startup.

[Racer97]

O2 sensor main purpose is to maintain catalyst efficiency.

[ELS]

The MAF measures flow, not mass. Whether you're in Miami or Denver, the MAF sees the same volume of air though the mass of air is very different. Air/fuel ratio is mass of air to mass of fuel, not volume. The O2 sensor gives a readout of the amount of oxygen in the exhaust gases which gets translated into an air/fuel ratio. With volume information from the MAF, it can determine the air density, and injector pulse width is adjusted to compensate. At least that's the case in closed loop. In open loop, the ECU delivers fuel based on preset fueling tables and basically ignores the MAF and O2 sensor. You usually only see open loop during cold startup.

Well that could be fixed by adding an atmospheric pressure sensor. But wouldn't it not need this? less dense air would cool the MAF sensor element less
And mass could be calculated by flow, the flow values for the MAF sensor, and time.
And well yeah a dirty MAF sensor would be a problem.

But then is there a way to fix the error in the O2 sensor value when running EGR?

[rebelrouser]

two types of fuel systems speed density and mass air flow. But both have O2 sensors, main reason is the O2 gives self-learning capability. Computer picks what fuel map to use by looking at all the sensors including MAF if it has one and then looks at the O2 to see if the air fuel is correct, if it is not the computer then shifts the fuel maps until they air fuel is correct. short-term and long-term fuel trims is the process as this happens, you can watch it on most scan tools. Short term fuel trims are the computer hunting back and forth, and the long term trims are when the computer decides to shift the fuel maps rich or lean. This happens in closed loop operation.
Either system can fuel the engine to run correctly in open loop without the 02's, many stock systems for example do not use the 02's at WOT or cold operation before the 02's warm up. Part of this came from the government emission requirement that the vehicle run for 100,000 miles with no major maintaince and still meet emission requirements. As the engine wears, carbon buildup etc. the computer can adapt and still provide the exact air fuel ratio to meet emissions.

[ELS]

two types of fuel systems speed density and mass air flow. But both have O2 sensors, main reason is the O2 gives self-learning capability. Computer picks what fuel map to use by looking at all the sensors including MAF if it has one and then looks at the O2 to see if the air fuel is correct, if it is not the computer then shifts the fuel maps until they air fuel is correct. short-term and long-term fuel trims is the process as this happens, you can watch it on most scan tools. Short term fuel trims are the computer hunting back and forth, and the long term trims are when the computer decides to shift the fuel maps rich or lean. This happens in closed loop operation.
Either system can fuel the engine to run correctly in open loop without the 02's, many stock systems for example do not use the 02's at WOT or cold operation before the 02's warm up. Part of this came from the government emission requirement that the vehicle run for 100,000 miles with no major maintaince and still meet emission requirements. As the engine wears, carbon buildup etc. the computer can adapt and still provide the exact air fuel ratio to meet emissions.

Also the ECU would have to default to using speed density or MAF when the O2 sensor fails.
Stock ECU's usually run in open loop mode until the engine gets up to nearly operating temp, Or if you press the throttle it goes into closed loop where it uses the O2 sensor to regulate the fuel to hold an AFR of 14.7.
But I'm still confused how would one make closed loop work correctly with EGR, since it would change the oxygen percentage in the exhaust so the O2 sensor would give a false reading that would make the ECU think the engine is running richer than it actually is.

[Chris_Hamilton]

O2 sensor reads the air fuel mixture result after the EGR has injected gas into the airstream. It reads the result of combustion with the EGR gas. Whatever the reading is how rich or lean it's running.

[ELS]

O2 sensor reads the air fuel mixture result after the EGR has injected gas into the airstream. It reads the result of combustion with the EGR gas. Whatever the reading is how rich or lean it's running.

Yeah but since there's a lot more burnt gasses while unburnt oxygen stays the same, wouldn't the O2 sensor see less oxygen content?

[Nut124]

The MAF measures flow, not mass. Whether you're in Miami or Denver, the MAF sees the same volume of air though the mass of air is very different. Air/fuel ratio is mass of air to mass of fuel, not volume. The O2 sensor gives a readout of the amount of oxygen in the exhaust gases which gets translated into an air/fuel ratio. With volume information from the MAF, it can determine the air density, and injector pulse width is adjusted to compensate. At least that's the case in closed loop. In open loop, the ECU delivers fuel based on preset fueling tables and basically ignores the MAF and O2 sensor. You usually only see open loop during cold startup.

I thought MAF does indeed measure mass flow - thus the acronym: Not so? Hot wire is MAF, paddle is volume.

[ELS]

The MAF measures flow, not mass. Whether you're in Miami or Denver, the MAF sees the same volume of air though the mass of air is very different. Air/fuel ratio is mass of air to mass of fuel, not volume. The O2 sensor gives a readout of the amount of oxygen in the exhaust gases which gets translated into an air/fuel ratio. With volume information from the MAF, it can determine the air density, and injector pulse width is adjusted to compensate. At least that's the case in closed loop. In open loop, the ECU delivers fuel based on preset fueling tables and basically ignores the MAF and O2 sensor. You usually only see open loop during cold startup.

I thought MAF does indeed measure mass flow - thus the acronym: Not so? Hot wire is MAF, paddle is volume.

The paddle would be more like MAP since it creates a restriction and then gets moved by the pressure difference that is created.
Then the paddle position can be directly translated into air velocity since the paddle position will be a result of the pressure drop, and the flow will be the pressure drop and known flow values for the paddle sensor. Pretty much the same system as speed density but here the restriction is the paddle instead of the throttle flap.

[Nut124]

The MAF measures flow, not mass. Whether you're in Miami or Denver, the MAF sees the same volume of air though the mass of air is very different. Air/fuel ratio is mass of air to mass of fuel, not volume. The O2 sensor gives a readout of the amount of oxygen in the exhaust gases which gets translated into an air/fuel ratio. With volume information from the MAF, it can determine the air density, and injector pulse width is adjusted to compensate. At least that's the case in closed loop. In open loop, the ECU delivers fuel based on preset fueling tables and basically ignores the MAF and O2 sensor. You usually only see open loop during cold startup.

I thought MAF does indeed measure mass flow - thus the acronym: Not so? Hot wire is MAF, paddle is volume.

The paddle would be more like MAP since it creates a restriction and then gets moved by the pressure difference that is created.
Then the paddle position can be directly translated into air velocity since the paddle position will be a result of the pressure drop, and the flow will be the pressure drop and known flow values for the paddle sensor. Pretty much the same system as speed density but here the restriction is the paddle instead of the throttle flap.

Hot wire MAF sensor is based on heat transfer/dissipation. That is mass not velocity nor volume.

[ELS]

I thought MAF does indeed measure mass flow - thus the acronym: Not so? Hot wire is MAF, paddle is volume.

The paddle would be more like MAP since it creates a restriction and then gets moved by the pressure difference that is created.
Then the paddle position can be directly translated into air velocity since the paddle position will be a result of the pressure drop, and the flow will be the pressure drop and known flow values for the paddle sensor. Pretty much the same system as speed density but here the restriction is the paddle instead of the throttle flap.

Hot wire MAF sensor is based on heat transfer/dissipation. That is mass not velocity nor volume.

Mass is directly related to volume.

[Nut124]

The paddle would be more like MAP since it creates a restriction and then gets moved by the pressure difference that is created.
Then the paddle position can be directly translated into air velocity since the paddle position will be a result of the pressure drop, and the flow will be the pressure drop and known flow values for the paddle sensor. Pretty much the same system as speed density but here the restriction is the paddle instead of the throttle flap.

Hot wire MAF sensor is based on heat transfer/dissipation. That is mass not velocity nor volume.

Mass is directly related to volume.

Not so. Just google it. Wikipedia is good.