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AIr flow meter and mass airflow

AIr flow meter and mass airflow

(OP)
It seems to be pretty common terminology to call an AFM a MAF especially in the automotive world. It's my understanding that on it's own there is no way for an AFM to calculate mass airflow.
An AFM could be used with other components like a temp sensor and MAP sensor to calculate mass airflow but on it's own it cannot. It seems to me akin to calling a hotwire a MAF. A hotwire MAF cannot calculate mass airflow without the other included components to calculate mass airflow and that make it a mass airflow sensor. A hotwire is nothing more without those things.
It is very hard to find information talking about technical details and technical function in regards to the airflow's effect on the vane on an AFM. I have thought in the past that a vane AFM measured volumetric airflow but in thinking about it I realize that's not entirely correct either. X CFM passing through the vane at the fringes of space would open the sensor much less than the same CFM flowing through it at sea level so the the AFM should respond to a reduction in mass airflow at the same volumetric airflow but not in a way that can calculate volumetric airflow without other sensors right?

Many AFM devices also have a temp sensor. This will help but it still seems to me you wold need a pressure sensor to calculate mass airflow.

So is the whole entire internet wrong? Or am I missing something?

RE: AIr flow meter and mass airflow

Hotwire airflow meters do in fact measure mass airflow. The filament loses heat in proportion to the number of air molecules interacting with it. This heat loss is proportional to average air velocity, and air density ie massflow. Air temperature is not so much "measured" as "used as a reference to maintain the filament at a fixed temperature above the airstream". This can be simply achieved by incorporating the filament and the reference (air temp) resistor as two of the four legs of a wheatstone bridge, using bridge current to heat the filament and maintaing the bridge in balance with a feedback cct.

je suis charlie

RE: AIr flow meter and mass airflow

gruntguru is quite right, though some may quibble with his first sentence. The hot wire element, together with the reference temperature element, allows a signal that is a function of the velocity of the stream passing the element to be generated. The velocity can be used for a surrogate of mass flow, assuming that the transfer function from velocity to mass flow is known for the unit in which the hot wire element is employed. This is going to be the case for a released production unit. In fact, the transfer function may well be developed directly from output signal voltage (or current or frequency, as may be the case) to mass flow, ignoring the intermediate step of velocity.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: AIr flow meter and mass airflow

(OP)
Okay well I guess I blew that part lol. I thought that hot wire sensors required more sensory input to calculate actual mass airflow.
This does still set the baseline though because a hotwire MAF does calculate mass airflow.

So the real point of the thread and what I was trying to get at is if a vane type air flow meter should be categorized as a mass airflow meter. I say no it shouldn't but almost everything you can find on them online puts them in the category of mass airflow sensors.

RE: AIr flow meter and mass airflow

Hemi.(I did mention velocity in my post.) Apart from that:

Name 5 sensors that directly measure the physical property stated on the box - without measuring some intermediate physical property. I will start you off. Tape measure.

For every one you can name, I will name ten that measure some intermediate physical property and infer the measurement of the stated physical property.

je suis charlie

RE: AIr flow meter and mass airflow

(OP)
Does a vane AFM infer the measurement of the stated property though? That is the whole question.
You can look at the information coming from a hotwire MAF and estimate within a fairly accurate margin the mass airflow right?
Looking at a vane type air flow meter without data on air pressure or temp can you actually know what the mass airflow was?

RE: AIr flow meter and mass airflow

No. You need to know air density as well.

je suis charlie

RE: AIr flow meter and mass airflow

(OP)
That was exactly my thinking. It could be used in conjunction with other sensors to be a mass airflow sensor but on it's own could not be considered to be one.

Then the question is what is an easy accurate title to describe what an AFM actually measures? It's not mass airflow, and it's not volumetric airflow.

RE: AIr flow meter and mass airflow

I doubt there is an easy title. I think it is safe to say that its output is volumetric and not sensitive to atmospheric pressure within a normal range.

je suis charlie

RE: AIr flow meter and mass airflow

Hi yoshimutuspeed,

By "vane AFM" do you mean the single disk, like in Bosch K Jetronic?
http://www.ffp.fi/contentimages/tekstit_kejet_2.jp...
Most charts show this system measures engine temp to determine if enrichening is required for starting and the first several minutes of running, but NO air temp sensor in the air stream to correct airflow > mass flow.

Or, the curious right angle flapper in L Jetronic?
http://www.alfabb.com/bb/forums/attachments/carbur...
This one does have an air temp sensor just before the flapper.

I was thinking some systems had a barometric pressure sensor, but on first glance these to vintage Bosch system had none.
Maybe they began relying on the Lambda sensor to make it all right.

RE: AIr flow meter and mass airflow

Those mechanical air-flow meters dated back to a different era in emission standards. I don't know of any current applications of these (that doesn't mean they don't exist - it's just been a long time since I've seen one.) They could probably get away with a lot back then that would not fly today.

Carburetors (before emission controls) have no temperature/barometric pressure correction, either, and the engines still (mostly) ran.

Also bear in mind that quite a number of modern, emissions-compliant engines have no MAF sensor at all. They infer the amount of airflow from barometric pressure, manifold pressure, intake temperature, throttle position, etc. The view of these manufacturers is likely that pressure and temperature and position sensors are more robust. From my own experience, I can't say that I disagree.

If you can get the system to work with no MAF, surely you can get it to work with an imperfect one!

RE: AIr flow meter and mass airflow

(OP)
Tmoose I am interested in the differences and defining characteristics of any unit that falls under the title of AFM.
I do not believe either of those could fall properly under the title of mass airflow meter. I am much more familiar with the L Jetronic style and I know that most have intake temp sensors. Some L Jjetronic cars do also have rudimentary MAP sensors. This is where I was saying an AFM could be part of a system that can estimate mass airflow but on it's own shouldn't be considered a mass airflow sensor.
You are correct about most systems using the O2 sensor as the final piece to do the fine tuning compensation. In this case the EFI system does know enough to calculate for mass airflow but the AFM is only one of the tools is uses to do so.
Interestingly enough most first generation MR2s had L Jetronic with O2 sensors but in at least some areas of Europe and possibly other design markets they had no O2 sensors. These required manual baseline setting of the tune. I think these may have used a rudimentary MAP sensor but I don't remember for sure.

If you search for airflow meter the most common thing you will come up with is something like a portable weather station. Now this unit should be able to calculate mass airflow because it is usually also going to be collecting temp and pressure readings. I would still argue that an airflow meter can be used to calculate mass airflow but that it should not be automatically labeled as a mass airflow sensor because many AFMs don't provide enough information to calculate mass airflow.
As another example a wind speed meter like a sniper might use will be a similar air flow meter but the sniper won't care about temp or pressure or the then achievable calculation of mass airflow. He only cares about the winds effect on the bullet and that will have the same effect on the meter as it will the bullet so those other variables are really irrelevant.

RE: AIr flow meter and mass airflow

Hi Yoshimitsuspeed,

What did you mean when on 15 Mar 15 01:07 you said "vane AFM" ??
regards,

Dan T

RE: AIr flow meter and mass airflow

(OP)
PrianPeterson
I completely agree with everything you said. My question is in regards to nomenclature and proper understanding of the principle and function. I wanted to make sure I wasn't missing something and that I understood within an acceptable level of accuracy what an AFM did. And apparently also had a little to learn on the basic operation of a hot wire MAF as well hehe.
I hate it when wikipedia is the best source I can find to cite but in this case I haven't really found anything that better describes the operation and differences between an AFM and a MAF and Wikipedia definitely has messed it up pretty bad. I really feel like there were sources I could easily find a couple years ago that made it clear that an AFM was not a MAF but I can't find them anymore.
Wikipedia lists vane type airflow meters under MAF and in a recent edit someone has suggested they be completely merged under mass airflow sensor.
I believe this is completely incorrect but can't really find anything that backs that in a substantial manner.
http://en.wikipedia.org/wiki/Mass_flow_sensor
http://en.wikipedia.org/wiki/Air_flow_meter

Quote (wikipedia)

An air flow meter, is a device that measures air flow, i.e. how much air is flowing through a tube. It does not measure the volume of the air passing through the tube, it measures the mass of air flowing through the device per unit time. Thus air flow meters are simply an application of mass flow meters for a special medium. Typically, mass air flow measurements are expressed in the units of kilograms per second (kg/s).


Now as far as motor operation these systems did work pretty dang well especially compared to the capability of the carbs and mechanical fuel injection that preceded them.
I can drive my MR2 from 4000 feet to 12000 feet and under cruise maintain stoichiometric AFRs under cruise the whole time. When I increased my injector size by 20% I really learned how much even basic 80s technology was able to adapt and what it's limits were. I was surprised to see that it was able to use closed loop AFRs to trim open loop operation and was able to largely adapt to the larger injectors. It never got all the way there though. The size increase was just outside of the ECUs range of adaptability.
I do beleive one of the biggest things that has allowed newer cars to get rid of the AFMs and MAFs is more advanced 02 sensors. Of course paired with the other technological advancement as well but I think the medium band and wide band sensors have done a lot in allowing cars to really tighten up their efficiency and emissions with less monitoring on the intake side.

RE: AIr flow meter and mass airflow

The O2 sensor and monitoring strategies are necessary for achieving the last bit of accuracy that is necessary for compliance with today's emission standards ... but the base tune that is used for open loop operation (cold start, full load, and in the event that the O2 sensor fails) is usually pretty darn close these days! It has to be close, because generally the O2 sensor is used to determine short-term and long-term trim maps, and these are only allowed to have a certain percentage before the diagnostics chime in with fault codes indicating that something is wrong. Your 1980s car probably has a wider allowance in its trim maps than newer models do.

Between one car (which is a DaimlerChrysler product), one van (which is a FiatChrysler product), and two fuel-injected motorcycles (one Kawasaki, one Honda), I own no MAF sensors. All but one bike is closed-loop EFI with 3-way catalyst. The one bike (which is from 2004) is open-loop EFI with secondary air injection and oxidizing catalyst.

I don't know how the cars do it, but the bike systems use a combination of both N-alpha (i.e. RPM and throttle position) at high engine load, and speed-density (i.e. RPM and intake manifold pressure) at light load, and some sort of blend in between. If a sensor conks out, it switches over to the other algorithm (and turns on the fault-warning lamp).

RE: AIr flow meter and mass airflow

In fact, thermal conductivity sensors cannot ever measure mass. Of course an ECU can infer mass flow from the operating point of a circuit that includes a conductivity sensor. If the flowing gas is air, the calculation is reliable. If anything other than air is flowing, the calculation will be wrong. For example, atmospheric humidity throws it off. Fortunately for fuel metering, the error in fuel metered due to humidity is in the same direction as the needed correction to the calculated air mass, so no correction need be made (at least at the current level of required accuracy).

I once debated a colleague about this and he cited a Mercedes Benz PR statement that made the claim that hot wire MAFs measured mass while the old vane style MAF did not. The curious thing is that the vane style sensor is an aerodynamic device where mass is expressly part of the force equation. Oh well...

RE: AIr flow meter and mass airflow

The point is that hot-wire MAF sensors generate a mass-flow-dependent output regardless of temperature and pressure variations. Vane style AFM's do not, their output being sensitive to both temperature and pressure.

A hot wire MAF is actually quite insensitive to Humidity - the output still delivering a fairly accurate MAF. The problem as humidity increases is that dry air flow (and therefore oxygen flow) is reduced - having been displaced by water vapor.

je suis charlie

RE: AIr flow meter and mass airflow

Hot-wire vs vane strikes me as either a brilliant bit of lateral thinking or the fruits of random experimentation and investigation of unexpected correlation. Either way, who have thought it could work so well?

Steve

RE: AIr flow meter and mass airflow

Gruntguru, are you saying that the output of an aerodynamic flow sensor is dependent on temperature and pressure in a different way than mass flow is dependent on temperature and pressure?

A 1% error in conductivity due to humidity would be a problem, IMHO, if it were not for the error largely overlapping the reduced air due to displacement.

The hot wire anemometer does not measure mass, but as said, since you know the gas is air, you can infer mass and then the device itself is more precise and reliable than the vane.

RE: AIr flow meter and mass airflow

The output of a hot wire AMM has a fixed relationship to air mass flow. This relationship holds even if air pressure or temperature vary.

The output of a Vane AFM has a relationship to air mass flow. Any variation of air pressure or temperature will affect that relationship. Compensation will be required.

je suis charlie

RE: AIr flow meter and mass airflow

Gruntguru, you said "compensation will be required" for the vane type MAF, but this is true of the hot wire meter and all of the transducers I know of. The hot wire anemometer was not the first choice in the early days because of non linearities and dynamic range problems. Compensation is built-into the circuit especially current control.
I am certainly not arguing that the hot-wire is not superior to the vane. Number one is that it has no moving parts. I am stating that the hot-wire has NO input from mass in its operation while the vane DIRECTLY measures mass.

RE: AIr flow meter and mass airflow

(OP)
If an AFM directly measured mass airflow then why do some even run a temp sensor? Is the amount the door opens directly related to the mass passing by regardless of density?
Here is an extreme scenario to think about. Some people put their AFM after the turbo on boosted builds or NA motors converted to turbo. This seems like a very bad idea to me because it doesn't seem to me like the AFM will read properly. If someone were to run 44 PSIA boost through the AFM would the AFM read the same as it would before the turbo having air flowing past it at one third the pressure and 3 times the volumetric flow rate?

Another thought that is much less related but operates on the same principles.
If an AFM is able to measure mass airflow simply by the air passing by it and that airs effect on the vane then shouldn't it be very easy to make a carburetor that never needs jetting? Yet as you change altitude you need to run leaner jets because the change in mass airflow is not directly linear to the force the air applies to draw the fuel out of the jet. Wouldn't the same factors change in relation to the mass airflow and the force applied to the metering plate?

RE: AIr flow meter and mass airflow

Yes, but the electronic systems to which these devices are connected nowadays are more than smart enough to deal with the difference.

RE: AIr flow meter and mass airflow

(OP)
Sure but then why not just call an O2 sensor a MAF because it often gets incorporated into systems that can rather accurately estimate mass airflow?
I mean heck with an O2 sensor and knowing injector duty cycle alone I would bet you could estimate mass airflow more accurately than an AFM on it's own.

RE: AIr flow meter and mass airflow

The sensor reacts based on the mass of the air going through it. Sure it is a combination of density, temperature, velocity, but the dominating factor is mass flow.

Will it read the same before a turbo and after? No, because the nature of the sensor is to take a sample with a small element in a known location inside a tube, use a calibration table that assumes a certain flow profile (at or near atmospheric pressure) and guess what the flow is through the entire tube. Based on the measurement from that one location.

The majority of the error is probably due to the higher density air at a lower velocity, not matching the calibration table that is expecting higher velocity/lower density for that raw value of the sensor output.

You can't denounce an entire sensor type for being inaccurate, when it's not calibrated for the situation it's being put in. It's less than ideal, but better than no sensor at all (some may disagree, that's fine, use speed density.) If you added another dimension to the calibration table that would pick a different value based on varying pressure, then it could be just as accurate as a naturally aspirated configuration. More so in fact, because it would compensate better for atmospheric conditions.

RE: AIr flow meter and mass airflow

I am referring only to hot wire anemometers (MAF sensors) to be clear. :)

RE: AIr flow meter and mass airflow

Quote (1gibson)

The sensor reacts based on the mass of the air going through it. Sure it is a combination of density, temperature, velocity, but the dominating factor is mass flow.

Will it read the same before a turbo and after? No, because the nature of the sensor is to take a sample with a small element in a known location inside a tube, use a calibration table that assumes a certain flow profile (at or near atmospheric pressure) and guess what the flow is through the entire tube. Based on the measurement from that one location.

The majority of the error is probably due to the higher density air at a lower velocity, not matching the calibration table that is expecting higher velocity/lower density for that raw value of the sensor output.
Actually, as explained by gruntguru in the second post, the hot wire concept [with a reference temperature element] is quite immune to changes in pressure or temperature, since it is functioning on a heat transfer principle, which depends on the number of air molecules passing the sensor per unit time, at a constant delta T. The reference temperature element is used to ensure that the delta T between the sensing element and the sample air flow is constant, removing the sampled air temperature as a variable.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: AIr flow meter and mass airflow

Oh absolutely the concept is bulletproof. The implementation is a different story when you try to use it outside the calibrated range.

RE: AIr flow meter and mass airflow

1gibson.
I think the main problem with comparing a MAF sensor before and after a turbo would be a) the possibility of turbulence or swirl. Either of these will increase the local velocity in the region of the hot wire. b) The duct geometry prior to the MAF. The velocity profile across the MAF is crucial. I recall (many years ago) installing an "orifice plate" to slightly reduce the cross section to correct a lean mixture problem. A very slight reduction in area and the resulting "vena contracta" produced a marked increase in MAF output and solved the problem.

je suis charlie

RE: AIr flow meter and mass airflow

Yoshimitsu, you asked "...If an AFM directly measured mass airflow then...?". The vane or swinging door meter is an aerodynamic device and directly measures mass. It is in the operational equation as a causal element. But, the equation often has non-linear components, like v^2. Compensation may be required. It can be built in like the side ports of a pitot tube, analogous to the compensating reference element of a hot wire meter.

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