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AFM question 
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DIP8 - Involved

Joined: Tue Aug 20, 2013 9:52 pm
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I have a question about the old style Vane air flow meters.

It's call an air flow meter not mass flow meter. However, will the temperature of the air change the reading any, assuming the same cfm flowing through it? The reason I wonder is that cold air flowing through the meter will have more mass than warmer air, so I assume it can push "harder" on the vane and thus hold it open more and thus a larger voltage output for colder air vs warmer air. Does that sound correct? For example: 1000cfm of an extremely dense fluid such as peanut butter (creamy not crunchy) should push the vane open further than 1000cfm of 200deg air. Therefore it's more of an air MASS sensor than an air FLOW sensor. I keep going back and forth on which it is.

These systems also have an air temp sensor in them that the ecu definitely uses to adjust fuel, so my above assumption may be partially wrong or totally wrong. I think there are these possibilities:

Given a fixed amount of CFM of air flowing, the AFM voltage output is proportional to:
1. ONLY air volume flow. It's a true volume flow measurement device regardless of IAT or density of fluid flowing.
2. mostly air volume flow, with a little extra vane movement for colder air but not enough to call it a true mass sensor.
3. air mass flow as the vane movement is primarily a function of the mass of air flowing past, not the volume.

Len


Fri Sep 27, 2013 3:23 am
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DIP8 - Involved

Joined: Tue Aug 20, 2013 9:52 pm
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After thinking about this some more, here are my thoughts.

Cold dense air will definitely push the vane farther than warm air at a given flow rate. However, the delta change reflected in the vane position is very small compared to the delta change when the actual flow rate varies. Think of it as a 1meg resistor in parallel with a 100ohm resistor. If you vary the 100 ohm the overall resistance changes quite a bit, if you vary the 1meg, the overall resistance will vary, just not as much.

So while the density of air will affect the position, it's not particularly sensitive to it. Therefore we can treat the output of the AFM as a volume measurement device. Then we apply air temp correction to turn the volume measurement into a mass measurement for use in fuel calcs. I suppose I'll have to do some testing to see just how "sensitive" it is to air temp.

Lenny


Fri Sep 27, 2013 10:25 pm
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Or you can through the ancient junk directly into the bin where it belongs :-) Put a film MAF in place IF you REALLY want MAF, but there are few reasons to use it, and no good ones.

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Fri Sep 27, 2013 10:48 pm
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DIP8 - Involved

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LOL. Don't hold back, Fred. Tell us how you really feel. :)

I agree that there are much better options today, but what I'm working on needs to support my old junky AFM too.

I did find the equations to calculate what the delta force on the flapper vane is when the density changes. Here is the relevant equation:

It's the equation for dynamic pressure as applied to a compressible fluid. (asciified of course - see the wiki entry for nicer pictures)

q = 1/2rpM^2

where:

q = pressure in Pascals
r = ratio of specific heats (1.4 for air @ sealevel- I haven't looked up creamy peanut butter yet)
p = static pressure in Pascals
M = Mach number

I'm still researching but it looks like the ratio of specific heats will change with air density so this is where the pressure will vary with density which varies with temperature.

So, using this equation I can calculate the pressure applied to the vane which multiplied the area of the vane will give the force on the vane. This of course in turn affects vane position as it works against the spring force. Note that I'm not actually trying to use all this math realtime in the ecu. I'm just trying to understand the physics to see how the AFM sensor responds in the real world.

Len


Fri Sep 27, 2013 11:38 pm
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The plot thickens...

Upon researching the ratio of specific heats I found this:

"The above value of 1.4 is highly consistent with the measured adiabatic indices for dry air within a temperature range of 0 to 200 °C, exhibiting a deviation of only 0.2% "

Therefore the ratio in the earlier formula is pretty much always 1.4. That makes the AFM vane output extremely insensitive to air temperature. So insensitive that for my purposes, I'll treat it as a perfect volume measurement device. The reason it's so insensitive is because it's a compressible fluid flowing through it. If we were using the vane to measure something INcompressible, it would be HIGHLY sensitive to the density.

Len


Fri Sep 27, 2013 11:51 pm
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Fred wrote:
Or you can through the ancient junk directly into the bin where it belongs :-) Put a film MAF in place IF you REALLY want MAF, but there are few reasons to use it, and no good ones.

What would you prefer over using a MAF then?


Sat Sep 28, 2013 3:12 pm
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MAP! Unless you have wild cams and/or ITBs, MAP is excellent :-)

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Sat Sep 28, 2013 5:01 pm
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Did someone say MAF?!


Sat Sep 28, 2013 8:40 pm
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