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Thread: Measuring AIR Volume ?

  1. #1
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    Measuring AIR Volume ?

    ok if a MAF sensor works by measuring Air volume
    does air volume stay the same after it has been
    compressed/pressurized through a turbo?
    or does the air volume change bcuz its been through
    the turbos compressor

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  3. #3
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    its doesn't work by measuring air volume...
    hint: MAF = MASS air flow

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    "The MAF is located between the air filter and the throttle body. Its usually a large box with a cable going to it. The MAF uses a delicate sensor inside to measure the volume of air passing through it. This info is sent to the DME and the DME adjusts the amount of fuel injected into the cylinder for the correct fuel/air ratio. Since the MAF is on the vacuum side of the engine it's easy to see that all vacuum hoses and connections need to be tight for it to work propely. If air enters through a leak in a hose or connection then its entering after the MAF and its not being measured. The DME only measures what passes through the MAF. If 10% of the air is getting into the engine after the MAF then you'll get 10% less fuel and the engine will run lean."
    ^^^^
    i read this on one a post but have a quik question. the MAF run on the vacuum side so it obviously a draw through type MAF correct? well my MAF is 6 pin bosch hotwire. my car is running turbo at 5 psi boost.if the MAF measure volume of air isnt that different from compressed air coming from the turbo? should the MAF be moved to before the turbo as air is not compressed yet?

    PLEASE EXPLAIN IM CONFUSING MYSELF NOW!!

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    Quote Originally Posted by brisbolo View Post
    if the MAF measure volume of air isnt that different from compressed air

    Read post #3 again.

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    Quote Originally Posted by BenFenner View Post

    Read post #3 again.
    MAF=MASS AIR FLOW
    so it measures MASS AIR FLOW and not air volume?
    so does it make a difference weather it is compressed or not?
    when measuring MASS AIR FLOW

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    I suggest you learn some basic physics, i.e. The definitions of volume and mass before you respond again.

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    ok i did some definition searching as stated above^^^^^
    i checked into volume and mass i understand how the MASS air flow sensor reads MASS now instead of volume...but what i still dont understand is when the turbo sucks in all air MASS before it compresses the air, is the mass still the same as before it was compressed or is it different now since it has been compressed and now pressurized?

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    Conservation of Mass would be the next thing to google. Basically, air mass is the same through the entire engine, intake to exhaust (plus fuel mass).

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    Quote Originally Posted by brisbolo View Post
    ok i did some definition searching as stated above^^^^^
    i checked into volume and mass i understand how the MASS air flow sensor reads MASS now instead of volume...but what i still dont understand is when the turbo sucks in all air MASS before it compresses the air, is the mass still the same as before it was compressed or is it different now since it has been compressed and now pressurized?
    If you understood how a MAF works then you should have the answer to your question

    If you take an aluminum pop can and crush it, does it change weight?

    But seriously google it and then move your MAF before your turbo. Not knowing your setup or what MAF your running, it's likely a plastic housing and your probably using some sort of clamp on the housing to hold a coupler to it. Most stock plastic MAF housings aren't built for clamped on couplers that see positive pressure and will like blow off.

    Quote Originally Posted by Nate P View Post
    Conservation of Mass would be the next thing to google. Basically, air mass is the same through the entire engine, intake to exhaust (plus fuel mass).
    Umm... Not really. Don't forget mass is density multiplied by volume and density of air is dependent on temperature and water content. So the mass of air is not truly the same through the entire engine, intake and exhaust. Also, I need to brush up on my conservation of mass, but not really thinking that was the right wording here. Since your introducing "energy" by burning fuel and compressing air with the turbo, I think conservation of mass in the scenario is a bit more than air mass in plus fuel mass = mixture out
    Last edited by cragg56; 01-19-2012 at 07:38 PM. Reason: Automerged Doublepost

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    Quote Originally Posted by cragg56 View Post
    Umm... Not really. Don't forget mass is density multiplied by volume and density of air is dependent on temperature and water content. So the mass of air is not truly the same through the entire engine, intake and exhaust. Also, I need to brush up on my conservation of mass, but not really thinking that was the right wording here. Since your introducing "energy" by burning fuel and compressing air with the turbo, I think conservation of mass in the scenario is a bit more than air mass in plus fuel mass = mixture out
    I am confused, are you saying mass is not conserved as it travels through the engine? Maybe I misunderstand your point, but if that is what you are saying where would it go?

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    Quote Originally Posted by cragg56 View Post
    If you understood how a MAF works then you should have the answer to your question

    If you take an aluminum pop can and crush it, does it change weight?

    But seriously google it and then move your MAF before your turbo. Not knowing your setup or what MAF your running, it's likely a plastic housing and your probably using some sort of clamp on the housing to hold a coupler to it. Most stock plastic MAF housings aren't built for clamped on couplers that see positive pressure and will like blow off.



    Umm... Not really. Don't forget mass is density multiplied by volume and density of air is dependent on temperature and water content. So the mass of air is not truly the same through the entire engine, intake and exhaust. Also, I need to brush up on my conservation of mass, but not really thinking that was the right wording here. Since your introducing "energy" by burning fuel and compressing air with the turbo, I think conservation of mass in the scenario is a bit more than air mass in plus fuel mass = mixture out
    so in other words, the air MASS stays the same whether compressed or not?
    because as far as i know if you crush as soda can like you said....the weight remains the same

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    Quote Originally Posted by rodger533i View Post
    I am confused, are you saying mass is not conserved as it travels through the engine? Maybe I misunderstand your point, but if that is what you are saying where would it go?
    Like I began my statement with. I may need to brush up on my conservation of mass. But no I was not trying to say that mass is not conserved. I just can't remember how energy/heat plays a factor in everything. The real point I was trying to make is that air mass through the system is really not the same since temperature/pressure changes.
    Last edited by cragg56; 01-19-2012 at 09:40 PM.

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    Compressed air has more density which means more mass. Density changes due to temperature and pressure differences.


    The MAF reading will be different with and without a compressor.

    Just trying to make it as basic as possible for you.
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    Quote Originally Posted by wazzu70 View Post
    Compressed air has more density which means more mass. Density changes due to temperature and pressure differences.


    The MAF reading will be different with and without a compressor.

    Just trying to make it as basic as possible for you.
    thanks i appreciate you trying to make it basic as possible for me. so whether the MAF is on the turbo pressure side or pre turbo it will be different reading either way?

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    m=d*v is as easy as its going to get. If you can't interpret the equation you have a lot of research to do.

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    Quote Originally Posted by cragg56 View Post
    Umm... Not really. Don't forget mass is density multiplied by volume and density of air is dependent on temperature and water content. So the mass of air is not truly the same through the entire engine, intake and exhaust. Also, I need to brush up on my conservation of mass, but not really thinking that was the right wording here. Since your introducing "energy" by burning fuel and compressing air with the turbo, I think conservation of mass in the scenario is a bit more than air mass in plus fuel mass = mixture out
    The high (and low) pressure gas exchange processes are just conversions, energy is already there.

    The point was, airmass is identical pre and post turbo (and on a veryyyyyyyyyyyyyyy basic level, at the exhaust)

    Quote Originally Posted by cragg56 View Post
    Like I began my statement with. I may need to brush up on my conservation of mass. But no I was not trying to say that mass is not conserved. I just can't remember how energy/heat plays a factor in everything. The real point I was trying to make is that air mass through the system is really not the same since temperature/pressure changes.
    They change, but in a giant happy relationship (or mass wouldn't be conserved?).
    Last edited by Nate P; 01-19-2012 at 11:41 PM. Reason: Automerged Doublepost

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    Quote Originally Posted by brisbolo View Post
    thanks i appreciate you trying to make it basic as possible for me. so whether the MAF is on the turbo pressure side or pre turbo it will be different reading either way?
    Pre and post compressor MAF placement will have effect as the MAF is setup to measure MASS flow at ambient conditions or very close to it.

    After the compressor compresses the mass it will remain the same just in a smaller volume.

    I.e 600CFM @1bar boost = 300CFM given same air temperature. Given the same mass it will scale with air temperature as density will change.
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    Exactly

    Quote Originally Posted by Nate P View Post
    They change, but in a giant happy relationship (or mass wouldn't be conserved?).
    I was trying to be polite above, but mass has to be the same. The only way it wouldn't would be a little nuclear reaction going on in our engines, ala a guy named Einstein and his e=mc^2 stuff. To understand the relationship a good place to start is the ideal gas law. But I realize that is slightly off topic.

    Now, I am not saying that a MAF sensor would not measure mass flow differently pre and post turbo because I don't completely understand the intricacies of how they work and defer to those with a wealth of MAF sensor knowledge on here. But theoretically a true mass sensor will measure the same rate regardless of placement, conditions, etc.

  20. #20
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    I think this whole conversation of mass thing has people going down the wrong road...plus, conversation of mass discussion are usually relegated to ideal closed systems which this is not...

    MAF (mass airflow) meter measure air mass. This is done by heating an element within the MAF to a fixed temperature. As air moves across this element it cools the heated element requiring more voltage to be applied to maintain the fixed temperature. In this way the MAF generates a 0-5.0 volt signal to the ECU. Now there is a relationship that converts this voltage into air mass into grams/second (g/s). Please keep in mind that the relationship between output voltage and air mass (in g/s) is not linear but is actually a curve which is established through testing of the MAF unit (element placed within a specific MAF housing)

    Mass is different pre and post turbo. You are compressing air into a fixed volume (intake side which includes intercooler and related piping) thereby increasing its density. So YES, the mass will increase which is why you can easily max out a MAF that is not properly calibrated for use in a turbocharged system.

    Placement of the MAF:

    Draw through:

    If you place the MAF before the turbo this is known as draw through. Now in this method you can't use a blow off valve that vents to atmosphere as the air has already been accounted for by the MAF. You have to recirculate it back into the intake system after the MAF. I would also argue that it is more difficult to account for the how the turbocharger will compress the air across different conditions relative to the compressor map so it would be a little more difficult to tune as you will be not be getting as accurate an idea of true air mass compared to blow through.

    Blow through:
    Placed after the turbo, but after the off valve and before the throttle body. This allows you to use a vent to atmosphere blow off valve because you are "reading" the air mass that goes into the engine and venting it to atmosphere has no conquences on the tune. As I stated above I would argue that its a better solution as you will be reading true amount of air mass after its been compressed by the turbo.

    Ultimately the MAF is only part of the equation. It must be properly matched to the airflow required and the ECU has to be retuned properly to use it.

  21. #21
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    Mass is different pre and post turbo. You are compressing air into a fixed volume (intake side which includes intercooler and related piping) thereby increasing its density. So YES, the mass will increase which is why you can easily max out a MAF that is not properly calibrated for use in a turbocharged system.
    How does a turbocharger create mass? (or additional mass then what the compressor has to begin with)

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    Quote Originally Posted by Nate P View Post
    How does a turbocharger create mass? (or additional mass then what the compressor has to begin with)
    in the strictest sense it is not "creating" mass...

    it's better to think of moving mass from one container to another...

    look at it this way...

    Preturbo atmosphere is one container of limitless volume with a particular density based on temperature, humidity, etc...

    Post-turbo is a closed container representing the intake system (intercooler, piping, intake mani).

    The turbo is moving air from one container into the other through compressor wheel. This compression does come with a temperature increase and we can view the relative efficiency of this compression through the use of compressor maps.

    We are increasing the mass because of this compression into a fixed volume. So we are not "creating" mass...just increasing it...

    for example if we made post turbo side "limitless volume" then we would get no change in density because as soon as the compressed molecules of air came out the turbo it would just expand back to the way it was. Kind of like what happens when you blow off an intercooler pipe and experience "no boost".

    Read Post #30 from this thread where i explain my theory on why supercharged cars experience different PSI reading in winter vs summer...
    It might help as well..

    http://forums.bimmerforums.com/forum...=395165&page=2

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