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Methode of measuring engine blow-by

  1. Jun 27, 2010 #1
    dear all,

    some literature which I'm read, measuring blow by of engine can be execute use the pipe. not use with pressure gauge. how to measuring and calculating this if we must be use by pipe (not by pressure gauge)....?

    here is the ilustration.....(attach)


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  2. jcsd
  3. Jun 27, 2010 #2


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    We measure the pressure drop over an orifice plate.

    Your device isn't going to work; it's just going to blow bubbles, or blow water right up the spout depending on how heavily your engine is breathing.

    If you must use a manometer, why not use a normal orifice plate and measure the pressure drop using an inclined manometer or some such?
  4. Jun 27, 2010 #3
    It doesn't matter how the manometer is designed, its just not going to work. The slowest you will get a normal IC engine to run is maybe at around 300 rpm. Thats five pressure fluctuations a second. Manometers are fine for steady state measurements but no manometer will have a response fast enough for something like this.
  5. Jun 28, 2010 #4
    That is a crankcase pressure tester. You place it in the dipstick tube, and if pressure is too high, there may be something wrong. It will not tell you which cylinder has the problem. For that you need to do a static and dynamic compression test. It can find other problems.

    It is one of many tools you can use:

  6. Jun 28, 2010 #5
    dear all.....

    thanks for replay, I'm sorry because my ilustration picture is uncorrectly.
    here is we attach the ilustration again....

    best rgrds

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  7. Jun 28, 2010 #6


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    Topher, it's not about measuring blowby of an individual cycle, but for the while engine.

    Wahyudi, see my first post. Your design will give you crankcase pressure, but to measure blowby you need a flow rate. As I stated in my first post, you can do this by measuring pressure differential over a restriction.
  8. Jun 28, 2010 #7
    Oh , ok. Why not just measure the compression in each cylinder? That way you measure blow-by from each individual cylinder and do it much more accurately as well.
  9. Jun 29, 2010 #8

    Ranger Mike

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    if you are going to all the trouble of measuring the engine for compression, why not take it one more step and measure for leak down. almost same amount of effort and same spark plug fitting. leak down will tell you if you have bad piston rings or valve seat problems...compression test just identifies good or bad cylinder but not whats bad..
  10. Jun 29, 2010 #9
    Mr brewnog,..
    yes right, what I mean is "crankcase pressure", because in Komatsu Diesel engine, the crankcase pressure = blow-by pressure. which is method to measuring compression pressure leak for while engine, not individually.

    Mr thoper,...
    thanks for suggest, in my application both the method is usually used.

    Mr ranger,...
    thnks for suggest too, but what I ask is method to measure the moving water level if the air pressure applyed from the engine breather to the plastic pipe?

    here is my 3rd ilustration (I'll be upload on the next post)......

  11. Jun 29, 2010 #10
    dear all....

    here is my 3rd ilustration. which one the method to measure blow-by/crankcase pressure are correctly?? the 'A' picture or the 'B' picture ilustration.

    note: - in Komatsu standard manual of engine, method 'B' is used
    - in other reference, which I see....method 'A' is used
    - so, which one the method are correct?

    we wait U'r suggest...thnks


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  12. Jun 29, 2010 #11


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    Because a compression test doesn't show you how much gas is getting past the rings under running conditions.

    So do you want to measure blowby (which is a volumetric flow rate) or crankcase pressure (which is a pressure)? They're different, you're treating them as the same term.
  13. Jun 29, 2010 #12
    thnks for replies Mr brewnog...

    if we look the blowby & crankcase pressure have a same units (mmAqua/mmH2O). the different is only on the measuring method (with pressure gauge or with plastic pipe).

    1. if measure with gauge it's read a gauge pressure/not connect to atmosphere
    2. if measure with plastic pipe (on my ilistration), it's read an absolute pressure/connect to atmosphere

    Mr....why I ask this case? because sometimes if we on job site there haven't a gauge, we usually use plastic pipe to read the blowby/crankcase pressure. but, the problem is how to read the movement of water level are correct? the 'A' one or the 'B' one on my 3rd ilustration above?

    here we upload the tools if we use gauge...


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  14. Jun 30, 2010 #13


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    So your question is "how do you read a U-tube manometer"?

    Just measure from one meniscus to the other, that gives you liquid gauge pressure. Add barometric pressure for absolute.

    Please still note that "blowby" is volumetric flow rate, you're actually measuring crankcase pressure. It's semantics but it's important...

    Looking at your drawing you need to be careful where your "adapter" is; because there is movement of gas between the sump and atmosphere you might (is your gauge is right at the exit of the breather system) just end up measuring atmospheric pressure.
  15. Jun 30, 2010 #14
    ok, thnks for advice...

    so, which one the method to read a actual value of blow-by Mr brewnog?
    the 'A' or 'B' picture? and please give me a reason for each reading (mmH2O) method.....

    note: - all this time we usually use the 'B' method

    we wait U'r advice again Mr....

  16. Jul 1, 2010 #15


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    B, as I described, assuming I interpret your drawing correctly. And it doesn't measure blow-by, it measures pressure at the point the feed for the manometer is taken (which could be meaningless depending on its location). For the third and final time, blowby is a volume flow rate (measured in litres per minute, millilitres per second, or cubic yards per fortnight), you're measuring a pressure (bar, kPa, mmH2O etc).
  17. Jul 1, 2010 #16
    ok Mr brewnog.....

    but, why not 'A' sir? what is the reason?
    because, the ilustration 'A' is a simple measurement than 'B'

    why the measured object is like in the picture 'B' (the parameter value is from 2nd water level (left side tube) to the peak of water level in U-tube (right side tube)/while the engine running in rated speed)??

    note: - if we calculate in mathematic, {mmH2O in 'B' = 2 x mmH2O in 'A'}

    why not the mmH2O in 'A' we're use to the standart value? so we must not to
    multiply the mmH2O in 'A' with '2'......give me a reason please..

    thnks, rgrds
  18. Jul 1, 2010 #17


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    Because you haven't stated that the "first" water level in A is just under barometric pressure. For all I knew it could be with the engine running at part load, in which case just measuring the delta from the "first" level would have been meaningless.

    Please read this, and if you're still confused come back and pose your question:

  19. Jul 1, 2010 #18
    What do you do if the dipstick tube descends to beneath the oil level in the sump, as I believe is the case with my engine? Oil will climb up the tube a little and that's it. No way to measure crankcase pressure.

    If you try to gauge leak down by removing the oil fill cap, how do you distinguish between ring blow by and air from the vacuum pump, which I believe exhausts to the valve cover.

    http://img686.imageshack.us/img686/4875/img1341p.jpg [Broken]

    Because it can be a hassle to get the glow plugs out and they may break or fracture while attempting to remove them.

    Pressure above atmospheric is as good an indicator of blowby as flow. The more blow by, the higher the crankcase pressure. The higher the crankcase pressure, the higher the flow.

    I should think pressure inside the crankcase would reach an equilibrium above atmospheric high enough to prevent more blowby or for crank case gasses to seep up past the rings on the intake stroke.

    But it's more likely it would start leaking pressure by the sump gasket or what not, first.

    Crankcase pressure and blow by are linked.

    Crankcase pressure and blow by are linked.
    Last edited by a moderator: May 4, 2017
  20. Jul 1, 2010 #19


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    But it's a function of how effective the breather system is. Measuring blowby is the way to measure blowby, inferring it from crankcase pressure doesn't tell the whole story.

    Not nearly; on one side you have a few hundred bar of cylinder pressure; and the other side a few kPa of crankcase pressure. Almost every other component below the piston would have destroyed itself before equilibrium. More fundamentally, rotation of the engine is driven my the pressure differential over the piston; so if equilibrium were (hypothetically) approached the engine wouldn't be turning round anyway. This is all academic and I don't see what it has to do with the question.

    Of course, I never asserted against this. But they're NOT THE SAME THING. The OP ostensibly asked how to measure blowby, I provided an answer. It turned out this wasn't what he wanted, but still didn't make the distinction between these two fundamentally different parameters. Besides (and this was the more important point I tried to draw attention to), getting an accurate measure of crankcase pressure relies on measuring it in the right location; measuring pressure at the breather outlet (as questionably depicted) will give a misleading figure.

    I'm sure you mean well and have engine knowledge to share, but have you considered that your apparent desire to try and pick holes in sound technical repostes is the reason you were banned from the other forum?
    Last edited: Jul 1, 2010
  21. Jul 1, 2010 #20
    Naturally, it's assumed that you measure crankcase pressure on a warm engine, at the PCV valve after blocking the crankcase breather.

    You are quite correct. That completely slipped my mind.

    It looked like the dipstick tube to me. I think he said as much too.
  22. Jul 3, 2010 #21
    dear all......

    thnks for advice me. some literature which I read on this page (informed by all),
    here is my conclusion :
    1. measuring blow-by/crankcase pressure can be use the basic principle of Bernoulli equation,
    where this applicated to manometer system
    P1 + h1 = P2 + h2...........(where the gravity & density of the water is be ignored
    because the value of gravity & density in the equation is same)
    2. the pressure of blowby which we measure is SAME with the differential pressure between
    P1 and P2
    P1 - P2 = h2 - h1 ......(h2-h1 = delta h/difference of height)
    3. thus, the higher the pressure difference between P1 and P2, the difference in height
    (h2 - h1) will be greater.
    4. so, the pressure of blow-by is not indicated by how much water movement, but is shown
    by how big the difference in height on the pipe surface water manometer.

    here we upload my referance...

    ok guys, thanks for a good disscuss......

    best regards

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    Last edited: Jul 3, 2010
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