Question about PCV system and fuel/air mix

Discussion in 'Gen 3 Prius Main Forum' started by ColoPriusV, May 18, 2020.

  1. ColoPriusV

    ColoPriusV Junior Member

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    With all the forum discussions about piston ring blow-by, oil consumption, oil catch cans etc. I started thinking about the PCV system and how blow-by contaminants could impact a proper stoichiometric fuel-air ratio in a highly efficient engine system:
    • the amount of blow-by gasses that enter the PCV system is a function of engine wear, rpm and load, and enters the plenum downstream/beneath the throttle body and mass air flow meter - is this correct?
    • the volume of flow is neither metered nor controlled - if it was controlled there would be a potential risk of blowing an engine seal.
    • the amount of unburned hydrocarbons and oil vapor in the PCV air flow is also not metered or taken into account by the engine fuel management system. This can lead to overly rich mixtures in a worn engine (from unburned hydro carbons, engine oil vapor, etc), and accelerating carbon deposits in the combustion chamber, spark plugs, EGR system, and catalytic converter.
    • PCV airflow may also contain inert gases, such as combustion blow-by, water vapor, etc. Again, how does the engine fuel management system account for this?
    So in general, how does the engine fuel management system account for a highly variable air flow with varying combustible content to prevent a significant decrease in engine performance and efficiency as the engine wears??

    Many thanks in advance for your thoughts ...
     
    #1 ColoPriusV, May 18, 2020
    Last edited: May 18, 2020
  2. Mendel Leisk

    Mendel Leisk Underfoot

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    I think you're right on most/all your points: the PCV input is downstream of the MAF, and is a bit of a dog's breakfast. An effective Oil Catch Can helps with that.
     
    #2 Mendel Leisk, May 18, 2020
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  3. ColoPriusV

    ColoPriusV Junior Member

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    Thank you for that spelling catch, Mendel.

    I thought of these questions as i installed an OCC last weekend, and just have to wonder why Toyota didn't engineer something similar to pull the varying amounts of garbage that enter the fuel/air mix from the PCV system as the engine wears. I realize oxygen sensors are downstream to adjust fuel/air mix, but they can also also get fouled by contaminants as wear occurs and more by-products enter the fuel-air system.
     
    #3 ColoPriusV, May 18, 2020
    Last edited: May 18, 2020
  4. Mendel Leisk

    Mendel Leisk Underfoot

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    Where the PCV valve screws in, is a plate roughly 6" square, with a rudimentary labyrinth. I think that helps slightly, but not much. It's not trivial taking it off btw, I wouldn't bother: it has from-in-place gasket around the perimeter, not easy to remove, tricky getting compatible FIP gasket to reinstall.

    Yeah I think the best you can do is install an effective Oil Catch Clean, and clean the intake manifold periodically.

    FWIW, since I had a second OCC on hand, I've added it in series, and it collected a wack more, more than the first one actually, though it appears to be about 90% water, when I drained both this spring. I suspect in fall the second one won't have much at all, since it's through winter we seem to get a lot more humidity in the system, summer's pretty much all oil.
     
  5. ColoPriusV

    ColoPriusV Junior Member

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    Interesting - your description reminds me of a video that compared Moroso and RX oil catch cans by hooking them up in series, measuring how much each one caught, and then swapping. Interesting that your second OCC caught more water - were both OCCs by the same manufacturer or different?

    Also have to wonder if there's more condensation in the winter due to the start-stop cycles of our hybrid engines. How would you estimate your mileage is split between city and highway in the winter?

    I've also wondered if it would be useful to install a catch can at the bottom of the intake manifold so oil that condenses and collects during the Atkinson cycle could be collected and not get sucked into the intake.
     
    #5 ColoPriusV, May 18, 2020
    Last edited: May 18, 2020
  6. Mendel Leisk

    Mendel Leisk Underfoot

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    Yeah both are the Moroso 85474, the one in that video. Long story, but I had two, so might as well...

    Previous years with just one, there was always about double in spring vs fall (I drain in conjunction with spring and fall oil changes), presumably due to rain, high humidity.

    When I put in the second can, it was about 15k kms and 2 years since first install and I had thoroughly cleaned intake at that time. I recleaned intake and while it was not bad it was getting some funky build up. EGR passages were part of it, and OCC can't really help with that.
     
    #6 Mendel Leisk, May 19, 2020
    Last edited: May 19, 2020
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  7. tvpierce

    tvpierce Senior Member

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    I believe the oxygen sensor accounts for that. In open-loop (while the engine comes up to operating temperature) the ECU is just using data from the MAF sensor -- so it just estimates the air/fuel ratio. Once the engine reaches operating temperature, it goes into closed-loop mode where the ECU gets data from the MAF sensor and the upstream oxygen sensor. By getting that data from the oxygen sensor, it knows what the result of combustion is, and adjusts the fuel delivery to maximize efficency.
     
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