Vortec 350 PCV positive crankcase ventilation question.

Discussion in 'Engine Performance + Maintenance' started by Pinger, Jul 31, 2020.

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  1. Schurkey

    Schurkey I'm Awesome

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    1. Try your hypothesis, see if it actually works. This debate will go on forever. Keep in mind that crankcase ventilation improves oil and bearing life; you may not have full results of your modification unless you do oil sampling on a regular basis, and the bearings may not show problems for a hundred-thousand miles.

    2. Study what propane/LPG forklifts or other industrial engine designers are using to "fix" this problem.

    3. Contact the folks who made your gas mixing valve, see what they recommend.
     
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  2. Pinger

    Pinger I'm Awesome

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    I will - as soon as the two new PCV valves from Rock Auto arrive.
    Thus far, just by playing with a GM PCV valve in my hands I can see how it behaves and, when I blocked off it's connection to the inlet manifold with the engine running I increased the signal to the mixer by some margin on opening the throttle - exactly the aim.
    In essence, what I am doing here is akin to setting up a carb (with all that entails) but with the luxury of final (but slower acting) electronic control. The better set up the 'carb' is, the less reliant on the electronic correction correct fuelling is. Basics first, correctives second.
    When I'm doing this stuff, I can read live data on a lap top connected to the LPG ECU and as I open the throttle from idle I can watch the mixture go lean and the actuator open to compensate and watch it go to correct mixture. But in that moment between opening the throttle and the above happening, I can 'feel' the hesitation and if provoked a backfire is the result. Who wouldn't look for a solution to that on their own engine?

    Yes, but remember that there will be little in the way of liquid fuel in the crankcase of this engine and as the oil film at the rings is undiluted with gasoline then I can expect a better seal there also so a cleaner crankcase. I very probably will do sampling as the oil I use is a 'long life' oil and the length of that 'life' is ascertained by oil analysis. There will I think still be enough circulation to remove water vapour. The oil I use has a tackifier which enables it to cling as it cools on shutdown so there's a degree of corrosion protection there.

    While I aim to preserve as much circulation as possible, it is worth noting that a turbocharged motor on boost when blow-by will be at its worst dispenses entirely with circulation as the check valve in the line to the manifold must close to prevent boost pressuring the crankcase, and breathing (but without circulation) from its crankcase inlet is all that is then available. That 'breathing' is into the vicinity of the air filter - just as was done for decades before emission legislation mandated otherwise. There is some latitude here in what I plan!



    If I can get near one.... Covid really has made access I once enjoyed a whole lot more difficult. Generally, here in the UK, indoor forklifts are electric.
    Standard practice for converted cars is as I have already outlined (and am trying to avoid). Worth mentioning that injection systems are becoming the norm for LPG conversions and they neatly side step the issue.

    This route I am investigating is on the advice of the company who sold me my new reducer/vaporiser. Their view is that I either accept routing LPG thorough the crankcase or devise an alternative arrangement as undoubtedly I am currently losing vacuum signal to the mixer on opening the throttle.

    On an informed LPG forum they suggest the mixer being too big (recognising the lack of signal on opening the throttle) so while there is confirmation of that loss of signal, there is no explanation as to how a venturi loses sensitivity with increased air velocity through it. It pulls the LPG at idle adequately then loses the signal when the throttle is opened. Venturis don't do that - they just don't!

    New PCV valves will be here in a couple of weeks - then we'll know. Cheers!
     
    Last edited: Aug 4, 2020
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  3. Schurkey

    Schurkey I'm Awesome

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    Mistake in logic.

    The venturi and airflow through it isn't the problem. The problem is mostly the same as with a carburetor--when the throttle is suddenly opened, an accelerator pump is needed because otherwise fuel flow lags behind air flow.

    This isn't a venturi problem, it's a fuel-metering problem. The metering circuits require some time to adjust to the new air flow requirements--time for the main circuit to begin flowing fuel into the airstream after the idle circuit is no longer active due to the changed (opened) throttle position.

    The problem is not your venturi, it's on the fuel-flow side. The fuel-metering isn't keeping up with throttle position changes fast enough.

    'Course, a liquid-fuel carbureted/TBI engine also has fuel falling out of suspension in the intake manifold when the throttle is suddenly opened, that a gaseous-fuel engine wouldn't have.
     
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  4. Pinger

    Pinger I'm Awesome

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    OK, to use the carb analogy (bear in mind though that vapourised LPG doesn't suffer the inertia problem that liquid gasoline does and, as currently set up is being supplied at slightly above atmospheric pressure) the way the PCV valve works is that on throttle opening, it too opens - and allows the manifold to ingest unmetered air to the detriment of signal to the venturi.
    So, with a carb, imagine as you open the throttle, yanking the hose to the brake servo off at the same time. Air flow through the venturi is reduced and hence a lesser amount of fuel is drawn than is needed. That's how my reducer/vaporiser supplier describes it and it's where I currently am. The lag comparable to a carb set up is due to the actuator having to wait for data from the O2 sensor - which will eventually arrive and correct the mixture - but not quickly enough.

    I'm continuing to give the whole thing thought and the biggest concern for now (and how it plays out will be entirely dependent on my proposed second PCV valve mounted on the passenger side's sensitivity to vacuum via its inherent anti-backfire facility) is subjecting the crankcase to prolonged periods of vacuum (if the vacuum at idle closes the additional PCV valve) when I only want/need it to momentarily.

    Anything pressure fed oil will be unaffected but possibly there will be tendency to draw oil out of the small end (wrist pin?) bearings. I'm going to try and find out how dry sump systems with scavenge pumps deal with this - if it all. I'm pretty sure that the preferred set-up for a high rpm race motor with dry sump is to run with high vacuum in the crankcase at all times. They run huge scavenge pumps that pull aerated oil so a great deal of air a well as oil. Blow-by would negate some of that vacuum (but only on'throttle) but a full on race motor will run piston clearances so tight it won't even turn over until pre-warmed.
    The other end of the scale from where I am for sure - but small end bearings are a common feature in both.

    I can though, ascertain the above with adhoc tests before deciding to finally commit to this - when the new PCV valves arrive.....
     
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  5. Pinger

    Pinger I'm Awesome

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    So, just to round this off.
    While waiting for the PCV valves to arrive I had a change of mind and decided to install the additional one the other way around to how I'd first intended. It is now installed inverted on the passenger side and doesn't make a hell of a lot of difference. In essence, I now have two PCV valves in series and flowing in the direction they were designed for. This is how it's going to stay for now at least as the throttle pick up from idle seems OK and there's no negative effect for the crankcase. I have, very slightly, moved in the intended direction merely through the additional resistance compared to an open inlet (and I am still avoiding circulating LPG) through the crankcase.

    I did try the PCV valve the other way up and immediately, the desired result was achieved ie, a more open LPG actuator at idle - virtually the same as out of idle. Then the advantage vanished. What surely happened was that the valve went initially to 'backfire mode' ie, fully closed and all crankcase circulation ceased also. Then, on throttle opening it opened - but didn't close again as it first had.
    Backfire mode is (when installed on drivers side) also the position it assumes when the engine isn't running and gravity just pulls it closed. There's nothing but the pressure differential to control it (ie no spring) so completely unpredictable with the pressure differential in the opposite from intended direction. It could stay shut and allow a large amount of vacuum to accumulate in the crankcase. It could stay open and contribute nothing. A bump in the road could decide which.

    Referring back to the first paragraph, I'm going to leave it as described. While it's not making much difference on opening the throttle from idle, it may help build more vacuum on the over-run which will help throttle response there as I have had to configure over-run fuel cut-off to stop the truck running away at closed throttle and after a prolonged period of over-run there can be a hesitation while the LPG flow re-establishes. Probably won't though as the drivers side valve is the one that controls the level of vacuum in the crankcase. If it does pull the additional valve to a more closed position then I've gained something. I'll know better when I drive it tomorrow.

    For sure though I've given up the idea of pulling massive vacuum in the crankcase (not least as I need the coolant in the oil pan like I need another orifice in my body). The only other alternative is to isolate completely the crankcase from the inlet manifold and consider an air pump to force circulation (directing the vapours to the air inlet upstream from the MAF and avoiding the backfire risk) above a certain throttle opening when blow-by is likely to have built to a level worth being concerned about. More trouble than any 'problem' warrants I think. I can always tweak the over-run fuel cut-off to get around any hesitation there - at the expense of engine braking and fuel.
    Ho hum - we don't know until we try.....
     
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