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Parallel vacuum sources

  1. May 16, 2018 #1
    I have two vacuum sources each pulling 1 inHG. If I connect the vacuum sources to a Y-connector, with identical sized hoses, so that each pulls on one tip of the Y, will I get 2 inHG vacuum on the remaining hose, or will I get 1 inHG with more cfm?

    Thank you,
    B
     
  2. jcsd
  3. May 16, 2018 #2

    Tom.G

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    1 inHG with more cfm!
     
  4. May 16, 2018 #3
    Thank you. But...Dang it. I don't know what to do with that information.
    So the application can be explained like this... The crankcase in an engine is constantly pressurizing from air leaking past the piston. There needs to be a constant vacuum source pulling this pressurized air out of the crankcase and bringing in fresh air from outside. So the crankcase should always be under negative pressure.
    It sounds like the most important thing in this situation is how much cfm the vacuum source can evacuate from the crankcase. Having a 15 inHG vacuum that only evacuates 1cfm is not as beneficial as 1 inHG that evacuates 5cfm. Does this sound like a correct statement for this example?
    I feel it is more complicated than that because air is compressible.
     
  5. May 16, 2018 #4

    Tom.G

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    The usual approach in an Internal Combustion engine is to vent to the atmosphere thru a rather coarse filter, then run a vacuum line (usually small) from the crankcase to the input manifold. Look up 'PCV Valve' (Positive Crankcase Ventilation) on the internet.

    Does this work for you?
     
  6. May 17, 2018 #5
    In this scenario the intake pulls less and less vacuum the more the throttle plate opens until it actually goes the other way and pressurizes the crankcase through the same port. The method you explain would allow pressure to build in the crankcase if the vent(s) it is using to evacuate to atmosphere are too restrictive. Thank you for the input, but I would like to work through the scenario I proposed.
     
  7. May 17, 2018 #6
    I don't see how the intake manifold pressure on a running engine could ever go above atmospheric, since in that case it wouldn't be able to draw in fresh air for combustion.
     
  8. May 17, 2018 #7
    I appreciate the participation but please let's stick to the original question proposed. FYI...The engine is a turbo. The intake pressurizes as you increase throttle.
     
  9. May 17, 2018 #8
    You'll get 1 inHG with more cfm.
     
  10. May 17, 2018 #9
    If I want to adequately evacuate the crankcase, by maintaining a constant negative pressure in it, by using multiple sources of vacuum in parallel, it sounds like the most important thing in this situation is how much cfm the vacuum source can evacuate from the crankcase. Having a 15 inHG vacuum that only evacuates 1cfm is not as beneficial as 1 inHG that evacuates 5cfm. Does this sound like a correct statement for this example?
    I feel it is more complicated than that because air is compressible. I am thinking I need to measure the amount of psi per minute that the crankcase naturally builds up and them convert that into cfm so I can determine how much cfm I need the vacuum to evacuate. So, back to the question above, does the inHG the vacuum pulls really matter as long as the cfm requirement is met?

    Thank you,
    B
     
  11. May 17, 2018 #10
    FYI, dry sump oil pumps achieve this effect. What kind of pumps are you using? Electric?


    Yes, you are on the right track, to pull any vacuum at all you need to be able to keep up with the blowby flow, to pull an actual vacuum, you need to "pull" many times more than the blowby flow.
     
    Last edited: May 17, 2018
  12. May 17, 2018 #11
    Thank you MRFMengineer, I hadn't heard of dry sumps. I have looked into mechanical vacuum pumps for installation under the hood primarily for running air tools on utility vehicles. They are not really designed to work for this purpose. There were others that need to be rebuilt every year which is not practical.
    Looks promising though and expensive, curious if they can be ran completely dry for evacuation purpose only or if at least some oil is required to function and not breakdown.
    Electric vacuum pumps apparently don't produce enough vacuum and are not dependable.
    I am using air venturi vacuums.
     
  13. May 18, 2018 #12
    The usual item that is used in this situation is a catch can. Turbo engines especially running higher boost can build up crankcase pressure quickly. Making sure that you vent this to atmosphere is important as you have already implied. A secondary issue is that this excess pressure (and high temperatures when operating at high boost states) can cause issues with lubrication due to oil loss and fouling due to the return via vacuum source to intake causing intake track and ignition plug pollution. An oil catch can is a large ( typically 1 liter+) can with internal baffles. These are designed with sufficient air flow capacity to prevent pressurization of the crankcase. They also have internal baffles that allow the expelled gasses to cool and thus oil vapors condense and return to either crankcase or a pool where they are captured and returned.
    It is very difficult to stay ahead of blow by gasses. as the throttle conditions vary at a level that cannot be regulated. What I am understanding from your posts is that you are attempting to evacuate the blowby and prevent the accumulation. Up to now other engineers have found it much easier to accept that blowby happens and needs to be accommodated.
    Are you searching for a new method of prevention or just for a revision of accepted practice? I believe that a little understanding of where you are leading would be helpful.
     
  14. May 18, 2018 #13
    Not allowing blow by to accumulate. Staying ahead of the blow by with constant negative pressure greater than the positive pressure of the blow by even in boost conditions.
     
  15. May 18, 2018 #14
    If you are just pulling, then regardless the amount/performance of vacuum pumps the crankcase will be always full of (low pressure) blow by gases. If you wants to prevent accumulation then what you need is not vacuum but venting.
    You already mentioned the "bringing in fresh air from outside", but this part seemingly got forgotten.

    Your additional air intake holes will be almost as important in determining the ratio of blow by gas and air inside the crankcase as the vacuum pumps.
     
    Last edited: May 18, 2018
  16. May 18, 2018 #15
    Certainly there must be fresh air that the vacuum draws in. What I am trying to determine is a solution that generates enough constant vacuum to stay ahead of the blow by. I am currently attempting using simple venturi vacuum and looking into how using multiple venturis might work.
     
  17. May 18, 2018 #16
    If you want to keep 90% fresh air ratio inside, then your intake holes will be nine times more important than the amount of blow by what you are focused on right now.
     
  18. May 18, 2018 #17

    Randy Beikmann

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    The solution to your problem depends on your goal, and your application.

    1) Goal: Are you wanting to run a vacuum to reduce aerodynamic drag (windage) inside the engine? This is what they do in racing engines with a "dry sump" (which is also there to avoid having oil flung around by the moving parts, which creates more drag, and aerates the oil). It's worth a few HP. In that case you don't really care about how much air is present vs. "blow-by" gases. Or is your goal to keep the concentration of blow-by low, to avoid contamination of your oil by gasoline?

    2) Application: Are you talking about an engine where you need to pass emissions laws? If you are, you won't be venting any gases from the crankcase to the atmosphere. It will need to be fed into the engine to be burned.

    As far as catch-cans are concerned, they can be part of a system that returns the blow-by into the engine. They will collect liquid content, and allow gases to pass back to the engine. Turbo guys don't like to have their intercoolers fouled by oil, which reduces their heat-transfer rates, and a catch-can can avoid that.
     
  19. May 18, 2018 #18
    Application primarily to vent crankcase to atmosphere and prevent from being recirculated in engine. Not looking for more hp. Not concerned with passing emissions. Just want to make sure whatever solution is implemented it performs better than stock so I don't put engine in more danger of catastrophic failure. Spoke to Moroso and their sumps require oil to operate and will seize if use only for fume evacuation.
     
  20. May 18, 2018 #19

    Randy Beikmann

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    If you're just venting to the atmosphere, I'm not sure you need anything more than the pressure that's built up from the blow-by gases. If you want to clear them out to reduce oil dilution from the condensing unburnt fuel, that's another story.

    There are automotive vacuum pumps that are used to boost brake systems in vehicles that have little manifold vacuum. I don't know their capacity, or how well they would tolerate pumping some amount of exhaust gases through them. But it's worth checking out. Maybe if you ran the crankcase gases through a catch-can or other separator on the way to the pump it would be OK.
     
  21. May 18, 2018 #20
    Want to avoid letting the blow by pressure be the force that evacuates the fumes to atmosphere. Prefer fresh air as much as possible in crankcase to prevent oil dilution. Want to pull vacuum through catch can on its way out to atmosphere to be a little more eco friendly.
     
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