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11-20-2012, 01:15 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by erich weaver
And as indicated in a previous post, use of a separator that returns oil to the sump prevents you from being able to monitor the amount of oil that is being discharged, which is a crude indicator of engine health.
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Don't think the statement made sense the first time. You would quantify belly oil how?
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Dan Horton
RV-8 SS
Barrett IO-390
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11-20-2012, 02:15 PM
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Join Date: Nov 2005
Location: AUSTRALIA
Posts: 749
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Can anyone explain why the oil returned to the crankcase by a separator would necessarily contain more contaminates than the rest of the oil in the crankcase?
I would have assumed that the oil that goes out the breather is simply small droplets of crankcase oil held in suspension in the air and the separator simply collects these droplets into what is essentially the same oil as in the crankcase.
Fin
9A
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11-20-2012, 03:26 PM
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Join Date: May 2005
Location: Tuttle, Oklahoma
Posts: 2,563
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Quote:
Originally Posted by Finley Atherton
Can anyone explain why the oil returned to the crankcase by a separator would necessarily contain more contaminates than the rest of the oil in the crankcase?
I would have assumed that the oil that goes out the breather is simply small droplets of crankcase oil held in suspension in the air and the separator simply collects these droplets into what is essentially the same oil as in the crankcase.
Fin
9A
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Well the air in the air/oil mixture from the crankcase vent is going to have a lot of the contaminants being discussed. The air/oil separator will allow those contaminants in the air to "fall" back into the oil that the separator will then return back to the sump.
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11-20-2012, 04:43 PM
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Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,682
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Quote:
Originally Posted by DanH
Don't think the statement made sense the first time. You would quantify belly oil how?
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Everyone take a deep breath and relax. Like I said, a CRUDE indicator of engine health. If you take a look at the belly of your plane before/after every flight and one day you see more than normal fresh oil, it might be good to evaluate further, no? Sometimes your eyeball alone IS good enough
erich
Last edited by erich weaver : 11-20-2012 at 04:58 PM.
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11-20-2012, 04:45 PM
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Join Date: Apr 2009
Location: USA
Posts: 1,499
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Quote:
Originally Posted by DanH
Don't think the statement made sense the first time. You would quantify belly oil how?
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During every preflight I inspect the bottom of my plane. After cleaning and waxing then a 2 hr trip to Savannah I have a good idea how much oil mist should be on the bottom and how far back the heavier buildup goes. If I show signs of an increased amount I know to investigate further.
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Good judgment comes from experience, and a lot of that comes from bad judgment.
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11-20-2012, 04:59 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by RVbySDI
Well the air in the air/oil mixture from the crankcase vent is going to have a lot of the contaminants being discussed. The air/oil separator will allow those contaminants in the air to "fall" back into the oil that the separator will then return back to the sump.
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Let's theorize a little here.
A good place to start might a categorization of the air/oil mixture into its component parts.....gas, liquid, solid, and vapor. The gas component will contain various materials in a vapor phase as well as liquid materials in droplet suspension. The liquid material will contain various liquid and gas solubles as well as solids in liquid suspension.
If I recall correctly, vapor will not condense to liquid droplets until there is a change in temperature and/or pressure. Although the breather system may allow some cooling it is not a design function. There is no pressure change. Most of the vapor phase contaminants would thus remain with the gas component and pass through the separator to the exit. The liquid materials in droplet suspension would be returned to the sump, the designed function of the separator.
I suspect almost all the water leaves the system as vapor. I'm pretty sure those contaminants limited to liquid transport (notably solids) get returned to the sump where they concentrate (suspended lead salts and carbon particles might be examples). The degree of concentration would be proportional to the quantity of oil normally lost and replaced with new oil. The rest of the chemical stew hard to define, but again, there's no reason to assume anything leaving the case as vapor get returned to the case as liquid.
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Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 11-20-2012 at 05:09 PM.
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11-20-2012, 05:57 PM
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Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,682
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Let me start by saying I have no idea how much bad stuff may exist in the vapor phase. That being said, just because the breather system is not designed to cool doesnt mean that it doesnt occur. In fact, it almost certainly IS occurring, wouldnt you agree? And so, I would argue that there is indeed likely to be some materials leaving in the vapor phase and returning in the liquid phase. Whether it is significant in the big picture or not, I dont know. All that aside, I dont mind rubbing my airplane's belly once in awhile. Makes her feel good
erich
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11-20-2012, 05:57 PM
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Join Date: Apr 2009
Location: USA
Posts: 1,499
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Add water, combustion byproducts such as unburned fuel, lead oxides, and nitrogen oxides into your oil bath, aerate with the crankshaft, heat until the water vaporizes and flows through your crankcase breather into the air/oil separator. Here it condenses, especially in cold wx. You now have this creme colored stuff sometimes found on your dipstick being pumped back into the engine along with the oil. The water and nitrogen oxides can form nitric acid. All of this contamination breaks down the oil additives and decreases the ph of your already acidic oil. I rerouted my breather tube to come out near the exhaust, some exhaust pipe staining is evident, but most still goes around it at cruise.
__________________
Good judgment comes from experience, and a lot of that comes from bad judgment.
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11-20-2012, 06:53 PM
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Join Date: Oct 2009
Location: Victoria, BC, Canada
Posts: 3,932
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Quote:
Originally Posted by DanH
I suspect almost all the water leaves the system as vapor. I'm pretty sure those contaminants limited to liquid transport (notably solids) get returned to the sump where they concentrate (suspended lead salts and carbon particles might be examples). The degree of concentration would be proportional to the quantity of oil normally lost and replaced with new oil. The rest of the chemical stew hard to define, but again, there's no reason to assume anything leaving the case as vapor get returned to the case as liquid.
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Note that both the gases and the liquids can carry solid contaminants in suspension, so some contaminants can leave with the gases, but most return with the liquids if you have a separator... The solids will stick to the "sticky" oily sides of the separator. You're right, water leaves as a gas. It can condense in your breather or separator, but by that point is probably beyond the return point anyway. And after 30 minutes of flight, you'd be hard pressed to find any water in your engine at all... Until you shut down, let fresh moist air in, and let it cool.
For the most part, what comes out your breather will be one of two things: First, overspray of the liquids, and "breathing" in and out of the gases, just from the mechanical splashing around that goes on inside. The concentration of contaminants in those liquids and gases won't be any different than the concentration of contaminants in the rest of your oil. The small quantity you lose through the breather won't change the effect the rest of them will have on your engine... ie. if you've got crappy contaminated oil in general, catching a tiny bit of it with your breather line isn't going to make things better in any measurable way.
The second thing that could come out your breather is a large slug of liquid oil, which will happen when you roll your RV inverted and get some negative G by accident. This is really what people want to catch and return to the engine, because losing all of your oil every time you fly upside down will prove to be much harder on the engine in the long run than just returning the oil to the sump where it can continue to lubricate.
If you don't fly aerobatics, and you're seeing any significant amount of oil coming out of your breather, you're either running your engine with too much oil in it to start with, or you've got other engine issues to investigate.
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Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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11-20-2012, 07:24 PM
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Join Date: Aug 2009
Location: Goodyear, Arizona
Posts: 877
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That large slug of oil that comes out of the breather during aerobatics is the thing I will be trying to fix with my new oil separator from Antisplat Aero. I was out doing a bunch of aerobatics the other day and started noticing a burning oil smell every time I did over the top maneuvers. That was a little disconcerting to say the least. I landed right away to investigate.
No, I do not have an inverted oil system, no I was not hanging upside down, and I was not doing anything less than about .5 G, certainly not any negative G.
When I got on the ground, my belly was covered with oil. I lost about a 1/2 quart of oil just from aerobatic maneuvering. Hmmm...
Hoping the oil separator helps.
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Karl, Goodyear, Arizona (KGYR) ATP, CFII
RV-14A, Flying
Extra 330LX, Flying
RV-8, Sold
RV-7, Sold
Bearhawk 4-Place, Sold
=VAF= donor 2020
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