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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. :confused: Fin 9A |
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erich |
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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. |
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 |
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.
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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. |
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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