What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Breather tube termination into exhaust

Webb

Well Known Member
Sponsor
I’m doing some research on terminating the breather tube into the exhaust to burn off the oil like a smoke system would do and do a better job of keeping the belly clean.

After seeing it installed on another plane, I wanted to query the gallery.

To install, an oval hole is made in the exhaust pipe to match the oval produced by cutting a 45 angle on a steel tube. The tube is ground to have a curve that matches the curvature on the exhaust pipe. Making sure there are no protrusions into the exhaust pipe and weld the tube to the exhaust to make a Y connection. A flex hose would then be used.

Install area would be determined after getting exhaust pipe in position and at least 12 inches from the end of the exhaust pipe.

I searched the forums and did not find anything other than complaints of dirty bellies and other ways to terminate a breather tube.

Has anyone done this or even considered doing? And is it a good or bad idea?

An interesting idea up for discussion.
 
The Y pipe will collect coked oil and plug -BAD. Need a way to routinely clean the Y tube of this coke. Also exhaust pressure will act negatively on case venting, so need a one way check valve to access the intermittent low pressure pulses. Do more searching. Dan H posted a part # for getting one of these valves at Napa.

Having to clean the oil off the belly is work, but not as much as rebuilding an engine if the breather blocks and blows out the front seal and loose all your oil.

Also consider that the breather is the only way for moisture to escape from the crankcase after shut down. A one way check valve closes off this path. If the moisture doesn't escape, it will condense into liquid water. Not saying that all of the moisture will escape from the breather, but some does and the more you can get out, the better.
 
Last edited:
I did the install that Dan describes in the article in Kitplanes. https://www.danhorton.net/Articles/Separator-or-Condenser.pdf

The primary goal was to keep the belly clean, as all the crankcase vapors burn inside the exhaust tube. So far so good and the tailpipes are nice and tan. The extra over-pressure valve is wrapped in foam so it's easy to grab and look for any blow by, as a way to monitor the status of the system.

It's overall just a much cleaner install with one less tube hanging down in front of the belly. Time will tell. Cheers
 
Breather termination......

I’m doing some research on terminating the breather tube into the exhaust to burn off the oil like a smoke system would do and do a better job of keeping the belly clean.
To install, an oval hole is made in the exhaust pipe to match the oval produced by cutting a 45 angle on a steel tube. An interesting idea up for discussion.

I had heard many things about welding it directly to the exhaust including those in post #2, among others. Not to mention how do you get that angle/position of the welded tube right without it being on the airplane?

I decided to try dumping the condensation/oil etc directly onto a part of the hot exhaust that would burn it off yet keep it open, both during its function and after shutdown. My belly stays very clean: the minute amount that gets on the belly is very manageable and MUCH less than if it was not being burned off by the exhaust. You can see in the pictures where the discharge hits the exhaust pipe: some burned-up junk there. I have had to adjust the angle of the discharge tube several times to get it in the optimal position.

No: that is NOT a crack in the engine mount! :eek: It is oil residue from the fitting itself. After I saw that in the photograph I had to go out and wipe it off just to make sure myself!:p
 

Attachments

  • IMG_8576.jpg
    IMG_8576.jpg
    365.1 KB · Views: 359
  • IMG_8577.jpg
    IMG_8577.jpg
    386.1 KB · Views: 384
What I Did

I too, wanted a way to "burn" off the oil mist that comes out of the breather hose, after the air/oil separator, without compromising either the breather line, or the exhaust pipe. Because of the high velocity of the exit air at that location on the exhaust pipe usually chosen for that purpose - I felt I needed to somehow "protect" that air, so the cooling exit air didn't carry away the "mist" I was trying to control.

What I did: I made a fitting that "straps" to the exhaust pipe with a hose clamp, that uses a double wall exit strategy to force the mist filled air to impinge on the inner wall, hopefully catching any mist in that air, then being burned off in a low velocity environment from the contact heat of the mating surfaces. The breather air now passes out of the inner chamber at a reduced velocity, passes through an annular outer ring through round holes. The area of the exit holes is ~ 3x the breather hose - to relieve any potential for back pressure buildup. The inner holes are triangular & the outer holes are round - just trying to give the air more time to deposit the oil mist on a surface before it exits as well.

It will "coke" up a bit over time - but never enough to worry about between CI's.

YMMV - but it has worked for me for more than 20 years.

HFS

Harmon Rocket II S/N 002
 

Attachments

  • Breather “Dump” on Exh. Pipe - CU.jpg
    Breather “Dump” on Exh. Pipe - CU.jpg
    134.9 KB · Views: 120
  • Breather “Dump” on Exh.jpg
    Breather “Dump” on Exh.jpg
    114.5 KB · Views: 147
Last edited:
...
I decided to try dumping the condensation/oil etc directly onto a part of the hot exhaust that would burn it off yet keep it open, both during its function and after shutdown. My belly stays very clean: the minute amount that gets on the belly is very manageable and MUCH less than if it was not being burned off by the exhaust. ...

Same here, Denver - I just routed the breather to drip onto the exhaust and my belly is very clean. No worries about coking up.

BTW, shame about that engine mount crack in the photo! :D
 
I did the install that Dan describes in the article in Kitplanes. https://www.danhorton.net/Articles/Separator-or-Condenser.pdf

The primary goal was to keep the belly clean, as all the crankcase vapors burn inside the exhaust tube. So far so good and the tailpipes are nice and tan. The extra over-pressure valve is wrapped in foam so it's easy to grab and look for any blow by, as a way to monitor the status of the system.

It's overall just a much cleaner install with one less tube hanging down in front of the belly. Time will tell. Cheers

Thank you for pointing me in the direction of Dan’s article. I would think a backup relief valve and routine cleaning of the tap, let’s say at each oil change which I typically do 3 times per 100 hours would be more than adequate for a margin of safety.

I am thinking of elimination of an oil separator/collector and just running directly.
 
I am thinking of elimination of an oil separator/collector and just running directly.

IIRC, there are some like that in the certified world. In theory, no separator may mean more oil content, which may mean more coking. However, coking appears to be location dependent also, so treat it as an experiment and inspect often until you have a handle on coke build rate.
 
IIRC, there are some like that in the certified world. In theory, no separator may mean more oil content, which may mean more coking. However, coking appears to be location dependent also, so treat it as an experiment and inspect often until you have a handle on coke build rate.

Dan, thanks for sharing your knowledge. Here are a few more questions.

Would coking be higher or lower the further away from the exit?
Would coking rate vary on size of orifice of tube into exhaust?
What size tube would be optimal?
 
I did find a service bulletin for the SR22 that cleaning should be done every 50 hours or oil change which every comes first..

And a picture of how it enters the exhaust.
 

Attachments

  • 66E9ED42-D696-48AC-ACCE-4CD22C1A44FA.jpg
    66E9ED42-D696-48AC-ACCE-4CD22C1A44FA.jpg
    230.5 KB · Views: 200
I have used the Anti-Splat crankcase vent on two of my airplanes - my current RV6, and my former RV4:

https://antisplataero.com/products/crankcase-vacuum-kit-complete

Here is a pic of my RV4 installation. I don’t have a clear picture of the one on my RV6.

71CB6023-B723-404A-96B7-AD809C65C404.jpg

I only have about 50 hours on my -6 so far, but I put about 200 hours on the -4, and the system performed as advertised. There is virtually nothing on the belly, except soot, which is hard to see unless you wipe with a clean wet paper towel. I do that about every 50 hours. That’s also when I pull the tube off the exhaust pipe to remove any coking. I have found as much as 25% of the opening coked after 50 hours. That whole cleaning process takes about 10 minutes. On the -4 I could look up the exhaust pipe with a flashlight to see if there is any coking. On my -6, I can’t quite do that, but I can use my boroscope to check it occasionally.
 
Would coking be higher or lower the further away from the exit?
Would coking rate vary on size of orifice of tube into exhaust?
What size tube would be optimal?

Strictly anecdotal, but closer to the cylinder head seems to be better.

It would seem a larger tube would take longer to coke shut, area being a squared function...but nobody knows.

Unknown.
 
Anti-Splat installation

With my Anti-Splat installation, where the exit hose from the Air oil separator is directly connected to a welded stub into the exhaust, I can confirm the fact that this stub keeps coking up.
Now that I know the rate of build up, I clean it out once a year during condition inspection.
To answer the op's question, I think I would just rather rout it on top of the exhaust pipe and put up with an occasional trace of oil from the breather tube.
The belly stays very clean with my installation, however getting the exhaust soot cleaned up was much easier when it was mixed up with a little oil.
 
Everyone doing the evacuator has the EXTRA, safety blow off EGR valve bypass like DH, right?

No one is just doing this on condition trend, without an extra safety bypass channel, right?

Crack a ring or increase blowby and your coking trend is invalidated.
 
IIRC, there are some like that in the certified world.

Correct...Turbo continental TSIO-360's have breathers piped directly into tailpipe, no valve.

dl0Evsd.jpg
 
Last edited:
right

Everyone doing the evacuator has the EXTRA, safety blow off EGR valve bypass like DH, right?

No one is just doing this on condition trend, without an extra safety bypass channel, right?

Crack a ring or increase blowby and your coking trend is invalidated.

I wanted to avoid thread drift and bypass the Anti-Splat installation but yes,
2 EGR valves, the whole installation is now a bit of cluster....
If I did it again, I really just might go with an above the exhaust pipe termination and avoid the potentially serious issues with a plugged up breather exit.
 
Where to dump breather junk.........

2 EGR valves, the whole installation is now a bit of cluster....
If I did it again, I really just might go with an above the exhaust pipe termination and avoid the potentially serious issues with a plugged up breather exit.

The system that I made works well. Simple. No worries about things getting plugged up. Breather exit is above the exhaust which is a pretty warm place and no chance of freezing.....:) Belly stays clean. :cool:
 
If a hole was put in the breather tube like is currently done allow eliminatin of check valves?

This is of course assuming the pressure was less at the entry point into the exhaust to draw the air. Is it?
 
Last edited:
If a hole was put in the breather tube like is currently done allow eliminatin of check valves?

This is of course assuming the pressure was less at the entry point into the exhaust to draw the air. Is it?

pretty sure it oscillates from positive to negative on each stroke, hence the check valve to get only negative pressure. If you are going to do this, you need to do more research. Many potential risks from home brewing a solution here. Plugging a breather outlet is a serious flight risk. Many, myself included, have had good success with terminating the breather 1/8" from the exhaust pipe. The separator pulls out the bulk of the oil and the remainder sticks to the pipe and burns off.
 
Last edited:
pretty sure it oscillates from positive to negative on each stroke, hence the check valve to get only negative pressure. If you are going to do this, you need to do more research. Many potential risks from home brewing a solution here. Plugging a breather outlet is a serious flight risk. Many, myself included, have had good success with terminating the breather 1/8" from the exhaust pipe. The separator pulls out the bulk of the oil and the remainder sticks to the pipe and burns off.

EDIT: Looking at a cut-away view of the inside of the crankcase, I see that there is a fairly small passage that connects the front cylinder cavity from the rear cylinder cavity, just a gap in the main bearing support bridges. It seems small enough that even though the total crankcase volume is constant, there must be pretty high velocities running back and forth through that passage as the front pistons move up and the rear pistons go down, or vise versa. (up,down meaning in the sense of compression stroke, power stroke). So I can see how there would be fairly large pressure pulses generated that would be felt in the accessory case.

I question the idea that the crankcase pressure "oscillates from positive to negative on each stroke". There are the same number of pistons going up as going down. The crankcase volume stays constant. No doubt there are pressure waves bouncing around causing small pressure fluctuations of all sorts, but the overall pressure should be pretty constant. And who knows how those pressure transients propagate through the small passageways into the accessory case where the breather vent port is.

Also, what is the natural frequency of the check valve? It presumably has a spring, or a mass, or both. 2400 RPM is 40 hz. If the natural frequency of the check valve is below, or near that, there could be a substantial phase lag and maybe even 180 degree phase lag, meaning it would be opening at the high-pressure pulse!

Bob Mills is getting set to actually measure the (average) pressure in the breather tube with the Antisplat exhaust pipe tap breather. I'd also like to see him test the more common curved breather that dumps 1/8" above the exhaust pipe right at the cooling air exit, where the pressure is locally below free-stream static pressure.

By the way, free stream static pressure is around 400 " H2O. Antisplat says they get 3--5" H20 below free stream (that's from their website). So do you really think you could see any performance change from "crankcase vacuum" with a drop in crankcase pressure of about 1%?
 
Last edited:
Steve, I think Larry was speaking of oscillating exhaust pipe pressure, which definitely goes positive and negative. See attached, from an old CAFE report.

Mr. Nimmo seems to feel the best approach to tapping negative pressure is based on velocity, a slash cut port in the lee of a tube projecting into the gas flow, typically at a tailpipe location well downstream where multiple cylinders contribute to more consistent flow.

I've had good results (-3" Hg or better) with a tap located in the downpipe of a single cylinder, where positive and negative peaks are larger. This approach relies on the check valve operating well at 23 hz or less

Sky Dynamics taps the center of a collector, and nets 1.5 to 2 psig (3 to 4" Hg) negative per a published dyno sheet. Pretty sure they too use a top hat check valve, sometimes at more than 3000 RPM. Tapping one collector of a 3 into one on a 540 means the check valve is hitting 75 hz.

Back in the bike racing days I ran a case evacuator incorporating a two stroke reed valve, which needed to operate up to 85 hz.

No approach is wrong if it works.
 

Attachments

  • CAFE EPG 4into1.jpg
    CAFE EPG 4into1.jpg
    93.7 KB · Views: 94
Last edited:
Steve, I think Larry was speaking of oscillating exhaust pipe pressure, which definitely goes positive and negative. See attached, from an old CAFE report.

Mr. Nimmo seems to feel the best approach to tapping negative pressure is based on velocity, a slash cut port in the lee of a tube projecting into the gas flow, typically at a tailpipe location well downstream where multiple cylinders contribute to more consistent flow.

I've had good results (-3" Hg or better) with a tap located in the downpipe of a single cylinder, where positive and negative peaks are larger. This approach relies on the check valve operating well at 23 hz or less

Sky Dynamics taps the center of a collector, and nets 1.5 to 2 psig (3 to 4" Hg) negative per a published dyno sheet. Pretty sure they too use a top hat check valve, sometimes at more than 3000 RPM. Tapping one collector of a 3 into one on a 540 means the check valve is hitting 75 hz.

Back in the bike racing days I ran a case evacuator incorporating a two stroke reed valve, which needed to operate up to 85 hz.

No approach is wrong if it works.

Ah! I see. I didn't really occur to me that the exhaust pressure would really go negative. I figured The tap was relying on the local flow change around the protruding tube, as you describe.

Taping a single cylinder would certainly reduce the frequency, and exploit the highest pressure fluctuation, if it really goes negative that far up the pipe.
3 " Hg is significant, about 10% reduction in windage. Might get an Hp or two?
 
Ah! I see. I didn't really occur to me that the exhaust pressure would really go negative. I figured The tap was relying on the local flow change around the protruding tube, as you describe.

Experiments suggest it's a little of both. Germaine to your skill set, what shape should we make the end of the tap for maximum low pressure due to local flow change?

Taping a single cylinder would certainly reduce the frequency, and exploit the highest pressure fluctuation, if it really goes negative that far up the pipe.

Definitely goes negative all the way up and down the pipe. It's a tuning fundamental. The CAFE exhaust pressure plot was recorded 1.25" below the cylinder exhaust flange.

3 " Hg is significant, about 10% reduction in windage. Might get an Hp or two?

I like the lack of oil leaks, the clean belly, and the higher evaporation rate of crankcase water, but the two chambers pumping back and forth through those small holes in the center web surely has to cost some power.

When you measure Bob's evac pressure, make sure the AS tap assembly is clamped air tight, and I suggest a new NAPA valve. If you still think it's not responding fast enough, make one with a two stroke tent reed. I guarantee it will keep up.
.
 

Attachments

  • 2012-10-14_14-37-33_138.jpg
    2012-10-14_14-37-33_138.jpg
    413.6 KB · Views: 132
EDIT: Looking at a cut-away view of the inside of the crankcase, I see that there is a fairly small passage that connects the front cylinder cavity from the rear cylinder cavity, just a gap in the main bearing support bridges. It seems small enough that even though the total crankcase volume is constant, there must be pretty high velocities running back and forth through that passage as the front pistons move up and the rear pistons go down, or vise versa. (up,down meaning in the sense of compression stroke, power stroke). So I can see how there would be fairly large pressure pulses generated that would be felt in the accessory case.

I question the idea that the crankcase pressure "oscillates from positive to negative on each stroke". There are the same number of pistons going up as going down. The crankcase volume stays constant. No doubt there are pressure waves bouncing around causing small pressure fluctuations of all sorts, but the overall pressure should be pretty constant. And who knows how those pressure transients propagate through the small passageways into the accessory case where the breather vent port is.

Also, what is the natural frequency of the check valve? It presumably has a spring, or a mass, or both. 2400 RPM is 40 hz. If the natural frequency of the check valve is below, or near that, there could be a substantial phase lag and maybe even 180 degree phase lag, meaning it would be opening at the high-pressure pulse!

Bob Mills is getting set to actually measure the (average) pressure in the breather tube with the Antisplat exhaust pipe tap breather. I'd also like to see him test the more common curved breather that dumps 1/8" above the exhaust pipe right at the cooling air exit, where the pressure is locally below free-stream static pressure.

By the way, free stream static pressure is around 400 " H2O. Antisplat says they get 3--5" H20 below free stream (that's from their website). So do you really think you could see any performance change from "crankcase vacuum" with a drop in crankcase pressure of about 1%?

Should have been more clear. The EXHAUST oscillates from positive to negative on an intersecting pipe. The high velocity of the gas as it is pushed out from the exh stroke of a cyl will create a negative pressure on an intersecting pipe greater than ~100* (whole point of headers is to tune this so one pipes high velocity pulse helps pull exh from another pipe that is in overlap i.e. low velocity and thus improves scavanging) and then it briefly reverses as the velocity comes to an abrupt stop inside a tube with only one opening, creating a slight positive pressure on that intersecting tube. The check valve insures that the breather tube goes from ambient to negative and not positive to negative, keeping that reversion spike out of the breather tube.

I would expect a crankcase to always be positive or ambient pressure due to the fact there is blowby constantly adding air and every upstroke has a corresponding downstroke, so in full balance. Many racers have shown a power increase by keeping the crankcase at a vacuum instead of neutral or positive and have no reason to doubt this works. That said, they are not using something with small gains like this exh approach. Instead, they are using powerful electric vacuum pumps. From what I have learned, keeping the breather open will generally keep the crankcase at ambient unless blowby is greater than the breathers volume capacity to release it.
 
Last edited:
Experiments suggest it's a little of both. Germaine to your skill set, what shape should we make the end of the tap for maximum low pressure due to local flow change?
.

Well, any abrupt barrier will create a separated flow region downstream, which will create lower pressure. Inside a pipe, there is also the simple effect that any flow restriction will cause the whole flow at that section to accelerate, like a venturi.

Perhaps the best conceptual model for the separated flow downstream is to think of a windscreen on an open cockpit airplane. If the windscreen is sloped, there is less drag, implying less pressure loss. With the windscreen perpendicular to the flow the pressure loss would be greater. Based on that argument, it would seem the best arrangement for the tap would be to be perpendicular to the exhaust pipe, with the intersecting tap tube cut to extend inside the tube a short distance on the "upwind" side, and trimmed flush on the downwind side.

This whole conversation has really changed my point of view on these systems. Thank you guys!

Also in fairness, looking at the AS website again, he said 3-5" of pressure reduction, but was not specific about the units. I assumed he meant " H2O, but looking at the data plot Dan posted, I was apparently wrong. It looks like there could be 3--5" Hg reduction, which is significant.
 
I had a SkyDynamics 4-into-1 pipe on a non-RV that was supplied with a stinger from the collector that was intended to have the breather attached. We did not experience any coking but did see a marked reduction in oil consumption and very little oil on the belly. This was in an airplane used for competition aerobatics and would expect some oil usage from pos-neg-pos transitions. The engine was a new parallel valve IO-360. The breather came from an aerobatic breather tank with a change-over weight. No data on exactly what was going on but we were happy with the result.

Perhaps the significant factor is routing a pipe to the centre of a 4 pipe collector will produce a low pressure due to a venturi? I have heard some owners even attached a vacuum hose to this pipe and ran an AH.
 
Found on Google... someone quoting from a Mag I don't have access to:

Nov. 2009 Drag Racer ran back to back dyno test changing pans, trays, e.vac, vacuum pumps, and oil pumps. Here's the results. All Moroso products.


Street-strip pan with flat tray, Std volume pump base line...........935 HP
Exhaust type Pan-E-Vac............................................... ........964 HP +29
Hi volume oil pump.............................................. ..................953 HP -11
Race pan with curved louvered tray and kickout.........................993 HP +40
Vacuum pump 3 vane(over Pan-E-Vac)....................................1018HP +25
vacuum pump 4 vane.............................................. ..............1024HP +6
Deeper sump race pan and shrouded oil pump to close off tray.....1025HP+1
Alumn pan with curved tray, & more louvers..............................1027HP+2


Pan-E-Vac - pretty sure is this:https://www.moroso.com/crankcase-evacuation-system25900/
great similarity to AS system concept
from the dyno test it gained 3%
which is more than you'd get going 9 to 10 CR.
Notable is you gain more HP from the extra drag of a vacuum pump than you lose in the later tests... but that also usually means changing rings.

https://www.yellowbullet.com/threads/curious-about-dyno-resluts-on-a-crankcase-evac-system.247494/
 
Well, any abrupt barrier will create a separated flow region downstream, which will create lower pressure. Inside a pipe, there is also the simple effect that any flow restriction will cause the whole flow at that section to accelerate, like a venturi.

Perhaps the best conceptual model for the separated flow downstream is to think of a windscreen on an open cockpit airplane. If the windscreen is sloped, there is less drag, implying less pressure loss. With the windscreen perpendicular to the flow the pressure loss would be greater. Based on that argument, it would seem the best arrangement for the tap would be to be perpendicular to the exhaust pipe, with the intersecting tap tube cut to extend inside the tube a short distance on the "upwind" side, and trimmed flush on the downwind side.

This whole conversation has really changed my point of view on these systems. Thank you guys!

Also in fairness, looking at the AS website again, he said 3-5" of pressure reduction, but was not specific about the units. I assumed he meant " H2O, but looking at the data plot Dan posted, I was apparently wrong. It looks like there could be 3--5" Hg reduction, which is significant.

Antidotally...I just installed ASA separator and PCV setup and it seems to have given me 2-3 KTAS (60-65% lean of peak range) There's enough of a vacuum on the case that my engine driven fuel pump pressure has dropped about 1 PSI.
 
A vacuum reduces the vapor pressure of water. Theoretically, this would improve elimination of crankcase moisture, correct?
 
Last edited:
A vacuum reduces the vapor pressure of water. Theoretically, this would improve elimination of crankcase moisture, correct?

Vapor pressure is a function of temperature.

To increase evaporation (the quantity of water molecules changing from liquid phase to vapor phase), reduce the local pressure (crankcase vacuum or altitude) or increase the vapor pressure (heat the liquid), or both.
 
Vapor pressure is a function of temperature.

To increase evaporation (the quantity of water molecules changing from liquid phase to vapor phase), reduce the local pressure (crankcase vacuum or altitude) or increase the vapor pressure (heat the liquid), or both.

I believe that the vacuum or reduced pressure lowers the liquid's boiling point and boiling creates an exponentially greater rate of evaporation than natural evaporation, assuming you can get the pressure low enough. Didn't know that low pressure on it's own increased the evaporation rate, but trust you. This the general philosophy for getting water out of refrigeration lines. The high vacuum causes the water to boil at low temps and gets sucked out as vapor. Takes only a few minutes and kind of cool, as you can see the vapor coming out of the pump almost immediately.
 
Last edited:
I am curious as to why we don’t plumb the breather back into the intake like we have done with automobiles, no oil on the belly and no worry about plugging due to coking?
 
I am curious as to why we don’t plumb the breather back into the intake like we have done with automobiles, no oil on the belly and no worry about plugging due to coking?

Because we care more about our airplane than our car.
But more seriously there’s lots of combustion byproducts in the exhaust gases that simply are not good for the internals of the engine and it’s best to simply get them out as quickly as possible .
 
I am curious as to why we don’t plumb the breather back into the intake like we have done with automobiles, no oil on the belly and no worry about plugging due to coking?

Because these systems were designed long before that became a practice due to environmental concerns. PCV systems require a bit of engineering due to the additional air intake and type certifications don't really allow for this kind of upgrade. PCV uses vacuum downstream from the throttle plate and you can't just bypass air around the carb or servo without modifications to the way the carb or servo meters fuel. There would be no concern routing the air output out of the separator into the FAB, but will make an oily mess of the FAB, as the separators are nowhere near 100% effective.
 
Last edited:
I am curious as to why we don’t plumb the breather back into the intake like we have done with automobiles, no oil on the belly and no worry about plugging due to coking?

I think this is a very good question. Under normal circumstances the stuff coming out of that tube would be a bit of oil suspended in hot air, some exhaust gases, some water, and some other byproducts of combustion.

None of these seem like they would cause a big problem, particularly since the air flow into the engine would be much higher than the flow of the breather. Also, all this air came into the air intake system originally.

What might concern me more is what happens if the engine starts pushing out more oil, like after negative G maneuver - will that interfere with the combustion process? What if there are bits of carbon - could they clog any carb or throttle body orifices?

I have no separator and have my breather exit just over an exhaust pipe - this provides me a bit of information about engine health, as would an oily belly.

It will be interesting if this idea is developed and we can see if there really are downsides, or if it's just something we in the aviation community have overlooked.
 
A vacuum reduces the vapor pressure of water. Theoretically, this would improve elimination of crankcase moisture, correct?

Nope. What DanH said in post 34 is correct. The vapor pressure of water is a function of temperature; at a fixed temperature, the vapor pressure is constant (doesn't change with pressure).
As you lower the pressure, more of the water will evaporate out of the liquid because the vapor pressure is a higher fraction of the total pressure. When the actual pressure equals the vapor pressure, the water will "boil", meaning that all of the water can exist as a gas at that pressure and temperature.

You can put a cup of water in a bell jar at room temperature and pump it down to vacuum. Yes, the water in the cup will boil. If you seal the bell jar, and there is enough water in the cup, the pressure in the bell jar will rise because of the added gaseous water, and will eventually reach an equilibrium where the water stops boiling because the pressure has risen to the vapor pressure at that room temperature.

The same thing happens inside the propane tank on your barbecue. The gas pressure in the tank is the vapor pressure of propane at the ambient temperature. As you use propane, more liquid propane evaporates to maintain the pressure at the vapor pressure.

I believe that the vacuum or reduced pressure lowers the liquid's boiling point and boiling creates an exponentially greater rate of evaporation than natural evaporation, assuming you can get the pressure low enough. Didn't know that low pressure on it's own increased the evaporation rate, but trust you.

The increase in vapor pressure with temperature is a smooth curve. The higher the temperature, the quicker water will evaporate to reach equilibrium where the partial pressure of water in gas state equals the vapor pressure. The curve is sort of exponential, but it's not like suddenly at the boiling point, there is a huge step-increase in evaporation rate. It is still just 'natural evaporation.' It is just that the gas bubbles that form in the liquid and float to the top to escape are big enough to see.
 
Last edited:
Packaging options??

Hey DanH - - - do you think the NAPA check valve has to be close to the exhaust or could it be 10-14" away? I it seems attractive to have something resembling the Cirrus design.
 
Last edited:
Back
Top