crankcase breather tube, (long)

[Tom Martin]

I have long been an advocate of dumping the crankcase vent directly on top of the exhaust pipe. This is not my idea, it is one that has been used by many RV types. This system does an excellent job of getting rid of nuisance oil drips from the breather tube to the hangar floor but based on some testing that I did a couple of weeks ago I believe we should reroute our crankcase breather aft of the cowling.
The particular engine that I am flying now seems to use more oil than I am used to; about six or hours per quart. Also there is more oil on the belly than I am used to. I have been doing quite a bit of work on engine cooling with this plane to the point that I have greatly reduced the outlet size of the cowling. After my last modification I felt that my oil consumption had increased. My theory was that as I increased the pressure in the lower plenum the air was moving faster in the area of the breather tube and sucking oil out. This turns out to be correct but not in the way that I thought. To test my theory I took some pressure readings in three places firewall forward. The first was at the dipstick, the second in the back of the upper plenum and the third in the bottom of the lower plenum, close to the breather tube opening.
I used a simple water manometer. What I was expecting was a negative pressure at the dipstick. What I got, at cruise speeds, was 7? of positive pressure! The upper plenum pressure was 12? and the lower cowling pressure 6?. At first I could not believe what I was seeing but subsequent tests confirmed the readings. Back on the ground I did a test at 2000 rpm with the cowling removed and found a pressure of 1? in the crankcase. Clearly, in flight, the crankcase was equalizing its pressure with the lower plenum. This is not a good thing and explained the small trip of oil I get around the front seal, but how could it explain excessive oil use and the fresh oil on my belly. My current theory is that relative to the lower cowling the engine crankcase pressure is neutral to slightly positive and this could explain why oil was going out the breather tube even though the crankcase was very positive in pressure relative to outside air.
I rerouted the breather tube, down and out the back of the cowling. It extends about three inches. A flight test now shows that I am making negative 2? of water column pressure.
After about six hours of flight I can say that my oil consumption is indeed lower, how much it is too early to tell. Also there seems to be very little oil on the belly of the aircraft. Is negative ok? I know the race car guys try to pull negative pressure on their crankcase but is it ok for a Lycoming? I asked some engine people at AirVenture. The Lycoming people had no real response. Both Bart Lalonde and Alan Barrett stated that negative pressures of that level should be no problem if oil was not being pulled from the crankcase. They both stated that positive pressures in the crankcase could lead to increased oil consumption. Mr Barrett confirmed my theory for oil getting out of the crankcase as the air inside the crankcase is very turbulent and that a fine oil mist is created which can easily move out the breather tube under my scenario.
My case is a bit unique in that I have greater lower cowling pressures than normal installations but I believe, based on my current information, that the oil breather tube should not be vented inside the cowling itself. This includes situations where oil separators are being used and the outlet dumped inside the cowling. Even if your engine is not using excessive oil it is not a good thing to have your crankcase pressurised as it increases the risk of blowing the front seal and seepage in other parts of the engine. Test your own installation, it is quite easy and costs next to nothing to do. According to my engine questions, there should be 5.5 to 6 inches of difference between the top and bottom plenum so I am in good shape there.
As always, a whistle slot, or small hole should be included in the system somewhere in the warm part of the cowling to provide pressure relief should the end of the vent line freeze. I will need to test the hole location to make sure that it does not pressurize the system as well.

[Wicked Stick]

Tom,

Got any pictures of how you setup and routed the testing equipment? If so, please post.

[Tom Martin]

Purchase 25 feet, or so, of plastic tubing, 1/4 ID at your local hardware store.
It is easy to make a water manometer. You will need a scrap of lumber, 1" by1" will do, about 18" long. Tape a length of tubing around the wood strip so that both ends come off the top and the bottom curve is not restricted. Now fill the tube with water so that it comes about half way up the board. This is point zero. Measure up and down the tube and lable each inch and half inch. Seven inches up and seven down should work.
Forward of the firewall I used 1/4" aluminium tubing to form an outlet inside the upper plenum and another down to the lower part of the lower plenum. These are tie wrapped to the engine mount. I took my heater hose off both sides of the firewall heater box and ran the tubing through there. Hook one end of the plastic to the metal tubing on the engine. A press fit is good enough. The other end is labelled and left open and accessable to the pilot. The manometer is tapped to the panel somewhere visible.
For the dipstick I made a little aluminium cover that was tapped with a brass fitting to hold the plastic tubing. This little cover was held to the dipstick tube with baffle tape scraps and two gear clamps. The dipstick is left out for the test. Note that this tube should clamped shut in the cockpit until you are ready to hook it to the manometer as fumes can pass through to the cockpit. (I know that now!)
I used aluminum furnace tape to seal around the open heater box for the duration of the test.
In the air you just swap hose connections to the manometer to get your readings. Note that the if there is suction and the water rises two inches above point zero on one side it also drops two inches on the other side. This would be called four inches of water pressure. Negative or positive depends on whether it is pulled in or out of manometer.
It is fun, give it a try. The manometer is also very useful for finding leaks in fuel tanks, static systems etc. Kevin Horten even has a discription on his RV8 web site of how to check your altimeter and airspeed using a manometer.

[airguy]

Keep in mind that this pressure reading is relative - not absolute. It will vary depending on cabin pressure (ram air from vents, etc), so the pressure reading in any particular spot may vary from day to day depending on cabin pressure. The RELATIVE pressure between multiple locations, though, should remain the same, since a cabin pressure change will affect all readings equally.

Unless you want to tie the other end of your manometer to the static port...

[Tom Martin]

Greg
You are correct in that the absolute pressure can vary, but all I am interested in is the relative pressure differences between the crankcase and the different plenums.
This information will be valuable through my next round of testing after I install a cowl flap.

[Alan Carroll]

Tom,

I'm intrigued by your findings, but also somewhat confused. If I understand correctly, in the original configuration you found that the lower cowling pressure was +6" (greater than cabin pressure), and that the the crankcase pressure was +7". Based on the measurement of +1" crankcase pressure during the ground run, it sounds like the original breather tube setup was applying +6" to the crankcase (1+6=7). If so, how could oil mist be escaping through the breather tube? Shouldn't any flow be the other direction, into the crankcase? Even if the crankcase is perfectly sealed (unlikely) and there is zero net flow, I don't understand why increasing the pressure should result in more oil going out the breather.

Is it possible oil was escaping somewhere else, such as past the piston rings? Have you observed any oil on the bottom plugs? I'm curious because I have a similar setup, and similar oil consumption.

Nice job designing the experiment by the way!

[Tom Martin]

Allan
All pressures noted are in comparison to other pressures and I have not adjusted for cabin pressure or altitude. The difference would be small. The upper plenum, doghouse over cylinders, had a relative pressure of 12" in flight.
The lower cowling about four inches from the outlet had an inflight pressure of 6". The crankcase had an inflight reading of 7". As noted the crankcase had a 1" pressure on the ground with the cowlings off. Therefore, if you remove the one inch that the engine is producing on the ground from the 7" noted in flight the crankcase and the lower plenum are at the same 6". However inflight the crankcase is still has one inch more pressure than the lower cowling. Thus there is still a positve flow out of the crankcase, relative to the lower cowling pressure. This plus the inner turbulent airflow and the flow of air in the lower cowling are causing this fine oil mist to exit the bbreather tube. Now the air inside the crankcase is still pressurized compared to the air outsidet the cowling and there are areas inside the cowling, around the spinner for example, that are probably not as high a pressure as the lower aft area due to leakage around the spinner. This is conjecture on my part but it would explain the small seepage that I have around the crankcase oil seal.
The crankcase would have a higher pressure under this system, relative to the top of the piston, than the engine was designed for. Thus oil could possibly being pushed past the rings but my plugs and exhaust seem to be oil free. This internal pressure is very likely robbing the engine of some power, how much would be just a guess on my part.
This is all new to me, and I may not be explaining it well, but there is no doubt that if you are dumping your vent into the lower cowling you are adding pressure to the crankcase and this is not good.
Try moving your vent aft, like I did, and see what difference it makes to your oil consumption. I know that I have made an improvement with my change.

[Alan Carroll]

Tom,

I realized right after submitting my post that I was doing the arithmetic wrong, and that in your original configuration there was in fact a pressure gradient from the crankcase into the lower cowl.

What I'm still confused about is why the oil stopped leaving after you moved the tube. It appears that you put the breather tube in an area with lower pressure than before, which resulted in the "negative" pressure you measured at the dipstick? If so, why didn't the loss through the breather increase rather than decrease? Something here I'm not getting.

I don't know for sure if its possible for high crankcase pressure to contribute to oil escaping past the rings, but his might be a question for Bart or other engine experts. Normally I'd think of this the other way around; leakage past worn rings pressurizing the crankcase.

My oil consumption seems to be in the 6-8 hours per quart range, and I do typically see some oil on the lower plugs. The engine is running strong though, and with only 300 hours I doubt the rings are worn. I asked Bart about this at Oshkosh and he suspected break-in issues. He suggested checking how fast the oil turned black (>5 to 10 hours would be considered normal).

Alan

[Tom Martin]

For fifty or sixty years certified aircraft had their breather tubes end outside the cowling. They still do. As is often the case someone in the experimental world tried something different by ending the tube in the cowling area just above the exhaust pipe. This does an excellent job of getting rid of those oil drips on the hangar floor and for the most part has not caused any apparent problems. Others copied this system and suddenly it becomes acceptable. In my case as I increased the pressure in the lower cowling my oil consumption increased as did the oil on the belly. As stated before I was amazed at how much I had presurized the crankcase. Both Bart Lalonde and Allan Barrett confirmed that this could lead to increased oil consumption due to either oil being pushed past the rings and or, as counterintuitive as it seems, increased bypass from the breather tube.
As stated before I would have a relative 1" postive pressure in the crankcase compared to the lower plenum. This is pushing the oil vapour out of the engine. The fact that my oil consumption is now lower must mean that either a 2" vacuum does not draw as much as a 1" push or that I am not pushing as much oil past the rings. Perhaps both come into play.
This is more complex than I first thought, but just maybe the traditional certified system had it right all along.

[Alan Carroll]

Tom,

I agree that this is not easy to figure out. I'm guessing that less oil is going out past the rings, but this is conjecture on my part. I'd be interested to know what your oil consumption is now, after you've had a few more hours.

In addition to controlling drips, the other reason I've heard for placing the breather tube outlet above the exhaust is to keep it from icing up.

Alan