Tom Martin
Well Known Member
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.
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.
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