Oil Breather Tube Orientation

Further random thoughts;

The mussing over an initial vertical orientation, of the bather pipe, occurred as I was removing oil smear from my aircraft belly.

In the past I have tried a home made oil catch-can on the breather, with little success - I do have a new untested design.

In an automotive engine the function of the crankcase breather is to ;

 # Allow crankcase pressure and volatile fractions/contaminates of the oil to escape, either to atmosphere (older engines) or be ducted into the air inlet system, to be consumed in the combustion process (new engines).

 # Retain oil (droplets/mist) within the engine.

In an automotive engine there is usually a plate, sometimes  + a gauze "filter", at the engine end of the breather pipe, to encourage heavier oil droplets to septate from the hot vapour and remain in the engine.

In Rotax 912, its much the same as in an older automotive engine, without any significant system for retaining airborne oil droplets/mist.

I suspect that just orientating the breather up would have little benefit.

If a metal coiled (copper/aluminium) pipe, were employed, as part of the vertical orientation, to assist the oil droplets to cool/coeles, oil may run back down to the tank. ???????

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Sean,

Neither my aircraft nor others I’m familiar with get any oil out the oil tank vent tube.  I’m curious what elevation your oil tank is mounted at as compared to the Rotax installation guidelines.  

Jeff Blakeslee wrote:

Sean,

Neither my aircraft nor others I’m familiar with get any oil out the oil tank vent tube.  I’m curious what elevation your oil tank is mounted at as compared to the Rotax installation guidelines.  

Hi Jeff,

I believe its well within Rotax specifications, relative to the oil pump.

I don't get a lot of oil - what I get is mainly on my center /ventral mounted antenna base - annoying!

Oil consumption is also well within Rotax specification's but noticeably more than my last 912ULS that did not require a top up between changes, also annoying.

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Sean, consider it may just be the location of your vent tube exit.  Sounds like you have a bit more low pressure at that point than you should.  Oil vapor is normal in the engine back to the oil canister.  Upon exit into the can generally there is very little to draw it out the vent.  On aircraft where this is a problem it is usually due to overfill systems.  This however can be an issue if the vent line has no breathing "notch" in the line inside the engine bay area, this is shown in the installation manual.  The notch should be in a neutral pressure zone, inside the cowling, to stop ice formation blocking the line AND to reduce any low pressure draw from the exit point on the line end.  

I can't say this is your problem, just a suggestion to check. A check of the pressure inside your cowling might reveal if that area is under low pressure.  ( a handheld altimeter, run your pressure line into the zone under your cowl you wish to check, it should match your aircraft altimeter if the pressure is in balance) 

Cheers

RW,

In my efforts to reduce/prevent this small oil contamination of my antenna base I have;

 # Tried a home made catch can - no benefit/didnt catch hardly any oil. Analysys - bad design. May try again.

 # In loo of notch - Have a vacuum reducing T fitting in end of hose. One end to atmosphere (bottom of cowling / exit air), the other, to top firewall, inside of cowling (high pressure?).  This idea is current. May help a little.

As for altimeter - don't have a spare of any description (will make enquiries of flight school) However I expect the exit air to be relatively high pressure/atmosphere. Reasons; This is the only cowling exit, which has been progressively enlarged to facilitate oil cooler (mounted in exit air) efficacy ie only just large enough to do the job in summer temperatures. Also, this is the underside of the fuselage, so normally a high pressure zone.

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