CASA 292/14 - Conditions and direction about Jabiru engines

[Keenaviator]

Mine and others run just on the tip of the dipstickAny more and ot gets pumped out

Excess oil makes them run hot oil temps too.

 

Id suggest if you run at the level seen after an hour of so flight oil use will be much lower.

 

Something you need to watch and be careful with

I normally use 2 quarts and have the oil about 5mm on the dip stick. This time I added another 200ml - higher oil temps and oil pressure very high. It appears to be happier with the lower level.

 

 

[Russ]

Doing a lot of xcountry stuff...long legs, it worries me starting with oil leval low on the dip stick, ( as appears to be preferred by some folks )

 

One time enroute, oil leval was only just showing, ( literally just wetting the tip ) since then I've run it 4...5 mm higher. I then get minimal blow back into the bottle, but retain a higher enroute leval.

 

Question....just how low can we go ??

 

Some time back I saw a mod ....chap fitted a smallish blowback tank atop his engine, inside this was fine mesh, engine pushed oil vapour etc into this, vapour re liquified, then gravitied by another line back into engine. Said it worked great.

 

 

[facthunter]

Returning that oil to the engine is not a good idea. By then it often has a lot of water condensed in it and is highly contaminated with blowby products .. I would never recommend it. IF you do a lot of long trips I would recommend being able to transfer a measured amount of new oil to match the known rate of usage. The total amount of oil in a Jabiru is not a lot. Nev

 

 

[biggles]

Russ , it takes some time for the oil to drain back to the sump . Best to leave it for at least 30 mins to get an accurate dip. As FH says forget about 'recycling ' oil blown out . Fill to level you are comfortable with . You know your aircraft . Personally I am not comfortable with running low on the stick , I tend to use a bit so fill a little higher (at least halfway between L and H ), if it gets blown out then so be it , oil is cheap . I get the impression you tend to operate this way also , which is good considering the country you transit ........ Bob

 

 

[AVOCET]

Regarding oil in the bottle ,

 

Fit a larger hose dia , 19 mm from the breather to the bottle , dont wory about the smaller outlet at the dipstick end ,

 

The the larger hose reducces the pressure in the pipe ie less oil in the bottle , works great , have done it to three jab engins , and you only end up with water in the bottle ,

 

Wyane Jons put me on to this .

 

 

[biggles]

That's interesting Mike . I think the pipes at both ends are about 10-12 mm . What sort of fittings do you use at each to increase inside dia. to suit 19 mm hose ....... Bob

 

 

[AVOCET]

That's interesting Mike . I think the pipes at both ends are about 10-12 mm . What sort of fittings do you use at each to increase inside dia. to suit 19 mm hose ....... Bob

I just use a hose clamp and the the rubber hose " crushes" on to the pipe ok ,

You could use a piece of original hose as a stop gap , but ive found just clamping the larger dia 17/19mm ID

 

works ok .

 

 

[frank marriott]

Regarding oil in the bottle ,Fit a larger hose dia , 19 mm from the breather to the bottle , dont wory about the smaller outlet at the dipstick end ,

The the larger hose reducces the pressure in the pipe ie less oil in the bottle , works great , have done it to three jab engins , and you only end up with water in the bottle ,

 

Wyane Jons put me on to this .

I don't understand the logic of this move? To me a hose with the same restriction at each end should provide the same pressure/flow? A long hose gives fluid restriction based on size, but we are talking about low pressure gas over 1 mtr. As you say you have proved it works, but I cannot see how.

 

 

[AVOCET]

Yes , it sounds like whichcraft , but it does work , I had a 2200 that spued oil , and it was suggested to me to try this mod .

 

Needles to say , it works , but how , i dont fully understand ,

 

Ive run the the Avocet for 60 hrs , 3 oilchanges , and not one drop of oil ,

 

Just water , which evaporates between flights .

 

Mike

 

Ps Its a simple mod to try , and see for yourself ,

 

 

[biggles]

I don't understand the logic of this move? To me a hose with the same restriction at each end should provide the same pressure/flow? A long hose gives fluid restriction based on size, but we are talking about low pressure gas over 1 mtr. As you say you have proved it works, but I cannot see how.

Fluid dynamics . As pipe diameter increases for the same mass flow, the gas pressure and velocity decreases , . ie slows down the oil vapor flow allowing it to 'plate out' on the inside of the pipe and drain back into sump ..... Bob

 

 

[frank marriott]

Fluid dynamics . As pipe diameter increases for the same mass flow, the gas pressure and velocity decreases , . ie slows down the oil vapor flow allowing it to 'plate out' on the inside of the pipe and drain back into sump ..... Bob

Bob

 

I can understand what you are saying there but if the restiction at the output end is the same wouldn't the velocity be the same, at least once the capacity of the bigger hose is filled?

 

So what you are saying is that it is achieving a similar action/result to the container with steel wool inside which catches the oil in the gas and drains it back.

 

I don't profess to have any expertise in this area but am having trouble getting my head around the principle when the input and output size remains the same.

 

 

[Oscar]

Frank, think about a hose attached to a sealed water tank, with another hose the same size out the side of that sealed tank somewhere near the top of the tank. |Assume that the water is heavily contaminated with particles of rust (for example). The velocity and pressure of the water in the hoses into and out of the sealed tank will be the same, (less pressure head effects) - but the water INSIDE the tank will move slowly and let the rust particles settle out onto the bottom of the tank.

 

Reclaiming blow-by oil that is contaminated with the products of combustion isn't a very good idea; however, since the blow-by exits into the sump anyway and the oil relief hose is carrying basically oil fumes that have already come from the sump, the nett effect of any reclaiming of oil from the breather pipe isn't going to contain much in the way of carbon / iron oxide particles, but if it returns oil that is reduced in viscosity from uncombusted petrol and any condensed water vapour / sulphuric acid, that would not be good for the engine.

 

Overly-low oil temps are bad for engine life...

 

 

[biggles]

BobI can understand what you are saying there but if the restiction at the output end is the same wouldn't the velocity be the same, at least once the capacity of the bigger hose is filled?

So what you are saying is that it is achieving a similar action/result to the container with steel wool inside which catches the oil in the gas and drains it back.

 

I don't profess to have any expertise in this area but am having trouble getting my head around the principle when the input and output size remains the same.

Frank , as you are aware the +Ve pressure is developed in the crankcase , and the vapor carrying oil particles undergo a velocity increase in the dip stick riser , then pass through a narrower pipe into the catch tank via the long hose . As the hose is only marginally smaller ID than the connecting pipes at each end ,the change in pressure and velocity is small , and hence the oil vapor is carried over into the tank . With the suggested mod , that same constant mass flow now enters a a larger space , causing a change in both pressure and velocity , giving the vapor time to condense and return to the engine by gravity . A small amount of oil/vapor mixture may still carry over , but will experience an increase in both velocity and pressure , as it passes through the narrower diameter pipe into the catch tank . Hope this helps , but not always an easy concept to grasp ...... Bob

 

 

[biggles]

Sorry Oscar , didn't realise you had posted , now Frank's probably really confused

 

Bob

 

 

[AVOCET]

The reason i thought the mod was a good one is no more oil streaking , and on the avocet that means oil over the tail feathers ,

 

It works that well , i dont know why they dont come out like that , unless the oil flowing back into the sump is deemed harmfull to the engine ?

 

 

[Oscar]

Bob - your version is MUCH better than mine! To me, excessive blow-by is indicative of a problem and trying to capture blow-by oil seems to be a bit of a 'turn up the radio to drown out the noise of the engine rattling' type of concept. The engine in question here does on average about 50 hours/year, which if an hour every weekend (ish) should keep it in good nick - but if it sits for many weeks between flights, is probably getting some corrosion in the bores and packing iron oxide into the rings causing the blow-by. Excessive restriction of the crankcase ventilation will for sure cause problems but a sweet-running engine shouldn't be pushing out much except stuff that shouldn't be in the oil...

 

 

[facthunter]

Look the jury is back. The verdict is. That stuff shouldn't go near the engine. If you don't like "Oily belly" empty the grunge out of the bottle and keep your plane clean by all means but don't contaminate your engine oil. Your engine is a precision bit of gear and water (even PURE water) is no good for it. That is why you try to get your oil temps at 85 degrees or a bit more to evaporate the water out of the oil. Nev

 

 

[AVOCET]

Seems like the catch can should be at the engine end, then the hose from the can to the outlet sloping back to let the oil drain back into the bottle .

 

Think ill try this .

 

Mike

 

 

[Guest john]

Seems like the catch can should be at the engine end, then the hose from the can to the outlet sloping back to let the oil drain back into the bottle .Think ill try this .

Mike

What we have proven over many years to be successful in the Comanches regarding blobuy fumes existing via the breather tube is to securely tie wrap a small aerosol can (opened at the top with a can opener) to the firewall directly underneath the end of the breather tube & this procedure prevents the blobuy fumes from discolouring the belly of the plane .

After say 50 hours of flying when an engine oil change is performed, the contents of the aerosol can is emptied & you also have a clean airframe belly.

 

 

[frank marriott]

Oscar & Biggles

 

Thanks for the input. I discussed the principle with a friend tonight who works in this area [nothing to do with aeroplanes]

 

and he explained what I was missing - although I wouldn't try to explain the actual process he satisified me to what was actually happening - simply put I didn't understand the difference between hydrolic and gas pressures.

 

It is refressing to see that the topic was not taken over by keyboard experts [as happens too often] and I can now understand what Avocet is saying. Whether or not it is a good idea aside, at least I now know what and why it does what it does.

 

Again thanks for the informative input.

 

 

[Jabiru Phil]

Frank,

 

In laymen terms. The velocity and pressure loss is determined by the size and LENGTH of the FITTING

 

and of course the diameter of the hose.

 

Say for instance, you had a 50mm hose going to the collector bottle/can, you would have very little velocity and flow.

 

Probably as Avocet says the oil would flow back to the engine and the vapour would continue on.

 

I have often thought the same for the oil delivery fittings, inside diameters are very small and they must be some restrictions here.

 

PHIL.

 

 

[Jabiru Phil]

Frank,

 

Posted during yours.

 

Phil

 

 

[frank marriott]

Make that word refreshing, I didn't reread my post until too late to edit!

 

 

[jetjr]

Theres a few designs of catch can, all sit on top of engine and collect oil and return back down stick. Even Jab have a small box one which ive fitted. Does stuff all.

 

I have a very nice barrel one but wont fit my engine if any one is interested.

 

ive spent significant time measuring collected oil and operating level. If you run extra 5mm higher, it blows out in less than 1 hr flight and almost exactly the volume of oil added to maintain this 5mm higher is in the bottle at change time.

 

Raising level on a long flight makes you comfortable but doesn't change situation in engine at all. Only solution is to work out accurate oil use in ml/hr and add this at end of flight. Or stop more often and top up. If your engine is using more than you have spare on the stick then theres an issue. Can always adjust dipstick to make yourself happy :)

 

 

[facthunter]

I admire your work jr, but adjusting the dipstick...... You are jesting. SOMEONE will do it. You just know it. Then the poo hits the prop. Nev