Frans,
I agree with your assessment. Except that burping right after shutdown does any
good. Right after shutdown, there's no oil in the crankcase to return to the
reservoir. While operating, the engine crankcase has been continuously
pressurized
returning any oil to the reservoir.
You do the "burp" on a cold engine prior to the start in order to return any oil
accumulated in the crankcase (or cylinders) back to the reservoir so the
quantity
may be properly checked. It also serves to provide some pre-lub to the
bearings and oil galleries.
The oil in the crankcase will have come from some residual drain down from the
engine, but mostly from the siphoning from the reservoir back into the
crankcase.
This si because in our aircraft the reservoir level is almost always above
the crankcase. My engine has been down for almost a year but I still "burp"
the engine every time I go to the hanger to work. And every time it takes a
fair number of blades to push about a half a quart of the oil back into the
reservoir.
It's down about half a quart prior to burp and is full after the burp.
This also pumps fresh oil into the galleries and bearings keeping things
coated with fresh oil. And probably adds to that half quart in the crankcase
as it drains down between visits.
Blue skies & tailwinds,
Bob Borger
Europa XS Tri, Rotax 914, Airmaster C/S Prop.
Little Toot Sport Biplane, Lycoming Thunderbolt AEIO-320 EXP
3705 Lynchburg Dr.
Corinth, TX 76208-5331
Cel: 817-992-1117
rlborger@mac.com
On Feb 2, 2012, at 4:34 PM, Frans Veldman wrote:
>
> On 02/02/2012 01:28 PM, Sidsel & Svein Johnsen wrote:
>
>> This leakage of compressed air past the piston rings will also blow away
>> some oil deposited on the cylinder walls during the down stroke,
>
> "Blow away the oil"??? The idea here is not to turn the prop like a mad
> man, but to slowly rotate the prop, and preferably wait at each
> compression point to let the air slowly leak out. There is no blowing
> here, just gentle movement.
>
>> I doubt that there is any meaningful buildup of oil pressure (i.e. enough
>
> You can actually achieve full operational oil pressure with hand
> cranking. Of course the oil will soon leak past the bearings so the
> pressure won't hold for long, but at least the bearings get lubricated.
> If you don't believe it, get an assistant to rotate the prop, switch on
> the engine instruments (NOT the ignition!) and observe the oil pressure.
> The engine needs to be cold for the pressure to build up, but this is of
> course what we are discussing, to burp a cold engine.
>
>> If there
>> were some pressure with resulting oil flow, it would mean that oil is
>> flowing back into the very crankcase that we try to drain, wouldn't it?
>
> Yes, but this amount is much less than the oil that dripped into the
> crankcase during the long time the engine was switched off. The bearings
> have become dry and all the oil is at the bottom of the engine. Sure you
> add some new oil to this, but the surpluss will be removed and the
> bearings receive a fresh load of oil.
>
> There is another thing you need to consider: oil will slowly leak back
> from the oil tank into the engine, if the level in the oil tank is
> higher than in the engine (which is usually the case). The amount
> depends on various factors but it will build up over time.
> It is even possible (and has happened) that the oil in the engine seeps
> into the cylinders. Since oil is not compressible it will wreck the
> engine during the compression stroke. This reason alone is worth the
> trouble to hand crank the engine just before starting, so you can feel
> in time if the engine "locks" due to oil in a cylinder.
>
>> I have attended the Rotax maintenance course first in Denmark, later in
>> Norway. The very experienced service agent for Scandinavia both times
>> recommended doing this turning-for-oil-level-check immediately after
>> stopping the engine - not prior to starting it.
>
> So instead off scraping away the oil or "blowing it away" just before
> starting, it is now done just after switching it off (which is still
> before the subsequent startup). Either way, the procedure is carried out
> somewhere in the time frame between the shut down and the next startup.
> Even if I follow the reasoning that burping the engine removes some oil,
> I still fail to see why removing this oil immediately after shut down
> would be better than just before the next startup. I would rather leave
> it there as long as possible.
>
> Bad reasoning sometimes spread very fast. I wonder whether anyone has
> conducted some serious research on this? Who came up with this reasoning?
>
>> The reason is that the
>> engine is easier to turn when warm,
>
> Well, if this is the most important consideration I will agree with it.
> But for me, I happily sacrifice some ease in favor of the longevity of
> my engine.
>
>> Risk of oil lock in a cylinder with our type of engine/lubricating system:
>> I do not know.
>
> See above. It is a realistic risk. I had a leaking return valve in my
> turbo oil return line, and in a few weeks of time my turbo was filled
> with oil. The same can happen with the crankcase (and subsequently the
> cylinders).
>
> All in all, burping the engine removes some oil, regardless whether this
> is done immediately after shut down or just before start up, but doing
> it just before startup has the benefit that the bearings are
> relubricated and a potential hydraulic lock is detected in time.
>
> Frans
>
>
>
>
>
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