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sidsel.svein(at)oslo.onli
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 Posted: Thu Feb 02, 2012 4:34 am Post subject: SV: Re: burping the 912 |
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The Rotax manual tells us to turn the engine prior to starting only with
reference to verifying the oil level in the reservoir tank (i.e. with empty
crankcase). The manual also explains how the turning brings the oil back to
the tank: Compressed air is leaking past the piston rings into the
crankcase, and this overpressure in the crankcase pushes the oil back to the
tank.
This leakage of compressed air past the piston rings will also blow away
some oil deposited on the cylinder walls during the down stroke, while the
scraping of the rings removes some oil both in the down and up stroke.
I doubt that there is any meaningful buildup of oil pressure (i.e. enough
for lubrication) during this short-lasting and slow hand cranking. 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?
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. The reason is that the
engine is easier to turn when warm, and the oil has lower viscosity - it
therefore flows easier back to the tank. This difference is noticeable on
cold winter days.
Risk of oil lock in a cylinder with our type of engine/lubricating system:
I do not know.
A different matter altogether is whether to turn the engine after it has
been preserved with special oil, e.g. for winter storage. Turning should
then NOT be done, as it will scrape/blow preservation oil off the cylinder
walls.
Regards,
Svein
LN-SKJ
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frans(at)privatepilots.nl
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| Posted: Thu Feb 02, 2012 2:45 pm Post subject: SV: Re: burping the 912 |
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On 02/02/2012 01:28 PM, Sidsel & Svein Johnsen wrote:
| Quote: | 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.
| Quote: | 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.
| Quote: | 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.
| Quote: | 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?
| Quote: | 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.
| Quote: | 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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rlborger(at)mac.com
Guest
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| Posted: Thu Feb 02, 2012 3:48 pm Post subject: SV: Re: burping the 912 |
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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(at)mac.com
On Feb 2, 2012, at 4:34 PM, Frans Veldman wrote:
| Quote: |
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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grahamsingleton(at)btinte
Guest
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| Posted: Thu Feb 02, 2012 4:35 pm Post subject: SV: Re: burping the 912 |
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Frans
that is sufficiant reason for me to think that doing it before start up makes good sense. Thanks for your patience to reason it all out!
Graham
From: Frans Veldman <frans(at)privatepilots.nl>
To: europa-list(at)matronics.com
Sent: Thursday, 2 February 2012, 22:34
Subject: Re: SV: Re: burping the 912
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
[quote][b]
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