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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