REFLECTOR: Blow by

Mark Magee edjonesbrady at gmail.com
Sat Mar 10 14:07:04 CST 2012


Hi Lou,
I have heard so many discussions on what level the oil is supposed to be in
recip engines, that is in the crankcase, and on the stick (which can be a
study unto itself). As I understand it if you run possibly more than the
optimum amount in your crankcase, irrespective of blow-by, you will blow
some engine oil out the crankcase breather everytime you encounter a little
negative G on the airplane. Unless you have an inverted engine oil system
(dry sump) then negative G such as even moderate turbulence will throw
engine oil to the top of the engine and your blow by (all aircraft recip
engines have -some- blowby) will blow relatively large slugs of engine oil
out the breather line. Unless you are recovering this via air/oil separator
it is gone for good. Even with an air/oil separator in heavy turbulence
your air/oil separator won't keep up with the volume of large slugs of pure
oil being vented overboard. Air/oil separators are only designed to make
oil vapor precipitate back into liquid form for recovery. So if you run too
much oil, whatever level that is, then the physical level will be -closer-
to the crankcase breather located at the top of the engine, and even less
negative G will force the oil up to the vent, and the blow-by will push it
right out.
So, it is important to find that level where you engine likes to run
without blowing excessive lube oil overboard, as even a new tight engine
will throw lube oil overboard in negative G conditions such as moderate to
better turbulence.
It seems to border on religion sometimes the discussions I listen to on oil
levels in LYC/CONT engines. I think the confusion can happen on the various
derivatives of the numerous 4, 6 and even 8 cylinder engines, accessory
cases and even different dipsticks.
We never want to push too hard on the stick forward for too long. Pilot
induced negative G as well will dump lube oil. Extended negative G's will
starve your oil pump and seize your non-aerobatic engine. I have never
liked the LYC/CONT method of crankcase breathing, but it does work. I have
an Air/Wolf air/oil separator in a box and after reading the instructions
to install I've moved it down the line in priorities.
Take a look at the inverted oil systems sold and you'll get an idea of the
potential to dump engine lube oil overboard.
Blow-by is only part of the problem.

On Sat, Mar 10, 2012 at 11:01 AM, Lou Stedman <stedmanlou at roadrunner.com>wrote:

>   Last week we were discussing the amount of oil different engines burn
> in a given time. I mentioned that I do not fill up my crankcase because the
> first quart will blow out within an hour if I do that . However if I fill
> to six quarts I don’t go through another quart for about four to five
> hours. After that I got a note from someone that I should not do as I
> described as it is a indication that there is a blow by problem with my
> engine (Lyc IO-360). If this is true, what is the cause of blow by?
>
> Lou Stedman
> Velocity SEFG
> N7044Q
> Olean, NY
>
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