Boat Forum / Cruising / June 2004

You should take up a new hobby.  Glassblowing can be relaxing.

correct

correct

The pumped fluid doesn't vaporize. At this point, the ambient air
pressure through the vent (e.g. the PCV on an engine crankcase) is
higher than the pressure in the line evacuated by the pump. The
differental pressure (even if only a couple of psi) pushes oil up to the
impeller or positive displacement device in the pump. The differential
pressure must be greater than the weight of the oil in the line to prime
the pump (ambient air pressure > inlet pressure plus weight of pumped
fluid). The greater the difference, the faster the prime and the greater
the available suction head (distance pump can be above steady state
fluid level). I believe in most conditions that the effect of viscosity
is limited to "how long" and not "if" priming takes place. This assumes
the pump can run dry indefinitely and ignores second-order effects like
friction of the fluid against the line.

dave

You posted the information that his was true in another post... please
read your own references.  Lycoming... your source... says 30 seconds
at normal temperatures and 60 seconds with very cold oil.  This has
nothing to do with "genee/rickie,"  this is data YOU, YOURSELF, posted
as proof.

No wonder this is confusing to you.  You are confusing an air
conditioner with a lubrication system.

Fact of the matter is that most heavy oils (50W in a typical A/C
engine and many boating applications) become much too viscous to be
pumped at low temperatures and the pump simply doesn't prime....or
pump.  (If it must, it will suck air from around the shaft and/or
housing... I know of one engine manufacturer that redesigned the oil
pump to make this situation less critical.) The oil doesn't vaporize
any more than the pump, lines, or anything else in the system.  The
engine runs dry.  Hence, that is why in many cold climates light oils
and preheating are necessary.

Bear in mind, too, that an oil pump is not a suction pump... and is
not meant to evacuate the sum total of atmospheric pressure 14.696#,
29.92 in. hg., etc... this isn't a physics laboratory... it is an
engine. I might remind you that that means not 100% efficient... or
that a positive displacement pump won't transfer 100% of it's
theoretical fluid capacity... that real world situation, which in
Jaxworld is termed negative displacement.... ok, whatever.  The
greater the task assigned to a pump beyond it's designed
characteristics the lower one might expect the efficiency to drop.  In
fact, a typical lubrication pump will never attain 100% because it
just isn't sealed or designed to close enough tolerances to do
so......   thus, a pump can lose it's prime if it draws too much
air... for whatever reasons.... there are ample resources on the net
to research this... I checked.

I know of no engines that are designed to run dry for any extended
period of time... 30 seconds or 60 seconds in really cold temperatures
are frequently quoted maximum figures.

If you are fishing around for whether or not prelubing is a good
idea.... it is, but using the engine you wish to start as the motive
power for the prelubing process defeats the whole purpose.

Prelubing should be done by some other means to lubricate those areas
accessible by pressure oil.  Bear in mind, however, that MUCH of most
engines is NOT lubricated by pressure oiling and, thus, conventional
methods of prelubing does a limited job, at best, of lubricating all
points needing oil.  Think assembly lube... and why. And how you put
assembly lube on parts of the engine not touched pressure oil.

http://www.valleypowersystems.com/emd_docs/infonotice42.pdf
http://www.federal-mogul.com/cda/content/front/0,2194,2442_897063_6837,00.html

Jax, I've led you in the right direction(s) so that you might educate
yourself.  I've tired of this thread..... and all of the splinter
threads you have created to keep this general topic going. I KNOW that
other posters are tired of reading this, too.... save those that just
care to follow, seeking to know just how irrational and illogical this
"discussion" will get.  

I, for one, seeing no point in continuing, will bow out now.........

Jax,

I could not bear to dig through the rest of the thread to see if the
complete story got put together, so here it is....

If you had ever in your short life built up an engine, you would know
that priming the oil pump is an important consideration for a new engine
only.  Most all of your little engine have the oil pump above the rest
oil level (shown by the full mark on the level indictaor stick).

Any worthwhile engine is designed so it will hold enough oil in the
sitting still pump to make it seal up on the next turn of the crank
shaft.  This is not hard to do, but is is why small engine lube oil pick
up tubes often go into the pump body above the pumping element.

Any Positive Displacement pump that is nearly sealed can easily achieve
a suction side depression (negative pressure) in the 29"Hg region.

As most lube oil components vapor pressures are very low (small mm Hg @
300F), flashing the lube oil is not one of the things that design people
worry about at all.

As a lab rat (consulting, contract or direct) for several major
manufactures including some automotive over multiple decades, the "time
to lube" is a function addressed early in phase zero or phase one
developement of most engine programs.  And, thirty seconds to have oil
on the valve gear is acceptable.  Remember, this is an engine like most
of the rest of the world and it has about a pound of oil in it you can
never drain out (ask anyone that though he was breaking down a dry
engine).

The only engines that I recall immediately that had the lube oil pump in
the oil are the older BMW and some recent Mitsubishi.  Those both had
chain drive oil pumps in the pan.  Most everything else does not.  Quite
a number of in-line engines have the lube oil pump mounted externally on
the the outside of the crankcase on the other end of the distributor drive.

Matt Colie  Lifelong Waterman, Licnsed Mariner and Pathological Sailor
Engine Development and Durability Dynomometer Laboratory Supervisor
Engine Component Development Engineer Supervisor  and
Participant in four "clean paper" engine programs  also
Chief Engineer of Steam or Motor Vessels - Any Horsepower