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


JAXAshby wrote:

genee/rickie claim that oil pumps have to spin for some time to draw a prime.

I am trying to visualize how an oil pump draws a prime, particularly on oil at
say 15 degrees. In order to draw a prime the pump would have to evacuate the
air above the oil and below the pump, the difference in air pressure on the
evacuated side vs ambient air pressure leaves no more than a few pounds of
pressure total (can't be more than 14.7# total, for that is atmospheric
pressure). Then the oil would have to vaporize and then be drawn into the
pump, then to be compressed back to liquid to then be pumped to the bearings
needing pressure oil.

At even normal room temperature engine starts -- let alone cold weather engine
starts -- it would seem an oil pump requiring a prime to work might take
several minutes engine run time to begin to pump even small amounts of oil.
Anyone know how the engine designers allow for this and still make the engines
last more than a minute or so? Anyone know of which engine designs have oil
pumps that have to pull oil up before pressurizing it? How do they get the oil
volume on such oil pumps?