dave, nice explanation of how an oil pump would fail to provide any oil at all,
let alone pressure oil, to even the first bearing in line upon engine startup.

think of the grinding, clashing, rubbing, clattering you would hear on the
bearing for a minute or two or five.

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,

correct

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).

correct

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.

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

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That is one way of doing it, and a usually excellent way to boot.

I once got so worried about start-up oil pressure on an engine I'd just
built that I made up a small accumulator with a solinoid valve run from the
ignition. The idea was to isolate about 100cc of oil under pressure when the
engine was turned off and re-introduce that back into the system when the
ignition was turned on next time.

Supposedly 40% of engine wear occures when the engine is cold but that
wasn't why I did it, the engine was required to go on full load immediately
on startup and I hoped the instant oil pressure would protect it better.

It did produce almost instant start-up pressure but I've got no idea how
much it would affected engine life, that engine has only done 200 Hrs in the
last 10 years or so.

Mark..


"Dave Skolnick" wrote in message
news:VIDDc.1384$fd3.81@lakeread04...
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,

correct

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).

correct

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.

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

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JAXAshby wrote:
dave, nice explanation of how an oil pump would fail to provide any oil at all,
let alone pressure oil, to even the first bearing in line upon engine startup.

think of the grinding, clashing, rubbing, clattering you would hear on the
bearing for a minute or two or five.

Thus the old practice of chucking a rod with appropriate gear on the end
in a drill motor, pulling the distributor, and spinning up the oil pump
on older engines after a rebuild.

A bit out of my field, but I believe most of the noise I have
encountered with engines at startup (at least those that haven't been
sitting too long) is top end noise (vice more expensive bottom end
issues) and usually associated with hydraulic lifters pumping up vice
bearing issues. Recall that there is a fair amount of oil retention in
the bearings for extended periods of time, and the separation of the
journal from the bearing on oil during operation is based on pressure
developed by the rotating journal, not the oil pump. I believe the pump
sustains the quantity of oil and therefore only indirectly the thickness
of the oil wedge supporting the journal. My college machine design notes
and textbooks are in the basement and I don't really want to go dig them
out, but I believe my recollection is accurate.

dave

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gene, you klutz. Look what you said:

too viscous to be
pumped at low temperatures and the pump simply doesn't prime

and

an oil pump is not a suction pump...

so which is it, gene? an oil pump needs a prime, or an oil pump is not a
suction pump.

Or ...

.... you don't have a clew what the word "prime" means?

gene, have you any idea what kinds of bearins **require** pressure oil?

it seems not.

nah, gene, **you** said an engine should not be prelubed.

*I* said if you don't, it can take upwards of 30 to 60 seconds for cold oil
under pressure to reach the last of the bearings requiring pressure oil.

That is a fact, gene, and if your ears were any good you would know that, or if
you understood what goes on in a engine you would know why they happens.

gene, you are a hammer mechanic.

Thus the old practice of chucking a rod with appropriate gear on the end
in a drill motor, pulling the distributor, and spinning up the oil pump
on older engines after a rebuild.

yup.

and also the reason race car engines and race motorcyle engines are spun up
with the ignition off to achieve pressure oil to all the bearings needing
pressure oil before lighting the fire.

Also the reason aircraft owners who care about their engines run the prop
through a number of times before turning the mags

Also the reason the US Navy has prelube precedures for piston engines.

I believe most of the noise I have
encountered with engines at startup ... noise ... associated with hydraulic
lifters pumping up

you can hear that alright, but listen to the other clattering going on. It is
pretty serious noise.

If one has a hard time recognizing the sounds of pressure oil bearings working
without pressure oil to them, start your engine right off, listening to the
sounds coming out, taking care to hear the change in sounds over the next ten
to sixty seconds (much quieter), let the engine run a couple minutes and shut
off. Then restart and notice the difference in sounds.

JAX,
Do you have a car? How do you "prelube" it? Put a socket and rachet on
the crank pully and pull it through several times before starting?
krj

JAXAshby wrote:
nah, gene, **you** said an engine should not be prelubed.

*I* said if you don't, it can take upwards of 30 to 60 seconds for cold oil
under pressure to reach the last of the bearings requiring pressure oil.

That is a fact, gene, and if your ears were any good you would know that, or if
you understood what goes on in a engine you would know why they happens.

gene, you are a hammer mechanic.

JAXAshby wrote:
If one has a hard time recognizing the sounds of pressure oil bearings working
without pressure oil to them, start your engine right off, listening to the
sounds coming out, taking care to hear the change in sounds over the next ten
to sixty seconds (much quieter), let the engine run a couple minutes and shut
off. Then restart and notice the difference in sounds.

Just remember that the pressure from the pump is only to supply oil to
the bearings (and maintain sufficient flow for cooling). The film the
journal "flys" on is extremely high pressure that is dynamically
generated by the spinning journal. Regardless of oil pressure from the
pump, there is metal-to-metal contact at start-up until the oil wedge
establishes itself.

All good things are on the web, and easier to find then my college notes:
http://www.roymech.co.uk/Useful_Tabl...ubrication.htm

I've never seen a hydrostaticly lubricated machine outside a lab (note a
total of four sentances in this discussion). For practical purposes,
only the hydrodynamic section is applicable; see the table in the link.
Note 1 MPa ~= 145 psi, so oil film pressure in most bearings we deal
with will run from 750 - 3000 psi. Obviously not pump pressure.

The oil must be there to develop the film, but the film thickness is
thousandths of an inch. There is going to be sufficient oil unless the
engine has been rebuilt or has been sitting a long, long time. The oil
pump maintains a flow of oil for cooling and replacement of displaced
oil. The start-up noises arise because it takes a finite period for the
oil wedge to develop.

dave

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