Motor Oil 101

[IraB]

Not saying that the film left on parts when the engine is not running prove much lube on start up but I am convinced it does provide some lube for critical moments.
Again, anyone ever assemble an engine dry?
If not, why not?

[B.D.F.]

I do not think that is quite right.

It is the spinning shaft that causes any hydrodynamic bearing to work, not oil pressure. In fact, oil pressure is not needed at all; a lot of machine tools use heavily loaded spindles that are lubricated simply by gravity feeding oil into the bearing. As long as the space between the shaft and the housing is full of oil, dynamic pressure takes care of the rest.

As to the part about parts actually touching when not moving, that can be correct. But modern oils have an extreme pressure package (additive) in them to prevent seizing even when parts actually touch each other. This also happens all the time between components such as gear teeth, even when the engine is actually running. Typical E.P. packages are ZDDP, molydisulfide and graphite although the last one is generally not used in internal combustion engines.

Brian

At rest there is metal-to-metal contact.
That contact continues until oil pressure can hydraulically
separate the components.
Hence the premise: "highest wear at start-up". 

The thin film that "clings" is insufficient to protect
against this wear.

[works4me]

BDF, you eloquently elaborated where my intention was omitted.
I meant to suggest contact would continue until the oil pump
could flow enough oil to provide lubrication.
Thanks for the clarification.

[timsatx]

I guess my question would be that if you do not think oil pressure is important then why not drain the oil from the engine and drive it?

[IraB]

I guess my question would be that if you do not think oil pressure is important then why not drain the oil from the engine and drive it?

Because that is where the vast majority of lubrication comes from. 

My only point is that I believe the oil film left on parts does also provide critical lubrication before oil pressure is established at start up.  The art of all this is in finding an oil that allows rapid build up of pressure on start up AND provides an adequate film for lubrication for the brief moment before pressure is established.

Anyone ever know an engine builder who assembled engines bone dry?
Me either.

[timsatx]

Because that is where the vast majority of lubrication comes from. 

My point exactly. The engine doesn't lay in oil, it requires oil pressure to lubricate it. 

[B.D.F.]

Irrelevant and illogical question. Same as asking if you like salt? If you say yes, I ask why you do not eat five pounds per day?

Loaded questions are different and convey meaning in what appears to be a question but is really an accusation: "Have you stopped beating your wife yet?" would be such a question.

Now, want to talk about how oil works?

 

I guess my question would be that if you do not think oil pressure is important then why not drain the oil from the engine and drive it?

[B.D.F.]

It does not, it only requires a film of oil to be present and that the rotating units in question do not overheat.

Lots of engines have hydro-dynamically lubricated bearings.... without a pressurized oiling system.

A pressurized oil system based engine requires pressurized only in that it has no alternative means to keep oil in the bearings. If such an engine were altered slightly, it would run perfectly well with nothing but slash lubrication and the occasional squirt (easy boys!) of oil where needed, such as at the bottom of the piston skirts and the bottom of the cylinder(s).

The key point here is that it is not the pressure provided by the oiling system that causes a hydro-dynamically lubricated bearing to work. Just do the pressure over area vs oil pressure and you will see that 40 PSI, 60 PSI is not sufficient to support either a main bearing or a rod journal bearing in any engine in any vehicle in any driveway. Speaking of mainstream America here- no telling what may be in the Australian outback for example :-)

Brian

My point exactly. The engine doesn't lay in oil, it requires oil pressure to lubricate it.

[Steve in Sunny Fla]

It does not, it only requires a film of oil to be present and that the rotating units in question do not overheat.

Lots of engines have hydro-dynamically lubricated bearings.... without a pressurized oiling system.

A pressurized oil system based engine requires pressurized only in that it has no alternative means to keep oil in the bearings. If such an engine were altered slightly, it would run perfectly well with nothing but slash lubrication and the occasional squirt (easy boys!) of oil where needed, such as at the bottom of the piston skirts and the bottom of the cylinder(s).

The key point here is that it is not the pressure provided by the oiling system that causes a hydro-dynamically lubricated bearing to work. Just do the pressure over area vs oil pressure and you will see that 40 PSI, 60 PSI is not sufficient to support either a main bearing or a rod journal bearing in any engine in any vehicle in any driveway. Speaking of mainstream America here- no telling what may be in the Australian outback for example :-)

Brian

where do you get this stuff? It's not just the oil pressure supplied by the pump , but the clearance at the bearing which restricts the oils flow off the bearing and provides the film that separates the parts. This is why proper clearance is so critical. open up a clearance .002", pressure drops, oil film flows off and doesn't separate parts, parts soon go boom. If you've noticed, crank clearances have tightened up since the advent of low viscocity oils.  Shell style bearing rely on zero parts contact, whereas roller bearings have constant contact. Shell bearings indeed do rely on oil pressure to separate the parts, case closed. Steve

[jworth]

Okay then.  Explain how 2 strokes work without oil pressure.

[Gitbox]

 

[SteveJ.]

Okay then.  Explain how 2 strokes work without oil pressure.

Ummm....roller bearings.

[B.D.F.]

Steve, I have absolutely no idea what you are addressing here. I said that oil pressure is not necessary for a hydro- dynamic bearing. You respond with 'where do you get this stuff? It's not just the oil pressure supplied by the pump.... '  Right, I think I just said that.

Where do I get it? Oh, various places. Bernoulli's principle works modestly well here. Shear principles work fantastic. The shear gradient between the oil that <must> be stopped at the crankshaft bearing for example, will pick up speed in an exponential curve until it reaches the highest speed that it travels around the bearing, and then begin an exponential decline in speed until it reaches the bearing shell, where again the velocity is zero. It is these two velocity gradients that cause a pressure rise as the shaft is moved from the center of the bearing (away from the force applied to it). Got a lot of it in tribology courses.

Oil pumped under pressure is not needed to make a hydro dynamic bearing function. It is not needed to make it function any better either.

Take the pressure of the oil, multiplied by the area of 1/2 of the bearing journal and multiply them. Then tell me how 40 PSI of oil pressure across a couple of square inches of area, yielding 80 pounds of force is resisting four tons (8,000 lbs. ) of downward pressure in a conventional 350 cubic inch V-8 engine.....

Just curious, but what do you think allows small engine bearings to work without oil pumps? Briggs and Stratton, along with many, many other manufacturers have been making them and we have been using them for decades. ??

Opps, I didn't see where you had closed the case, sorry. 

Brian

where do you get this stuff? It's not just the oil pressure supplied by the pump , but the clearance at the bearing which restricts the oils flow off the bearing and provides the film that separates the parts. This is why proper clearance is so critical. open up a clearance .002", pressure drops, oil film flows off and doesn't separate parts, parts soon go boom. If you've noticed, crank clearances have tightened up since the advent of low viscocity oils.  Shell style bearing rely on zero parts contact, whereas roller bearings have constant contact. Shell bearings indeed do rely on oil pressure to separate the parts, case closed. Steve

[B.D.F.]

Not to anyone in particular but you know, science really does explain quite well how all of this works. Really easy to find this information printed in texts all over the place. There is really no need to reinvent the wheel or tribology all over again. I would suggest we make use of what resources are available before venturing out on our own to 'explain' how things work.

"A little learning is a dangerous thing;
drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain,
and drinking largely sobers us again."
-Alexander Pope

Brian

[T Cro ®]

Okay then.  Explain how 2 strokes work without oil pressure.

Roller Bearings support the crankshaft in small 2-Smokes and the oil in the gas lubes the piston walls; there is more than enough oil splashing around in the crankcase to keep the roller bearings supplied with the small amount of oil that they need...

[Steve in Sunny Fla]

BDF, I dunno, maybe you're an engineer and I'm a lowly engine builder, but I get the feeling that you're trying to "baffle by bullsh*t"

Here's some info on how oil separates parts on what I'm calling "shell bearings" http://en.wikipedia.org/wiki/Fluid_bearing

If your theories are sound and correct, all modern engine automotive designs  are incorrectly designed with shell bearings, because according to you, oil pressure is apparently not needed to separate the parts, but merely provide lubrication. If you are correct, then why are modern engines designed with pressure pumps at all? It's well known that there are power losses due to pumping loss, and any effort to achieve best efficiency would want to rid pumping loss all together. Yet we still have presurized oil systems and shell bearings as the common method of design. what are all these engine designers missing? Steve

[B.D.F.]

First off Steve, these are not 'my' theories, anymore than gravity or relativity is 'my' theory. That said, I do subscribe to them and try my best to follow along.

I have no idea how engine building would school one in the ways of fluid dynamics anymore than carrying rocks for a living would give one a solid understanding of physics. But in any event, you are the one who brought up what each of us may do or may be; I am happy to leave that alone and talk about the subject at hand. Frankly, I do not care what you do or have done unless it has some bearing (moderate pun intended) on the operation of.... well, bearings. And I do not mean you have tightened the main bearing caps of a gazillion engines down correctly; looking at a battery, even for a very long time or perhaps many, many batteries, will not help one understand how or why they produce electricity.

I can understand your comment about the bafflement. Complex things often seem as if they could be simplified; sometimes that is true, sometimes not. In this particular case, you seem to want the oil pump to be providing the mechanism by which plain bearings work; that would surely be the simple answer but unfortunately it is not correct. Rotational speed of the shaft, shear forces and the velocity gradient w/in the oil layer are the sources of the mechanism by which plain journal bearings work.

That is a fine link you provided and I am in agreement that it well illustrates how fluid dynamics work within a plain journal bearing. From that page, in addition to the key references to the Reynolds number, these bullet points alone describe, as simply as I believe can be done, how a fluid filled bearing works to resist the forces trying to force the shaft sideways w/in the bearing housing:

Begin sited page quote:

Hydrodynamic condition – Load:
Increase in load decreases minimum film thickness
Also increases pressure within the film mass to provide a counteracting force
Pressure acts in all directions, hence it tends to squeeze the oil out of the ends of the bearing
Increase in pressure increases fluid viscosity

End sited page quote:

Now notice that an increase in load increases pressure withing the film mass to provide a counteracting force, as well as the sentence 'increase pressure increases fluid viscosity'. That is the entire key is to why plain journal wet bearings work. Notice the lack of reference to any externally applied pressure; it simply does not apply to the mechanics at work here.

Brian

BDF, I dunno, maybe you're an engineer and I'm a lowly engine builder, but I get the feeling that you're trying to "baffle by bullsh*t"

Here's some info on how oil separates parts on what I'm calling "shell bearings" http://en.wikipedia.org/wiki/Fluid_bearing

If your theories are sound and correct, all modern engine automotive designs  are incorrectly designed with shell bearings, because according to you, oil pressure is apparently not needed to separate the parts, but merely provide lubrication. If you are correct, then why are modern engines designed with pressure pumps at all? It's well known that there are power losses due to pumping loss, and any effort to achieve best efficiency would want to rid pumping loss all together. Yet we still have presurized oil systems and shell bearings as the common method of design. what are all these engine designers missing? Steve

[IraB]

Problem here is that everyone is correct.

Properly designed crank bearings lubricated by oil under pressure are far superior and will last much longer than similar ones that are submerged in oil or splash lubed.  That said, I do have a heck of a lot of hours on my Briggs powered push mower.

Steve is spot on that  high revving engines such as those we ride really call for oil pressure in the bearings.  Because these motors have zero oil pressure on cranking and start up, I also bet he lubes the crap out of them when he assembles engines because that provides some lubrication at a critical moment too. 

[Steve in Sunny Fla]

I feel like this is another conversation with a politician. Why do I bother?

So for a clarification to the onlookers, let me respond in this manner.

Your original contention (or at least how I understood your point) was that forced (pumped) oil pressure was unnecessary for separating the rotating parts of an engine. My point is that forced oil is necessary in this instance, if using shell style (not ball or needle) style bearings.

  I also pointed out the necessity for controlling clearances, oil film flow speed, etc.

  The elements of how the film creates a wedge are all part and parcel of the function of the rotation of the parts. How much oil reaches the bearing is set by viscosity, oil system design, clearances in other parts of the engine, etc. What happens on one single shell style bearing - lets say a rod bearing - is going to be a function of the oil pressure, viscosity, clearance, rotational speed, width of bearing, side clearances, etc.

  To think that this bearing system doesn't rely on forced oil as the first part of the design would be ludicrous. Without forced oil, this system could not function, well, not long enough to provide any service.

  Let's also look at heat build-up on the bearing and shaft. Forced oil, flowing off the side clearances, carries away the heat. This is again another place where viscosity plays a key role. If the film is to thin, it flows off the bearing sufficiently, but the wedge thickness can be crushed by the forces applied at a given point in the rotation - think "power stroke" and you'll see what I mean. If the oil is to thick, it provided better protection in terms of separating the parts, but the slow flow off the surface results in excessive heat build up in the parts.

  Crankshafts are drilled so that the bearing gets a fresh shot of oil from the system just as the highest loads are being applied through the rod. If pumped oil was really unnecessary, then why insure the highest amount of fresh, cool (relative to the parts being oiled) , pressurized oil is applied as the power stroke occurs, and that the top shell of a rod gets this oil during said power stroke?

  One of the failures that can occur to engine rod bearings occurs when pre ignition takes place. Basically the piston / rod is slammed down onto the crank while the crank is still reaching or at TDC. The result isn't to rotate the crank, but hammer the rod bearing top shell into the crank. This is one place where the hydrostatic fluid wedge often gets overwhelmed and is pushed out. Bearing hits crank, and permanent damage is the result. Not just from heat but from deformation. Have you ever seen a shell bearing lose it's crush?
 This happens because of the bearing being hammered into the crank, from oil film loss, not heat.

  Of course, all my ramblings are moot if you take the politician's approach, and try to fall back on "the oil film strength is provided by rotational force, viscosity, etc" sure, true, to an extent - but the big problem with that theory is that if forced oil had not been introduced to the journal / bearing to start the process then the film strength from rotation, etc could never develop regardless, and therefore engines with shell style bearings rely on forced oil, period. Steve
 

[Steve in Sunny Fla]

BDF, I have an idea- rather than operate in acedemia, let's work in the real world.

I'll use my engine in my c-10, you use yours in your c-14.

I'll use my understanding of oiling, and leave my engine as I built it. Seems good, it's lasted about 40k of beatings so far.

Now for your c-14, let's see if forced oil is unnecessary. It will be simple to do. simply connect a hose from a tap into the oil system, and route said hose back into the oil pan. perhaps include a valve, so we can set the oil pressure in the engine to 1# pressure, which will insure lubrication to all parts. Now go for a ride. 

 Since we'll probably both be at the National this year, I suggest we do this experiment there. Heck we'll go ride together. Me, with 40# oil pressure, you, with 1#. Let the real world results prove who's correct.  I'm looking forward to it.

 Oh, before I forget... bring a trailer so you can get your bike home.    Steve