Pressure is still more important than flow in a journal bearing where separation of the parts is dependent upon a hydrodynamic "wedge" of oil. Very little heat should be generated there if the parts are kept separated. Oil picks up most of its heat circulating in the head area and from the cylinder walls. The last generation BMW M3 needs 10W-60 because of the rod bearing loads. Im not saying that thicker is always better, just that thinner is not always better either.
oil thickness and bearing clearances
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Jimbo, this is one of the parts I don't understand...the hydrodynamic wedge. Supposedly, the higher the pressure an oil is subjected to, the thicker the oil behaves. So, how thick is thick enough? I know it must depend upon the application. But, since this is all a dynamic condition, perhaps increased flow to the area by using a thinner oil (to a point) would make up for shear (pun intended) thickness. Or would it? I certainly don't claim to know. I certainly agree thinner is not always better, just like I agree thicker is not always better.
Dynamic pressure is inversely proportionate to volume. In other words when a fluid is put into motion the dynamic pressure is inversely proportionate to dynamic volume or flow. The more fluid forced through an opening or line, the faster a fluid will flow~velocity. The faster a fluid flows, the more friction incurred, and the flow~velocity is reduced in a proportionate manner.
This is one benefit of synthetics IMO. Synthetics favorably change the inverse proportion law with respect to pressure, volume and velocity.
So we can reduce friction by using a thicker oil which flows slower?
Hydrodynamic pressure due to a shafts rotational speed and dynamic pressure as related to flow are two different situations and not related.
Paul
The inverse proportion law dictates behavior of water, but It may well apply to all fluids.
It would make sense that the less turbulence a fluid incurs while moving, the less drag will be imposed on the fluid. Water velocity is calculated in feet per second. Slower moving water will always have less friction loss than water moving at a higher velocity for a given application which is desirable from a hydraulic perspective.
If you can reduce oil turbulence within an engine, the result should be that the oil will move through the engine more efficiently and therefore reduce internal heat and lubricate better. This would be done by polishing and optimizing all internal pathways, determining the correct oil volume, velocity, and pressure from the pump for the appropriate viscosity. This is an advantage of true synthetic I would think~ they move more uniformly through the engine.
Even if we could theoretically reduce the turbulence within an engine, most of it is beyond our control. All that we have to work with is oil viscosity and type. To answer your question, no I don't believe that a thicker oil will have less drag or friction loss than a thinner oil moving at the same rate. Again, that's where synthetics come in, it flows in a more uniform manner whereby you can use a thicker oil and approximate the same drag of a thinner dino IMO.
Because it is not economically feasible for engine manufacturers to polish and tweak all oil pathways in an engine, the oil companies have made their oils more slippery to reduce drag and flow more efficiently.
So the correct viscosity is one that is thin enough to get from the pan to the oil pump. Then thick enough to keep the parts separated ,but not too thick as to take too much power to turn the pump? Depending on the ambient temperatures and operating conditions.
That's about it Steve. Also consider the rate of flowback from the top end down as well as the distance the pump needs to push the oil and you get an ideas of how much viscosity increases pumping effort and if the flowbaxk at cool temperatures and higher RPMs is adequate.
Some(most) engine designs seems virtually unafected by a wide range of viscosity while others are extra sensitive due to these dynamics.
In mild climes I have a hard time noticing any difference at all in my test mule/science project.
Some(most) engine designs seems virtually unafected by a wide range of viscosity while others are extra sensitive due to these dynamics.
In mild climes I have a hard time noticing any difference at all in my test mule/science project.
It has 185K miles on it now and has a hot oil pressure of around 40 psi at 1500 - 4000 rpm with a 12.2 Cst 0w-30 oil. This is a little lower than it used to be.
Jim, I would use a 10-30 or 40 oil, but this will make precious little difference, IMO. Having an oil pressure gage is good(I have no gage) and a oil temperature gage is even better..
Do you have the manufacturers specs ??
Jim, I would use a 10-30 or 40 oil, but this will make precious little difference, IMO. Having an oil pressure gage is good(I have no gage) and a oil temperature gage is even better..
Do you have the manufacturers specs ??
I mis-spoke myself two posts back. I meant hydrodynamic lubrication, NOT hydrostatic. Too bad the initiator of a posting cannot go back at any time and correct errors on this board.
Regards, Gary in Sandy Eggo
Regards, Gary in Sandy Eggo
Plain bearings (connecting rod) are protected from wear by a cushion of oil forming between the moving surfaces (the crankshaft journal rotating inside the connecting rod big end). The higher the rpm, the thicker (wider) this cushion will be (up to the maximum clearance of the bearing (approximately 001"-003"). A lower revving higher torque engine will require a higher viscosity oil to help build this oil cushion. A higher revving engine can achieve the same cushion thickness with a lower viscosity oil, and since power with a higher revving engine is more a product of rpm, the bearing loads at lower rpm's are not as significant as in a low speed, high torque engine. Hence, a lower viscosity oil will suffice in a higher revving engines plain bearings.
Connecting rod bearings are full of oil at startup. This is oil that was present at shutdown. This oil is sufficient to protect the rod bearings at no-load idle rpm until pumped oil starts being delivered (which happens in seconds regardless of viscosity). Oil pump pressure at operating temperatures beyond 5-10 lbs is of no consequence to connecting rod bearings. They achieve their protection from hydrostatic (rotationally generated) oil pressure between the mating surfaces.
Connecting rod clearances have always been designed to work best at operating temperatures with oil viscosity in the SAE-30 range.
Cylinder walls are lubricated by excess oil thrown off the crankshaft journal bearings. This lubrication IS directly proportional to oil pressure and INDIRECTLY proportional to oil viscosity. A higher viscosity oil will throw less oil through the journal bearings to the cylinder walls.
A higher viscosity oil will internally generate more heat as it shears than will a lower viscosity oil.
Cam lobes do not enjoy the same sorta closure as a connecting rod journal, consequently, the hydrostatic function is not as helpful to lubrication here. Cams LOVE high viscosity oil. The higher the better.
Roller and ball bearings ARE the "lubrication" between the shaft and the housing it rides in. Oil is only needed to lubricate the relative (sliding) movement between the balls (or rollers) and the cage that retains the balls (or rollers). If there is no cage, but just a lot of rollers, it's the sliding movement between the rollers (or balls) that requires oil lubrication. Since loads between the rollers/balls is slight, viscosity is not as important here.
Regards, Gary in Sandy Eggo
Connecting rod bearings are full of oil at startup. This is oil that was present at shutdown. This oil is sufficient to protect the rod bearings at no-load idle rpm until pumped oil starts being delivered (which happens in seconds regardless of viscosity). Oil pump pressure at operating temperatures beyond 5-10 lbs is of no consequence to connecting rod bearings. They achieve their protection from hydrostatic (rotationally generated) oil pressure between the mating surfaces.
Connecting rod clearances have always been designed to work best at operating temperatures with oil viscosity in the SAE-30 range.
Cylinder walls are lubricated by excess oil thrown off the crankshaft journal bearings. This lubrication IS directly proportional to oil pressure and INDIRECTLY proportional to oil viscosity. A higher viscosity oil will throw less oil through the journal bearings to the cylinder walls.
A higher viscosity oil will internally generate more heat as it shears than will a lower viscosity oil.
Cam lobes do not enjoy the same sorta closure as a connecting rod journal, consequently, the hydrostatic function is not as helpful to lubrication here. Cams LOVE high viscosity oil. The higher the better.
Roller and ball bearings ARE the "lubrication" between the shaft and the housing it rides in. Oil is only needed to lubricate the relative (sliding) movement between the balls (or rollers) and the cage that retains the balls (or rollers). If there is no cage, but just a lot of rollers, it's the sliding movement between the rollers (or balls) that requires oil lubrication. Since loads between the rollers/balls is slight, viscosity is not as important here.
Regards, Gary in Sandy Eggo
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