Thought I might modify a few Stribeck curves to demonstrate what happens to engines when running cold, with cold oil.
For reference, hydrodynamic lubrication is where a lubricant, the load, and the relative motion produce sufficient separation forces to keep the parts away from each other...it's the definition of zero wear, and when established, parts, and their asperities cannot touch.
The mixed regime is high friction, very high wear, with no fully developed hydrodynamic lubrication...the development of Anti Wear additives produces the lower line, where tribofilms protect surfaces and reduce wear, by making glassy layers of substrate, and additive components...a wee bit of parent metal is consumed, but overall wear is reduced to the point that your cams last (virtually forever)
Here's a standard curve, with an Anti Wear additive line imposed...it's representative of a running, fully warmed engine through it's load profile.
Points of notice...
Bearings are hydrodynamic. Low speeds, high loads are to the left of the range, high speeds, light loads are to the right...more to the right, more parts separation, but more friction and oil induced drag. Manufacturers are moving more to the left, into mixed lubrication, as evidence by what they are saying, what they are requesting for GF6 certification, and moving towards thinner lubricants at operating temperatures.
Cams and followers...completely the opposite, generally at operating temperature operate in mixed regimes, and are demonstrated to have very very long life, testimony to the additives and the tribofilms.
Pistons and rings. Hydrodynamic and EHD...the ends of the strokes are mixed, the middle hydrodynamic due to surface speeds. More load moves the curve left, more speed moves it right, and more viscosity moves it right.
Next pic has numerical overlays of where the ranges would be with cold oil after start-up at 0C. Given that the Stribeck curve posted is not representative of an actual design, I've taken the liberty of Using M1 5W30 ESP at operating temp (100C) as the baseline, and DEMONSTRATING where the curve goes at other temps.
at 0C, the 5W30 is 52 times the viscosity of the lubricant at 100C, and what it does, is move EVERYTHING to the right...massively...can't fit it on the scale (would require logarithmic scale)
Bearings become more hydrodynamic, but as hydrodynamic is zero contact, can't be any less wear than zero.
Camshafts are pushed to the right, into EHD, and well into full hydrodynamic for part of the range.
Pistons and rings again are moved into EHD, and full hydrodynamic.
Clearly, there's a huge increase in friction, but it's not at the expense of wear, parts not touching aren't wearing.
It's the bit in the middle, the warm-up that's the problem. The oil is thinning with temperature and the anti wear additives haven't kicked in as yet.
This is the point of the Sequence IVA wear test. The engine (Nissan KA24E) is operated between an idle speed and 1500RPM, with the oil temperature controlled to 49-59C. There is plenty of oil volume, just at the wrong temperature. The choice of temperature and engine speed ensures that the situation is the "perfect storm" for camshaft wear, in mixed mode, but the Anti-Wear additives not functional...it still takes 100 hours to do the test...
What does the Stribeck Curve look like at that operating point ?
Cam and followers are clearly back into the mixed range, and with AW additives not replacing the tribofilms at the rate that they are removed...
Bearings (still not wearing, they are hydrodynamic).
Pistons and rings, still have a lot more hydrodynamic than at full temperature, but again, the additives aren't as strong.
For reference, hydrodynamic lubrication is where a lubricant, the load, and the relative motion produce sufficient separation forces to keep the parts away from each other...it's the definition of zero wear, and when established, parts, and their asperities cannot touch.
The mixed regime is high friction, very high wear, with no fully developed hydrodynamic lubrication...the development of Anti Wear additives produces the lower line, where tribofilms protect surfaces and reduce wear, by making glassy layers of substrate, and additive components...a wee bit of parent metal is consumed, but overall wear is reduced to the point that your cams last (virtually forever)
Here's a standard curve, with an Anti Wear additive line imposed...it's representative of a running, fully warmed engine through it's load profile.
Points of notice...
Bearings are hydrodynamic. Low speeds, high loads are to the left of the range, high speeds, light loads are to the right...more to the right, more parts separation, but more friction and oil induced drag. Manufacturers are moving more to the left, into mixed lubrication, as evidence by what they are saying, what they are requesting for GF6 certification, and moving towards thinner lubricants at operating temperatures.
Cams and followers...completely the opposite, generally at operating temperature operate in mixed regimes, and are demonstrated to have very very long life, testimony to the additives and the tribofilms.
Pistons and rings. Hydrodynamic and EHD...the ends of the strokes are mixed, the middle hydrodynamic due to surface speeds. More load moves the curve left, more speed moves it right, and more viscosity moves it right.
Next pic has numerical overlays of where the ranges would be with cold oil after start-up at 0C. Given that the Stribeck curve posted is not representative of an actual design, I've taken the liberty of Using M1 5W30 ESP at operating temp (100C) as the baseline, and DEMONSTRATING where the curve goes at other temps.
at 0C, the 5W30 is 52 times the viscosity of the lubricant at 100C, and what it does, is move EVERYTHING to the right...massively...can't fit it on the scale (would require logarithmic scale)
Bearings become more hydrodynamic, but as hydrodynamic is zero contact, can't be any less wear than zero.
Camshafts are pushed to the right, into EHD, and well into full hydrodynamic for part of the range.
Pistons and rings again are moved into EHD, and full hydrodynamic.
Clearly, there's a huge increase in friction, but it's not at the expense of wear, parts not touching aren't wearing.
It's the bit in the middle, the warm-up that's the problem. The oil is thinning with temperature and the anti wear additives haven't kicked in as yet.
This is the point of the Sequence IVA wear test. The engine (Nissan KA24E) is operated between an idle speed and 1500RPM, with the oil temperature controlled to 49-59C. There is plenty of oil volume, just at the wrong temperature. The choice of temperature and engine speed ensures that the situation is the "perfect storm" for camshaft wear, in mixed mode, but the Anti-Wear additives not functional...it still takes 100 hours to do the test...
What does the Stribeck Curve look like at that operating point ?
Cam and followers are clearly back into the mixed range, and with AW additives not replacing the tribofilms at the rate that they are removed...
Bearings (still not wearing, they are hydrodynamic).
Pistons and rings, still have a lot more hydrodynamic than at full temperature, but again, the additives aren't as strong.