| M.D. Holloway |
10-15-2008 11:52 AM |
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SAE Technical Paper Series. Film-Forming Properties of Zinc-Based and Ashless Antiwear Additives. 2000-01-2030
1. It is increasingly important to understand the formation of films with the reduction of phosphorus in oils while maintaining engine durability.
2. Paper to describe technique for measuring both anti-wear film thickness and distribution.
3. Above technique to compare film formation of conventional and ash-less oils.
4. The reason for the reduction of phosphorus (ash) in oils is that it damages emissions devices (catalytic converter).
5. To address protection of after-treatment devices is to find ash-less anti-wear additives that can replace the ZDDP (zinc dithiophosphate) used in conventional motor oils.
6. ZDDP serves both as an oxidation inhibitor and anti-wear additive.
7. ZDDP films form a thin lower sulfur-laden inorganic substrate (can be iron sulfide) with a top organic layer of short-chain polyphosphate/phosphate material.
8. Phosphate layer hardness proportional to contact pressure applied during the formation of anti-wear film on surface.
9. Primary ZDDP forms solid films 10 to 20 nm thick at temperatures above 100C (212F). Secondary films form at lower temperatures.
10. ZDDP films form unevenly up to 1000 nm thick overall, with thin sulfide layer 10-20 nm thick, followed by an iron or zinc phosphate layer 50 to 200 nm thick, then with a top polyphosphate layer up to 800 nm thick which can be easily removed by detergents.
11. There is a growing importance for friction reduction with the end effect of improving fuel economy to meet CAFÉ standards. This study also evaluated the effect of different AW additives on friction.
12. Compared to all alternative AW additives (VG48, ZDTC, AP, DDP), the ZDDP formed the thickest films by order of magnitude of 200% and those ZDDP films formed the fastest.
13. ZDDP films resulted in lowest wear track width with the slowest increase in width where other additives very quickly reached higher wear track widths early on then maintained those higher levels, compared to ZDDP.
14. ZDDP film thickness increases with oil temperature.
15. ZDDP film roughness has poor friction characteristics compared to other AW additives.
16. ZDDP film formation reaction caused by shear or “asperity contact,” not sliding or because of temperature.
17. Amine phosphate or didoecylphosphite AW additives show some promise in reducing wear while providing lower friction than ZDDP in “mixed lubrication conditions.”
18. ZDDP is the only phosphorus containing AW additive shown to be film-forming even at low temperatures.
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SAE Technical Paper Series. Design Considerations in Formulating Gasoline Engine Lubricants for Improving Engine Fuel Economy and Wear Resistance Part 1 Base Oils and Additives. 2007-01-4143
1. Gains in fuel economy can be made by reducing friction between moving surfaces.
2. The majority of frictional losses can be accounted for from the piston/ring system, crankshaft, oil pump, and valve-train.
3. The top ring on the pistons with its sliding contact operates in a mixed lubrication regime consisting of both boundary (metal to metal direct contact) as well as hydrodynamic (oil film between moving surfaces) lubrication.
4. Majority of non-corrosive wear occurs where boundary lubrication exists, like the top piston ring, cam lobe, tappets, and rockers.
5. High friction can result in areas with boundary lubrication or where high viscous friction forces and drag may occur with hydrodynamic lubrication in bearings, oil pumps, and with piston skirts.
6. The lubricant components most important to controlling friction and wear are the base oil (BO), viscosity modifiers (VM), and friction modifiers (FM).
7. A variety of group I, II, III, and IV base stock were used in their analysis with a kinematic viscosity at 100C of ~4 cSt.
8. The viscosity modifiers employed were olefin co-polymer (OCP) and molecular weight (MW).
9. Several Moly based friction modifiers (FM) were used not to exceed 1 wt% in a completely formulated ISLAC GF-4 engine oil.
10. Friction based efficiency losses can be characterized as 20% from mechanical losses, 8% internal friction losses, 32% heat loss in coolant, 35% heat loss in exhaust, and 5% from parasitic losses from aux devices.
11. The area of greatest opportunity for improvement is with reduction of friction in piston and rings followed by crankshaft and bearings.
12. Industry standard sequences VIB, IIIG, and VG attempt to represent a wide array of engine platforms in ILSAC GF-3 and 4 specifications.
13. There is criticism that the engines used in the above sequences do not represent the industry as a whole and performance of ILSAC lubricants in engines other than those tested.
14. The sequence VIB engine shows significant response to the use of friction modifiers and HTHS (high temperature/high shear @ 150C) values as well as to base oil (BO) viscosity and VM and AO (anti-oxidant) type.
15. The sequence VIA is also highly responsive to HTHS viscosity.
16. The sequence VIB engine in mixed lubrication shows valve-train is composed of 75% boundary and 25% mixed; rings and pistons have 25% mixed, 25% elasto-hydrodynamic (EHL), and 50% hydrodynamic (HL); crankshaft and bearings have 25% EHL and 75% HL; and oil pump is 100% HL.
17. The sequence IIIG also is influenced by BO volatility (Noack%) as well as VM and AO type.
18. Sequence VG is used to evaluate the lubricant under severe oxidation and sludge forming conditions.
19. Adequate performance for a lubricant in the VIB sequence is most important to most OEMs as this is a good indicator of fuel economy, after which sequences to evaluate the wear performance are then considered.
20. These new lubricants must balance the fuel economy improvement FEI% and good wear resistance.
21. Oils capable of forming sacrificial tribofilms in boundary or mixed lubrication to reduce friction can create a competitive adsorption scenario where more robust anti-wear films are prevented from forming, leading to high wear.
22. ZDDP can increase the coefficient of friction.
23. As speed increases, the coefficient of friction decreases, with Group IV oils having the lowest and Group II oils the highest but has little effect of the FEI %.
24. The greatest decrease in the traction coefficient was between Group II and IV oils of 40% at an entrainment velocity of .3 m/s which translates to less than a 1% increase in fuel economy.
25. A base oil with more than 67% methylene groups, less than 27% methyl groups, and less than 15% napthene groups would minimize the traction coefficient in elasto-hydrodynamic lubrication conditions.
26. With the Mo-based friction modifiers, a steady state coefficient of friction of .05 -.08 was recorded.
27. The Mo interactions form a glassy, polyphosphate anti-wear film in conjunction with a sulfurized metal surface (from ZnDTP and various other Zn/P species) to form the final tribofilm.
28. Film formation for Mo FM’s varies from 500 to 800 seconds.
29. The shear strength of the Mo enriched tribofilm is much less than the ZDDP based anti-wear pads alone.
30. The COF of the FM tribofilm is directly linked to the shear strength and hardness of the tribofilm.
31. The COF was the least with MoAmine and MoDTC-2 FM’s, with the MoAmine taking the longest to form (>800 sec) and the MoDTC-2 being the fastest at less than 500 sec.
32. The Mo FM’s with the lowest COF did not directly correlate to having the lowest wear.
33. MoEster with nearly the highest COF between all the Mo FM species had the lowest wear, then followed by MoAmine which is tied with MoDTC-2.
34. The poor performance of MoDTP and MoDTC-1 has to do with the interaction of ZDDP which leads to a decrease in tribofilm hardness even though these tribofilms were the thickest and fastest to form.
35. The majority of friction in the engine is in the hydrodynamic lubrication scenario, where a reduction in viscosity has the highest measurable improvement in fuel economy improvement, hence the push for 0w20 and 5w20 lubricants.
36. The base oil has no significant relationship on wear other than a slight improvement with oils with more methyl groups, which help to contribute carbon to the anti-wear tribofilm with a significantly lower wear coefficient.
37. But base oil does have an effect on hydrodynamic friction which viscosity and film thickness which is good for wear, bad for FEI.
38. No major increases in fuel economy improvement can be expected by adding FM alone.
39. Minimizing napthene groups and maximizing methylene groups lower the traction coefficient.
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