1987-11-01 Relationship of Engine Bearing Wear and Oil Rheology 872128
Member OVERKILL referred to this SAE paper, another Taxi test. It is from a 1987 published paper but I felt it was worth reviewing:
One premise they stated was: ...it is not clear that thicker oil films will lead to less bearing wear or longer bearing lifetimes. The relationship between HTHS and MOFT in journal bearings is conflicting with no clear conclusion. (keep in mind this is a 35 year old study).
The first arm of the study ran a Buick 3.8L V6 on a dyno. They tested two different 20 grade, and a 30 and 40 grade oil that were “thought” to be API SF quality. The 40 grade was the reference or control oil. (? fully formulated, commercially available oil?). Oils were changed at 2,500 miles but samples for oil analysis were done at 1,250 miles, mid drain interval samples. Direct engine wear was performed by measuring bearing weights before and after the test. The study could have been made better by first breaking in the engines, then tearing them apart to get before and after weights. I admired their attention to details as measing each half of the bearings revealing that more wear ocurred on the high load sides.
Let me digress. Regarding experimental oils vs commercial oils. A friend who worked in the oil section of Shell said the various sub-departments were compartmentalized and secretive. Almost as though keeping information separate so nobody would know the whole truth and formula of the final product but a select few. And we rarely hear how they did the research in development of these oils. Like Coka Cola, some may have no patent because then everybody would know exactly how they were formulated and produced.
The experiment:
Kinematic viscosities at 100C were: 5W20, oil#1=7.0. 5W20, oil#2=8.6. 10W30, oil#3=10.5 and 10W40, oil#4=15.6.
HTHS at 150C were: oil#1=2.1. oil#2=2.5. oil#3=3.0. oil#4=3.3.
Volatilities were interesting, at 375C: oil#1=44. oil#2=26. oil#3=17. oil#4=17.
If I read it right they calculated the load of pushing the car down the road at 70 MPH to be 35 HP. The test runs were hard acceleration, cruise, breaking, repeat...Sump temperatures were kept at 115C (240F) and bearing temperatures were thought to be 150C.
Idle oil pressure, PSI: oil#1=8. oil#2=11. oil#3=8. oil#4=11.
At 65 MPH, PSI: oil#1=35. oil#2=40. oil#3=30. oil#4=38.
MOFT at 120C oil temp in the gallery/ 1,500 RPM, 80 ft-lb load: oil#1=0.70. oil#2=0.88. oil#3=1.05. oil#4=0.95.
MOFT at 120C oil temp in the gallery/ 3,000 RPM, 80 ft-lb load: oil#1=0.72. oil#2=0.93. oil#3=1.13. oil#4=0.70.
Results:
The main and connecting rod bearings had essentially the same amount of wear. The “thicker” 20 grade oil and the 30 and 40 grade oils had the same amount of wear. There was a “non dramatic increase” in wear of the thinnest oil. (They say this but comparing weights the thinnest oils had 3 - 4 times the wear of the others with my interpretation of the data. Remember that 3 or 4 times a really low amount of wear is still a low amount of wear). Later in the paper they described the difference as “much higher” with the thinnest experimental oil.
Also interesting is that the oil analysis data reflected the same pattern of wear as the engine breakdowns, in these dyno tested cars. This goes against what many people on this web site think. This test arm is evidence that UOA is a worthwhile means of measuring engine wear.
The Second Arm of the study was in New York City taxi cabs.
There were 9 different oils, each oil was run in 5 different cars. Cabs were a Chevrolet Caprice with a 4.3L V6 that were spec’ed for a 5W30 motor oil. Each cab was run for 100,000 mi.
Variables were HTHS levels from a low of 2.2 (KV at 100C=6.48) to a high of 3.0 (KV at 100C=9.14). Another variable was the use of viscosity index improvers of two types. One was shear-stable and the other was shear-thinning. Bearing wear was determined by weight loss of the bearings and used oil analysis.
Interesting that UOA for this arm of the experiment did not discriminate between oils, nor with bearing wear. This indicating that used oil analysis is of no benefit to evaluate engine wear. Maybe the true value is somewhere between being of no use and being a reliable indicator.
Taxi Cab Results:
They had quite the variety of experimental oils. They stated that "poor quality" ones can show low wear in one engine and high wear in another. But "good oils" show low wear in all engines. Wear was elevated in a third of the thin oil taxis. No abnormal wear was in taxis with higher HTHS. Wear was always comparable in the main and connecting rod bearings. Average vehicle speed and the fraction of time idling did not correlate with wear. They stated that the critical HTHS viscosity is 2.3.
In general, shear-thinning oils performed better than shear stable oils. They kind of wrote this off hinting that commercial formulators are better. Wear was “marginally” higher for the lower viscosity oils and they recommended that oils should have at least a HTHS of 2.6. Oils with VI improvers performed better than without VII. Also, polymer VI improvers had less wear. There was a statistically insignificant increase in wear at the connecting rod in some of the thinner oils.
They concluded that Minimum Oil Film Thickness MOFT does not discriminate oil performance as good as the laboratory viscometric HTHS measurement. All but the two thinnest oils, out of the nine tested, provided adequate protection.
My Dig:
Once again we are investigating before the thrust of the 20 grade requiring vehicles. The 20 grade oils have come a long way since this experiment. The oils used here were experimental and even the investigators admit that they cannot duplicate the performance of fully formulated commercial oils.
For those who keep referring to the “Taxi Study” proving that an oil must have a HTHS of at least 3.5 to be usable, this is not showing that. The critical value based on these thinnest of experimental, non fully formulated, API SF-ish oils is a HTHS of 2.3.
If your application is always lighter than the maximum design level, you should be able to use a thinner modern oil. Certainly one grade in either direction is not a big change. Who wants to try 4 or 5 grades lower?
ali
My previous Taxi review:
https://bobistheoilguy.com/forums/threads/the-frequently-quoted-taxi-study-sae-2005-01-3818.364101/
Member OVERKILL referred to this SAE paper, another Taxi test. It is from a 1987 published paper but I felt it was worth reviewing:
One premise they stated was: ...it is not clear that thicker oil films will lead to less bearing wear or longer bearing lifetimes. The relationship between HTHS and MOFT in journal bearings is conflicting with no clear conclusion. (keep in mind this is a 35 year old study).
The first arm of the study ran a Buick 3.8L V6 on a dyno. They tested two different 20 grade, and a 30 and 40 grade oil that were “thought” to be API SF quality. The 40 grade was the reference or control oil. (? fully formulated, commercially available oil?). Oils were changed at 2,500 miles but samples for oil analysis were done at 1,250 miles, mid drain interval samples. Direct engine wear was performed by measuring bearing weights before and after the test. The study could have been made better by first breaking in the engines, then tearing them apart to get before and after weights. I admired their attention to details as measing each half of the bearings revealing that more wear ocurred on the high load sides.
Let me digress. Regarding experimental oils vs commercial oils. A friend who worked in the oil section of Shell said the various sub-departments were compartmentalized and secretive. Almost as though keeping information separate so nobody would know the whole truth and formula of the final product but a select few. And we rarely hear how they did the research in development of these oils. Like Coka Cola, some may have no patent because then everybody would know exactly how they were formulated and produced.
The experiment:
Kinematic viscosities at 100C were: 5W20, oil#1=7.0. 5W20, oil#2=8.6. 10W30, oil#3=10.5 and 10W40, oil#4=15.6.
HTHS at 150C were: oil#1=2.1. oil#2=2.5. oil#3=3.0. oil#4=3.3.
Volatilities were interesting, at 375C: oil#1=44. oil#2=26. oil#3=17. oil#4=17.
If I read it right they calculated the load of pushing the car down the road at 70 MPH to be 35 HP. The test runs were hard acceleration, cruise, breaking, repeat...Sump temperatures were kept at 115C (240F) and bearing temperatures were thought to be 150C.
Idle oil pressure, PSI: oil#1=8. oil#2=11. oil#3=8. oil#4=11.
At 65 MPH, PSI: oil#1=35. oil#2=40. oil#3=30. oil#4=38.
MOFT at 120C oil temp in the gallery/ 1,500 RPM, 80 ft-lb load: oil#1=0.70. oil#2=0.88. oil#3=1.05. oil#4=0.95.
MOFT at 120C oil temp in the gallery/ 3,000 RPM, 80 ft-lb load: oil#1=0.72. oil#2=0.93. oil#3=1.13. oil#4=0.70.
Results:
The main and connecting rod bearings had essentially the same amount of wear. The “thicker” 20 grade oil and the 30 and 40 grade oils had the same amount of wear. There was a “non dramatic increase” in wear of the thinnest oil. (They say this but comparing weights the thinnest oils had 3 - 4 times the wear of the others with my interpretation of the data. Remember that 3 or 4 times a really low amount of wear is still a low amount of wear). Later in the paper they described the difference as “much higher” with the thinnest experimental oil.
Also interesting is that the oil analysis data reflected the same pattern of wear as the engine breakdowns, in these dyno tested cars. This goes against what many people on this web site think. This test arm is evidence that UOA is a worthwhile means of measuring engine wear.
The Second Arm of the study was in New York City taxi cabs.
There were 9 different oils, each oil was run in 5 different cars. Cabs were a Chevrolet Caprice with a 4.3L V6 that were spec’ed for a 5W30 motor oil. Each cab was run for 100,000 mi.
Variables were HTHS levels from a low of 2.2 (KV at 100C=6.48) to a high of 3.0 (KV at 100C=9.14). Another variable was the use of viscosity index improvers of two types. One was shear-stable and the other was shear-thinning. Bearing wear was determined by weight loss of the bearings and used oil analysis.
Interesting that UOA for this arm of the experiment did not discriminate between oils, nor with bearing wear. This indicating that used oil analysis is of no benefit to evaluate engine wear. Maybe the true value is somewhere between being of no use and being a reliable indicator.
Taxi Cab Results:
They had quite the variety of experimental oils. They stated that "poor quality" ones can show low wear in one engine and high wear in another. But "good oils" show low wear in all engines. Wear was elevated in a third of the thin oil taxis. No abnormal wear was in taxis with higher HTHS. Wear was always comparable in the main and connecting rod bearings. Average vehicle speed and the fraction of time idling did not correlate with wear. They stated that the critical HTHS viscosity is 2.3.
In general, shear-thinning oils performed better than shear stable oils. They kind of wrote this off hinting that commercial formulators are better. Wear was “marginally” higher for the lower viscosity oils and they recommended that oils should have at least a HTHS of 2.6. Oils with VI improvers performed better than without VII. Also, polymer VI improvers had less wear. There was a statistically insignificant increase in wear at the connecting rod in some of the thinner oils.
They concluded that Minimum Oil Film Thickness MOFT does not discriminate oil performance as good as the laboratory viscometric HTHS measurement. All but the two thinnest oils, out of the nine tested, provided adequate protection.
My Dig:
Once again we are investigating before the thrust of the 20 grade requiring vehicles. The 20 grade oils have come a long way since this experiment. The oils used here were experimental and even the investigators admit that they cannot duplicate the performance of fully formulated commercial oils.
For those who keep referring to the “Taxi Study” proving that an oil must have a HTHS of at least 3.5 to be usable, this is not showing that. The critical value based on these thinnest of experimental, non fully formulated, API SF-ish oils is a HTHS of 2.3.
If your application is always lighter than the maximum design level, you should be able to use a thinner modern oil. Certainly one grade in either direction is not a big change. Who wants to try 4 or 5 grades lower?
ali
My previous Taxi review:
https://bobistheoilguy.com/forums/threads/the-frequently-quoted-taxi-study-sae-2005-01-3818.364101/