Comments on Dr. Ali Haas’ “Introduction to Motor Oil"
Wow- I just sat here for two hours reading that. Thank you for the link. Richard Clark, where are you? Your thoughts?
i asked our head of engineering at ZPlus Howard Hoyt for his comments:
Comments on Dr. Ali Haas’ “Introduction to Motor Oil”
In general, this is one of the most common sense explanations of oil viscosity I have read, and is on the money with its description of optimum viscosity. Engines are designed with bearing clearances and other parameters optimized for specific viscosity oil AT OPERATING TEMPERATURE, and the author has an excellent grasp on this fact. There are some parts I would correct:
Chapter 1
“It is time to introduce the concept of lubrication. Most believe that pressure = lubrication. This is false. Flow = lubrication. If pressure was the thing that somehow lubricated your engine then we would all be using 90 weight oil. Lubrication is used to separate moving parts, to keep them from touching. There is a one to one relationship between flow and separation. If you double the flow you will double the separation pressure in a bearing. The pressure at the bearing entrance is irrelevant.”
Each system in an IC engine depending on lubrication has a different mode of lubrication, but since this section seems to be dealing just with crankshaft bearings which employ hydrodynamic lubrication, let’s address that system. The fact is that pressure and flow are inextricably related. You cannot have flow without pressure, and given a fixed system, they have a simple and fixed relationship. The author is correct in summarizing that pressure does not equal lubrication, but neither does flow.
The relationship between flow and separation pressure that the author speaks of is only applicable to hydrostatic bearings. Flow is NOT needed for hydrodynamic lubrication in a plain bearing. This is a common misconception. Short of starvation, there is little relationship between engine oiling system pressure OR flow through the bearing and the resulting hydrodynamic wedge oil film thickness. There IS a functional relationship between loading, viscosity, and oil film thickness in any hydrodynamic bearing, which is well illustrated by the Stribeck Curve (see ZPlus Tech Brief #11 – Internal Combustion Engine Lubrication). The only reason an engine bearing needs flow is to replace the oil lost due to side leakage. A sealed crankshaft bearing full of oil could still be able to keep a hydrodynamic film established and stay lubricated with zero flow from the engine oiling system. Of course, flow IS desirable in order to remove heat due to oil shear. The heat added to oil in a plain bearing is almost entirely due to the shear between the layers of oil with differing velocity as they slide by each other.
The film thickness in a hydrodynamic bearing is inversely proportional to load, and directly proportional to viscosity. Shear heating is proportional to differential bearing surface speed, oil film thickness and oil viscosity. Therefore, the main reason to flow oil in these bearings is to keep cool oil in the bearing. Within these constraints, the author is correct that the thinnest oil will have the highest oil flow rate as well as generate the least shear heat. The thinnest oil you use will also give the thinnest oil film thickness in the bearing, which is NOT a good thing. Unfortunately the author’s statement is a bit open-ended, and I am sure he did not mean to infer that if you could get an oil that had, say, <1 cSt viscosity at 100°C, it would be better than one with 10 cSt. Oil film thickness is directly proportional to viscosity, and that is the design criteria which manufacturers use to specify oil viscosity as it applies to bearings.
It is our opinion that the manufacturer’s recommendation of viscosity is the gold standard for maximum engine protection. Regardless of any discussion of oil performance and our understanding of the physics involved, we would only trust an OEM gauge so far. To adjust choice of oil viscosity in a $20,000 dollar car let alone a $200,000 car based on readings from a $50 gauge is ludicrous. Only after very careful consideration of every possible factor (including actual laboratory calibration of the OEM oil gauge) should any change from the manufacturer’s recommendation ever be made. We have tested oil, boost, and fuel pressure gauges against a laboratory gauge and regularly found them to be off as much as 10lbs!
Chapter 2
“Synthetic oils are a whole different story. There is no VI improver added so there is nothing to wear out. The actual oil molecules never wear out. You could almost use the same oil forever. The problem is that there are other additives and they do get used up. I suppose if there was a good way to keep oil clean you could just add a can of additives every 6 months and just change the filter, never changing the oil.”
There are two problems with this statement: Many synthetic oils do indeed have VI improver, and much of what is now marketed as synthetic is actually a very highly refined Group III oil, called VHVI (Very High Viscosity Index) oil. (For more information on this issue, please read the section on Castrol vs. Mobil in ZPlus tech Brief #10 – Oil Base Stocks) Castrol Syntec is an excellent example of high quality oil which is advertised as “synthetic” but which is Group III base oil, not PAO Group IV oil. This merely means that the characteristics that the author attributes to true synthetics do not necessarily apply to these VHVI oils. In some respects the VHVI oils are even superior to Group IV PAO synthetics, but they are not equivalent. It is extremely difficult to get information from oil manufacturers as to what the ZDDP level is, never mind whether or not the formula included VI improvers or not. The only reason we know that Castrol Syntec is not a Group IV PAO base is because of documents associated with the suit brought against them by Mobil.
Chapter 4
“As it turns out synthetic oils do cling to parts better as they have higher film strength than mineral oils.”
This statement confuses two different characteristics: adhesion, which is one substances attraction to another, and cohesion, a substances attraction to itself. Film strength is due to oil’s cohesion, which is due to molecular structure as well as chemical properties, but its ability to cling to metal engine parts would be a function of its adhesion, which is mainly due to polar characteristics. In reality, if we are talking PAO Group IV oil, the adhesion is poorer than many mineral oils due to its lower polarity and the fact that it is more chemically inert, and bonds less well to other substances. This is one of the reasons it is so stable at high temperatures and in oxidizing environments. Most Group IV PAO base oils are formulated with other highly polar oils and additives in an effort to make them more chemically solvent to additive packages. This also has the desirable characteristic of making them adhere to metal better. They do have excellent film strength in most cases due to the uniform physical structures that their molecules can take which promotes cohesion.
“Synthetics are thinner overall. They have greater slipperiness. Yet they stick better to engine parts. Again, this concept is the opposite of normal thinking.”
Once again, if by synthetic we are talking Group IV PAO oil, then of course, “thinner” can only really mean viscosity, and that is a design criteria of any given weight oil, conventional or synthetic. If this statement is referring to the low temperature viscosity vs. high temperature viscosity, or VI, there are some Group III VHVI oils which have higher VI than Group IV PAO.
Due to synthetics cohesion characteristics, they form strong films that separate surfaces well, which is the definition of slippery. This is one reason why engine break-in of flat-tappet motors is best performed with regular mineral oils, whose lower cohesion and film strength more strongly promotes rotation of the lifters. Again, in general PAO synthetic bases by themselves do not adhere more strongly to motor parts than do mineral oils. As a matter of fact, the least refined and most highly polar mineral oils, Group I, can display the greatest adhesion to metal of all base mineral oils. This being said, with the inclusion of highly polar additives, synthetics can exhibit improved adhesion as well.
Chapter 5
“The temperature of oil on your gauge is not as hot as it really gets. This temperature is an average with oil from different parts of the motor. Some parts are hotter than others. It is said that some of the oil gets as hot as 400 or 500 F in these racing situations..”
Excellent point which many people do not realize. This pinpoints the advantage of synthetic oils in a high-performance application. Under extreme shear, such as the rings at high RPM mid-stroke or crankshaft and crankpin bearings under severe loading, oil molecules can reach 500°F or hotter. At these temperatures, PAO molecules are more stable than most of the molecules in the mix of a mineral-derived oil. This shear is proportional to load and RPM, and greater flow will allow for quicker return to the sump and hopefully cooling.
Chapter 6
“Synthetic Class:…Castrol Syntec…”
Castrol Syntec is a Group III VHVI mineral oil. It is great oil, but not synthetic in the traditional chemical definition. This probably applies to many competing oils claiming to be “synthetic” as well, but getting accurate information about what base stock or mix of stocks was used in the manufacture of a specific oil can be difficult. (For more detail see ZPlus TechBrief #10 – Oil Base Stocks).
Chapter 7
“As one can see just going from the previous SJ to the current SL rating is a significant improvement. I cannot wait to get the upcoming SM oil into my cars.”
The oil base stock performance has continually been improved with succeeding API ratings. The current SM oils are indeed the highest quality oils in general that have been offered to the consumer. However, to state unequivocally that the most modern SM oil is best for all applications is to misstate the situation. The factor in addition to quality that determines suitability is formulation. All of the comments in the author’s paper apply to the hydrodynamic conditions in crankshaft bearings, but not to the boundary lubrication conditions found in a high-performance flat-tappet engine’s cam and lifter contact. The formulation of today’s SM oils is designed for the engines of today. It is explicitly NOT designed for all older vehicles. Specifically for proper lubrication of flat-tappet cam and lifter engines, the formulation needs to have sufficient antiwear additive. Maybe the author literally means that he just really wants SM oil in HIS car, not everyone else’s?
The only vehicles specifically designed for SM oil are vehicles designed in anticipation of the 2004 adoption of the SM oil. Likewise with SJ oil and SF oil. The API tests attempt to ensure backward compatibility, but their tests are conducted using engines which are low-performance. The largest change has been the reduction in phosphorus due to potential catalytic converter problems. In the mid-1980s, when SF oil was the current API spec, there was no upper limit on phosphorus, and many of the heaviest duty oils contained upward of 1400ppm of phosphorus in the form of ZDDP. This was necessary to combat cam and lifter wear problems with high-performance flat-tappet engines. Once the EPA mandated the extension of catalytic converter warranty coverage, the car manufacturers had no choice but to redesign engines to use less ZDDP. The current crop of engines utilize roller cam followers and other technologies to deal with the lowered ZDDP levels, which now hover around the 600ppm range.
Specifically, the API SJ rating requires passing the Sequence IIIE, IIIF, or IIIG tests for cam and lifter wear. The API SL rating requires passing the Sequence IIIF or IIIG test. The difference between the Sequence IIIE, IIIF, and IIIG tests is interesting. They all are performed using a low-performance GM 3800 V6 modified with flat tappet lifters, and the results obtained do not necessarily equate to the oil’s performance in a high RPM, high spring-pressure, and high lift application. The Sequence III tests were designed to provide accelerated results of an oil’s performance in an average engine application.
If an oil company chooses to have their oil tested for SJ rating using the IIIE or IIIF, but it contains less than 0.08% phosphorus, they also need to pass the Sequence VE test for cam and lifter wear. If they choose to use the IIIG test, and have at least 0.06% phosphorus, then that additional test is waived.
If they choose to have their oil tested for SL rating using the Sequence IIIF, it must also pass the Sequence VE test for cam and lifter wear. If they choose and pass the IIIG test, then that additional test is waived.
What these test requirements show is there is a large body of testing and resulting faith in the ability of ZDDP to control cam and lifter wear. Competing antiwear technologies have yet to reach these performance standards, and are required to undergo additional testing.
If a person has a vehicle designed before 1996 which has flat lifters, and especially in the case of an older high-performance pushrod engine with flat lifters, they MUST raise the level of ZDDP to match the oil that the manufacturer specified at the time of manufacture. To not do so is to court increased cam and lifter wear.
For a more detailed explanation of engine lubrication, please refer to the Tech Briefs on the zddplus.com website. Specifically papers #10 and 11(to be posted shortly) deal with the issues of viscosity and boundary lubrication.
But very interesting none the less!
