Here is
BOBISTHEOILGUY himself:
"How do tbn numbers and sulfated ash levels relate to engine oil performance?
Sulfated ash and TBN are two of the physical and chemical measurements made on the engine-oil. They are not a direct measure of an engine oil's performance. The higher the number is not necessarily better.
Sulfated ash is a direct measure of the amount of oil additives that contain metal. These metallic base additives make up the detergent and anti-wear additive systems that prevent deposits and wear. They generally contain metals such as calcium, magnesium and zinc.
The total base number (TBN) is a measure of the engines oil's ability to neutralize any acids that may be formed during the combustion of the diesel fuel. Historically, the higher the engine oil's total base number the better its ability to neutralize any acids that may be formed. However, starting out with a high TBN DOES NOT GUARANTEE that the engine oil will RETAIN its high level of TBN during service. With the drive by users and equipment manufactures toward extended engine oil drain intervals and the future of exhaust gas recirculation to reduce emissions there is a greater need for the engine oil to contain an effective detergent/dispersant additive system that will not only effectively neutralize any acidic components that may be formed, but also retain its TBN throughout the engine oil's drain interval.
An engine oil's tbn will reduce over time and use. Today's diesel engines are designed to have lower oil consumption rates and smaller sump capacities. With the desire for longer oil drain intervals and engine oil that has a tbn of 8, 10 or even higher could actually have a tbn of 4 or 5 by the time the oil is either refreshed though makeup or changed. Therefore, it is critical that an engine oil exhibit very good to excellent tbn capability in order to prevent engine wear and bearing corrosion.
The ability of an engine oil to retain its tbn throughout the engine oil's drain interval is dependent upon the type of detergent/dispersant additive chemistry used in it formulation.
An all calcium based detergent/dispersant additive package that not only effectively neutralizes any acidic components that may be formed but also can provide excellent tbn retention over the engine oil's drain interval.
Most heavy duty diesel engine oils contain either a high magnesium based or a calcium/magnesium based detergent dispersant additive system. Magnesium base detergent systems though they will neutralize acids as the are formed, do not have the same effective acid neutralizing capabilities as an all calcium based additive system. Magnesium based detergent additive systems do not neutralize any of the weak acids that may have been formed by the combustion of the diesel fuel. These weak acids over time can build up in the engine oil eventually causing the engine oil to rapidly lose its tbn retention.
Magnesium based detergents are primarily used in the formulation of car engine oil formulation because of their ability to allow these engine oils to pass the various engine sequence tests used to measure wear protection. However, in heavy duty diesel engine oils magnesium base detergents can cause higher wear in the ring-belt area of a diesel engine. Magnesium based detergents from magnesium oxides when the neutralize acids, a by product of the acid neutralization reaction. Magnesium oxide is very hard in consistency and can abrade and polish cylinder walls.
Calcium based detergents on the other hand form calcium sulfate when they neutralize acids. Calcium sulfate is very soft and fluffy in consistency and is soluble in oil. Calcium based detergents in addition have been found to enhance a diesel engine oil's ability to provide excellent high temperature piston cleanliness, provide excellent protection against bore polishing and enhance the soot handling capabilities of the engine oil.
How high of a TBN is really necessary and how critical is the number?
This question comes up continually, and it deserves some clarification. TBN is not a measure of an engine oil's performance, and an engine oil with a higher TBN number such as 13 or 14 is not necessarily better than an engine oil that has a tbn of 10. There are two things that are more critical to know about TBN than its starting point: how it measured and the rate at which it depletes.
The ASTM has tree methods for measuring TBN. If these three different test methods were used to test the same new oil, each method would give three different answers. In other words, the TBN rating of an engine oil is affected by the test method used to measure it. It is Schaeffer Mfg's experience and the experience of many other oil analysis lab's that the ASTM d-2896 test method will produce a higher TBN rating and the D-4739 test method will result in a lower number. The third method ASTM D-664 is now an obsolete test method and is only used to measure the total acid number of lubricating fluids such as hydraulic fluids. Most companies use the ASTM D-2896 test method to measure the TBN of a new oil.
However, when testing used engine oil, it is the ASTM D-4739 test method that provides the most reliable results. The ASTM-D4739 test method more accurately reflects the level of neutralizing power left in the engine oil after use. The ASTM D-2896 test method, because ir measures the high end of alkalinity, may indicate that more base is left in the engine oil than what may truly be left in the engine oil or is advisable for continuing service.
Most used oil analysis programs use the ASTM-D2896 method unless another test method is requested, because This test method is easier and faster to run. Since TBN is a very critical factor especially with the advent of EGR containing engines, which will produce and introduce more acidic components into the engine oil and you're trying to extend oil drain intervals, the ASTM D-4739 test method should be used and specified by you.. The ASTM D-4739 test method provides a more accurate indication of serviceability, especially if trying to extend oil drain intervals.
The second important feature of TBN, more critical than how high high it is rated, is the rate at which it depletes during use. As we stated in the previous question on tbn some engine oil start with a high tbn and then drop and lose their neutralizing ability quickly. Other engine oils can start with a lower tbn, deplete at a slower rate and maintain their alkalinity for a much longer period of time. Naturally, what is important is how well an engine oil can maintain its tbn during service. The real value of a tbn number is not determining an engine oils quality; it is in making sure an engine oil is not becoming corrosive at the engine of a drain period.
This is another case of requirements changing with the times, and the old tradition of a tbn level of greater than 10 for otr vehicles cannot be used as a rule of thumb anymore. Engine designs have drastically changed over the years and fuel sulfur levels have dropped significantly during the last decade, so that even in off road application where fuel sulfur can be as high as 0.5%, the demand for a very high tbn rating cannot really be justified. What is more important is how the engine oil's tbn will last and maintain it neutralizing ability over the entire oil drain interval."