All Caterpillar-Built Diesel Engines
The availability of fuel supplies around the world is changing. To meet the current demand, refineries are now purchasing crude oil from different worldwide sources which include more high sulfur crude oils than before.
The sulfur contents of the refined fuels vary geographically throughout the world from an average of .27 percent by weight to 1.92 percent by weight.
The sulfur content of refined diesel fuels is dependent on the amount of sulfur in the crude oils and on the economics of and/or the refiners’ abilities to remove it. For this reason, the sulfur content of some of today’s diesel fuels is increasing.
The Fuel Sulfur Problem
When diesel fuel is burned in an engine’s combustion chamber, the fuel sulfur is chemically converted to sulfur oxides. These compounds, in turn, react with water vapor to form sulfurous and sulfuric acids. As the vapors condense in the valve guides and in the piston ring area, the acids can chemically attack the metal surfaces and cause corrosive wear.
Neutralizing The Acids
One function of the lubricating oil is to neutralize the acids and, thus, retard the corrosive damage. Certain additives used in lubricating oils contain alkaline compounds which are formulated to neutralize these acids.
The measure of this reserve alkalinity in a lubricating oil is known as its Total Base Number or TBN. The Total Base Number is measured by one of two procedures: American Society For Testing Materials (ASTM) D-2896 or ASTM D-664. Caterpillar’s recommendations are based on ASTM D-2896.
The higher the initial TBN value generally indicates more reserve alkalinity or acid-neutralizing capacity. To minimize corrosive wear caused by increased fuel sulfur levels, engine oils which have higher TBN values are essential.
The New Oil Recommendations
Caterpillar’s new recommendations provide a means of combating the undesirable effects of high sulfur fuels. New guidelines have been developed for the selection of oils that permit STANDARD OIL CHANGE INTERVALS (as per the applicable Maintenance Guide) when using diesel fuels with sulfur contents of up to 1.5 percent.
Caterpillar previously based its standard oil change intervals on a fuel sulfur content of 0.4 percent and recommended shortened change intervals when fuels with greater than 0.4 percent sulfur content were used.
In the past, Caterpillar has recommended specific service classifications of oil, such as API (American Petroleum Institute) Class CD, but has not specified or published any alkalinity reserve (TBN) information. Because the range of TBN values is wide and because there is an increase in the sulfur content of some diesel fuels, it is important that TBN information be readily available. To satisfy this need, the TBN information is now included in the Engine Manufacturers Association’s “Lubricating Oils Data Book For Heavy Duty Automotive and Industrial Engines.” This is available from Caterpillar by Form No. SEBU5939.
The Correct TBN
The first step to determine the correct TBN value is to find the fuel sulfur content. This information should be available from the fuel supplier. If it is not, a fuel sample should be analyzed by a competent, independent laboratory.
When the sulfur content is known, use the graph shown in the illustration to determine the necessary TBN value. For fuels with sulfur content above 0.5 precent by weight, the TBN value should be 20 times the measured fuel sulfur content. The upper broken line is used to determine the necessary TBN value for new oil. New oils having the recommended TBN values will provide acceptable neutralizing performance through the standard change interval.
The lower solid line on the graph provides information needed to determine the minimum TBN value for used oil. The TBN of used oil must be established using the ASTM D-2896 procedure.
Note that the limits increase proportionately with the fuel sulfur contents. Controlled laboratory tests have demonstrated that alkalinity concentrations in critical areas having only small amounts of oil, such as valve guides and piston ring belt areas, must be proportionately higher to effectively neutralize the higher concentration of acids in these areas.
If the fuel sulfur content is not available, use an oil with a TBN of 10 in the United States, in Canada and in the countries where the fuel sulfur content is regulated to 0.5 percent or less by law. In all other areas of the world, including offshore applications, use an oil with a TBN of 20.
Monitoring The Oil
Coping with the effects of fuel sulfur is not a simple task.
Even though the use of proper lubricants and correct change intervals reduces the degree of corrosive damage, engine wear will increase when fuels with higher sulfur contents are used.
Oils which have larger concentrations of acid-neutralizing compounds also have larger ash contents. This may increase deposits on exhaust valve heads and on turbo-charger nozzle rings.
All oils of the same TBN may not perform identically. Oil alkalinity can be achieved through a variety of additives, but some additives simply are more effective against acids than others.
For this reason, the engine should be closely monitored through Scheduled Oil Sampling. If the oil recommendation is followed, but the S.O.S. analysis indicates excessive wear through unacceptable levels of iron (Fe) or chromium (Cr) particles, an oil which has a higher TBN value should be used.
Indications of other elements such as copper (Cu), aluminum (Al), tin (Sn) or silicon (Si) must not be overlooked. Acid corrosion is not the only cause of engine wear. Infrared analysis can determine the degree of sooting, the oxidation level and the amount of sulfur products in the engine oil, which can contribute to engine wear.
Summary Of The Recommendations
For many years, Caterpillar has recommended reduced oil drain intervals as a means of combating the adverse effects of high fuel sulfur. However, data and research now indicate that excessive wear can still occur even when using shortened drain intervals.
For the aforementioned reasons, Caterpillar has developed new recommendations to provide acceptable engine life with the use of higher sulfur fuels. These new recommendations supersede all previously-published oil drain data.
To effectively combat the effects of high sulfur fuels, Caterpillar endorses the following recommendations:
1. Know the fuel sulfur content. Periodically request this information from the supplier, or have the fuel analyzed for sulfur content by an independent laboratory. The fuel sulfur content can change with each bulk delivery. If the sulfur content can not be determined, use the guideline in the second paragraph of Step 2.
2. Select an American Petroleum Institute (API) Class CD engine oil which has the correct TBN value for the fuel sulfur content. Use the graph to determine the correct TBN value.
If the sulfur content is not available, use the following guideline. In the Unites States, in Canada and in the countries where the fuel sulfur content is regulated to 0.5 percent or less by law, use an oil which has a TBN of 10. In all other areas of the world, including offshore applications, use an oil with a TBN of 20.
3. Follow the recommendations and the standard change intervals in the appropriate Maintenance Guide.
4. Maintain a sound Scheduled Oil Sampling program. Monitor the iron (Fe) and chromium (Cr) levels for indications of the lubricant performance. Infrared analysis is an excellent method with which to determine the condition of used oil, along with the ASTM D-2896 procedure to measure the reserve alkalinity (TBN).
Graph for determination of necessary TBN. Find the fuel sulfur percentage on bottom of the graph. Find point where the new oil TBN line intersects the sulfur content line, and read the required TBN at the left side of the chart.