LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

S- and C-Series American Petroleum Institute Oil Classification
Two fundamental classifications of oils are recognized by API performance specifications for either spark or compression ignition engines. S-series oils, where the S designates spark ignition systems, are designed to meet performance standards for spark ignition systems in engines burning gasoline, propane, or natural gas fuels. C-series oils meet compression ignition engine performance requirements for diesels. S- and C-series oils are each uniquely formulated and use a different additive package to adapt to contamination from combustion byproducts of gasoline and diesel fuel. For the same reason, natural gas engines require a unique oil and additive formulation, but oils for natural gas engines are a subset of S-series oil.

These API S and C standards have progressively evolved since they were first introduced in the 1940s to meet demands for advances in engine technology and engine manufacturers’ performance requirements. The S and C are followed by another letter to indicate what level of revision the oil is certified to meet. For example, the earliest API classification was C followed by the letter A. This classification is now obsolete but was followed by successive standards indicated by progressive alphabetic designations: CB, CC, CD, and so on.

Today’s oils are highly refined petroleum products with a package of chemicals mixed into the oil called additives that enable lubricating oil to meet unique technological operating requirements of engines (Figure 12 and Figure 13) . In response to a request from a group of manufacturers, the API develops any new specification for oils from what is called a temporary proposed category (PC) . In January 2016, the specification that was called PC-11 became CK-4, replacing the older CJ-4 specification needed for engines built in 2007 and later. Since 2007, diesel engines that use exhaust particulate filters required CJ-4 oil that did not contain additives that could harm the catalysts in the aftertreatment system. The next PC-12 will improve oil performance qualities beyond the current API CK-4 engine oils that are compatible with new biofuels and have properties to reduce engine wear for enhanced engine durability.

FIGURE 10-12: API oil performance standards have evolved to help meet emissions standards and changes to fuel quality. Earlier oils had to tolerate 5% sulfur content in fuel, plus high amounts of combustion particulate and acid-forming byproducts of oxides of nitrogen (NOx).

FIGURE 10-13: Changes in technology and emissions control systems have driven changes to oil performance standards.

The number 4 in the API classification indicates that the oil has high levels of detergent. This feature was introduced in the late 1990s to help cope with additional soot loading of engine oil caused by retarded injection timing and to extend oil drain intervals. Typically, the engine manufacturer makes the recommendation to the API for any new classification of oil based on upcoming technology. Engine manufacturers also have their own specifications for oil appearing alongside an API of ACEA classification. This information can be found in the vehicle’s operator manual or manufacturer’s service manual.