Applied Failure Analysis
OIL and your engine
Bearing failure, piston ring sticking, and excessive oil consumption are classic symptoms of oil-related engine failure. How do you avoid them? There are numerous ways, three of the most important being Scheduled Oil Sampling (S·O·S SM), regular maintenance of the lubrication system, and the use of correct lubricants. Following these recommendations can mean the difference between experiencing repeated oil related engine failure and benefiting from a productive and satisfactory engine life. This booklet attempts to tell the story of oil: what it is composed of and what its functions are, how to identify its contamination and degradation, typical consequences, and some preventive measures to help you protect your engine against the devastating effects of oil related engine failure.
Understanding Oil
Function
Engine oil performs several basic functions in order to provide adequate lubrication. It works to keep the engine clean and free from rust and corrosion. It acts as a coolant and sealant; and it provides an oil film cushion that keeps metal-to- metal contact to a minimum, thereby reducing friction and wear. But these are only the basic functions of oil. It is the particular demands of a given application and the special conditions under which an oil is used that largely determine the numerous additional functions oil must perform. These additional functions make choosing the correct oil for the job vital.
The selection of a suitable lubricating oil should be based on the engine performance requirements as specified by the manufacturer, as well as the application and the quality of the available fuel. Diesel engines, for instance, normally operate at lower speeds but higher temperatures than gasoline engines, making conditions exceptionally conducive to oil oxidation, deposit formation and corrosion of bearing metals. Under these conditions, the oil is expected to function in an expanded capacity. This is where additives are noticed. The final performance characteristics of the oil depend on the base oil and the additives used. The amount or types of additives used vary according to the properties of the base oil and the environment in which the oil will function.
Base Stocks
Lubricating oil begins with base oil or base stock. Base stocks are mineral (petroleum) or synthetic origin, although vegetable stocks may be used for specialized applications. The base stock provides the basic lubricating requirements of an engine. However, unless it is supported with additives, base oil will degrade and deteriorate very rapidly in some operating conditions. Depending on the type of base stock, petroleum, synthetic or others, different additive chemistries are used.
Mineral Oils
Mineral stocks are refined from petroleum crude oils. The crude oil source and the refining process will determine the base stock characteristics. The crude oils used for diesel engine lubricants are primarily made up of paraffin, napthene, and aromatic compounds. The crude oils with higher paraffin content are most frequently used in blended engine oils.
The refining process begins with vacuum distillation. Vacuum distillation separates the oil into products with a similar boiling range and similar viscosities. After vacuum distillation, the oils must be purified to remove or modify undesirable compounds. Base oil purification is usually done by solvent extraction and hydrofinishing or by hydrocracking and hydrofinishing. Both of these processes are used to limit or eliminate wax, sulfur, and aromatics. Variations in these refining process produce base oils with different characteristics.
Mineral base stocks are most prevalent for diesel engine oil formulation because they exhibit proven characteristics and are readily available at a reasonable cost.
Synthetic Oils
Synthetic base stocks are formed by processes that chemically react materials of a specific chemical composition to produce a compound with planned and predictable properties. These base stocks have viscosity indexes much higher than HVI mineral base stocks, while their pour points are considerably lower. These characteristics make them valuable blending components when compounding oils for extreme service at both high and low temperatures. The main disadvantage of synthetics is the significantly higher price and the somewhat limited supply. The group of synthetic oils known as esters causes greater seal swelling than mineral oils. The possible use of ester synthetic oils requires that component design be carefully considered for seal and ester oil compatibility. The use of synthetic base stocks lubricants in Caterpillar engines and machines is acceptable if the oil formulation meets the specified viscosity and Caterpillar performance requirements for the compartment in which it will be used. For very cold ambient conditions, the use of synthetic base stock oils is necessary.
Additives
Additives strengthen or modify certain characteristics of the base oil. Ultimately, they enable the oil to meet requirements beyond the abilities of the base oil.
The most common additives are detergents, oxidation inhibitors, dispersants, alkalinity agents, anti-wear agents, pour-point depressants and viscosity index improvers.
Here is a brief description of what each additive does and how.
Detergents help keep the engine clean by chemically reacting with oxidation products to stop the formation and deposit of insoluble compounds. The detergents in use today are metallic salts called: sulfonates, phenates, phosphonates or salicylates.
Alkalinity agents help neutralize acids. The detergents are also strong acid neutralizers, changing combustion and oxidation acids into harmless neutralized salts.
Oxidation inhibitors help prevent increases in viscosity, the development of organic acids and the formation of carbonaceous matter. These anti-oxidants are the following chemicals: zinc dithiophosphates, phenate sulfides, aromatic amines, sulfurized esters, and hindered phenols.
Depressants help prevent sludge formation by dispersing contaminants and keeping them in suspension. Common dispersant types include polyisobutenyl succinimides and polyisobutenyl succinic esters.
Anti-wear agents reduce friction by forming a film on metal surfaces and by protecting metal surfaces from corrosion. The principal types of agents are alkaline detergents, zinc dithiophosphates and dithiocarbamates.
Pour-point depressants keep the oil fluid at low temperatures by preventing the growth and agglomeration of wax crystals. Pour point depressant types are polymethacrylates; styrene- based polyesters, crosslinked alkyl phenols and alkyl naphthalenes.
Viscosity Index improvers help prevent the oil from becoming too thin at high temperatures. Viscosity index improvers (VI improver) are chemicals which "improve" (reduce) the rate of viscosity change with temperature change. Chemicals used as VI improvers are polyisobutenes, polymethacrylates, styrene-based polyesters, styrene-based copolymers and ethylene propylene copolymers.
Total Base Number (TBN)
Understanding TBN requires some knowledge of fuel sulfur content. Most diesel fuel contains some amount of sulfur. How much depends on the amount of sulfur in the crude oil from which it was produced and/or the refiner's ability to remove it. One of the functions of lubricating oil is to neutralize sulfur by-products, namely sulfurous and su.lfuric acids and thus retard corrosive damage to the engine. Additives (primarily detergents) in the oil contain alkaline compounds which are formulated to neutralize these acids. The measure of this reserve alkalinity in an oil is known as its TBN. Generally, the higher the TBN value, the more reserve alkalinity or acid- neutralizing capacity the oil contains.
Ash or Sulfated Ash
The ash content of an oil is the noncombustible residue of a lubricating oil. Lubricating oil detergent additives contain metallic derivatives, such as barium, calcium, and magnesium compounds that are common sources of ash. These metallo-organic compounds in the oils provide the TBN for oil alkalinity. Excessive ash content will cause ash deposits which can impair engine efficiency and power.
Viscosity
Viscosity is one of the more critical properties of oil. It refers to its resistance to flow. Viscosity is directly related to how well an oil will lubricate by forming a film to separate surfaces that would contact one another. Regardless of the ambient temperature or engine temperature, an oil must flow sufficiently to ensure an adequate supply to all moving parts.
The more viscous (thicker) an oil is, the thicker the oil film it will provide. The thicker the oil film, the more resistant it will be to being wiped or rubbed from lubricated surfaces. Conversely, oil that is too thick will have excessive resistance to flow at low temperatures and so may not flow quickly enough to those parts requiring lubrication. It is therefore vital that the oil has the correct viscosity at both the highest and the lowest temperatures at which the engine is expected to operate.
Oils change viscosity with temperature, becoming less viscous as their temperatures increase. Refining techniques and special additives increase the Viscosity Index (VI) of oil. The higher the VI number of the oil, the lower its tendency to change viscosity with temperature.
The Society of Automotive Engineers (SAE) standard oil classification system (SAE J300) categorizes oils according to their viscosity (via a number system such as SAE 10W, SAE 30, SAE 15W40, etc.).
Each of the viscosity grades or numbers has limits on the viscosity of the oil at given temperatures. For viscosity grades specified with a "W" the oil viscosity is defined by both viscosity at 100°C and at maximum low temperature for cranking and pumping. In other words, the oil's viscosity has been tested to verify the oil's flow under specified low temperatures. Therefore the "W" in an oil viscosity grade is commonly understood to mean that the oil is suitable for winter service. For grades without the W, the oil viscosity is defined at 100°C. only. The attached chart indicates the viscosities for the various oil viscosity grades.
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API Engine Oil Classifications
The gasoline and diesel engine oil performance classifications are defined by the American Petroleum Institute (API) service classifications established jointly by API, SAE and ASTM (American Society of Testing Materials).
API gasoline engine oil classifications have two letter designations that start with the letter "S." The current active designations are API SJ, and API SL.
API diesel engine oil classifications have two letter designations that start with the letter "C." The current active four-stroke cycle diesel engine oil classification designations are API CF, API CF-4, API CG-4, and API CH-4.
API CH-4 oils were developed in order to meet the requirements of the new high performance diesel engines. Also, the oil was designed to meet the requirements of the low emissions diesel engines. API CH-4 oils are also acceptable for use in older diesel engines and in diesel engines that use high sulfur diesel fuel. API CH-4 oils may be used in Caterpillar engines that use API CG-4 and API CF-4 oils. API CH-4 oils will generally exceed the performance of API CG-4 oils in the following criteria: deposits on pistons, control of oil consumption, wear of piston rings, valve train wear, viscosity control, and corrosion.
Three new engine tests were developed for the API CH-4 oil. The first test specifically evaluates deposits on pistons for engines with the two-piece steel piston. This test (piston deposit) also measures the control of oil consumption. A second test is conducted with moderate oil soot. The second test measures the following criteria: wear of piston rings, wear of cylinder liners, and resistance to corrosion. A third new test measures the following characteristics with high levels of soot in the oil: wear of the valve train, resistance of the oil in plugging the oil filter, and control of sludge.
In addition to the new tests, API CH-4 oils have tougher limits for viscosity control in applications that generate high soot. The oils also have improved oxidation resistance. API CH-4 oils must pass an additional test (piston deposit) for engines that use aluminum pistons (single piece). Oil performance is also established for engines that operate in areas with high sulfur diesel fuel.
All of these improvements allow the API CH-4 oil to achieve optimal oil change intervals. API CH-4 oils are recommended for use in extended oil change intervals. API CH-4 oils are recommended for conditions that demand a premium oil. Your Caterpillar dealer has specific guidelines for optimizing oil change intervals.
API CG-4 oils were developed primarily for diesel engines that use a 0.05 percent level of fuel sulfur. However, API CG-4 oils can be used with higher sulfur fuels. The TBN of the new oil determines the maximum fuel sulfur level. See Illustrations 1 and 2 on pages 31 and 32.
API CG-4 oils are the first oils that are required to pass industry standard tests for foam control and viscosity shear loss. API CG-4 oils must also pass tests that were developed for corrosion, wear and oxidation.
API CF-4 oils service a wide variety of modern diesel engines. API CF-4 oils provide more stable oil control and reduced piston deposits in comparison to API CF and the obsolete CE and CD classifications of oil. API CF-4 oils provide improved soot dispersancy in comparison to API CF and obsolete CD oils. The API CF-4 classification was developed with a 0.40 percent sulfur diesel fuel. This represents the type of diesel fuels that are commonly available worldwide.
NOTE: Do not use single grade API CF oils or multigrade API CF oils in Caterpillar Direct Injection (Dl) Diesel Engines (except Caterpillar 3600 Series Diesel engines).
NOTICE: API CF is not the same classification as API CF-4. API CF oils are only recommended for Caterpillar 3600 Series Diesel Engines and Caterpillar engines with precombustion chamber (PC) fuel systems.
Some commercial oils that meet the API classifications may require reduced oil change intervals. To determine the oil change interval, closely monitor the condition of the oil and perform a wear metal analysis. Caterpillar's S·O·S Oil Analysis Program is the preferred method.
NOTICE: Failure to follow these oil recommendations can cause shortened engine service life due to deposits and/or excessive wear.
