LO 10-7 Outline lubrication system maintenance and service procedures

1.1. Limiting Factors to Engine Oil Life

Limiting Factors to Engine Oil Life

Today’s diesel engine technology requires that lubrication oils meet a variety of operating conditions and provide for long service intervals and engine durability. To understand what additives are needed to provide acceptable service, and when oil should be changed, it is helpful to consider the main factors that can limit oil life.

The Percentage of Solids
The percentage of solids refers to the total amount of particles that are suspended in the oil. Combustion soot, dirt, and oxidized oil are typically the major solids found in oil. The percentage of solids becomes a limiting factor to oil life when the particles interfere with the oil’s lubricating abilities. Engine manufacturers suggest an oil change to remove dissolved solids before they rise above 5% by weight. However, improvements in wear reduction are best achieved if the solids are kept below 2%. Detergents are added to the oil to prevent the solids from clumping and forming damaging deposits inside the engine. Detergents surround the particles and suspend them in the oil until it is drained or until the filter removes the particles. Synthetic oils require change intervals, just like conventional oils, if the percentage of solids is the limiting factor to engine oil life.

Sulfur and Combustion Acids
Sulfuric acids are formed in engine oil from two sources. One source is from sulfur in the diesel fuel, the other is from recirculating exhaust gases containing fuel-borne sulfur back into the cylinders. Because sulfur combines with the water produced during combustion, highly corrosive acids are formed in the oil. Additionally, oil oxidation can change the pH of oil. To counteract this condition, oil manufacturers add alkali or chemically based substances, such as calcium and ash, to the oil to neutralize the effect of sulfuric acids. The use of these additives distinguishes oils used in SI engines from oil used in diesel engines. The total acid number (TAN) refers to the acidity of an oil, whereas the total base number (TBN) is the measurement of a lubricant’s reserve alkalinity, which aids in the control of acids formed during the combustion process. The higher an oil’s TBN, the more effective it is in preventing the formation of wear-causing contaminants. Diesel engines will use a TBN of between 6–10 points. When the TBN is below half its new value, the engine oil should be changed.

Technicians Tip     The latest oil classifications for diesel engines are formulated to be compatible with the use of EGR and several different types of exhaust system catalysts. Sulfated ash, the additive once used to increase the TBN of diesel lubrication oil, is now almost eliminated. The removal of sulfur from fuel, as has occurred in recent years, is an important factor in extending the life of the catalyst material in the DPF. Less ash and other alkaline substances that can plug these catalysts are now used in engine oil. As the engine uses oil, the ash, which normally does not burn and, instead, deposits in the DPF, is now gone. As a result, the DPF requires fewer scheduled cleanings (not regeneration) when CJ-4 or CK-4 oil is used. Note that algorithms used by OEMs estimate ash loading of DPFs based on the use of the oil classification recommended at the time of engine manufacturing.     Oxidized Oil Oil is oxidized when it is exposed to air and oxygen combines with oil molecules. Conventional oils generally thicken, turn brown, and form deposits if highly oxidized. This process occurs more rapidly at high temperatures. The problem with oxidation is that oil loses some of its lubricating properties as it oxidizes, thus accelerating engine wear. Oxidation has shown strong correlations to lead corrosion in engine bearings. Synthetic oils are at an advantage because they do not oxidize as quickly as conventional oils. Measurements of oil oxidation during oil analysis are helpful because oxidation is the most sensitive and repeatable measure available of oil degradation. Therefore, if oil oxidation due to high engine operating temperatures is the limiting factor of oil life, the use of synthetic oil can extend the oil change interval. CK-4 oils have an improved additives package to limit oil oxidation Figure 54.

FIGURE 10-54: Oil chemistry changes depend on the type of driving cycle the vehicle encounters.

Other Oil Life-Limiting Factors
Engine problems, such as coolant leaks into the oil from sources such as leaking heads, require an oil change after a repair. Defective air filters that permit dirt to enter the engine, fuel leaking into the oil, wrong fuel (i.e., gasoline contamination of diesel fuel), water in the oil, or a mechanical failure producing internal debris are all circumstances that require an oil change subsequent to engine repair. Failing to change the oil has resulted in many engines experiencing catastrophic failures shortly following seemingly minor repairs.

When oil turns milky white, brown, or gray, it is usually a sign that coolant has leaked into it. Minor internal coolant leaks are not easily detected, and an oil sample should be obtained for analysis TABLE 10-6. Conventional antifreeze leaks will show up on an oil analysis as high silicate levels in the oil. Long-life antifreeze has other chemical signatures. Fuel contamination of oil can often be detected by simply smelling the fuel in the oil or performing an oil analysis after obtaining an oil sample.