LO 10-3 Identify and describe the purpose and function of oil additives

1. Oil Additives

Oil Additives

Motor oils are typically comprised of 75–85% base stock, which is the raw mineral oil processed from crude oil. The remainder of the oil volume is made of chemical additives Figure 22. Additives improve the original properties of base stock oil. Enhancing the performance of motor oil base stocks is necessary to adjust the performance of the oil to suit its intended application. For example, diesel engines produce higher levels of soot than gasoline engines, which is why engine oil turns black more quickly in diesels than in gasoline engines. This soot tends to clump together, leaves deposits inside the engine, and even thickens the oil. To prevent this, more detergent is added to diesel engine oil, causing the soot to stay suspended in the oil Figure 23. To understand the function of detergents, consider washing dishes with and without soap detergent. Dishwater will stay dirty if it contains detergent to keep the dirt suspended in the water. Without detergent, the dirt eventually settles out of the water to the bottom of the sink. By keeping soot (dirt) suspended, the filters can remove more soot from the oil and leave the engine less likely to build sludge deposits.

FIGURE 10-22: Relative values of viscosity improvers and detergents in diesel engine oil.

FIGURE 10-23: Diesel engine oils are formulated to tolerate the increased quantity of soot from diesel combustion. Retarded injection timing and the use of EGR contribute to higher soot levels in diesel-engine lube oil.

Additives also add performance characteristics to base-stock oils to suit a particular application. The use of VI improvers is an example previously mentioned. Antifoaming additives are critical to oil used in engines with hydraulically actuated electronically controlled unit injectors (HEUI) to prevent the aeration of engine oil. Aerated oil causes changes to injection timing and the pressurization of fuel, resulting in low power and poor fuel economy. Aerated oil does not lubricate parts very well and interferes with the delivery of the correct oil volume to engine components.