1. Purpose and Function of the Lubrication System

1.7. Oil Viscosity

LO 10-02: Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils.

 

Oil Viscosity
Oil viscosity is an important oil property and a term used to describe the difference between how well liquids flow or do not flow. For example, to compare the flow characteristics of maple syrup and water, the property of viscosity is most useful. A commonly used oil viscosity rating is the Society of Automotive Engineers (SAE) viscosity rating, established by the SAE and not the API. Until API standards were first developed in 1947, viscosity was the only way oil was rated. The term viscosity refers to a liquid’s resistance to flow Figure 14. A numerical designation and test standard developed earlier by the SAE is still used to measure oil viscosity. The numbers basically indicate whether a particular oil flows quickly or slowly (i.e., thicker or thinner viscosity). Engine lubricating oils having a viscosity with low numbers, such as SAE 0, 5, or 10, flow more easily than viscosities of SAE 20, 30, or 50. In other words, the higher the number, the thicker the viscosity and the more resistant the oil is to flow.

   Fig10_14

FIGURE 10-14: Viscosity is a liquid’s resistance to flow. Low-viscosity oil flows easier than high-viscosity oil.

 

Viscosity is measured using two methods. The simplest and oldest method is kinematic viscosity, which is a measurement of a fluid’s resistance to flow while acted upon by only the forces of gravity TABLE 10-1. It is determined by measuring the time, in seconds, required for a specific volume of fluid to flow a standard distance through a calibrated orifice at a closely controlled temperature. The second method, now used for the latest oils, is dynamic viscosity, which is a measurement of a fluid’s resistance to flow when acted upon by different bearing loads. Under varying pressure conditions, such as between a rotating shaft and a bearing, the viscosity of fluids can change. Dynamic viscosity measurement is considered the best predictor of oil viscosity in real-world operating conditions.

  TABLE 10-1: Comparing Kinematic Viscosity of International Standards Organization (ISO) and Society of Automotive Engineers (SAE) Grade Oils  
ISO Viscosity Grade Average Flow rate mm2 / second @40° C SAE Viscosity Grade Average Flow rate mm2 / second @100° C
15 15    
22 22    
32 32    
46 46    
68 68    
100 100 20 5.6
150 150 30 9.3
220 220 40 12.5
320 320 50 16.3
460 460    
680 680    

 

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