¨ Unit: ¤ Saybolt Universal Seconds (SUS). ¨ Conversion to cST 9 195 0.226 , when 32 100 135 0.220 , when 100 cST SUS SUS SUS cST SUS SUS SUS = × − ≤ ≤ = × − >
Viscosity vs. temperature ¨ Viscosity decreases as temperature increases. 10 Logarithmic scale Viscosity Index ( VI ) ¨ An arbitrary measure for the change of viscosity with variation in temperature. ¨ VI scale: set up by the Society of Automotive Engineers ¨ The lower the VI, the more viscosity drops as oil warms up. 11 Viscosity Index Classification < 35 Low (napthalene: 0) 35 – 80 Medium (paraffin: 100) 80 – 110 High > 110 Very High Automotive engine single-grade oils ¨ Single-grade oils: no VI improvers. ¨ Winter & non-winter grade oils. ¤ For winter grade oils, dynamic viscosity (resistance to flow) is measured with two methods at decreasing cold temperatures. ¤ Grade is assigned at lowest temperature the oil passes the tests. ¤ E.g, a 5W must have viscocity ≤ 6600. 12 1 ASTM D3244 2 ASTM D5293 3 ASTM D4684 4 ASTM D445 5 ASTM D4683.
Automotive engine single-grade oils ¨ For non-winter grade oils, the kinematic viscosity (flow rate) and high-shear rate are measured. ¨ The higher the viscosity, the higher the SAE viscosity grade is. 13 1 ASTM D3244 2 ASTM D5293 3 ASTM D4684 4 ASTM D445 5 ASTM D4683. Automotive engine multi-grade oils ¨ For most single-grade oils, the difference in viscosities between extremes temperatures is too large. ¨ Multi-grade oils (e.g., 5W-30): special polymer additives called viscosity index improvers , or VIIs are added to the oil. ¨ VIIs cause the multi-grade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot. 14 Automotive engine multi-grade oils ¨ The first number (15W) refers to the viscosity grade at low temperatures (W from winter) ¨ The second number (40) refers to the viscosity grade at high temperature.
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- Fall '08
- Internal combustion engine, ASTM