Viscosity and Viscosity Index of Lubricating Oils
Viscosity
Definition: Viscosity is the measure of a fluid's internal resistance to flow. It is essentially the "internal friction" of the liquid. In lubricating oils, it determines the thickness of the oil film formed between moving surfaces.
- Measurement: It is usually measured as Kinematic Viscosity using instruments like the Redwood Viscometer or Saybolt Viscometer.
- Units: The common unit is Centistoke (cSt) or the SI unit Pascal-second (Pa·s).
- Impact: If viscosity is too low, the lubricant film will break under load, causing wear. If it is too high, it leads to excessive power consumption due to fluid friction.
Viscosity Index (V.I.)
Definition: The Viscosity Index is an arbitrary, unitless scale that indicates the degree of variation of viscosity with changes in temperature.
Key Principle: All lubricating oils thin out (viscosity decreases) as temperature increases. The V.I. measures how much it thins out.
- High V.I.: Indicates a small change in viscosity with temperature. These oils are preferred because they stay thick enough to protect at high heat but thin enough to flow during cold starts.
- Low V.I.: Indicates a large change in viscosity. The oil becomes very thin at high temperatures and very thick at low temperatures.
Mathematical Calculation of V.I.
The Viscosity Index is calculated using the following formula:
$$VI = \frac{L - U}{L - H} \times 100$$
Where:
- U: Viscosity of the test oil at 40°C.
- L: Viscosity of a standard 0-VI oil at 40°C (having the same viscosity as the test oil at 100°C).
- H: Viscosity of a standard 100-VI oil at 40°C (having the same viscosity as the test oil at 100°C).
Summary Comparison
| Feature | Viscosity | Viscosity Index |
|---|---|---|
| Core Concept | Fluidity/Resistance to flow. | Temperature-Viscosity relationship. |
| Units | Centistokes (cSt), Poise. | Unitless (Dimensionless). |
| Dependence | Depends on molecular attraction. | Depends on chemical structure (paraffinic vs. naphthenic). |
