- Practice · 8 marks
Lubricant properties, additives and SAE grading
Short answer
Important properties are viscosity, high viscosity index, oiliness, flash and pour points, oxidation stability. Lubricants may be liquid (mineral, synthetic), greases, or solid (graphite, MoS₂). Additives include VI improvers, detergents, anti‑wear agents, anti‑oxidants. SAE grading is based solely on viscosity: single‑grade (SAE 30,40) and winter (5W); multigrade (10W‑30) meets both low‑ and high‑temperature requirements.
List and explain the important properties of engine lubricating oils. State the types of lubricants and the additives used in them. Explain the SAE system of grading lubricating oils.
Properties of a good lubricant
- Viscosity: resistance to flow. It must be high enough to hold a film under load, and low enough to keep friction low and allow cold starting.
- Viscosity index (VI): a measure of how much the viscosity changes with temperature. A high VI (above 100) means a smaller change. Multigrade oils have a high VI.
- Oiliness (lubricity): ability to form a film on metal under boundary conditions.
- Flash and fire point: must be higher than the operating temperature (flash point above about 200 degrees C), for safety against fire and evaporation.
- Pour point: lowest temperature at which the oil still flows; must be low for cold climates.
- Cloud point: the temperature at which wax crystals first appear and the oil turns cloudy; it must be low.
- Oxidation (chemical) stability: resistance to oxidation, sludge and carbon formation at high temperatures.
- Detergency, anti-foam, corrosion resistance and low volatility.
Types of lubricants
| Type | Examples | Note |
|---|---|---|
| Liquid (oils) | Mineral oil from petroleum, synthetic (PAO, esters), vegetable | Engine oils |
| Semi-solid | Grease (oil + soap) | Wheel bearings, chassis |
| Solid | Graphite, molybdenum disulphide | Very high temperature or load |
Additives
| Additive | Purpose |
|---|---|
| Viscosity index improver (polymers) | Reduce viscosity change with temperature |
| Pour point depressant | Lower pour point |
| Detergent and dispersant | Keep deposits and sludge suspended and clean |
| Anti-oxidant | Prevent oil ageing |
| Anti-wear (ZDDP), extreme-pressure | Protect surfaces under high load |
| Anti-foam, corrosion inhibitor | Stop foam, protect bearings from acids |
SAE grading
The Society of Automotive Engineers classifies oils only by viscosity, not by quality.
- Single grade: SAE 20, 30, 40, 50 (viscosity measured at 100 degrees C). A higher number means a thicker oil.
- Winter grades have a W: SAE 0W, 5W, 10W, 15W (measured at low temperature for cranking and pumping).
- Multigrade: SAE 10W-30 behaves like a 10W oil when cold and like a 30 grade oil when hot, so the same oil serves winter and summer.
Quality is given separately by the API service classification (for example SJ, SN for petrol; CF, CK for diesel).
✓ Answer checked by Claude Opus 5.5 (high effort)
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