--- title: "Lubricant properties, additives and SAE grading" subject: "Internal Combustion Engines" chapter: 6 key: "semester_6/internal_combustion_engines:lubricant-properties-additives-sae-grading" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/lubricant-properties-additives-sae-grading site: BCT Engineering --- # Lubricant properties, additives and SAE grading Internal Combustion Engines · Chapter 6: Engine lubrication systems - Asked in: year not given, 8 marks - Video: [SAE RATING OF Lubricating ऑयल | PROPERTIES OF LUBRICANT | FLASH POINT | FIRE POINT | VISCOSITY |](https://www.youtube.com/watch?v=R12TjmWGiDE) — AUTOMANIAC THE MECHANICAL CLUB - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/lubricant-properties-additives-sae-grading - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-6 ## Question 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. ## 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. ## Answer #### 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a high viscosity index (VI) mean? A. Viscosity changes a lot with temperature B. Viscosity changes little with temperature C. The oil is very thick D. The oil has a high flash point Answer: B. VI measures the rate of change of viscosity with temperature; a high VI means a smaller change, as in multigrade oils. (Key point: Viscosity index) 2. On what basis does the SAE system grade oils? A. Quality and additives B. Viscosity only C. Flash point D. Detergency Answer: B. SAE classifies only by viscosity; quality is given separately by the API service classification. (Key point: SAE grading basis) 3. What does SAE 10W-30 mean? A. Behaves like 30 when cold and 10 when hot B. Behaves like a 10W oil when cold and a 30 grade oil when hot C. Contains 10% water and 30% additive D. Can be used only in winter Answer: B. A multigrade oil has the cold behaviour of the W grade and the hot behaviour of the second number, so it serves summer and winter. (Key point: Multigrade meaning) 4. What is the purpose of a pour point depressant? A. Raise the flash point B. Lower the pour point so the oil flows in cold weather C. Prevent foaming D. Keep sludge suspended Answer: B. Pour point depressants lower the lowest temperature at which the oil still flows; detergents keep sludge suspended and anti-foam stops foam. (Key point: Additive purpose) ## Related in Internal Combustion Engines - [IP, BP and efficiencies of a petrol engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aindicated-brake-power-efficiency-numerical) — Internal Combustion Engines, chapter 8 - [Engine lubrication systems](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-lubrication-systems-types) — Internal Combustion Engines, chapter 6 ## Related in other subjects - [Engine lubrication systems and SAE grades](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Alubrication-systems-and-oil-grades) — Automotive Technology, chapter 4 - [Lubricant functions; solid lubricants](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricant-functions-solid) — Engineering Chemistry, chapter 8 - [Lubricating oil functions and lubricating system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-lubricating-system) — Power Plant Equipment, chapter 5 - [Lubricants and their functions](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alubricants-functions) — Engineering Chemistry, chapter 8, asked 4×