--- title: "Lubricating oil functions and lubricating system" subject: "Power Plant Equipment" chapter: 5 key: "semester_7/power_plant_equipment:diesel-lubricating-system" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/diesel-lubricating-system site: BCT Engineering --- # Lubricating oil functions and lubricating system Power Plant Equipment · Chapter 5: Diesel Power Plant - Asked in: 2070 Asar, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/diesel-lubricating-system - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-5 ## Question Write down the functions of lubricating oil in diesel engine. Sketch lubricating system of a diesel power plant and explain its working. ## Short answer Lubricating oil reduces friction and wear, cools components, seals piston rings, cleans away contaminants, prevents corrosion, and absorbs shock. A forced-feed system pumps oil from sump through filter and cooler to engine bearings, then drains back. ## Answer **Lubrication** in a diesel engine means supplying a film of oil between surfaces that move relative to each other, so that metal-to-metal contact is avoided. #### Functions of lubricating oil 1. **Reduces friction and wear** of pistons, rings, liners, bearings, cams and gears; this also reduces friction power loss. 2. **Cooling:** carries heat away from bearings and from the underside of piston crowns. 3. **Sealing:** forms a seal between piston rings and cylinder liner, preventing blow-by of gas. 4. **Cleaning:** washes away carbon, metal particles and dirt to the filters. 5. **Corrosion protection** of metal surfaces against acids formed by combustion. 6. **Shock absorption / noise reduction** in bearings and gears. #### Sketch: forced-feed (pressure) lubrication system ```text +--------------- Engine ---------------+ | main brgs, crank pins, gudgeon pins, | | camshaft, rocker arms, cyl. walls | +--------------------------------------+ ^ | drains | oil gallery v Oil cooler <- Fine filter SUMP / oil tank ^ ^ | | | strainer +---- Relief --+-- PUMP <+ valve (gear) raw cooling water through cooler ``` #### Working 1. A gear pump driven by the engine draws oil from the **sump** (or a separate oil tank in dry-sump systems) through a coarse **strainer**. 2. The oil is pumped at about 2–4 bar through a **fine filter** and an **oil cooler** (cooled by water from the cooling system). A **pressure relief valve** returns excess oil to the sump. 3. Oil enters the main **oil gallery** and is distributed through drilled passages to the main bearings, then through holes in the crankshaft to the crank-pin bearings and up the connecting rod to the gudgeon pin. 4. Branches feed the camshaft bearings, valve gear and rocker arms. Cylinder walls are lubricated by oil splash from the crank or by separate **cylinder lubricators** in large engines. 5. Oil drains back by gravity to the sump; in large plants it is periodically purified in a **centrifuge** to remove water and sludge. 6. A pressure gauge and temperature gauge (with low-pressure alarm/trip) protect the engine. Other lubrication methods are **splash** and **mist** (oil mixed with the fuel, used in small two-strokes) systems. A forced-feed system may be **wet sump** or **dry sump**; power plants use forced-feed systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which function does lube oil perform at the piston rings? A. Seals between rings and liner to prevent gas blow-by B. Ignites the fuel C. Raises compression temperature D. Meters the fuel Answer: A. The oil film seals the ring-liner gap. (Key point: Sealing function) 2. Besides reducing friction, how does lube oil help cooling? A. Cools the exhaust gases B. Carries heat away from bearings and the underside of piston crowns C. Replaces jacket water D. Cools the intake air Answer: B. Oil circulates and takes heat to the oil cooler. (Key point: Cooling by oil) 3. What does the pressure relief valve do in the forced-feed system? A. Filters the oil B. Cools the oil C. Returns excess oil to the sump to limit pressure D. Drains water from oil Answer: C. It protects the circuit from over-pressure by bypassing oil back. (Key point: Pressure relief valve) 4. What is the correct oil path? A. Sump, cooler, pump, bearings, filter B. Pump, gallery, strainer, sump, cooler C. Gallery, pump, bearings, strainer, sump D. Sump, strainer, pump, fine filter, cooler, gallery, bearings, back to sump Answer: D. The pump draws oil through a coarse strainer, then it is filtered, cooled and distributed from the main gallery; it drains by gravity. (Key point: Oil path) ## Related in Power Plant Equipment - [Diesel components, lubricating and cooling functions](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-components-lube-cooling) — Power Plant Equipment, chapter 5 - [Fuel supply system of diesel plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-fuel-supply) — Power Plant Equipment, chapter 5, asked 3× - [Diesel fuel supply and lubrication functions](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-fuel-supply-lubrication) — Power Plant Equipment, chapter 5 - [Layout of diesel power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-plant-layout) — Power Plant Equipment, chapter 5, asked 3× - [Diesel plant lubrication and cooling systems](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-lubrication-cooling) — Power Plant Equipment, chapter 5, asked 2× - [Diesel engine 75 kW: fuel consumption and efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-75kw-fuel-consumption) — Power Plant Equipment, chapter 5 ## Related in other subjects - [Lubricating greases and their functions](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Agreases-functions) — Engineering Chemistry, chapter 8, asked 3× - [Ideal diesel engine CR 20: efficiency and MEP](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cr20-max-2200k) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Heat balance sheet of a diesel engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-engine-heat-balance-numerical) — Internal Combustion Engines, chapter 8 - [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3