---
title: "Engine lubrication systems and SAE grades"
subject: "Automotive Technology"
chapter: 4
key: "semester_5/automotive_technology:lubrication-systems-and-oil-grades"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/lubrication-systems-and-oil-grades
site: BCT Engineering
---

# Engine lubrication systems and SAE grades

Automotive Technology · Chapter 4: Engine Cooling and Lubrication

- Asked in: year not given, 4+4 marks
- Video: [Lubrication System in Engine || Engine Lubrication System](https://www.youtube.com/watch?v=wrLGxNwpbt8) — Gear Institute Mechanical Engineering Videos
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/automotive_technology/lubrication-systems-and-oil-grades
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-4

## Question

(a) Name the functions of an engine lubricating system and describe the splash, pressure (wet sump) and dry sump systems. (b) What is meant by SAE 15W-40? Explain viscosity index.

## Short answer

Engine lubrication reduces friction, cools, seals, cleans and prevents corrosion. Splash system dips oil; wet sump uses a pump and filter; dry sump stores oil externally with scavenge pump. SAE 15W-40 multigrade oil has cold viscosity 15W and hot viscosity 40. Viscosity index (VI) measures viscosity change with temperature; high VI indicates minimal thinning when hot.

## Answer

#### (a) Lubrication

**Functions:** reduce friction and wear, cool the parts (piston underside, bearings), seal the piston rings, clean (carry dirt away to the filter) and protect against corrosion.

**1. Splash system:** a dipper on the connecting rod big end splashes oil from the sump onto the cylinder walls and bearings. It is simple and used in small engines (e.g. lawn mowers), but the oil supply varies with speed and oil level.

**2. Pressure (wet-sump) system:** a gear pump picks up oil from the sump, passes it through a filter and feeds it at 2 to 4 bar through the main gallery to the main bearings, big-end bearings (through drillings in the crankshaft), camshaft and valve gear. A relief valve limits the pressure, and the oil drains back to the sump.

```text
 Sump -> strainer -> pump -> filter -> main gallery
                        |  (relief valve)   |-> crank bearings
                        +<------------------ |-> cam, valves
```

**3. Dry-sump system:** oil drains to a small collector; a scavenge pump moves it to a separate external tank, and a pressure pump feeds the engine. It is used in racing and sports cars and in aircraft, because the engine can be mounted lower and there is no oil starvation during hard cornering or on steep inclines.

| Point | Wet sump | Dry sump |
|---|---|---|
| Oil stored | In crankcase sump | In separate tank |
| Pumps | One | Two (pressure + scavenge) |
| Cost / height | Cheap, taller | Costly, engine can sit lower |

#### (b) SAE grade and viscosity index

**SAE viscosity grade:** the Society of Automotive Engineers rates oil by viscosity. In **15W-40**, "15W" (W for winter) shows that the oil behaves like an SAE 15 oil when cold (it can be cranked and pumped down to about -20 degrees C), while "40" shows that at 100 degrees C it behaves like an SAE 40 oil. It is a multigrade oil for use all year: thin when cold for easy starting, but thick enough when hot to protect the engine.

**Viscosity index (VI):** a number that shows how little the viscosity changes with temperature. A high VI means the oil thins less when hot. Multigrade oils use polymer additives to give a high VI.

Oil must also meet quality grades such as API SN (petrol) and CK-4 (diesel).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does the pressure (wet-sump) system work?
   A. A dipper splashes oil from the sump
   B. A gear pump feeds filtered oil at 2 to 4 bar to bearings, with a relief valve for safety
   C. A scavenge pump moves oil to an external tank only
   D. Oil is mixed with petrol
   Answer: B. The pump draws from the sump, through the filter and main gallery to bearings and valve gear; the relief valve limits pressure. (Key point: Pressure system)

2. Why is a dry-sump system used in racing cars?
   A. It is cheaper
   B. It has one pump
   C. No oil starvation in high cornering and the engine can sit lower
   D. It needs no filter
   Answer: C. Oil is held in a separate tank with a scavenge pump and a pressure pump, so supply continues under inclination and the engine can be mounted lower. (Key point: Dry sump)

3. What does 15W-40 mean?
   A. Flows like SAE 15 when cold (W = winter) and behaves like SAE 40 at 100°C
   B. 15 % water, 40 % additive
   C. Works only from 15 to 40°C
   D. Weight 15 kg per 40 L
   Answer: A. It is a multigrade oil, thin enough when cold for starting and thick enough when hot for protection. (Key point: SAE 15W-40)

4. What does a high viscosity index indicate?
   A. Viscosity changes little with temperature
   B. Oil is very thick
   C. Oil burns easily
   D. Oil is petrol-compatible
   Answer: A. VI shows how little viscosity changes with temperature; multigrade oils use polymer additives to achieve a high VI. (Key point: Viscosity index)

## Related in Automotive Technology

- [Engine management system and ADAS](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-management-and-adas) — Automotive Technology, chapter 11
- [Engine displacement, compression ratio and power](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-displacement-bmep-numerical) — Automotive Technology, chapter 1
- [Phases of combustion in a CI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Acombustion-ci-engine-phases) — Automotive Technology, chapter 3
- [BMS functions and EV range numerical](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abms-ev-range-numerical) — Automotive Technology, chapter 12
- [Battery ignition system and spark timing calculation](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abattery-ignition-spark-numerical) — Automotive Technology, chapter 3
- [Manual and power steering systems](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Amanual-and-power-steering) — Automotive Technology, chapter 6

## Related in other subjects

- [Engine lubrication systems](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-lubrication-systems-types) — Internal Combustion Engines, chapter 6
- [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
- [Types of lubrication and viscosity](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Alubrication-types-viscosity) — Machine Design I, chapter 7
- [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