What are the functions of engine oil and why must it be changed at the specified interval?

A. Oil change interval is fixed at 12 months regardless of engine hours

B. Modern synthetic oils never degrade and need not be changed if the engine is used infrequently

C. Engine oil only lubricates — cooling and cleaning are performed by the coolant system

D. Engine oil: lubricates all moving metal surfaces; cools internal components; cleans (detergent oil suspends combustion by-products); seals (helps piston rings seal); protects against corrosion. Must be changed because it oxidises, breaks down, and becomes saturated with acidic combustion by-products that attack bearing surfaces

A. Compression ignition: diesel fuel is ignited by the heat of highly compressed air (compression ratio ~22:1) — no spark plugs are used. Fuel is injected into the compressed air at the moment of ignition

B. Diesel engines work at lower compression ratios than petrol engines for safety

C. Diesel engines use spark plugs but fire in a different sequence to petrol engines

D. Diesel engines use carburettors to mix fuel and air before compression

A. Compression, ignition, induction, exhaust — the compression happens before air enters

B. Induction (air in): piston down, air drawn into cylinder. Compression: piston up, air compressed to high temperature. Power (ignition): fuel injected, combustion drives piston down. Exhaust: piston up, exhaust gases expelled

C. Ignition, compression, induction, exhaust

D. Induction, power, compression, exhaust