What is the fundamental operating principle of a diesel engine that distinguishes it from a petrol engine?

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

A. Glow plugs are fitted to 2-stroke diesel engines only

B. Glow plugs are the same as spark plugs — they ignite the fuel on every compression stroke

C. Glow plugs are only used in generators — not in marine auxiliary diesels

D. Glow plugs are electric heaters in the pre-combustion chamber or cylinder head that pre-heat the air to assist cold starting — critical in cold weather (below 5°C) when compression alone may not generate sufficient heat for reliable ignition