Question 1

Match the exhaust smoke colour to its likely cause: (1) Black smoke, (2) White/grey smoke when warm, (3) Blue smoke

Accepted Answer

(1) Black: over-fuelling, blocked air filter, restricted exhaust — too much fuel or insufficient air for complete combustion. (2) White/grey when warm: coolant burning in cylinders (head gasket failure or cracked head). (3) Blue: oil burning in combustion chamber (worn piston rings, valve stem seals) Smoke diagnosis is a key skill from the RYA Diesel Course. Black smoke at idle/low revs: incomplete combustion from air restriction (dirty air filter) or faulty injector. Black smoke at all revs: over-fuelling (injection pump fault). White steam on start in cold weather: normal condensation. White/grey smoke when fully warm with sweet smell: coolant in cylinders — head gasket suspect. Blue smoke at startup clearing as engine warms: minor valve seal wear. Blue smoke continuously: significant oil consumption. Stop the engine if white smoke appears when warm — continued running will worsen head gasket damage.

Question 2

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

Accepted Answer

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 Diesel (compression ignition): air is compressed to ~22:1 ratio, reaching ~500-700°C. Diesel fuel (which ignites at ~250°C) is injected directly into this hot compressed air and spontaneously ignites. Petrol engines use ~8:1 compression and require a spark to ignite. The high compression ratio makes diesel engines more thermally efficient (~40% vs ~25% for petrol) and safer (diesel is much less volatile than petrol).

Question 3

Describe the four strokes of a diesel engine cycle in the correct order

Accepted Answer

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 The four-stroke diesel cycle (suck, squeeze, bang, blow): 1) Induction — intake valve open, piston moves down drawing in air only (no fuel). 2) Compression — both valves closed, piston moves up, air compressed to ~22:1. 3) Power — injector fires, combustion, piston driven down. 4) Exhaust — exhaust valve open, piston moves up expelling gases. Two complete crankshaft revolutions per power stroke per cylinder. Multiple cylinders provide smoother, overlapping power strokes.