Question 1

What do fuel injectors do and how do you identify a failing injector?

Accepted Answer

Injectors atomise diesel into a fine spray at very high pressure (200-2000 bar depending on system) directly into the combustion chamber at the correct moment. A failing injector: rough running on one cylinder; white/blue smoke at idle; increased fuel consumption; distinct "knock" from that cylinder; engine temperature higher than usual. Diagnosis: cut-out test (momentarily cut fuel to each cylinder in turn — rough running gets smoother when the bad cylinder is isolated) Modern common-rail injectors operate at 1600-2000 bar. Traditional mechanical injection (most older marine auxiliaries): 200-400 bar. The injector nozzle must produce a precise spray pattern — a partially blocked nozzle produces an uneven spray, causing poor combustion, smoke, and engine knock. Injectors are precision components requiring specialist testing equipment. At sea, the symptom recognition (rough running, smoke, knock) allows the skipper to identify the problem and manage the engine appropriately until professional repair.

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.