Question 1

When must a diesel engine fuel system be bled and how is it done?

Accepted Answer

Bleed when: fuel filters changed; fuel tank ran dry; new fuel line fitted. Method: open bleed screws at the primary filter, secondary filter, and injection pump (in that order); use the manual priming pump or hand primer to pump fuel through each point until bubble-free fuel flows; close screw; repeat upstream to downstream. Air in the high-pressure side requires specialist attention Air in the low-pressure fuel circuit prevents the engine starting — the injection pump cannot create the required pressure with air in the system. Bleeding sequence: work upstream to downstream. 1) Primary filter bleed screw — pump until fuel flows without bubbles. 2) Secondary filter — same. 3) Injection pump bleed screw. 4) Some engines: individual injector bleed. Most marine auxiliaries have an accessible hand priming pump on the primary filter head. Carry the relevant engine manual and practice the bleed procedure before you need it at sea.

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.