Question 1

During a pre-season check, the fuel tank is found to contain water and black sludge. What is the procedure?

Accepted Answer

Do not start the engine until the tank is cleaned. Pump out all fuel (use a hand pump or deck pump into approved containers). Inspect the tank interior (use a torch through the deck filler or access plate). Clean sludge from the bottom. Dose the clean tank with diesel biocide. Polish the fuel (filter through a fine filter) or replace entirely. Clean or replace primary filter. Replace secondary filter. Bleed the fuel system. Then restart A contaminated tank requires complete remediation before first use. Adding biocide to an active diesel bug infection kills the microorganisms but the dead cell mass remains and continues to block filters. Tank cleaning is the only permanent solution. Polishing the fuel (passing it through a fine filter) removes both water and particulate. After cleaning: new filters are mandatory — the old filters are saturated with contaminated fuel and will reintroduce contamination. This is a half-day job that prevents engine failure 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.