Question 1

What maintenance does a marine diesel heat exchanger require and what are the signs of a blocked exchanger?

Accepted Answer

Clean the raw water tubes annually (scale and marine organism deposits restrict flow); check and replace the internal zinc anode (sacrificial zinc rod inside the exchanger body, prevents galvanic corrosion of the aluminium shell); check hose connections for leaks. Signs of blockage: engine running warmer than normal; coolant temperature rising gradually over a passage; overheating alarm with adequate raw water flow Heat exchanger maintenance: the raw water (seawater) side of the heat exchanger accumulates scale (mineral deposits), barnacle larvae, and biological growth that restricts heat transfer. Annual fresh water flush and periodic acid descaling (dilute citric acid through the raw water circuit) prevents blockage. The internal zinc anode is critical — without it, galvanic corrosion of the aluminium shell from the brass tube bundle occurs rapidly. Replacement every 1-2 years. Blocked exchanger overheating is a slow process that may not trigger the temperature alarm until head gasket damage has occurred.

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.