Question 1

Most marine diesels use a two-circuit cooling system. Explain how it works

Accepted Answer

Raw water circuit: seawater is pumped in through the hull via the raw water seacock and impeller pump, passes through a heat exchanger, and is discharged through the exhaust. Fresh water circuit: a closed loop of treated coolant circulates through the engine block and head, cooled by the raw water in the heat exchanger — protects the engine from salt and corrosion Two-circuit cooling: the heat exchanger transfers heat from engine coolant (antifreeze/water mix in the primary circuit) to raw water (seawater in the secondary circuit) without the two ever mixing. The engine block sees only treated coolant — no salt, no corrosion. The raw water is expelled through the wet exhaust (mixes with exhaust gases, creating the characteristic "tick, tick, tick" of cooling water). The impeller pump (raw water) is the most common cooling failure point.

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.