Question 1

What coolant should be used in a marine diesel and how often should it be changed?

Accepted Answer

Use the engine manufacturer's specified coolant — typically ethylene glycol antifreeze/coolant mixed 50:50 with fresh water, providing freeze protection to -37°C and corrosion inhibitor protection. Change every 2 years — the corrosion inhibitor package depletes over time even without use. Test concentration with a refractometer. Never use plain water alone in a marine engine (no corrosion protection, freezes) Coolant quality is often neglected. OAT (Organic Acid Technology) and HOAT coolants are preferred in modern engines — check the engine manual. Do not mix different coolant types (OAT + conventional silicate coolant = gel formation that blocks the cooling system). A refractometer (cheap, available from chandlers) tests antifreeze concentration accurately. Old coolant becomes acidic and corrodes aluminium components — the water pump, thermostat housing, and heat exchanger are at risk. Change every 2 years regardless of usage.

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.