Question 1

A diesel-engined yacht is to be laid up afloat over winter. What engine winterisation is required?

Accepted Answer

Change engine oil and filter (acids in used oil corrode bearings over winter); flush raw water circuit with fresh water or antifreeze (prevents salt corrosion and freezing in the impeller pump); replace raw water impeller; check coolant antifreeze concentration; fog the cylinder bores with oil (prevents rust on cylinder walls); treat fuel with biocide; check anodes; cover air intake Winterisation priorities: 1) Oil change — acidic combustion by-products eat bearings if left in contact all winter. 2) Raw water flush — residual seawater in the impeller housing and heat exchanger corrodes aluminium and brass, and freezes causing cracking. 3) Fuel treatment — diesel bug thrives in winter tanks during temperature cycling. 4) Antifreeze check — most marine engines specify -15°C protection minimum; top up if needed. Fogging cylinders is less critical on diesels than petrol engines but still recommended. A thorough autumn service pays for itself in spring.

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.