Question 1

The engine has developed increased vibration that was not present last season. What should be checked first?

Accepted Answer

Check engine mounts first — rubber anti-vibration mounts deteriorate over time and can collapse, changing the engine alignment. Correct alignment critical: shaft coupling should be within 0.1mm alignment tolerance — misalignment causes vibration, stern gland wear, and gearbox damage. Also check propeller for damage (bent/missing blade), cutlass bearing play, and coupling bolts Engine vibration causes: 1) Collapsed/degraded engine mounts — very common after 5-7 years; 2) Propeller damage — inspect underwater or when hauled; 3) Shaft misalignment from mount collapse — use a feeler gauge at the coupling; 4) Cavitation from fouled bottom — barnacles on the propeller can cause severe vibration; 5) Cutlass bearing wear; 6) Flexible coupling wear. Engine mount collapse is the most common missed diagnosis — the mounts look intact but have softened and lost their damping. Press down on the engine by hand — more than ~5mm vertical movement suggests mount failure.

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.