Question 1

How do you confirm the alternator is charging correctly at sea?

Accepted Answer

With engine running: battery voltage should read 13.8-14.4V (for a 12V system) on a voltmeter or battery monitor — higher than the 12.6V resting voltage. Ammeter shows positive charge current. Belt tension correct and not slipping. Warning light out. A reading below 13.5V suggests alternator or regulator fault Alternator output: a 12V system alternator typically outputs 14.2-14.4V (absorption charging) when the battery is depleted, dropping to 13.6-13.8V when float charging. Check with a voltmeter at the battery terminals. If voltage is below 13.5V with engine running: belt slipping, diode pack failure, regulator fault, or connection problem. An ammeter shows whether current is flowing. Most modern boats have a battery monitor — the alternator state should show clearly after engine start.

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.