Question 1

The raw water pump impeller is often called the most important consumable on a marine diesel. Why and how often should it be replaced?

Accepted Answer

The impeller is a flexible rubber vane wheel that pumps seawater through the cooling circuit. It wears, cracks, and loses vanes over time — especially if run dry even briefly. Failure causes immediate overheating. Replace every 1-2 years or 200h regardless of appearance. Carry a spare. If vanes are found missing, check they have not lodged downstream in the heat exchanger (blocking cooling) The raw water impeller is behind a small cover plate on the pump body — accessible in most engines in 15-30 minutes with a screwdriver and circlip pliers. Spares typically cost £20-50. Impeller failure caused by: running dry (even for seconds), debris ingestion, age-related rubber degradation. After failure: overheating within minutes leading to head gasket failure and engine seizure — potentially a £3,000-10,000+ repair. The most cost-effective maintenance item on a marine diesel is a £35 impeller replaced every season.

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.