Question 1

What does a turbocharger do and what is the main maintenance concern on a turbocharged marine diesel?

Accepted Answer

A turbocharger uses exhaust gas energy to spin a turbine that compresses intake air, increasing air mass in the cylinder and allowing more fuel to be burned — increasing power output 40-60% without increasing engine size. Main concern: oil quality and level (turbos rotate at 100,000+ RPM on a thin oil film — degraded oil causes premature bearing failure); also allow engine to idle for 2-3 minutes before shutdown to let the turbo cool Turbocharger lubrication: the bearing rotates on oil film from the engine lubrication system. Stopping a hot engine abruptly ("hot shutdown") leaves oil in the hot turbo bearing, which cookes onto the bearing surfaces ("coking"). This degrades bearings over time. Correct procedure: idle for 2-3 minutes before shutdown. A turbo with degraded bearings will whistle, vibrate, and eventually destroy itself. Signs of turbo failure: blue smoke (oil burning), loss of power, unusual whine. Turbo replacement is expensive — prevent it with oil changes and correct shutdown procedure.

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.