Question 1

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.

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

What are glow plugs in a diesel engine and when do they become critical?

Accepted Answer

Glow plugs are electric heaters in the pre-combustion chamber or cylinder head that pre-heat the air to assist cold starting — critical in cold weather (below 5°C) when compression alone may not generate sufficient heat for reliable ignition Glow plugs (or heater plugs in some designs) preheat the combustion chamber for 5-15 seconds before cranking in cold conditions. Modern direct-injection diesels are better cold-starters and may have minimal glow plug dependency. Indirect-injection engines (older design, common in some auxiliaries) rely on them more. Signs of failed glow plugs: hard starting in cold weather only, eventually starts after extended cranking. Test by measuring resistance (~1-2 ohms) across each plug with a multimeter.