Question 1

The engine oil level is dropping noticeably between oil changes — approximately 0.5 litres per 20 hours. What are the possible causes?

Accepted Answer

Possible causes: 1) Oil leaks — check gaskets, sump plug, rocker cover seal, oil cooler hoses; 2) Valve stem seal wear — blue smoke on startup; 3) Piston ring wear — blue smoke continuously; 4) Crankcase breather blocked — pressure forces oil past seals; 5) Turbocharger bearing seal failure (turbocharged engines). 0.5L/20h is marginal — investigate; more than 1L/20h requires urgent attention Oil consumption investigation: check for external leaks first (simplest to fix). A UV dye test kit added to the oil confirms leak locations that are too small to see by eye. Internal consumption (burning oil) shows as blue smoke — check if this is at startup only (valve seals) or continuous (rings). A crankcase breather (PCV valve) blocked with oil sludge is a very common cause of excessive consumption — clean or replace it. Track oil consumption accurately (litres per hour or per 100nm) to detect trends before the oil level becomes critically low.

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.