Automotive Engine Fundamentals
This domain checks whether you understand how a reciprocating engine turns fuel energy into crankshaft rotation, and whether you can handle the standard terms - bore, stroke, swept volume, clearance volume and compression ratio - numerically.
The four-stroke cycle
- Induction: the piston moves down from top dead centre (TDC) with the inlet valve open, drawing in air or an air–fuel mixture.
- Compression: both valves closed, the charge compressed; petrol engines run about 8:1 to 10:1, diesels 16:1 to 22:1.
- Power: the spark plug fires (petrol, spark ignition) or fuel is injected into hot compressed air (diesel, compression ignition); this is the only stroke that produces work.
- Exhaust: the exhaust valve opens and burnt gas is pushed out. Bore is cylinder diameter; stroke is the TDC-to-BDC distance.
Firing order and valve timing
Four-cylinder engines commonly fire 1-3-4-2, spreading the power impulses and balancing the crankshaft. Valve clearance is set cold with a feeler gauge; too little clearance holds a valve off its seat and burns it.
Worked example
Bore 80 mm, stroke 90 mm, clearance volume 50 cm³. Swept volume of one cylinder = π/4 × 8² × 9 = 452.4 cm³. Compression ratio = (swept + clearance) ÷ clearance = (452.4 + 50) ÷ 50 = 10.05:1, a normal petrol figure. Keep bore and stroke in the same unit throughout.
Traps
Reporting compression ratio as swept ÷ clearance, which gives 9.05 and is wrong; quoting diesel compression ratios below petrol ones; placing the piston at TDC at mid-stroke; mixing bore with stroke; forgetting that only the power stroke delivers work.