Automobile Engine Fundamentals and Operation
Engine Fundamentals and Operation
This domain examines how a reciprocating internal-combustion engine converts chemical energy into crankshaft rotation, and how geometry and cycle events affect performance. The exam expects correct use of bore, stroke, swept volume, compression ratio, valve timing and firing order, plus a clear distinction between spark-ignition (SI) and compression-ignition (CI) operation.
Core rules
- In a four-stroke SI engine the piston makes intake, compression, power and exhaust strokes; the crankshaft turns twice per cycle while the camshaft turns once.
- A CI engine draws air only, compresses it to high pressure and temperature, then the injector sprays diesel near top dead centre; ignition is by heat, not by a spark.
- Swept volume per cylinder = (π/4) × bore² × stroke. Compression ratio CR = (swept volume + clearance volume) ÷ clearance volume.
- Firing order distributes power strokes; valve overlap is the period when inlet and exhaust valves are both slightly open near TDC.
Worked example
A four-cylinder petrol engine has bore 80 mm, stroke 90 mm and clearance volume 50 cm³ per cylinder. Convert to cm: bore 8.0 cm, stroke 9.0 cm. Swept volume = (π/4)(8.0)²(9.0) = 452.4 cm³. CR = (452.4 + 50) ÷ 50 = 10.05:1. The reasoning is that CR compares the total volume above the piston at bottom dead centre with the clearance volume at TDC; the clearance volume is included once in the numerator and is the denominator.
Traps
- Using bore instead of radius in the area formula; (π/4)d² is correct.
- Calling swept volume the compression ratio, or forgetting clearance volume.
- Saying the camshaft turns twice per cycle; it turns once.
- Describing diesel ignition as spark-initiated.