Internal Combustion Engines: Construction, Operation and Diagnosis
Domain focus
This domain examines the construction and working cycle of spark-ignition and compression-ignition engines, and the tests used to separate mechanical faults from fuel, ignition, and air-management faults.
Core rules
- A four-stroke cycle is intake, compression, power, and exhaust, completed in two crankshaft revolutions.
- Main components include block, cylinder head, piston, rings, connecting rod, crankshaft, camshaft, valves, and bearings.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume.
- Valve timing and overlap affect breathing; incorrect timing can cause hard starting, poor idle, or low power.
- Diagnosis uses compression test, cylinder leak-down test, cylinder balance, vacuum gauge, exhaust gas analysis, and scan data.
Worked example
An engine has an 80 mm bore, 90 mm stroke, and 50 cm³ clearance volume. Swept volume = π/4 × 8.0² × 9.0 = 452.4 cm³. Compression ratio = (452.4 + 50) ÷ 50 = 10.05:1, normally stated as 10:1. If the measured compression is low and rises after adding oil, worn rings are likely; if it remains low, suspect valves, head gasket, or cracked head.
Traps
Using bore diameter instead of radius in area calculations, mixing cm³ with mm³, and treating a dry compression result as conclusive. A leak-down test must use absolute pressure and correct cylinder position. Do not confuse a misfire from ignition with a mechanical compression fault without testing.