Engine Systems and Overhauling
This domain examines the four-stroke cycle, engine construction, valve timing, clearances, and overhauling procedure. The exam expects you to explain how air, fuel, and combustion gases are managed and why measurement and torque discipline matter during assembly.
Core rules
- Four-stroke petrol engine: induction, compression, power, exhaust. The crankshaft turns twice per cycle; the camshaft turns once.
- Valve timing connects piston position to valve opening. Incorrect timing causes poor starting, low power, or valve-piston contact.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume.
- Overhauling includes dismantling, cleaning, measuring wear, replacing gaskets and worn parts, and reassembling to specified torque and clearances.
- Pistons, rings, cylinder bores, bearings, and valve guides are measured for taper, ovality, and clearance against the manufacturer’s limits.
Worked example
A single-cylinder engine has a swept volume of 400 cm³ and a clearance volume of 50 cm³. Compression ratio = (400 + 50) ÷ 50 = 450 ÷ 50 = 9:1. If the head is skimmed and clearance volume falls to 40 cm³, the new ratio is 440 ÷ 40 = 11:1. The reasoning is that swept volume stays fixed while clearance volume controls the ratio; too high a ratio can cause detonation unless fuel and ignition timing suit it.
Traps that cost marks
- Setting valve clearance hot when the specification is cold, or vice versa.
- Ignoring timing marks and assuming TDC from the pulley alone.
- Tightening cylinder-head bolts in a circle rather than the specified sequence.
- Measuring bore taper without cleaning carbon or using the correct micrometer setting.
- Reusing torque-to-yield bolts without checking the manual.