Motor Vehicle Mechanics and Automotive Engineering
Domain 2 covers the mechanical systems a TST/MST must understand: engine cycles, transmission, braking, steering, suspension and auto electricals.
Engine and transmission
- Four-stroke cycle: intake, compression, power, exhaust; the crankshaft makes two revolutions per cycle.
- Swept volume per cylinder = π/4 × D² × L. Engine displacement = that value × number of cylinders.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume. Diesel engines typically run 16:1–22:1; petrol 8:1–12:1.
- The clutch connects and disconnects engine and gearbox; a slipping clutch shows rising engine speed without matching road speed.
Brakes and electricals
- Hydraulic brakes use Pascal's law; air brakes are common on heavy buses. Braking efficiency depends on friction, tyre condition and load transfer.
- The alternator charges the battery; the starter cranks the engine; fuses protect circuits.
Worked example
A 4-cylinder engine has bore 100 mm and stroke 120 mm. Convert to cm: D = 10 cm, L = 12 cm. Swept volume per cylinder = π/4 × 10² × 12 = 942.48 cc. For four cylinders: 942.48 × 4 = 3,769.9 cc ≈ 3,770 cc (3.77 litres). If clearance volume per cylinder is 62.8 cc, CR = (942.48 + 62.8) ÷ 62.8 ≈ 16:1, a diesel-like ratio.
Traps
Using radius instead of diameter; forgetting to multiply by the number of cylinders; adding clearance volume to the numerator only; and confusing engine displacement with combustion-chamber volume.