Engine Systems and Components
This domain tests the four-stroke cycle, the function of engine parts, engine terms and simple capacity calculations.
The four-stroke cycle
- Induction draws the air-fuel mixture into the cylinder.
- Compression raises pressure and temperature; the spark plug fires near the end of compression.
- Power: burning gas pushes the piston down.
- Exhaust pushes burnt gas out.
- The crankshaft makes two revolutions per complete cycle, and the camshaft turns once.
Components and terms
- Block, cylinder head, liner, piston, compression rings, oil ring, gudgeon pin, connecting rod, crankshaft, flywheel, camshaft, tappets and valves.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume.
- Firing order for an inline four-cylinder engine is commonly 1-3-4-2; for an inline six, 1-5-3-6-2-4.
- Valve clearance is set with a feeler gauge, normally with the engine cold.
Worked example
Find the swept capacity of a four-cylinder engine with bore 70 mm and stroke 80 mm. Use centimetres: bore 7.0 cm, stroke 8.0 cm. One cylinder = π/4 × 7.0² × 8.0 = 0.7854 × 49 × 8 = 307.9 cm³. Four cylinders = 307.9 × 4 = 1231.5 cm³, about 1232 cc. The reasoning is to calculate one cylinder first, then multiply by the number of cylinders.
Traps
- Swapping bore and stroke in the formula.
- Dividing swept volume by clearance volume only, instead of adding both volumes.
- Setting valve clearance on a hot engine when the specification is cold.
- Confusing camshaft speed with crankshaft speed.