Engine Fundamentals and Construction
This domain tests how a reciprocating engine converts fuel energy into rotary motion, and how its major parts are constructed and timed. Questions cover the four-stroke cycle, firing order, compression ratio, valve clearance and petrol-versus-diesel differences.
Core concepts
- A four-stroke engine completes intake, compression, power and exhaust in 720° of crankshaft rotation. The camshaft turns at half crank speed, so one cam revolution equals two crank revolutions.
- The piston moves between top dead centre (TDC) and bottom dead centre (BDC). Swept volume is the volume displaced by one piston stroke.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume. Petrol engines are typically about 8:1 to 12:1; direct-injection diesel engines are higher, often about 16:1 to 22:1, because ignition is by compression heat.
- Main parts include the cylinder block, cylinder head, piston, compression and oil-control rings, gudgeon pin, connecting rod, crankshaft, flywheel, valves, camshaft and timing belt or chain.
- A four-cylinder engine commonly fires 1-3-4-2, giving a power stroke every 180° of crank rotation.
Worked example
A single-cylinder engine has a swept volume of 400 cm³ and a clearance volume of 50 cm³. Reasoning: compression ratio = (400 + 50) ÷ 50 = 450 ÷ 50 = 9:1. If the clearance volume were reduced to 40 cm³, the ratio would rise to 440 ÷ 40 = 11:1, increasing compression pressure and heat.
Traps
Confusing swept volume with total cylinder volume, setting valve clearance when the engine is at the wrong stroke, ignoring timing marks, and assuming diesel engines use spark plugs. Ring end gaps must be staggered, and valve clearance is usually checked when the engine is cold unless specified otherwise.