Automotive Engine Systems and Components
Automotive Engine Systems and Components
The exam tests the four-stroke cycle, SI and CI engine differences, the function of major components, engine terminology and simple engine calculations. Expect labelled-part questions on the cylinder head, piston assembly and valve train.
Core rules
- Four strokes: intake, compression, power, exhaust. In an SI engine a spark plug ignites a petrol–air mixture; in a CI engine fuel is injected into hot compressed air and ignites spontaneously.
- Main parts: cylinder block, cylinder head, piston, piston rings (compression and oil control), gudgeon pin, connecting rod, crankshaft, flywheel, camshaft, valves, timing belt or chain, sump.
- Valve clearance is measured with a feeler gauge between the valve stem and rocker or cam follower, with the engine cold unless the manual says hot. Too little clearance holds the valve open and causes misfire or burning.
- Firing order for a four-cylinder in-line engine is commonly 1–3–4–2 or 1–2–4–3; set the timing marks before adjusting valves.
- Engine displacement = π/4 × bore² × stroke × number of cylinders.
Worked example
A four-cylinder engine has a bore of 75 mm and a stroke of 80 mm. Convert to cm: 7.5 cm and 8.0 cm. Swept volume of one cylinder = 0.7854 × 7.5² × 8.0 = 353.4 cm³. For four cylinders, 353.4 × 4 = 1413.7 cm³, quoted as 1414 cc or about 1.4 litres. If the clearance volume per cylinder is 44.2 cm³, the compression ratio is (353.4 + 44.2) ÷ 44.2 = 8.99:1, commonly written 9:1.
Traps
- Confusing swept volume with total cylinder volume.
- Adjusting valves on the wrong cylinder because the firing order was ignored.
- Mixing up SI and CI ignition methods.
- Measuring valve clearance with the cam on the peak of the lobe instead of the base circle.