Automotive Engine Systems and Principles
What this domain tests
The engine domain of the RBSE Prav Vocational Automotive examination tests how well you can explain the conversion of fuel energy into useful torque in a reciprocating petrol or diesel engine, and how confidently you can handle the numbers behind it: bore, stroke, swept volume, compression ratio, valve timing and firing order. Questions are usually a mix of labelled-diagram recall, short explanation and one small calculation.
Core rules and concepts
- A four-stroke cycle needs two crankshaft revolutions: induction, compression, power and exhaust. The camshaft turns at half crankshaft speed.
- Swept volume of one cylinder = (π/4) × bore² × stroke. Engine capacity is that figure multiplied by the number of cylinders.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume, calculated for one cylinder only.
- Petrol engines use a spark plug and a compression ratio of roughly 8:1–12:1; diesel engines rely on compression ignition at roughly 16:1–22:1.
- Valve overlap is the period when inlet and exhaust valves are both slightly open around top dead centre; it helps scavenging but too much hurts idle quality.
- Firing order (commonly 1-3-4-2 for a four-cylinder inline engine) determines ignition sequencing and engine balance.
Worked example
A four-cylinder engine has a bore of 80 mm and a stroke of 90 mm; clearance volume is 50 cm³ per cylinder. Bore = 8 cm, stroke = 9 cm. Swept volume per cylinder = 0.7854 × 8² × 9 = 452.4 cm³. Engine capacity = 452.4 × 4 = 1,810 cm³, about 1.81 litres. Compression ratio = (452.4 + 50) ÷ 50 = 10.05:1. The reasoning matters: the ratio is per cylinder, so you must not add the other three swept volumes into the formula.
Traps that cost marks
Using bore instead of radius in πr², quoting engine capacity inside the compression-ratio formula, describing a diesel engine as having a spark plug, and confusing valve overlap with tappet clearance.