Describe the four strokes of a petrol engine.

The four strokes are: Intake (piston moves down, intake valve opens, air-fuel mixture enters cylinder), Compression (piston moves up, both valves closed, mixture is compressed), Power (spark plug ignites mixture, expanding gases push piston down), and Exhaust (piston moves up, exhaust valve opens, burnt gases expelled). This completes two crankshaft revolutions per cycle.