The document discusses valve timing diagrams for 4-stroke and 2-stroke petrol and diesel engines. It provides details on the opening and closing of intake, exhaust, and transfer ports during each stroke. For 4-stroke engines, it describes the intake, compression, power, and exhaust strokes. The actual valve timings are given for 4-stroke diesel and petrol engines. For 2-stroke engines, it explains the expansion and compression strokes and provides actual valve timings. Diagrams are included to illustrate the valve timing events during each stage of the engine cycles.