Final answer:
The intake valve opens as the piston moves down, creating a vacuum that allows air (and fuel) to enter the cylinder for the intake stroke of a four-stroke cycle. This precedes the compression, power, and exhaust strokes, completing one engine cycle.
Step-by-step explanation:
The intake valve opens on a pump when the piston goes down because this movement creates a vacuum inside the cylinder which subsequently leads to a drop in air pressure relative to the outside atmosphere. This difference in pressure causes the intake valve to open, allowing air (and in the case of a combustion engine, fuel as well) to flow into the chamber to fill the vacuum. When the piston descends during the intake stroke, air is mixed with fuel, and this constitutes the beginning of the four-stroke cycle in internal combustion engines. After this intake stroke, the piston then moves up to compress the air-fuel mixture in a nearly adiabatic process, with the valves closed, where work is done on the gas.
In a four-stroke engine, this cyclical process involves the intake stroke, the compression stroke, the power stroke (ignition and expansion), and the exhaust stroke. During the power stroke, the ignited gas expands and does work on the piston by exerting force and moving it back down. Finally, during the exhaust stroke, the piston moves up once again with the exhaust valve open to expel the combustion gases and complete the cycle.