What is produced when the piston moves downward during the intake stroke?

During the intake stroke of an engine, the piston moves downward in the cylinder. This movement creates a space within the cylinder that results in a decrease in air pressure inside the combustion chamber. As the piston descends, it effectively lowers the pressure relative to the outside atmosphere, leading to a condition known as negative pressure or vacuum.

This negative pressure facilitates the flow of air (and fuel, in a fuel-injected engine) into the cylinder from the environment, as air rushes in to equalize the pressure difference. This process is crucial for efficient engine operation, as it ensures that the right mixture of air and fuel enters the combustion chamber for optimal combustion and power generation.

Thus, the correct understanding of the intake stroke is that it relies on the creation of negative pressure, which is essential for drawing in the necessary air-fuel mixture for the next stage of the engine cycle.