Why do 4 stroke engines not require an external blower?

In a four-stroke engine, the downward movement of the piston during the intake stroke creates a vacuum within the cylinder. This vacuum is crucial because it facilitates the drawing in of air (and fuel, in the case of carbureted engines) into the combustion chamber. As the piston moves down, it reduces the pressure in the cylinder compared to the atmospheric pressure, allowing external air to be drawn into the engine naturally without the need for an external blower or supercharger.

This process efficiently fills the cylinder with the air-fuel mixture required for combustion during the subsequent compression stroke. Since four-stroke engines utilize this vacuum effect to draw in air, there is no necessity for mechanical assistance from an external blower to force air into the combustion chamber, as is common in some two-stroke engines, which may require such assistance due to their design and operational principles.