What characterizes a normally aspirated engine?

A normally aspirated engine is characterized primarily by its lack of any kind of forced induction, which means it operates without a supercharger or turbocharger. This natural aspiration allows the engine to draw in air solely through the atmospheric pressure surrounding it. The correct answer is significant because supercharging or turbocharging increases the amount of air entering the engine, leading to more power and efficiency. However, in a normally aspirated engine, the air-fuel mixture enters the combustion chamber purely by the pressure differential created when the piston moves down in the intake stroke.

While it's often true that normally aspirated engines can have four or more cylinders, the number of cylinders is not a defining characteristic of their operation. Additionally, normally aspirated engines can be air-cooled or liquid-cooled, depending on their design, contradicting the idea that they are "never air-cooled." Therefore, the essence of a normally aspirated engine fundamentally lies in the absence of supercharging.