In an internal combustion engine, during operation, which event pulls the fuel/air into the combustion chambers?

• A. When the pistons move up
• B. As the pistons move down, they create a vacuum
• C. During compression
• D. At top dead center

Answer: (B)

The air–fuel mixture is pulled in during the intake stroke. As the piston moves downward, the chamber volume increases and pressure inside falls below the surrounding atmosphere. With the intake valve open, air (and fuel, where applicable) is drawn into the cylinder to fill the expanding space. This suction happens before the piston reaches the compression phase. The other events don’t create this inhaling action: moving up compresses the mixture, compression increases pressure, and top dead center is simply a timing point rather than the event that pulls in the charge.