In a gasoline engine, what event draws air and fuel into the combustion chambers?

• A. Sparks ignite
• B. Fuel is injected
• C. Pistons move down creating a vacuum
• D. Exhaust valve opens

Answer: (C)

During the intake stroke of a four-stroke gasoline engine, the piston moves downward, increasing the cylinder’s volume and creating a vacuum. With the intake valve open, this lower pressure draws in air from outside, and in many engines fuel is introduced by the fuel system as the mixture flows in. This vacuum-driven inflow is what fills the combustion chamber with the air-fuel mixture. The moment of ignition happens after compression, not during intake, and the exhaust valve opens later to push out burnt gases. So the downward piston creating a vacuum that pulls in air (and fuel) is the mechanism that brings the mixture into the cylinder.