In a four-stroke internal combustion engine, what is the purpose of the intake stroke?

• A. Power the piston
• B. Compress the air
• C. Exhaust waste gases
• D. Draw in air-fuel mixture

Answer: (D)

The intake stroke’s purpose is to admit the charge into the cylinder. As the piston moves down and the intake valve opens, the rising volume creates a low pressure that draws in air—and, in most engines, fuel is mixed with that air in the intake tract to form the air-fuel mixture. This filling sets up the next step, compression, where the mixture is pressed for efficient combustion. The other strokes have different roles: compression strengthens and heats the mixture, the power stroke uses the combustion to push the piston, and the exhaust stroke expels burnt gases. So drawing in the air-fuel mixture is what the intake stroke accomplishes.