When does fuel/air get pulled into the combustion chambers?

The correct timing for when fuel and air are drawn into the combustion chambers occurs when the piston moves down in the power cycle of an internal combustion engine. This process is part of the intake stroke.

During the intake stroke, as the piston descends, a vacuum is created in the combustion chamber. This vacuum pressure allows the intake valves to open, permitting the air-fuel mixture to flow from the intake manifold into the combustion chamber. The downward movement of the piston is essential because it creates the negative pressure required for this intake of the air-fuel mixture.

Understanding this mechanism is critical in the context of engine operation and efficiency, as it directly affects the engine's ability to draw in the necessary components for combustion, ultimately impacting performance and fuel economy.