The air-fuel mixture is forced into the intake manifold by ____________.

The air-fuel mixture is forced into the intake manifold primarily by atmospheric pressure. This process occurs during the intake stroke of the engine cycle when the piston moves down in the cylinder. As the piston descends, it creates a vacuum that allows air to be drawn into the cylinder from the intake manifold. The atmospheric pressure outside the engine pushes air (along with the fuel that is mixed with it) into the intake manifold and subsequently into the combustion chamber.

When the piston moves down, it creates a lower pressure environment, enabling the higher atmospheric pressure surrounding the engine to force the mixture into the manifold. This is essential for combustion, as a proper air-fuel mixture is required for the engine to run efficiently.

Other factors such as engine temperature, vacuum pressure, and mechanical force can influence the overall performance and efficiency of the engine, but atmospheric pressure is the crucial element that propels the air-fuel mixture into the intake manifold during normal operation.