How does Intake work?

During the Intake stroke, the piston opens up to let both air and fuel flow into the cylinder. The  cylinder is at maximum capacity to allow for an even distribution of both fuel and air. There does not have to be a lot of fuel in the cylinder for the engine to complete it's cycle because it will be able to react even if there is just a single drop of fuel. This does not mean the fuel-air ratio should be off, because it should always be in stoichiometric proportions to allow for maximum efficiency.  If the ratios are proportional, then the reaction SHOULD go to completion, but everyone knows that is not actually possible in the real world. The air flows into the cylinder because at that point it is a low pressure system, having just expelled all the reactants from the previous cycle.  Fluids naturally flow towards a system with a lower pressure to achieve an equilibrium of a constant pressure system.