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Now, let's examine why this intake manifold vacuum exists at all. After all, Diesel cycle engines apparently get away without it, so why should it exist here?
Intake manifold vacuum exists as the way for a basic Otto cycle gasoline engine to vary the amount of oxygen being fed into the cylinder for the combustion stroke.
For purposes of this argument, I am not going to address the various strange and wonderful chemical processes associated with combustion, nor any temperature differences associated with combustion that may occur outside of stoich, nor details about catalytic converter operation or O2 sensor operation, or EPA regulations, or any of that. For purposes of this argument, it's not important.
That being said, assume that on the combustion stroke of our hypothetical cylinder here, within this cylinder, that will be some set amount of fuel that will will combine exactly with some set amount of oxygen. If there is more fuel than oxygen present (rich), then there will be unburned fuel at the end of the combustion stroke. Similarly, if there is more oxygen than what is needed by the fuel (lean), then there will be excess oxygen at the end of the combustion stroke. Let's take as an extreme example of lean, the case of no fuel at all.
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