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Techanic; as I understand it,
The volume of the cylinder is constant, so the same volume of air enters the cylinder no matter what the throatle position. So what does the throatle do? it changes the mass of air entering the cylinder. So the density [mass/volume] of the air in the cylinder when the intake valve closes is controled by the throatle. So what else changes the density of air? Temperature, as the temperature goes up the density goes down. there are two other benifits to higher air temperature; mixing of fuel:air and combustion efficiancy. when looking for fuel economy we drive at very low power, so the throatle is almost closed, so by heating the air the thoatle can be opened a little more thus redusing pumping loses, caused by the throatle being almost closed. High power needs high density air, and lots of fuel, low power needs low density air, and very little fuel.
I hope this helps
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