While an engine might be described as an air pump, it's source for power is gasoline. Before gasoline can be used as fuel for an engine, it must be atomized so that it can be mixed with air to form a combustable mixture. Contrary to popular belief, gasoline in its liquid state is not combustable; only gasoline vapor will burn. Vaporization is the act of changing from a liquid state to a gasseous state and only occurs when the liquid absorbs enough heat to boil. The temperature-pressure relationship dictates that as pressure is reduced, the boiling point is also reduced.
Since the pressure in the intake manifold is far less than atmosphere, the boiling point of the gasoline is lowered considerably. At this reduced pressure, latent heat absorbed from the many air particles surrounding each fuel particle causes some vaporization, which is further aided by heat on the intake manifold floor.
Because complete fuel vaporization is the result of many factors (ambient temperature, fuel temperature, manifold vacuum, and intake manifold temperature), it is easy to see that anything which reduces any one of these factors will adversely affect vaporization and thus reduce fuel economy (and increase exhaust emissions). Some examples would be cold weather, an inoperative exhaust heat control valve, and high overlap camshafts and/or heavy throttle demands.
While the effects from lower temperatures are obvious, reduction of manifold vacuum either by valve timing or heavy throttle operation are highly detrimental to fuel economy due to the higher pressures (and boiling points) resulting in the intake manifold which reduces the amount of fuel vaporization which will occur by the time the charge enters the combustion chamber. Fuel not vaporized at the time of induction, is to a large extent, exhausted unburned from the combustion chamber ( and can cause high hydro-carbon exhaust emissions).
I am having a hard time with this crankcase pressure stuff...isn't the crankcase pressure dealt with by either a road draft tube or better yet the Positive Crankcase Ventilation system? A properly working PCV system "reclaims" unburned blow-by gasses by returning them to the intake manifold and, this being a combustable mixture, is added to the new air-fuel charge. Wouldn't the increased gasoline economy of the PCV system at least counteract the crankcase pumping losses?
