This door kept slamming shut by itself whenever we tried to air the place out. We got a door stopper, but I realized that the layout of the apartment is the same as that of a carburetor and the reason behind the moving of the door is the same as behind the moving of the fuel in a carburetor.

Both the carburetor and the apartment feature large open spaces which are connected by a narrow passage between them. This narrow passage results in an increased air velocity and an increase in air velocity results in a decrease in air pressure. A decrease in air pressure leads to atmospheric air pressure working for us and pushing the fuel into the engine and moving doors in an apartment in ghostlike fashion.

The atmosphere in which we live has an air pressure (1 bar or 14.5 psi) because all the air above us has a mass. This mass pushes down on us and creates pressure. Pressure is higher at sea level because there is more air mass above as at sea level than at the top of a mountain.

If we manage to reduce air pressure at any point then atmospheric air pressure will push things towards the point of low pressure. Air always moves from high to low pressure because air aims to equalize pressure everywhere.

Now we can reduce air pressure at a desired point by increasing air velocity at that point. A narrow passage between two large spaces is a great and simple way to increase air velocity at the passage because the narrowing space acts as a constriction or bottleneck and so air builds behind this constriction and pushes air or another fluid through the narrow passage with greater velocity. You have probably experienced this yourself if you ever blocked off half a garden hose with your thumb to increase the velocity of the water coming out the hose.

Now increased air velocity results in reduced air pressure because the fast moving air molecules they bump away the stillstanding air, they disperse it or reduce it's concentration if you will. This why doors in a draft will often slam shut by themselves. Air velocity increases across the front of the door so air pressure reduces on the front of the door but atmospheric air pressure remains behind the door. This pushes the door and moves it into a position where the air drag can grab it and slam it shut.

The same thing happens to the fuel inside the carburetor. One side of the fuel is exposed to atmospheric air pressure but the other side of the fuel is exposed to reduced air pressure that occurs due to the increased air velocity in the narrow section in the middle of the carburetor. This is how atmospheric air pressure managed to push the fuel up through the tube that leads to the fuel hole in the narrow passage. The fuel slightly protrudes above the level of the hole and then the air drag grabs it and takes it into the engine.

But this does raise an important question. If we rely on the throttle slide or butterfly to provide air flow through the carburetor how do we get fuel into the engine when there is no airflow i.e. the throttle slide is released and the engine idles? We do this by leveraging the power of the vacuum generated by the engine and using the idle air bypass. The throttle slide acts like a barrier between the atmosphere and the engine internals. The downward motion of the pistons generates a vacuum and when the throttle slide is released it prevents the atmosphere from entering the engine so the vacuum or the pressure difference between the inside and the outside of the engine remains. In reality we perceive this vacuum as air being sucked into the engine and when the throttle slide is released and the engine idles vacuum is strong. So the engine can suck in air through the idle air bypass. But as soon as the throttle slide is raised atmosphere enters, vacuum weakens and so the task of fueling the engine gets transferred from the pilot jet and the idle air bypass to the main jet.

A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Cole Philips

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