Before the dawn of fuel injected vehicles there lived the carburetor. The carburetor was what used to be responsible for getting the fuel that the engine needed to the intake manifold in order for the vehicle to run. There are still a good number of vehicles today that run strictly off of carburetors, though these are typically reserved to older cars or racing cars. In fact NASCAR still uses carburetors over fuel injection to this day.
The basic principle of carburetors is really not all that complex. When gasoline is in its liquid form it will not burn and in order to do so it must be vaporized. Carburetors will introduce vaporized or atomized fuel to the air flow above the intake manifold using a sophisticated network of internal fuel and air passages that use rudimentary physics. This in turn will vaporize the gasoline before it reaches the intake valves.
This process is accomplished when fuel is pumped into the carburetor’s bowls via the needle and seat assembly and then is taken into the intake because of the pressure differential caused by what is known as the venturi effect. The venturi has an hourglass shape and pressure is increased where the hourglass necks down which in turn helps create a low pressure spot or area. This reduction in pressure is what allows the fuel to move from the fuel bowls to the intake manifold.
One way to look at a carburetor is as though it is a fuel injection system with only one psi of actual pressure. Contrary to the belief that manifold vacuum is what pulls fuel out of the carburetor; it is actually the pressure differential that is responsible for the push.
In the fuel bowls of a carburetor you will find air vents. That means that the normal atmospheric pressure will be present at 14.7 psi. This is what will push down on the fuel. The corresponding venturi effect will reduce the overall pressure to around 13.5 psi. That little more than one psi of difference is all it takes to move the fuel through the carburetor.
However, the fuel that would be in droplet form at this stage and would be too big for proper atomization without some way of air being introduced into the mix via emulsification. This is where the carburetor’s air bleeds come into play. Tubes called “emulsion tubes” will carry the fuel that is in the fuel bowls directly to the venturis and atomize the fuel beforehand by mixing it with air that is derived from the air bleeds. This is sort of like when you are drinking liquid through a straw that has a small hole in it. This will ensure that the atomization of the fuel is thorough and that complete vaporization occurs which will make for better combustion, more power output, and lower overall emissions.
For all the developments in fuel injection over the past years, there are still many who still prefer a good set of carburetors. Perhaps it is because they are easier to work with and to tune versus their fuel injection counterparts, but whatever the case may be, the story of the carburetor is far from over when it comes to vehicles.