Carburetors: Adding Fuel to Fire
Carburetors in conjunction with an internal combustion engine have been in use a very long time, with the first carburetors appearing in the late 1890’s. Although they are no longer common place in large car or plane engines, they still remain a popular choice for smaller applications such as power tools and lawn mowers. [Wikipedia] To begin to understand how a carburetor works, we must first understand what function it serves. The carburetor is responsible for blending fuel and air into a combustible mixture. It must ensure that the engine receives an appropriate blend of air and gasoline in order to produce proper detonation times inside of the engine. Mixtures can be described as too lean or too rich due to a respective lack or surplus of gas being blended with air. On top of that, the carburetor controls the engines speed at ranges from idle to full throttle. It also needs to allow the engine to start hot or cold. [How Stuff Works] The main principle that allows a carburetor to work is actually quite simple, although the design of carburetors can become complicated depending on engines requirements.
In its simplest form, we can imagine a carburetor strictly as a tube. The engine pulls in air from outside through this tube creating a flow. Bernoulli’s principle, derived from the conservation of energy, states that when a flow is altered, its velocity or pressure must change to compensate for equilibrium. This means that when the flow is forced through a narrow opening, it will move faster and its pressure will drop. Therefore, when air inside the tube moves quickly, the carburetor will have a lower pressure inside it than the surrounding atmosphere. This low pressure makes the tube behave much like a vacuum as it sucks gasoline at atmospheric pressure into the tube through small jets.
Now that we know why a carburetor works, we can analyze the path air takes before it can be used in the engine. To begin, air is drawn from the atmosphere through an air filter which is attached to the carburetors intake. This ensures that no debris goes into the engine. When air first enters the carburetor, it is already undergoing the effects of Bernoulli’s principle and has lower pressure then it did outside. The air continues its path towards the engine where it is passed over a narrowing in the tube called the venturi. There is even more constriction of flow here which furthers an increase in air speed, lowering the pressure more. The lack of pressure at this point creates a strong vacuum like effect and draws fuel from a jet. The jet is designed to distribute a measured amount of gas into the air stream. Once air and gas are blended, the mixture continues towards the engine where it is compressed and burned.
By looking at the individual components of a carburetor, we can further our knowledge of how each part affects the device as a whole, giving us greater insight into how the device works. Below is a simple diagram [Wikipedia]:

The first component we will look at is the throttle valve. This valve is responsible for adjusting the speed at which air is allowed to enter the engine. When this valve is closed, the engine is said to be at idle. During idle, the engine only needs enough fuel to allow it to keep turning over. Some valves have small holes drilled in them that still allows air to be sucked through the carburetor while other throttle valves don’t close completely. [ezine articles] When the throttle valve is completely open, the carburetor is allowing the engine to take in as much fuel as it can. By manipulating this throttle valve, one can limit the engines maximum potential to produce power.
While the jet is responsible for distributing gas into the air stream, a float chamber is required to act as a reservoir that always contains enough fuel to meet the demands of the engine. When the fuel level becomes too high inside of the chamber, the float arm raises and blocks the valve. When the fuel level becomes too low, the float arm lowers allowing the fuel level in the chamber to rise. [How Stuff Works] This prevents the jet from constantly dumping gas into the air stream and flooding the engine. While a float chamber is a very common fuel reservoir system, it cannot be used upside down. There are many alternatives that can be used depending on the carburetors application, but they all provide the same basic function.
The design of the venturi, sometimes referred to as a barrel, can greatly affect the way the carburetor behaves. A more constrictive narrowing allows more accurate blends at low engine speeds, but is less effective at delivering maximum performance at high engine speeds. [Ninja250] The profile does not always appear as a perfect semi-circle either. For engines with a higher displacement, multiple venturi may be used in the carburetor to make more efficient use of the large volume of air travelling through it.
Cold starts require a large amount of fuel to get the engine to begin turning over. Carburetors have a choke that allows them to provide the engine with more fuel than usual. While there are different methods of creating a choke in the carburetor, the pictured diagram uses a choke valve which is shown in the off position. When used, the choke valve blocks the path of air entering the carburetor. This causes a very strong vacuum to occur in the venturi and the engine begins to draw in a very rich fuel mixture. Once the choke is turned off, the flow inside the carburetor is returned to normal.
Although there are many variations of carburetors available, they all operate on the same basic mechanics provided by Bernoulli’s principle. Increasing the air’s speed entering an engine allows the carburetor to draw in more gas as well, creating more power. Although carburetors are now obsolete on large scale engines due to more accurate fuel injection systems, they still remain an inexpensive option for a fuel delivery system in small engines.