More from today's class

The Bernoulli Principle:

The pressure in a "moving stream" of fluid is LESS than that of the surrounding fluid.  Recall the demonstrations:

- blowing between the 2 soda cans
- the beach ball suspended by the air from the shop vac
- imagine putting your thumb over the nozzle from a garden hose

The last example relates to the "equation of continuity" -

A1 v1 = A2 v2

Meaning:  As the area (of the tube) gets smaller, the speed of the fluid gets greater.

Videos for the carburetor:

http://www.youtube.com/watch?v=4MnOgR4_jaE

http://www.fz1-forum.com/forum/gen-i-garage-tech-help/13659-how-carburetor-works-short-animation.html

Carb image

Carburetor

Carburetor

Carburetor Theory

The carburetor has one main function: to regulate engine speed. It accomplishes this by adjusting the amount of air and fuel that reaches the engine chamber, which then sustains combustion.

For a low-speed idle you have a small amount of air and fuel entering the engine. This lowers the chemical mixture entering the combustion chamber, and thus lessens engine power and subsequently lowers the RPM (revolutions per minute). As we open the throttle, the carb allows more air and fuel into the combustion chamber, thus increasing engine power and RPM's.

The Venturi Effect

What allows the carb to pull fuel from the fuel tank is the Venturi effect, a phenomena related to the Bernoulli principle. This states the velocity of a fluid entering a tube increases as it passes through a smaller diameter region.

Fluids are generally incompressible, so the same amount of fluid (volume) is going to try to get through a volumetric cross-section of tube - if you make the diameter smaller, the fluid has to move faster to get through.

In a carburetor, this increase in air speed allows more fuel to be drawn from the float chamber (a fuel reservoir) through the jet, where it is somewhat atomized.

This low pressure region is precisely where fuel enters the carburetor throat. This is what allows the engine to "suck" fuel from the gas tank.

http://static.howstuffworks.com/mpeg/chainsaw3.mpg

http://auto.howstuffworks.com/question377.htm