Fuel Systems Bernoulli’s Principle, Atomization, And Stoichiometry

Carbureted Fuel Systems  Many different designs & configurations  All use similar principles  Basic purpose is to regulate fuel to air ratio  Atomization through air flow control

Basic Components  Bowl  Float  Needle valve  Jet  Venturi  Throttle Valve/Plate  Choke Valve/Plate  Primer Bulb

Bowl  Maintains a consistent supply volume  Must have a fuel input and output access points  Vented to maintain a constant atmospheric pressure  Fuel tank may act as the bowl for the carburetor

Emulsion Tube  “Straw-Like” tube that extends from the bowl to the venturi  Controls the atomization of fuel  Must remain free of obstruction

Needle Valve  Metal, rubber, or plastic valve that controls the input of fuel  Attached to the float  Opens and closes due to float movement  Will not allow fuel movement upside- down

Float  Metal or plastic floatation device inside the carburetor bowl  Fuel level inside of the carburetor moves the float  The float controls the needle valve  Together the float and needle valve maintain a constant volume of fuel in the bowl

Venturi  Named for Giovanni Battista Venturi  Device that creates the area of low pressure over the jet  Main tubular body of carburetor  Contains: Throttle valve, Jet End, and Choke

Throttle Valve/Plate  Thin metal/plastic plate on the ENGINE side of the carburetor  Regulate air flow over the venturi  DOES NOT REGULATE FUEL!!!

Choke Valve/Plate  Similar metal/plastic plate on the AIR-BOX side of the carburetor  Restricts air flow to the venturi  Causes a significantly lower pressure over the jet  Creates a “Rich” fuel mixture  Used for engine starting

Primer Button  Manually injects fuel through the carburetor  Creates a “Rich” fuel mixture  Acts like a manual “Fuel Pump”  Used for engine starting Primer Button

How It All Works  Fuel enters the carburetor past the needle valve  The float slowly rises, closing the needle valve until fuel is used  As air passes through the venturi, an area of low pressure allows atmospheric pressure to “Push” the fuel out the main tube  The fuel is atomized into a fine mist as it exits the jet

Atomization  Atomization – Process of converting a liquid into a fine mist  Idea is to create the smallest particles of fluid possible  Small particles are considered atomized particles  Smaller particles of fuel creates more efficient operation

Carburetor Diagram

Throttle Control  As the throttle plate opens, more air travels through the venturi  As the pressure in the venturi drops more and more, the amount of fuel atomized increases – adjusting the air to fuel ratio – A.K.A. Stoichiometry  Increasing the amount of atomization increases engine RPM’s and engine power

Things To Remember:  Jet control amount of fuel that can enter the venturi  Neither the throttle valve nor the choke valve control fuel flow – only air flow  Throttle valve is always on the engine side of the carburetor  Atmospheric pressures “Pushes” the fuel through the carburetor  Carburetor purpose is to regulate fuel to air ratio

Bernoulli’s Principle  Based on the principles of pressure  High Pressure vs. Low Pressure  Can create lift  air foils/wings  Fast moving air over wing creates lift  Wing “Lifts” upward on plane

Bernoulli’s Principle (Continued)  Principles apply to automobile designs  Shape is very important  Fast moving air over the vehicle creates lift  Vehicle gets “Lighter”  Vehicle gets better fuel mileage  Less control of vehicle

Bernoulli’s Venturi Application  Fast moving air creates an area of low pressure in the venturi  Low pressure over jet causes atmospheric pressure “Pushes” fuel into the venturi

Bernoulli’s Fluid Effects  The area of low pressure in the venturi is moving quickly over the jet  Atmospheric pressure of 14.7 PSI (pounds per square inch) “Pushes” fuel up through the jet  The fuel is atomized as it exits the jet

Carburetor Diagram

Mixtures  15.0 part of air to 1 part fuel  Most efficient engine operation  Less than 15.0 parts of air  “RICH” fuel mixture  More Power  More than 15.0 parts of air  “Lean” fuel mixture  Less power  Deceleration  *NOTE*  14.7 is the same as normal atmospheric pressure