Principles of Carburetion Automotive Technology 1 Mr. Wasacz

Objectives You will be able to list and explain the principles of carburetion Identify the types of carburetion Explain the float carburetor system Define manual throttle controls List the basic functions of a governor Describe the purpose of an air cleaner

Introduction In order for an engine to work, what must happen to the fuel?? Liquid form gasoline will not allow the engine to run!

Carburetors function The primary function is to produce an air fuel mixture to operate the engine This process can be done by using a simple mixing valve, however it is not as efficient The carburetor is a much more complex system that allows for the operation of the engine during these conditions

Carburetion Con’t Cold or Hot Starting Idling Part Throttle Acceleration High Speed Operation

The Intake of Air Remember, atmospheric pressure draws air/fuel through the carburetor and into the cylinder This drawing in of air takes gasoline atoms with it creating an air fuel mixture

Air / Fuel Mixture The amount of air needed for combustion is far greater than the amount of fuel needed The usual weight ration is 15 parts of air to 1 part of fuel For example, if we had 1 cubic foot of gasoline, it would require 9000 cubic feet of air to meet this ratio This ration will change depending on engine operation

Pressure differences A carburetor is a device that is operated by pressure differences These terms refer to those pressure differences Vacuum Atmospheric pressure Venturi Principles

Vacuum An absolute vacuum is an area that is completely free of air or atmospheric pressure This is difficult to obtain and never reached within an engine Any pressure that is less than atmospheric pressure is referred to as a vacuum Basically, a vacuum is an empty space

Atmospheric Pressure This is the weight of air molecules above earth This varies with altitude because they higher up you go, the less atmosphere is on you! If a vacuum is created, an equilibrium will want to be established, so the atmospheric pressure will fill that space (this is how engines work)

Venturi Principles The carburetor creates a partial vacuum itself by using this principle By restricting the passage that the air is flowing through, the velocity of the air will increase This increase in velocity will decrease the pressure behind the venturi This low pressure will be filled by fuel Basically, a venturi is a restriction in an air flow passageway

Venturi

Types of Carburetors There are three different types of carburetors These carburetors are named after the direction of airflow from the outlet to the engine manifold They are as follows Natural (Side) draft Updraft Downdraft

Natural Draft Carburetor Used to when there is little space in top of the engine Air flows horizontally into the manifold This type will be the most common on the small engines that we are working on.

Updraft Carburetor Placed low on an engine Use gravity feed fuel supply Air / Fuel Mixture must be forced upward into the engine Needs high air pressure Uses smaller passages

Downdraft Carburetor Operates with lower air velocities Uses larger passages Gravity assists the air fuel mixture flow into the engine Can provide large volumes of fuel for high speed and high power operation

Float Carburetors A Float is a small sealed vessel made of brass or plastic. It maintains a constant level of fuel in the float bowl. The float rises and falls with the fuel levels As the float rises to the top, it pushes a needle valve closed This needs then shuts off fuel flow to the carburetor

Float Bowl Carburetors

Choke System Since cold fuel is hard to vaporize, the choke is used during cold engine starts to provide a rich mixture to the carburetor in order to get the engine started. The choke is a round disc mounted on a shaft located at the intake end of the carburetor.

Throttle System The throttle is a round disc mounted on a shaft beyond the main fuel nozzle in the carburetor. It regulates the amount of air-fuel mixture entering the cylinder. The throttle can be connected to a governer The governer will open and close the throttle to regulate engine speed

What it looks like all together

Load Adjustment The amount of fuel entering the main discharge nozzle is sometimes regulated by a load adjusting needle. In some carburetors, there is no adjustment needle. These have a fixed flow that is designed for that specific engine

Primer Many small engines have hand operated plunger called a primer. When depressed it forces additional fuel through the main nozzle prior to starting a cold engine.

Diaphram carburetors This type does not have a float, rather the difference between atmospheric pressure and the vacuum created in the engine pulsates a flexible diaphragm The pulsation of the diaphragm takes place on every intake and compression stroke.

Air Cleaners / Filters As an engine operates, it needs an enormous amount of air passing through the carburetor into the combustion chamber This incoming air needs to be clean (this means that all particulate needs to be removed!) If this grit were to enter the combustion chamber, it would cause rapid wearing and scoring of the precision machined parts

Air Cleaners / Filters Under severe dust conditions, engine life can be reduced to minutes To keep this dust from entering the engine, we use different types of cleaners / filters Oil wetted Dry types Dual element

Oil Wetted Air Cleaner Uses a filtering element (crushed aluminum and polyurethane foam) dampened with engine oil Air is drawn through the oil wetted filter The filter in return removes any debris or particulate This type of filter can be reused! Clean with soapy water Dry Re-oil

Dry Type Air Cleaner Air passes through Treated paper Felt Fiber Flocked screen These are designed for 1 time use! You can only clean large particulate from the filter Most dust however will stay in between the fibers of the filter

Dry Type Cleaner Cont A new style is the pleated paper design. This style uses a porous paper filter material to allow air flow to the engine, while allowing the filtration of debris This design provides great surface area to collect particles This is common to most automobiles

Dual Element Air Cleaners These filters are used on engines that operate in dustier than normal conditions Can you guess why its called a dual element air cleaner?

Dual Element Air Cleaners Its because they use both an oil wetted cleaner foam filter, and a dry type filter This offers more protection to dust and particulate The oil wetted filter acts as a pre cleaner The paper cartridge then cleans out any missed debris These come in many shapes and sizes, the most common is a cylidrical filter

Summary What is the main point of the carburetor? The average weight ration of air to gasoline is 15:1. What does this mean? How do carburetors operate? (think of the venturi!) What are the three types of carburetors, and what is the difference?

Summary What are the two ways of regulating gasoline within the carburetor? What is the function of the throttle? What is the function of the choke? What is the function of the primer? What is the main purpose of a filtering system?