Principles of Carburetion

Automotive Technology 1Mr. Wasacz

Objectives

You will be able to list and explain the principles of carburetion

Identify the types of carburetionExplain the float carburetor system Define manual throttle controlsList the basic functions of a governorDescribe 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 StartingIdlingPart ThrottleAcceleration 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 MixtureThe 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 PrinciplesThe carburetor creates a partial

vacuum itself by using this principleBy 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 fuelBasically, a venturi is a restriction in an

air flow passageway

Venturi

Types of CarburetorsThere are three different types of

carburetorsThese 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 CarburetorUsed 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 CarburetorsA 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

levelsAs the float rises to the top, it pushes a

needle valve closedThis 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 / FiltersAs 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 CleanerUses a filtering element (crushed

aluminum and polyurethane foam) dampened with engine oil

Air is drawn through the oil wetted filterThe filter in return removes any debris

or particulateThis type of filter can be reused!

Clean with soapy water Dry Re-oil

Dry Type Air CleanerAir passes through

Treated paper Felt Fiber Flocked screen

These are designed for 1 time use!You can only clean large particulate

from the filterMost 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?

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

Dual Element Air Cleaners

SummaryWhat 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?