1. Principles of Carburetion
Automotive Technology 1
Mr. Wasacz

2. 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

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

4. 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

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

6. 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

8. 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

9. 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

10. 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

11. 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)

12. 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

13. Venturi

14. 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

15. 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.

16. 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

17. 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

18. 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

19. Float Bowl Carburetors

20. 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.

21. 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

22. What it looks like all together

23. 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

24. 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.

25. 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.

27. 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

28. 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

29. 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

31. 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

32. 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

34. 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?

35. 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

37. 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?

38. 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?