1. Carburetion
Chapter 9

2. Objectives List and explain the principles of carburetion.
Identify the three basic types of carburetors.
Explain the float-type carburetor operation.
Explain the operation of the diaphragm-type carburetors.
Define manual throttle controls.
List the basic functions of a governor.
Adjust and maintain common governors.
Describe the purpose of an air cleaner.

3. Principles of Carburetion
Produce an air-fuel mixture
Engines cannot run on liquid gas
Carb must vaporize the fuel and mix it with the air in the proper proportion for varying conditions
Cold/hot starting
Idling
Part throttle
Acceleration
High speed

4. Carburetion
Air enters the top of carb and mixes with liquid fuel sprayed into the airstream
The air-fuel mixture is forced into the intake manifold by atmospheric pressure
Figure 9-1
Video:
Video 2:

5. Air-Fuel Mixture About 15 parts air to 1 part fuel
About 9000 cu ft of air to 1 cu ft of gas
Due to Weight Difference
An engine needs different ratios at different speeds…figure 9-2

6. Carburetor Pressure Differences
Vacuum
Absolute vacuum
Vacuum in a small gas engine
Atmospheric Pressure Fig. 9-3
The pressure produced by the weight of air molecules above the earth
If air molecules are removed, what happens?
What creates a vacuum in the engine?

7. Venturi and Bernoulli Venturi’s Principle: Bernoulli’s Principle:
If the pathway of a fluid in constricted, the velocity of the fluid will increase.
Bernoulli’s Principle:
If the velocity of a fluid increases, it pressure will decrease
Any Examples?

8. Carburetor Venturi (Page 131)
Venturi: A restriction in the carburetor’s throat.
What will that lead to?
Time for an experiment!
Cup, Straw, and Water

10. Venturi When the gasoline enters the air stream it atomizes.
Why is this necessary?
Air Bleed 9-6:

11. Types of Carburetors Natural Draft (Side Draft): 9-8 Updraft: 9-9
Used when there is little space on top of the engine
Updraft: 9-9
Placed low on the engine (gravity fed).
Require high air speed.
Downdraft: 9-10
Can provide large volumes if fuel and air for high speed and high power output.

12. Float Type Carburetors
Float Valve
Bowl
Float Bowl Vent

13. Choke System Provides a rich air-fuel mixture for cold starting.
Cold fuel does not vaporize easily
Relies on manifold pressure rather than the venturi to suck fuel from the main nozzle.
Manual
Electronic Temperature
Bimetal Spring
Vacuum

14. Primer
Forces fuel through the main discharge nozzle for cold starting

15. Throttle
Adjusts how much air is allowed to flow through the carburetor.
More Air = More Fuel
More Fuel = More Speed

16. Main Nozzle Adjustment
Load Adjusting Needle
High Speed Needle Valve
Adjustable Main Jet
Whatever the name, it regulates the amount of fuel that enters the main discharge nozzle.
Some engines may have a fixed jet or orfice.

17. Acceleration
Mechanical plunger pump.
Acceleration Well: 9-18

18. Idle and Transition Circuits
Main Nozzle is inactive
Venturi is created around the throttle plate
As the throttle position opens other discharge holes are uncovered.

19. Idle Adjusting Needle Low Speed Needle Valve
Idle or Throttle Stop Screw

20. Diaphragm Type Carburetor
No Float, No Bowl
Why?

22. Vacuum Carburetors
9-32

23. Governor Systems
Governor: Maintains set engine speed regardless of load.
Give examples of why this is important 1 2 3 4 5 6

24. Pneumatic (Air Vane) Governors Figs. 9-36 to 9-42
Use air created by flywheel to regulate engine speed.
As the air speed increases the throttle is closed
As the air speed decreases the throttle is opened.
This is all about a balance of spring tension.

25. Centrifugal or Mechanical Governor Figs. 8-31 to 8-46
Let me show you.
Hunting:
Sensitivity:

26. Vacuum Governor
Figure 9-47

27. Governor Features
Stability:
Sensitivity:

28. Air Cleaners
Oil-wetted
Dry-Type
Dual Element

29. Objectives List and explain the principles of carburetion.
Identify the three basic types of carburetors.
Explain the float-type carburetor operation.
Explain the operation of the diaphragm-type carburetors.
Define manual throttle controls.
List the basic functions of a governor.
Adjust and maintain common governors.
Describe the purpose of an air cleaner.