Carburetion

Chapter 9

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.

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

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: http://www.youtube.com/watch?

v=9BYm0HnLGRU&safe=active Video 2: http://www.youtube.com/watch?

v=oIU-lGc3DL4&safe=active

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

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?

Venturi and Bernoulli

Venturi’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?

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

Venturi

When the gasoline enters the air stream it atomizes. Why is this necessary?

Air Bleed 9-6:

Types of Carburetors

Natural Draft (Side Draft): 9-8 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.

Float Type Carburetors

Float

Float Valve

Bowl

Float Bowl Vent

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

Primer

Forces fuel through the main discharge nozzle for cold starting

Throttle

Adjusts how much air is allowed to flow through the carburetor.

More Air = More Fuel

More Fuel = More Speed

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.

Acceleration

Mechanical plunger pump.

Acceleration Well: 9-18

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.

Idle Adjusting Needle

Low Speed Needle Valve

Idle or Throttle Stop Screw

Diaphragm Type Carburetor

No Float, No Bowl

Why?

Vacuum Carburetors

9-32

Governor Systems

Governor: Maintains set engine speed regardless of load.

Give examples of why this is important 1 2 3 4 5 6

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.

Centrifugal or Mechanical Governor Figs. 8-31 to 8-46 Let me show you.

Hunting:

Sensitivity:

Vacuum Governor

Figure 9-47

Governor Features

Stability:

Sensitivity:

Air Cleaners

Oil-wetted

Dry-Type

Dual Element

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.