Carburetion Ch 8 Explain theory of operation of a basic carburetor ID and understand the function of all carburetor parts Understand the effect of atmospheric pressure on the fuel system of a small gas engine ID: 460519
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Small EngineCarburetion
Ch 8
Explain theory of operation of a basic carburetor.I.D. and understand the function of all carburetor parts. Understand the effect of atmospheric pressure on the fuel system of a small gas engine.Identify 3 basic types of carburetors.
Chapter Objectives - Upon completion of this chapter you should be able to:
Revised 10/01Slide2
Principles of Carburetion
The amount of air needed for combustion is far greater than the amount of fuel. One pound of air would take up a much greater space than one pound of fuel.Air-fuel by volume:
One cubic foot of gas would have to be mixed with 9000 cu ft of air to be at a 15 to 1 weight ratioCh 8Slide3
Principles of Carburetion
Primary purpose is to produce a mixture of fuel and air to operate an engine.Engines cannot run on “liquid” gasoline.
Must be able to maintain air/fuel mixtureCold or hot startingIdlingPartial throttleAccelerationHigh Speed operation
Ch 8Slide4
Principles of Carburetion
Ch 8
Air-fuel
mixture
1) 14.7-1 Ratio (
15 parts
air
to 1 part
fuelSlide5
Carburetor Pressure Differences
A carburetor is a device that ……Vacuum- Any pressure less than atmospheric pressure
Atmospheric Pressure- the pressure produced by the weight of air molecules above the earth. 1. 29.92 HgCh 8Slide6
Carburetor Pressure Differences
Ch 8
Elevation and its effect on Atmospheric PressureHg-MercurySlide7
Carburetor Pressure Differences
Venturi PrincipleA restriction in a passage, which causes air to move faster.
Pressure in the passageway is reduced3. Vacuum created by the decreased pressure which allows fuel to be drawn into the airstreamCh 8Slide8
Small EngineCarburetion
Ch 8
Venturi PrincipleSlide9
Types of Carburetors
A. Natural Draft Carburetor 1. Used when there is little space on top of the engine 2. Air flows horizontally into the manifold
Ch 8Slide10
Types of Carburetors
B. Updraft Carburetor
Ch 8
Placed low on the engine and uses gravity fed fuel supplyAir-fuel mixture must be forced upwardSlide11
Types of Carburetors
C. Downdraft Carburetor
Ch 8
Operates with lower air velocities and larger passages
Air-fuel must flow downwardSlide12
Small EngineCarburetion
Ch 8
Carburetor Draft Types
Slide13
Float-type Carburetors
Float SystemFloat-a small sealed vessel made of brass or plastic
The purpose of the float is to maintain a constant level of fuel in the float bowl3. As the fuel is used in the float bowl, a needle valve is unseated to allow more fuel in the bowl. As the float rises, the needle valve seals the line to shut off the fuel.
Ch 8Slide14
Float-type Carburetors
B. Choke System 1. The choke is a round disc mounted at the intake end of the carburetor
2. When closed, the choke cuts off air to allow a richer (higher fuel to air ratio 7-1), air-fuel mixture,….Ch 8Slide15
Float-type Carburetors
Throttle System 1. The throttle is a round disc mounted beyond the main fuel nozzle
2. The purpose of the throttle is to regulate the amount of air-fuel mixture entering the cylinder(s). 3. Allows operator to vary engine speedCh 8Slide16
Float-type Carburetors
D. Load Adjustment 1. Regulates the amount of fuel entering the main discharge nozzle
a. Load adjust needle b. Fixed jet or orifice (non- adjustableCh 8Slide17
Ch 8
Small Engine
CarburetionSlide18
Float-type Carburetors
Acceleration System 1. On some systems, when the throttle is opened quickly, a large amount of air passes through the carburetor. A pump is used to force fuel into the carburetor to allow proper air/fuel mixture and maintain running engine.
Ch 8Slide19
Float-type Carburetors
Acceleration System 2. Acceleration Well is a reservoir of fuel, during idle fuel rises inside nozzle and flows into acceleration well. When throttle valve is opened quickly fuel rushes through the holes in nozzle. This provides a richer mixture under sudden need for power.
Ch 8Slide20Slide21
Float-type Carburetors
Economizer Circuit 1. Designed to retard fuel to the engine during part throttle, by lower pressure in float bowl
Ch 8Slide22
Float-type Carburetors
G. Idling Circuit 1. Throttle valve is closed 2. Fuel transported through an idle circuit
pathwayThrottle valve closed-idle system supplies just enough air-fuel to keep engine runningCh 8Slide23
Float-type Carburetors
Part-Throttle, Full Throttle Sequence 1. Other circuits outside of idling speed that have secondary discharge holes opened for more air/fuel mixture to open.
Ch 8Slide24
Float-type Carburetors
G. Primer 1. Hand operated plunger 2. Forces addition fuel up the nozzle and/or pumps air pressure onto the float bowl which forces fuel into nozzle
Ch 8Slide25
Ch 8Slide26
Diaphragm-type Carburetors
Does not contain a float systemAtmospheric pressure and the vacuum created in the engine pulsates a flexible diaphragm
The diaphragm draws fuel into the chamber of the carburetor from which it is readily drawn into the ventureCh 8Slide27
Ch 8Slide28
Negative pressure pulses from crankcase
Fuel pushed to carburetor
Fuel pulled from tankSlide29Slide30Slide31Slide32
Small EngineCarburetion
Ch 8
Carburetion Fuel Systems
Suction TypeB&S = (Vacu-jet)
Pump TypeB&S = (Pulsa-jet)
Gravity Feed Type
B&S = (Flo-jet)Slide33
Small EngineCarburetion
Ch 8
Carburetor Components and OperationSlide34
Small EngineCarburetion
Ch 8
Old style carburetorwith an adjustable needle valve
Newer style carburetor
with a fixed Jet
(Jet must be replaced to for altitude adjustment)Slide35
Small EngineCarburetion
Ch 8
When performing an altitude adjustment the jet is removed and replaced with a jet that has a smaller orifice.Slide36
Small EngineCarburetion
Ch 8
Main Jet LocationSlide37
Small Engine
Carburetion
Ch 8
Main Jet LocationSlide38
Small Engine
Carburetion
Ch 8
Main Jet LocationSlide39
Small Engine
Carburetion
Ch 8
Main Jet LocationSlide40
A
A
B
C
DSlide41
Ch 8
Small Engine
Carburetion What other components are related to carburetion???Slide42
Ch 8
Determine proper Air Fuel Ratio under load
by sparkplug colorationSmall EngineCarburetionSlide43Slide44
Small EngineCarburetor
Ch 8
Pneumatic GovernorSlide45
Small EngineCarburetion
Ch 8
Mechanical GovernorSlide46
Mechanical Governor OperationSlide47
Mechanical Governor OperationSlide48
Mechanical Governor OperationSlide49
Small EngineCarburetion
Ch 8
Air Cleaner
Dry Element