carburated].pdf

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• List and explain the principles of carburetion.
• Distinguish between natural draft, updraft, and
downdraft carburetors.
• Explain float-type carburetor operation.
• Explain the operation of diaphragm-type
carburetors.
• Explain vacuum carburetor operation.
Learning Objectives

• Differentiate between wet-bulb and dry-bulb
primers.
• Explain how manual throttle controls work.
• List the basic functions of a governor.
• Explain the operation of air-vane, centrifugal, and
vacuum governors.
Learning Objectives

Principles of Carburetion
• Engine cannot run on liquid fuel
– Carburetor vaporizes fuel
• Conditions
– Starting
– Idling
– Part throttle
– Accelerating
– High speed

• 15:1 by weight
• Fuel weighs more
than air
• 9000 ft3 air/1 ft3 fuel
• Proper ratio varies
– Speed
– Load
Air-Fuel Mixture

Carburetor Pressure Differences
• Different pressures inside carburetor
– Mix fuel and air
• Vacuum
• Atmospheric pressure
• Venturi principle

Vacuum
• Less than atmospheric pressure
• Created by downward stroke of piston
– Volume increase
– Pressures equalize if possible
• Absolute vacuum
– Never reached in small engine

Atmospheric Pressure
• Weight of air
• Varies with altitude
• Forces air to fill vacuum
– Rate of flow varies with difference

• Venturi increases air
speed
• Causes proportional
pressure drop
Venturi Principle

Main Nozzle
• Carburetor main nozzle
– Low-pressure area of venturi
– Vacuum draws fuel out of nozzle
– Air bleed helps atomize fuel
• Pressure drop decreases fuel’s boiling point
– Causes fuel to flash

Main Nozzle Operation

Types of Carburetors
• Classified by direction of airflow
• Natural draft carburetors
– Require less space at top of engine
• Updraft carburetors
– Have smaller passages
• Downdraft carburetors
– Provide larger volumes of air-fuel mixture

Natural Draft Carburetor
(Deere & Co.)

Updraft Carburetor

Downdraft Carburetor

Float-Type Carburetors
• Float
– Seats needle valve when fuel level is high
– Unseats needle valve when fuel level is low
– Come in many shapes
– Made from brass or foam
• Float bowl ventilation
– Provides atmospheric pressure to bowl

Choke
• Located on intake side of carburetor
• Closes to provide rich mixture
• Increases vacuum

• Located on outlet side
of carburetor
• Regulates flow of
mixture to cylinder
• Controls engine
speed
– Operator control
– Governor
Throttle

Load Adjustment
• Regulates fuel entering main nozzle
• Load adjustment needle
– Adjustable
• Fixed jet or orifice
– Nonadjustable

Acceleration System
• Open throttle allows more airflow
– Extra fuel needed
• Plunger-type pump used on large engines
• Acceleration well
– Stores fuel for acceleration
– Becomes air bleed when empty

Economizer Circuit
• Passage on engine side of open throttle
• Engine draws air from passage
• Reduces float bowl pressure
• Less fuel through main nozzle

Idling Circuit
• Provides fuel to keep engine running
• Mixture adjusted with idle-adjusting needle
• Bypass air pressurizes circuit

• Main discharge orifice
exposed to vacuum
• No airflow
• Main nozzle
inoperative
Idle
(Zenith Div., Bendix Corp.)

• Main and secondary
discharge orifices
exposed to vacuum
• Limited airflow
• Main nozzle
inoperative
Part Throttle

• Idle circuit inoperative
• Full airflow
• Main nozzle supplies
fuel
Full Throttle

Diaphragm-Type Carburetors
• Built-in impulse diaphragm fuel pump
• Intake stroke
– Fuel enters lower chamber
• Compression stroke
– Spring pressure return diaphragm
– Pushes fuel into upper chamber
• Fuel enters airflow through venturi

• Throttle open
• Choke closed
• High vacuum
• Fuel from idle
discharge ports and
main nozzle
Diaphragm Carburetor Starting

• Choke open
• Throttle closed
• No airflow
• No vacuum in venturi
• Fuel from primary idle
discharge port only
Diaphragm Carburetor Idle

• Choke open
• Throttle partly open
• Low vacuum in
venturi
• Fuel from primary and
secondary idle
discharge ports
Diaphragm Carburetor Part Throttle
(Rupp Industries, Inc.)

• Choke open
• Throttle open
• High airflow
• High vacuum in venturi
• Fuel from idle
discharge ports and
main nozzle
Diaphragm Carburetor Full Throttle

Vacuum Carburetors
• Simple design
• Draw fuel directly from tank
• Atmospheric pressure at top of tank
• Fuel metered by holes exposed by throttle
• Less fuel delivered as tank empties
– Modern units have fuel pump and fuel cup
– Provide consistent fuel delivery

Vacuum Carburetor with Integral Fuel Pump

Primers
• For cold starting
• Force fuel into carburetor
• Wet bulb primers
• Dry bulb primers
• May be installed in carburetor
• May be remotely mounted

Manual Throttle Controls
• Open and close throttle valve
• May be used alone
– Chainsaw, motorcycle, outboard engine
• May be used with governor
– Lawnmowers, power washers, etc.
– Manual throttle control sets speed
– Governor maintains desired speed

Governor Throttle Controls
(Deere & Co.)

Types of Governors
• Air vane (pneumatic) governor
• Centrifugal (mechanical) governor
• Vacuum governor
• All types perform same function
• Each type uses different sensing method

• Airflow from flywheel
cooling fins
• Air vane attached to
throttle valve
• Governor spring acts
to open throttle
• Air vane acts to close
throttle
Air Vane (Pneumatic) Governor
(Tecumseh Products Co.)

Air Vane (Pneumatic) Governor
• Spring tension must be calibrated
– Replace/fix damaged springs and linkages
– Mower blades speed less than 19K fpm
• Fixed speed governors
– Limited range of adjustment
• Variable speed governors
– Throttle control adjusts spring tension

Centrifugal (Mechanical) Governors
• Governor gear spins with camshaft
– Centrifugal force moves flyweights outward
– Lever moves to close throttle valve
• Governor spring acts to close throttle valve
– Adjusted by changing spring tension

Centrifugal (Mechanical)
Governor Operation

• Fixed speed
• Operator controlled
• Adjustment knob
• Spring position on
governor lever
Governor Speed Setting
(Briggs and Stratton Corp.)

Vacuum Governors
• Farm and industrial engines
• Vacuum diaphragm operates lever
– High vacuum closes throttle valve
• Low vacuum allows spring to open throttle

Vacuum Governor

Governor Features
• Stability
– Instability results in hunting
– Excessive stability results in dead-beat governor
• Sensitivity
– Excessive sensitivity results in hunting
– Inadequate sensitivity results in inadequate
correction

Review
Summarize the venturi principle.
A restriction in an air passage speeds up airflow and
causes a proportional decrease in pressure.

Review
Of updraft, downdraft, and natural draft carburetors,
which type operates with the lowest air velocities, and
why?
Downdraft carburetors. Gravity helps the air-fuel
mixture to flow into the cylinder.

Review
Why is the carburetor float bowl vented to the
atmosphere?
The vent applies atmospheric pressure to the fuel in
the bowl. Since the carburetor venturi is at low
pressure, the pressure difference forces fuel through
the main nozzle.

Review
In a diaphragm carburetor, what returns the
diaphragm to its original position, forcing fuel into the
carburetor?
Spring pressure

Review
Summarize the differences between a wet-bulb and
dry-bulb primer.
A wet-bulb primer is full of fuel, and acts like a fuel
pump to force fuel into the carburetor. A dry-bulb
primer pumps air into the float bowl, increasing air
pressure, which forces fuel up the main nozzle.

Review
List several applications that require the operator to
have sole control of the throttle.
Chainsaws, snowmobiles, motorcycles, or string
trimmers.

Review
What is the purpose of a governor?
It maintains a constant engine speed under changing
load conditions.

Review
Which type of governor uses pivoted flyweights?
Centrifugal (mechanical) governor

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