Transcript Carburetion Systems Notes

Carburetion Systems
Overview
 Purpose
 Types
 Gas,
of Carburetor
of Carburetors
Oil, & Air
The “BIG” Picture
 One
of the “Big 3” systems
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Ignition
–
Compression
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Carburetion
What is a Carburetor?

A carburetor’s primary purpose is to produce a
mixture of fuel and air to operate the engine.
Purpose of the Carburetor
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Secondary Purposes & Requirements Change air fuel ratio and amount of fuel that is
correct for different circumstances such as:
Cold or hot starting
Idling
Part throttle
Acceleration
High speed operation
Carburetor Theory
 Atmospheric
Pressure
–
a constant downward force of air on the Earth
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usually varies between 13 and 15 lbs per sq. in.
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areas of low pressure must be created in the
carburetor to create air flow
Carburetion
 Gasoline
engines cannot run on liquid
gasoline. It must be vaporized and mixed
with air in the proper proportions for
varying conditions.
How does it work?

Air enters the top of
the carburetor and is
mixed with liquid fuel.
How does it work?

The air fuel mixture is
forced into the intake
manifold by
atmospheric pressure
and burned in the
combustion chamber
of the engine.
Air-Fuel Mixture

The mixture will vary
depending on the
conditions. The
proportion is given as
the number of pounds
of air compared to the
number of pounds of
gasoline.
Air-Fuel Mixture

At normal operating
speed a small engine
will use an air-fuel
mixture of about 15
pounds air to 1 pound
of gasoline
Pressure Differences
 Carburetors
work on the principle of air
pressure differences. When discussing
pressure differences we will talk about
 Vacuum
 Atmospheric Pressure
 Venturi
Vacuum

An absolute vacuum is an area completely free of
air or atmospheric pressure.

Although an absolute vacuum is not reached in a
small engine, any pressure less that atmospheric
pressure is generally referred to as a vacuum
Atmospheric Pressure

Atmospheric pressure is the pressure
produced by the weight of air molecules
above the earth.

A partial vacuum is produced by the piston
on the intake stroke. When the intake
valve opens, atmospheric pressure forces
air through the carburetor to fill it.
Venturi Principle

A venturi is a
restriction in an air
passage that increases
air speed or velocity.
Venturi
This increase in
velocity reduces
pressure causing fuel
to be drawn into the
air stream.
 Particles of fuel are
vaporized by air
rushing through the
venturi.

Carburetor Theory (con’t)
 Airfoil
low pressure
airfoil
high pressure
Because the main discharge nozzle is extended into the
airstream, an airfoil is also created, further lowering the
pressure “above” the nozzle.
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If an engine ran at only one speed under ideal
conditions, the story of carburetion would end here.
The carburetor must be able to adjust the air/fuel mixture to
the conditions presented.
It must run smoothly and economically at widely varying
speeds.
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Richer mixture--more gas added to mixture.
Leaner mixture--less gas added to mixture.
 To
make these adjustments, a threaded needle valve
is added at the bottom of the main discharge nozzle.
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This regulates the amount of fuel that gets to the venturi
 To
further regulate the mixture, two “air regulators”
or butterfly valves are also added:
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These restrict the amount of air flow through the
carburetor--either manually or automatically.
This action decreases the power and speed and the
richness of the mixture within the engine.
–Throttle
valves restrict air movement at all
speeds and are generally manually controlled.
 Throttle
Open
Throttle Closed
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Choke valves restrict air movement at start-up to allow for
a richer mixture and can be manually or automatically
engaged.
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To further regulate the air/fuel mixture at lower
speeds, another threaded needle valve is added.
 This
allows the engine to run smoothly and
economically at very low speeds by allowing a
slight bit of fuel to slip past the throttle valve.
 Note: At low speeds, the throttle valve is fully
closed.
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Idle speed should be 1750 RPM on all B&S engines.
Vaporization
 Although
the venturi breaks the fuel into
fine particles, it is further vaporized by the
heat of the engine in the intake manifold
and by the swirling action of the air in the
combustion chamber.
Combustion

Cold fuel is difficult to vaporize, this is
why we choke or prime a cold engine to
help get it started.

Over choking or priming can cause raw
fuel to be pulled into the combustion
chamber resulting in bypass or a condition
known as flooding.
Summary

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Since a gasoline engine does not operate on
liquid fuel, it is the responsibility of the
carburetor to provide it with an air-fuel
mixture. The carburetor operates on the
principles of the following:
Vacuum
Atmospheric Pressure
Venturi
Types of Carburetors
 Gravity
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Feed
called a “Flo-Jet” by Briggs & Stratton
 Vacuum
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called “Vacu-Jet”
 Pulse
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Feed
Feed
called “Pulsa-Jet”
 Flo-Jet-
Gravity feeds or float type.
 Vacu-Jet-Suction feed-Tank is usually
below the carburetor and must have
atmospheric pressure to operate.
 Pulsa-Jet- Utilizes a fuel pump to distribute
the fuel.
Flo-Jet Carburetors
 Gravity
feed system
 fuel
flows by gravity to the carburetor
 gas tank must have a vent hole to provide
atmospheric pressure to “push” fuel to carburetor
 The float is found in the bowl.
– As fuel flows into the bowl, it raises
and lowers the float.
Flo-Jet Carburetors (con’t)
 The float is attached to a needle valve.
– The needle, along with its seat, work together to
turn on
and off fuel flow to the main discharge nozzle.
 Float level
– should be
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high enough to allow an ample supply of fuel at
full throttle, yet remain low enough to prevent flooding
and/or leaking.
should be set at 5/16 of an inch on our engines.
Flo Jet Float Type
Another type of Float Carburetor
Pulsa-Jet Carburetors
 Only
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“true” fuel system
contains a diaphragm type fuel pump and a
“constant level” fuel chamber
 Newest
design of carburetors for small
engines
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can obtain just as much (or more) horsepower
as older, more complicated float-type
carburetors
 This
is due to the fact that it provides a constant fuel
level directly below the venturi
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very little “lift” is needed to get the fuel to the carburetor
Pulsa-Jet Carburetors (con’t)
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The venturi can be made larger, allowing more air/fuel
mixture into the engine allowing an increase in
horsepower within the same sized engine.
See diagram for operating principals
Pulsa Jet
Vacu-Jet Carburetors
 Fuel
tank is below the carburetor
 Again,
atmospheric pressure is employed to help
get the fuel to the carburetor.
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Air pushes down on the fuel in the tank, when the piston
starts downward on the intake stroke, it creates an area of
pressure that is lower than that of the atmospheric
pressure. This causes the fuel to rise through the pickup
tube and travel toward the main discharge nozzle.
Vacu-Jet Carburetors (con’t)
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Vacu-Jet carburetors require a richer mixture
setting because the fuel system “lags” behind
the fuel requirements of the engine at the high
speeds that most small engines run at.
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 As
the throttle closes to idle, the leading edge takes
a position between 2 different sized discharge holes
that are found on the main discharge nozzle.
 The larger of the holes now becomes an area of high
air pressure and the flow of fuel ceases.
Vacu-Jet Carburetors (con’t)
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The smaller hole now becomes an area of lower pressure
and fuel continues to flow--just enough for the engine to
idle.
Choke valve is also different:
 There
are many problems with this choke system
including sticking open and closed, especially when
remotely controlled.
Linkage
 There
are many set ups for the throttle
linkage and the governor
 Always note and record the linkage before
dis-assembly
Carburetor Adjustments
 The
following applies to many Briggs and
Stratton Carburetors, however, many
carburetors have no adjustment
Carburetor Adjustments
 Adjustment
of the needle valve for
maximum power
 Causes
overheating, early or late detonation, and
short valve life
 No accelerator pump in Flo-Jet and Vacu-Jet models
so engine will “kill” if throttle is opened suddenly.
 “Floods” the engine causing “raw” fuel into the
combustion chamber (cylinder).
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dilutes the crankcase oil washing away the film on the
cylinder wall causing “scuffing”and lost compression.
Carburetor Adjustments (con’t)
 Throttle
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Adjustment
To adjust for maximum power and efficiency:
 turn
the screw in (clockwise) until the engine speed
decreases.
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This indicates a lean mixture.
 Turn
the screw out (counter-clockwise) until the
speed increases and again decreases
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This indicates a rich mixture.
Carburetor Adjustments (con’t)
 Slowly
turn the main needle valve back clockwise to
a point midway between the 2 readings
 This
method can also be used to set the maximum
speed the engine will run by first opening the
throttle all the way.
Carburetor Adjustments (con’t)
 Idle
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Speed Adjustment
Very similar to Throttle Adjustment
 IS
NOT necessarily the slowest speed at which the
engine will run.
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Can be any speed you choose
Carburetor Adjustments (con’t)
 Again,
turn the screw in (clockwise) until the engine
speed decreases.
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Lean mixture
 Turn
the screw out (counter-clockwise) until the
speed increases and again decreases
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Rich mixture
 Usually,
the idle adjustment needs to be reset with
each new application of the engine.
Gas, Oil, & Air
 Gas-–
use fresh, clean, unleaded gasoline with a
minimum of 77 octane
 In
Minnesota, the minimum octane allowed by law
is 87.
 Purchase an amount that can be used up within 30
days.
 Ethanol an/or methanol is fine to use in today’s
small engines.
Gas, Oil, & Air (con’t)
 Oil- Detergent
oils keep the engine free of gum and
varnish deposits and generally keep the engine
cleaner.
 No other additives should be used in the oil.
Gas, Oil, & Air (con’t)
 Air-–
Clean air is an extremely important part of the
carburetion system.
 New
technologies in foam materials make these air
cleaners somewhat obsolete.
THE END
Carburetor Theory (con’t)
 All
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carburetors work basically the same:
As piston moves from TDC to BDC on the
intake stroke, the intake valve opens, air is
“sucked” through the air cleaner into the
airhorn and . . .
Venturi creates an area of low pressure that
draws fuel from the fuel supply through the
main discharge nozzle.
 Venturi
also causes the air flow to rapidly
accelerate--thus vaporizing and mixing air and fuel
together very efficiently.