Carburetor
The carburetor works on Bernoulli's principle: the faster air moves, the
lower its static pressure, and the higher its dynamic pressure.
A carburetor is a device that blends air and fuel for an internal
combustion engine. It was invented by Karl Benz before 1885 and patented
in 1886. To carburetor means to combine with carbon. In fuel chemistry,
the term has the more specific meaning of increasing the carbon (and
therefore energy) content of a fuel by mixing it with a volatile
hydrocarbon.
Principles
Beginning in the 1930s, downdraft carburetors were the most popular type
for truck parts use in
the United States. In Europe, the side draft carburetors replaced
downdraft as free space in the engine bay decreased and the use of the
SU-type carburetor (and similar units from other manufacturers)
increased. Some small propeller-driven aircraft engines still use the
updraft carburetor design, however many use more modern designs such as
the Constant Velocity (CV) Bing(TM) carburetor.
The
accelerator linkage does not directly control the flow of liquid fuel.
Instead, it actuates carburetor mechanisms which meter the flow of air
being pulled into the engine. The speed of this flow, and therefore its
pressure, determines the amount of fuel drawn into the airstream.
Most carbureted (as opposed to fuel-injected) engines have a single
carburetor, though some engines use multiple carburetors. Older engines
used updraft carburetors, where the air enters from below the carburetor
and exits through the top. This had the advantage of never "flooding"
the engine truck parts, as any liquid fuel droplets would fall out of the
carburetor instead of into the intake manifold; it also lent itself to
use of an oil bath air cleaner, where a pool of oil below a mesh element
below the carburetor is sucked up into the mesh and the air is drawn
through the oil covered mesh; this was an effective system in a time
when paper air filters did not exist.
Beginning in the 1930s, downdraft carburetors were the most popular type
for truck parts use in
the United States. In Europe, the side draft carburetors replaced
downdraft as free space in the engine bay decreased and the use of the
SU-type carburetor (and similar units from other manufacturers)
increased. Some small propeller-driven aircraft engines still use the
updraft carburetor design, however many use more modern designs such as
the Constant Velocity (CV) Bing(TM) carburetor.
Operation
* Fixed-venturi, in which the varying air velocity in the venturi alters
the fuel flow; this architecture is employed in most downdraft
carburetors found on American and some Japanese cars
* Variable-venturi, in which the fuel jet opening is varied by the
slide. In "constant depression" carburetors, this is done by a vacuum
operated piston connected to a tapered needle which slides inside the
fuel jet. A simpler version exists, most commonly found on small
motorcycles and dirt bikes, where the slide and needle is directly
controlled by the throttle position. These types of carburetors are
commonly equipped with accelerator pumps to make up for a particular
shortcoming of this design. The most common variable venturi (constant
depression) type carburetor is the side draft SU carburetor and similar
models from Hitachi, Zenith-Stromberg and other makers. Other similar
designs have been used on some European and a few Japanese automobiles.
These carburetors are also referred to as "constant velocity" or
"constant vacuum" carburetors. An interesting variation was Ford's VV
(Variable Venturi) carburetor, which was essentially a fixed venturi
carburetor with one side of the venturi hinged and movable to give a
narrow throat at low rpm and a wider throat at high rpm. This was
designed to provide good mixing and airflow over a range of engine
speeds, though the VV carburetor proved problematic in service.
Under all engine operating conditions, the carburetor must:
* Measure the airflow of the engine
* Deliver the perfect amount of fuel to keep the fuel/air mixture in the
proper range (adjusting for factors such as temperature)
* Mix the two finely and evenly
This job would be simple if air and gasoline (petrol) were ideal fluids;
in practice, however, their deviations from ideal behavior due to
viscosity, fluid drag, inertia, etc. require a great deal of complexity
to compensate for exceptionally high or low engine speeds. A carburetor
must provide the proper fuel/air mixture across a wide range of ambient
temperatures, atmospheric pressures, engine speeds and loads, and
centrifugal forces:
* Cold start
* Hot start
* Idling or slow-running
* Acceleration
* High speed / high power at full throttle
* Cruising at part throttle (light load)
Modern carburetors are required to do this while maintaining low rates
of exhaust emissions.
Main open-throttle circuit
As the throttles are progressively opened, the manifold vacuums are
lessened since there are less restriction on the airflow, reducing the
flow through the idle and off-idle circuits. This is where the venturi
shape of the carburetor throat comes into play, due to Bernoulli's
principle (i.e., as the velocity increases, pressure falls). The venturi
raises the air velocity, and this high speed and thus low pressure sucks
fuel into the airstream through a nozzle or nozzles located in the
center of the venturi. Sometimes one or more additional booster venturis
are placed coaxially within the primary venturi to increase the effect.
As the throttle is closed, the airflow through the venturi drops until
the lowered pressure is insufficient to maintain this fuel flow, and the
idle circuit takes over again, as described above.
Bernoulli's principle is ineffective at idle or slow running and in the
very small carburetors of the smallest model engines. Small model
engines have flow restrictions ahead of the jets to reduce the pressure
enough to suck the fuel into the air flow. Similarly the idle and slow
running jets of large carburetors are placed after the throttle valve
where the pressure is reduced partly by viscous drag, rather than by
Bernoulli's principle. The most common rich mixture device for starting
cold engines was the choke, which works on the same principle.
Power valve
For open throttle operation a richer mixture will produce more
power, prevent detonation, and keep the engine cooler. These are usually
addressed with a spring-loaded "power valve", which is held shut by
engine vacuum. As the throttle opens up, the vacuum decreases and the
spring opens the valve to let more fuel into the main circuit. On
two-stroke engines, the operation of the power valve is the reverse of
normal - it is normally "on" and at a set rpm it is turned "off". It is
activated at high rpm to extend the engine's rev range, capitalizing on
a two-stroke's tendency to rev higher momentarily when the mixture is
lean.
Alternative to employing a power valve, the carburetor may utilize a
metering rod or step-up rod system to richen the fuel mixture under
high-demand conditions. Such systems were originated by Carter
Carburetor in the 50's for the primary two venturis of their four barrel
carburetors, and step-up rods were widely used on most 1-, 2-, and
4-barrel Carter carburetors through the end of production in the 1980s.
The step-up rods are tapered at the bottom end, which extends into the
main metering jets. The tops of the rods are connected to a vacuum
piston and/or a mechanical linkage which lifts the rods out of the main
jets. When the step-up rod is lowered into the main jet, it restricts
the fuel flow. When the step-up rod is raised out of the jet, more fuel
can flow through it. In this manner, the amount of fuel delivered is
tailored to the transient demands of the engine. Some 4-barrel
carburetors use metering rods only on the primary two venturis, but some
use them on both primary and secondary circuits, as in the Rochester
Quadra jet.
Accelerator pump
The greater inertia of liquid gasoline, compared to air, means that if
the throttle is suddenly opened, the airflow will increase more rapidly
than the fuel flow, causing a temporary "lean" condition which causes
the engine to "stumble" under acceleration (the opposite of what is
normally intended when the throttle is opened). This is remedied by the
use of a small mechanical pump, usually either a plunger or diaphragm
type actuated by the throttle linkage, which propels a small amount of
gasoline through a jet, wherefrom it is injected into the carburetor
throat. This extra shot of fuel counteracts the transient lean condition
on throttle tip-in. Most accelerator pumps are adjustable for volume
and/or duration by some means. Eventually the seals around the moving
parts of the pump wear such that pump output is reduced; this reduction
of the accelerator pump shot causes stumbling under acceleration until
the seals on the pump are renewed.
The accelerator pump is also used to prime the engine with fuel prior to
a cold start. Excessive priming, like an improperly-adjusted choke, can
cause flooding.
Choke
When the engine is cold, fuel vaporizes less readily and tends to
condense on the walls of the intake manifold, starving the cylinders of
fuel and making the engine difficult to start; thus, a richer mixture
(more fuel to air) is required to start and run the engine until it
warms up. A richer mixture is also easier to ignite.
To provide the extra fuel, a choke is typically used; this is a device
that restricts the flow of air at the entrance to the carburetor, before
the venturi. With this restriction in place, extra vacuum is developed
in the carburetor barrel, which pulls extra fuel through the main
metering system to supplement the fuel being pulled from the idle and
off-idle circuits. This provides the rich mixture required to sustain
operation at low engine temperatures.
Moreover, the choke is connected to a cam (the fast idle cam) or other
such device which prevents the throttle plate from closing fully while
the choke is in operation. This causes the engine to idle at a higher
speed. Fast idle serves as a way to help the engine warm up quickly, and
give a more stable idle while cold by increasing airflow throughout the
intake system which helps to better atomize the cold fuel.
In most carbureted cars produced from the mid 1960s onward (mid 1950s in
the United States) it is usually automatically controlled by a
thermostat employing a bimetallic spring, which is exposed to engine
heat. This heat may be transferred to the choke thermostat via simple
convection, via engine coolant, or via air heated by the exhaust. More
recent designs use the engine heat only indirectly. A choke unloaded is
a linkage arrangement that forces the choke open against its spring when
the vehicle's accelerator is moved to the end of its travel. This
provision allows a "flooded" engine to be cleared out so that it will
start.
Typically used on small engines, notably motorcycles, enricheners work
by opening a secondary fuel circuit below the throttle valves. This
circuit works exactly like the idle circuit, and when engaged it simply
supplies extra fuel when the throttle is closed.
Fuel supply
Float chamber
To ensure a ready mixture, the carburetor has a "float chamber" (or
"bowl") that contains a quantity of fuel at near-atmospheric pressure,
ready for use. These reservoirs are constantly replenished with fuel
supplied by a fuel pumps. The correct fuel level in the bowl is
maintained by means of a float controlling an inlet valve, in a manner
very similar to that employed in toilet tanks. As fuel is used up, the
float drops, opening the inlet valve and admitting fuel. As the fuel
level rises, the float rises and closes the inlet valve. The level of
fuel maintained in the float bowl can usually be adjusted, whether by a
setscrew or by something crude such as bending the arm to which the
float is connected. This is usually a critical adjustment, and the
proper adjustment is indicated by lines inscribed into a window on the
float bowl, or a measurement of how far the float hangs below the top of
the carburetor when disassembled, or similar. Floats can be made of
different materials, such as sheet brass soldered into a hollow shape,
or of plastic; hollow floats can spring small leaks and plastic floats
can eventually become porous and lose their flotation; in either case
the float will fail to float, fuel level will be too high, and the
engine will not run well unless the float is replaced. Conversely, as
the fuel evaporates from the float bowl, it leaves sediment, residue,
and varnishes behind, which clog the passages and can interfere with the
float operation. This is particularly a problem in automobiles operated
for only part of the year and left to stand with full float chambers for
months at a time; commercial fuel stabilizer additives are available
that reduce this problem.
Usually, special vent tubes allow air to escape from the chamber as it
fills or enter as it empties, maintaining atmospheric pressure within
the float chamber; these usually extend into the carburetor throat.
Placement of these vent tubes can be somewhat critical to prevent fuel
from sloshing out of them into the carburetor, and sometimes they are
modified with longer tubing. This is not necessary in installations
where the carburetor is mounted upstream of the supercharger, which is
for this reason the more frequent system. However, this results in the
supercharger being filled with compressed fuel/air mixture, with a
strong tendency to explode should the engine backfire; this type of
explosion is frequently seen in drag races, which for safety reasons now
incorporate pressure releasing blow-off plates on the intake manifold,
breakaway bolts holding the supercharger to the manifold, and
shrapnel-catching ballistic nylon blankets surrounding the
superchargers.
Instead, a diaphragm chamber is used. A flexible diaphragm forms one
side of the fuel chamber and is arranged so that as fuel is drawn out
into the engine the diaphragm is forced inward by ambient air pressure.
The diaphragm is connected to the needle valve and as it moves inward it
opens the needle valve to admit more fuel, thus replenishing the fuel as
it is consumed. As fuel is replenished the diaphragm moves out due to
fuel pressure and a small spring, closing the needle valve. A balanced
state is reached which creates a steady fuel reservoir level, which
remains constant in any orientation.
Multiple carburetor barrels
Colombo Type 125 "Testa Rossa" engine in a 1961 Ferrari 250TR Spyder
with six Weber two-barrel carburetors inducting air through 12 air
horns; one individually adjustable barrel for each cylinder.
While basic carburetors have only one venturi, many carburetors have
more than one venturi, or "barrel". Two barrel and four barrel
configurations are commonly used to accommodate the higher air flow rate
with large engine displacement. Multi-barrel carburetors can have
non-identical primary and secondary barrel(s) of different sizes and
calibrated to deliver different air/fuel mixtures; they can be actuated
by the linkage or by engine vacuum in "progressive" fashion, so that the
secondary barrels do not begin to open until the primaries are almost
completely open. These advantages may not be important in
high-performance applications where part throttle operation is
irrelevant, and the primaries and secondaries may all open at once, for
simplicity and reliability; also, V configuration engines, with two
cylinder banks fed by a single carburetor, may be configured with two
identical barrels, each supplying one cylinder bank. In the widely seen
V8 and 4-barrel carburetor combination, there are often two primary and
two secondary barrels.
Carburetor adjustment
Too much fuel in the fuel-air mixture is referred to as too rich, and
not enough fuel is too lean. The mixture is normally adjusted by one or
more needle valves on an automotive carburetors, or a pilot-operated
lever on piston-engined aircraft (since mixture is air density
(altitude) dependent). The (stoichiometric) air to gasoline ratio is
14.7:1, meaning that for each weight unit of gasoline, 14.7 units of air
will be consumed. Stoichiometric mixture are different for various fuels
other than gasoline.
Ways to check carburetor mixture adjustment include: measuring the
carbon monoxide, hydrocarbon, and oxygen content of the exhaust using a
gas analyzer, or directly viewing the color of the flame in the
combustion chamber through a special glass-bodied spark plug sold under
the name "Colortune" for this purpose. The flame colour of
stoichiometric burning is described as a "bunsen blue", turning to
yellow if the mixture is rich and whitish-blue if too lean.
Many American-market vehicles used special "feedback" carburetors that
could change the base mixture in response to signals from an exhaust gas
oxygen sensor. These were mainly used to save costs but eventually
disappeared as falling hardware prices and tighter emissions standards
made fuel injection a standard item.
Catalytic carburetors
The catalytic carburetor mixes fuel fumes with water and air in the
presence of heated catalysts such as nickel or platinum. This breaks the
fuel down into methane, alcohols, and other lighter-weight fuels. The
original catalytic carburetor was introduced to permit farmers to run
tractors from modified and enriched kerosene. While catalytic
carburetors were made commercially available in the early 1930s, two
major factors limited their widespread public use. First, the addition
of additives to commercial gasoline made it unsuitable for use in
engines with catalytic carburetors. Tetra-ethyl lead was introduced in
1932 to raise gasoline's resistance to engine knock, thereby permitting
the use of higher compression ratios. Second, the economic advantage of
using kerosene over gasoline faded in the 1930s, eliminating the
catalytic carburetor's primary advantage.
Manufacturers
Some manufacturers of carburetors are/were:
* The AMAL Carburetter Company, supplier to the British motorcycle
industry[1]
* Argelite, producer of Holley and Magneti Marelli carburetors for the
Argentine market
* Autolite, a division of the Ford Motor Company from 1967 to 1973.
* Bendix Stromberg and Bendix Technico carburetors, used on vehicles
made by Chrysler, IHC, Ford, GM, AMC, and Studebaker
* Bing Carburetor (used on motorcycles, mopeds, aircraft, boats)
* Briggs & Stratton, small engines (e.g. powered mowers)
* Carter carburetor, (used on numerous makes of vehicles, including
those made by Chrysler, IHC, Ford, GM, AMC, and Studebaker, as well as
on industrial and agricultural equipment and small engines
* Dell'Orto carburetors from Italy, used on cars and motorcycles
* Edelbrock performance carburetors
* Hitachi, Ltd. Hitachi carburetors, found on Japanese automobiles
* Holley, with usage as broad as Carter and Weber.
* Keihin, also common on Japanese and other motorcycles, a keiretsu
group company affiliated with Honda
* Lectron carburators
* Mikuni, common on Japanese motorcycles, especially in the 1980s
* Motec Engineering - high performance updraft carburetors
* Pierburg carburetor, in Volvo, VW and Audi
* Rochester Products Division, USA (A General Motors subsidiary; also
sold Weber/Magneti Marelli carburetors under license)
* Solex carburetor
* SU carburetor widely used on British Commonwealth and
European-designed vehicles, presently manufactured by Burlen Fuel
Systems
* Tecumseh Products Company, small engines (e.g. lawn mowers, snow
blowers)
* Villiers UK Motorcycle and small engines
* Walbro and Tillotson carburetors for small engines Info
* Weber carburetor, Italian, owned by Magneti Marelli
* Zenith UK, Also produced the Zenith-Stromberg Carburetors
