working of carburetor and its parts
carburetor, also spelled carburettor ,
device for supplying a spark-ignition engine with a mixture of fuel
and air. Components of carburetors usually include a storage chamber
for liquid fuel, a choke, an idling (or slow-running) jet, a main jet, a
venturi-shaped air-flow restriction, and an accelerator pump. The
quantity of fuel in the storage chamber is controlled by a valve actuated by a float. The choke,
a butterfly valve, reduces the intake of air and allows a fuel-rich
charge to be drawn into the cylinders when a cold engine is started. As
the engine warms up, the choke is gradually opened either by hand or
automatically by heat- and engine-speed-responsive controllers. The fuel
flows out of the idling jet into the intake air as a result of reduced
pressure near the partially closed throttle valve. The main fuel jet comes into action when the throttle valve is further open. Then the venturi-shaped
air-flow restriction creates a reduced pressure for drawing fuel from
the main jet into the air stream at a rate related to the air flow so
that a nearly constant fuel-air ratio is obtained. The accelerator pump
injects fuel into the inlet air when the throttle is opened suddenly.
In the 1970s, new legislation and consumer preferences led automobile manufacturers to improve fuel efficiency and lower pollutant emissions. To accomplish these objectives, engineers developed fuel injection
management systems based on new computer technologies. Soon, fuel
injection systems replaced carbureted fuel systems in virtually all
gasoline engines except for two-cycle and small four-cycle gasoline
engines, such as those used in lawn mowers.
Operation
- Fixed-venturi,
in which the varying air velocity in the venturi alters the fuel flow;
this architecture is employed in most carburetors found on cars.
- Variable-venturi, in which the fuel jet opening is varied by
the slide (which simultaneously alters air flow). 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. The most common variable venturi (constant
depression) type carburetor is the sidedraft SU carburetor and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and Zenith-Stromberg
companies helped these carburetors rise to a position of domination in
the UK car market, though such carburetors were also very widely used on
Volvos and other non-UK makes. Other similar designs have been used on some European and a few Japanese automobiles.
Basics
A carburetor basically consists of an open pipe through which the air passes into the inlet manifold
of the engine. The pipe is in the form of a venturi: it narrows in
section and then widens again, causing the airflow to increase in speed
in the narrowest part. Below the venturi is a butterfly valve
called the throttle valve — a rotating disc that can be turned end-on
to the airflow, so as to hardly restrict the flow at all, or can be
rotated so that it (almost) completely blocks the flow of air. This
valve controls the flow of air through the carburetor throat and thus
the quantity of air/fuel mixture the system will deliver, thereby
regulating engine power and speed. The throttle is connected, usually
through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.
Off-idle circuit
As the throttle is opened up slightly from the fully closed position,
the throttle plate uncovers additional fuel delivery holes behind the
throttle plate where there is a low pressure area created by the
throttle plate blocking air flow; these allow more fuel to flow as well
as compensating for the reduced vacuum that occurs when the throttle is
opened, thus smoothing the transition to metering fuel flow through the
regular open throttle circuit.
Power valve
For open throttle operation a richer mixture will produce more power, prevent pre-ignition detonation,
and keep the engine cooler. This is 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.
Accelerator pump
Liquid gasoline, being denser than air, is slower than air to react to a force
applied to it. When the throttle is rapidly opened, airflow through the
carburetor increases immediately, faster than the fuel flow rate can
increase. This transient oversupply of air causes a lean mixture, which
makes the engine misfire (or "stumble")—an effect opposite what was
demanded by opening the throttle. This is remedied by the use of a small
piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet 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.
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