Carburettor Additional Systems (Automobile)


Carburettor Additional Systems

One or more add-on devices are be used with the carburettor to improve economy, driveability, and emission control. Most commonly used devices are discussed below.

Hot-idle Compensator Valve

High air temperature at carburettor inlet causes gasoline to evaporate rapidly, which intern-can produce highly rich idle mixture. In order to overcome this problem, many carburettors use
a hot idle compensator valve (Fig. 9.64), which is a thermo­static valve consisting of a bimetal spring, a bracket, and a small poppet. The compensator valve is usually located either in the carburettor barrel or in a chamber on the rear the of carburettor bowl. A dust cover is placed over the chamber. The hot-idle compensator valve is normally closed by spring tension and engine vacuum. As tempera-”~ ture rises, the bimetal strip bends. This uncovers an
auxiliary air passage, or air bleed, through which air enters the carburettor below the throttle plate. As this extra air mixes with excess fuel to lean out the idle mixture, it prevents stalling and rough idling. Once the carburettor temperature returns to normal, the compensator valve closes to shut off the extra air supply.
Hot-idle compensator.
Fig. 9.64. Hot-idle compensator.


Idle Enrichment Valve

In order to reduce emissions, carburettors are adjusted for leaner mixtures. But for cold-engine operation, the idle mixture must be enriched, and to achieve this, an idle enrichment system is used in some cars. This works opposite to a hot idle compensator valve. A small vacuum diaphragm mounted near the carburettor top (Fig. 9.65) controls the circuit air. When control vacuum is applied, the diaphragm reduces idle system air due to which the mixture is enriched with fuel. Diaphragm vacuum is controlled by a temperature switch in the radiator. As the
engine warms, this switch stops the vacuum sig­nal, consequently the mixture returns to its nor­mal strength.

Fast-idle Pull-off (Choke Pull-off)

Choke and fast-idle operation for a long time can damage catalytic converter with rich air-fuel mixture. Some catalytic converter-equipped cars use fast-idle pull-off to avoid converter overheat­ing. In one system, manifold vacuum acts on a vacuum-back diaphragm at the rear of the car­burettor. This diaphragm drops the fast-idle cam to a lower step. Vacuum to the diaphragm is controlled by a vacuum solenoid operated by a coolant temperature switch and delay timer. In some engines, an electric solenoid instead of a
A typical idle enrichment valve.
Fig. 9.65. A typical idle enrichment valve.
vacuum diaphragm is used to open the choke. This pulls the first-idle screw off the cam whenever the engine is started with the coolant below a specified temperature.
Regardless of the system used, fast idle pull off has no effect on engine warm-up during ordinary operation because normal throttle movement disengages the fast-idle cam. The fast idle pull off system only operates when the car is warming
up while parked. carburettor

Throttle-stop solenoid.
Fig. 9.66. Throttle-stop solenoid.

Throttle Stop Solenoid

Engine dieseling or after-run results after the ignition is turned off if the combustion chamber temperatures keep the chamber hot enough to ignite an idle air-fuel mixture. Dieseling is, however, caused by several factors like higher operating temperatures, faster idle speeds, retarded idle ignition timing and lean air-fuel mixtures. Dieseling is prevented by closing the throttle more than required for normal slow idle speed. A throttle stop solenoid (Fig. 9.66) holds the throttle open for normal slow idle and allows the throttle to close further when the engine is shut off. When ignition is turned on the solenoid is energised, and its plunger moves out to contact the idle speed adjusting screw or a power brake on the throttle shaft. This holds the throttle open, until the ignition is shut off and consequently the solenoid is de-energised. This retracts the plunger and then closes throttle to block air flow.


Dashpots (Fig. 9.67) are small chambers fitted with a spring-loaded diaphragm and plungers. These are widely used over years on some cars using automatic transmission and on a few with manual transmission for slow throttle closing. A link from the throttle touches the dashpots plunger as the throttle closes. As force is applied to the plunger, air slowly bleeds out
of the diaphragm chamber through a small hole. Dashpots were originally used to prevent an excessively rich mixture on dash pot deceleration, which can cause stalling. They are now working as emission control devices on late-model cars, reducing HC emissions on deceleration.
Fig. 9.67. Dashpot

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