Carburetion
The three general stages involved in carburetion are metering, atomization, and vaporization. Metering is another term for measuring. In the process of carburetion, fuel is metered into the air passing through the barrel of the carburetor. The ideal air/fuel ratio at which all the fuel blends with all the oxygen in the air is called the stoichiometric ratio. This ratio is about 14.7:l . If there is more fuel in the mixture it is called a rich mix. If there is less fuel it is called a lean mix. The amount of fuel metered into the air is varied in relation to the amount of air passing through the carburetor. Atomization is the stage in which the metered fuel is drawn into the air stream in the form of tiny droplets. The droplets of fuel are drawn out of passages called discharge ports. The surface area of an atomized droplet is in contact with a relatively large amount of surrounding air. In addition, the venturi is a low-pressure area. The boiling point of the fuel is greatly reduced by lower pressures; therefore liquid gasoline can become a vapor with low amounts of heat. These factors combine to create a fine mist of fuel below the venturi in the bore. This is called vaporization-the last stage of carburetion. It occurs below the venturi, in the intake manifold, and within the cylinder. Swirl, turbulence, and heat within the intake manifold and cylinder also enhance vaporization.
BASIC CARBURETOR CIRCUITS
Variations in engine speed and load demand different-amounts of air and fuel (often in differing proportions) for best performance-and present complex problems for the carburetor. At engine idle speeds, for example, there is insufficient air velocity to cause fuel to be drawn from the discharge nozzle and into the air stream. Also, with a sudden change in engine speed, such as rapid acceleration, the venturi effect (pressure differential) is momentarily lost. Therefore, the carburetor must have special circuits or systems to cope with these situations. The basic circuits of a typical carburetor include the float, idle, main metering, power enrichment, accelerator pump, and choke:
The float circuit or fuel inlet system stores fuel and holds it at a precise level.
The idle circuit supplies the richer air/fuel mixture to operate the engine at idle and low speeds.
The main metering circuit provides the air/fuel mixture during open throttle conditions.
The power enrichment circuit supplies a richer mixture during heavy loads. In many carburetors, metering rods placed in the main jets provide power enrichment.
The accelerator pump circuit mechanically adds fuel to the mixture when the throttle is quickly opened.
The choke circuit provides a richer mixture during cranking and cold start-up by restricting the air flow into the carburetor.
CARBURETOR SYSTEMS
To meet complex fuel economy and emission requirements, carburetors were fitted with a variety of auxiliary controls. The following describes some of the more common assist devices you are likely to encounter when servicing a carburetor.
Choke Qualifier
Once the engine has started, a leaner mixture is needed. If the choke stays shut, the rich mixture floods the engine and causes stalling. Therefore, the automatic choke has a choke qualifying mechanism to open the choke plate slightly after the engine has started.
Dashpot
The dashpot is used during rapid deceleration to retard the closing of the throttle. This allows a smooth transition from the main metering system to the idle system and prevents stalling due to the overly rich air/fuel mixture. It also controls the level of HC in the exhaust during deceleration.
Hot-Idle Compensator (HIC) Valve
When the engine is overheated, a hot-idle compensator opens an air passage to lean the mixture slightly. This increases idle speed to help cool the engine (by increased coolant flow) and prevents excess fuel vaporization with-in the carburetor. The hot-idle compensator is a bimetal, thermostatically controlled air bleed valve. Vacuum Break Some carburetors are equipped with a fuel vacuum break system, which includes a primary diaphragm and a secondary diaphragm. The primary vacuum diaphragm opens the choke valve slightly as soon as the engine starts to keep the engine from over-choking and stalling. The secondary vacuum diaphragm, which is also attached to the choke lever opens the choke valve slightly wider in warm weather or when a warm engine is being started.
Choke Unloader
To be able to start a cold engine that has been flooded with gasoline, a choke unloader is required. The choke unloader is throttle-linkage-actuated and opens the choke whenever the accelerator pedal is floored. At wide-open throttle, the partially opened choke allows additional air to lean out the mixture and reduce fuel flow.
Throttle Position Solenoid
The throttle position solenoid is used to control the position of the throttle plate. It can have several functions, depending on its application. when the basic function is to prevent dieseling, the solenoid is called a throttle stop solenoid or an idle stop solenoid. when the engine is started, the solenoid is energized and touches the throttle plates that open slightly to the curb idle position. when the ignition switch is turned off, the solenoid allows the throttle plate to close completely, and shuts off the air/fuel supply to prevent dieseling or run-on. The throttle position solenoid is also used to increase the curb idle speed to compensate for extra loads on the engine. when this is its primary function, the solenoid might also be called an idle speed-up solenoid or a throttle kicker. This feature is most often used on cars with air conditioners.
TYPES OF CARBURETORS
Many types of carburetors have been built in order to accommodate different load conditions, engine designs, and air/fuel requirements. Carburetor Barrels A carburetor barrel is a passageway or bore used to mix the air and fuel. It consists of the throttle plate, venturi, and air horn. A one-barrel carburetor is used on small engines that do not require large quantities of air and fuel. A two-barrel carburetor has two throttle plates and two venturis. The area where the air comes into the carburetor is common on both barrels. A two-barrel carburetor may have one barrel that is smaller in diameter than the other one. shows a four-barrel carburetor. It has four barrels to mix the air and fuel. The engine operates on two barrels during most driving conditions. When more horsepower and torque is needed, the other two barrels add air and fuel to increase the output of the engine. When all throttle plates open and close simultaneously, the carburetor is called a single-stage carburetor. Some carburetors have two stages, called primary and secondary stages. In the primary stage, one or two throttle plates operate normally as in a single stage carburetor. The secondary stage throttle plates, however only open after the primary throttle plates have opened a certain amount. Thus, the primary stage controls off-idle and low cruising speeds, and the secondary stage opens when high cruising speeds or loads require additional air and fuel. The secondary throttle plates can be opened mechanically or by a vacuum source. Mechanically actuated secondary throttle plates are opened by a tab on the primary throttle linkage. After the throttle primary plates open a set amount, the tab engages the secondary throttle linkage, forcing the plates open. Vacuum-actuated secondaries have a spring-loaded diaphragm. Vacuum is supplied to the diaphragm from ports in the primary and secondary throttle bores. When the vacuum in the primary bore reaches a specific level, the vacuum supplied to the diaphragm overcomes the spring and opens the secondary throttle plates. The vacuum created in the secondary throttle bore increases the vacuum signal to the diaphragm, opening the secondary throttle valves still farther. nozzle I only. When the throttle valve is opened further, more suction moves cap E still further subjecting nozzle 2 also to engine suction along with nozzle 1. Now both nozzles I and 2 will deliver the fuel. But nozzle 2 is so adjusted that it gives lesser amount of fuel than nozzle l, thus providing compensation. Similarly, nozzles 3 and 4 which deliver still lesser amount of fuel are put into operation by further throttle opening.
(e) Suction controlled devices. These devices are operated by means of engine suction, which is applied to a sliding piston. The suction effect is increased as the engine speed is increased, which controls the movement of the piston which further controls the venturi area. Sometimes the engine suction is also used to actuate a needle which decreases the effective nozzle area as the speed is increased, thus providing compensation. This method is used in the S.U. Carburettor, which has been discussed in detail in Art 23 .
The three general stages involved in carburetion are metering, atomization, and vaporization. Metering is another term for measuring. In the process of carburetion, fuel is metered into the air passing through the barrel of the carburetor. The ideal air/fuel ratio at which all the fuel blends with all the oxygen in the air is called the stoichiometric ratio. This ratio is about 14.7:l . If there is more fuel in the mixture it is called a rich mix. If there is less fuel it is called a lean mix. The amount of fuel metered into the air is varied in relation to the amount of air passing through the carburetor. Atomization is the stage in which the metered fuel is drawn into the air stream in the form of tiny droplets. The droplets of fuel are drawn out of passages called discharge ports. The surface area of an atomized droplet is in contact with a relatively large amount of surrounding air. In addition, the venturi is a low-pressure area. The boiling point of the fuel is greatly reduced by lower pressures; therefore liquid gasoline can become a vapor with low amounts of heat. These factors combine to create a fine mist of fuel below the venturi in the bore. This is called vaporization-the last stage of carburetion. It occurs below the venturi, in the intake manifold, and within the cylinder. Swirl, turbulence, and heat within the intake manifold and cylinder also enhance vaporization.
BASIC CARBURETOR CIRCUITS
Variations in engine speed and load demand different-amounts of air and fuel (often in differing proportions) for best performance-and present complex problems for the carburetor. At engine idle speeds, for example, there is insufficient air velocity to cause fuel to be drawn from the discharge nozzle and into the air stream. Also, with a sudden change in engine speed, such as rapid acceleration, the venturi effect (pressure differential) is momentarily lost. Therefore, the carburetor must have special circuits or systems to cope with these situations. The basic circuits of a typical carburetor include the float, idle, main metering, power enrichment, accelerator pump, and choke:
The float circuit or fuel inlet system stores fuel and holds it at a precise level.
The idle circuit supplies the richer air/fuel mixture to operate the engine at idle and low speeds.
The main metering circuit provides the air/fuel mixture during open throttle conditions.
The power enrichment circuit supplies a richer mixture during heavy loads. In many carburetors, metering rods placed in the main jets provide power enrichment.
The accelerator pump circuit mechanically adds fuel to the mixture when the throttle is quickly opened.
The choke circuit provides a richer mixture during cranking and cold start-up by restricting the air flow into the carburetor.
CARBURETOR SYSTEMS
To meet complex fuel economy and emission requirements, carburetors were fitted with a variety of auxiliary controls. The following describes some of the more common assist devices you are likely to encounter when servicing a carburetor.
Choke Qualifier
Once the engine has started, a leaner mixture is needed. If the choke stays shut, the rich mixture floods the engine and causes stalling. Therefore, the automatic choke has a choke qualifying mechanism to open the choke plate slightly after the engine has started.
Dashpot
The dashpot is used during rapid deceleration to retard the closing of the throttle. This allows a smooth transition from the main metering system to the idle system and prevents stalling due to the overly rich air/fuel mixture. It also controls the level of HC in the exhaust during deceleration.
Hot-Idle Compensator (HIC) Valve
When the engine is overheated, a hot-idle compensator opens an air passage to lean the mixture slightly. This increases idle speed to help cool the engine (by increased coolant flow) and prevents excess fuel vaporization with-in the carburetor. The hot-idle compensator is a bimetal, thermostatically controlled air bleed valve. Vacuum Break Some carburetors are equipped with a fuel vacuum break system, which includes a primary diaphragm and a secondary diaphragm. The primary vacuum diaphragm opens the choke valve slightly as soon as the engine starts to keep the engine from over-choking and stalling. The secondary vacuum diaphragm, which is also attached to the choke lever opens the choke valve slightly wider in warm weather or when a warm engine is being started.
Choke Unloader
To be able to start a cold engine that has been flooded with gasoline, a choke unloader is required. The choke unloader is throttle-linkage-actuated and opens the choke whenever the accelerator pedal is floored. At wide-open throttle, the partially opened choke allows additional air to lean out the mixture and reduce fuel flow.
Throttle Position Solenoid
The throttle position solenoid is used to control the position of the throttle plate. It can have several functions, depending on its application. when the basic function is to prevent dieseling, the solenoid is called a throttle stop solenoid or an idle stop solenoid. when the engine is started, the solenoid is energized and touches the throttle plates that open slightly to the curb idle position. when the ignition switch is turned off, the solenoid allows the throttle plate to close completely, and shuts off the air/fuel supply to prevent dieseling or run-on. The throttle position solenoid is also used to increase the curb idle speed to compensate for extra loads on the engine. when this is its primary function, the solenoid might also be called an idle speed-up solenoid or a throttle kicker. This feature is most often used on cars with air conditioners.
TYPES OF CARBURETORS
Many types of carburetors have been built in order to accommodate different load conditions, engine designs, and air/fuel requirements. Carburetor Barrels A carburetor barrel is a passageway or bore used to mix the air and fuel. It consists of the throttle plate, venturi, and air horn. A one-barrel carburetor is used on small engines that do not require large quantities of air and fuel. A two-barrel carburetor has two throttle plates and two venturis. The area where the air comes into the carburetor is common on both barrels. A two-barrel carburetor may have one barrel that is smaller in diameter than the other one. shows a four-barrel carburetor. It has four barrels to mix the air and fuel. The engine operates on two barrels during most driving conditions. When more horsepower and torque is needed, the other two barrels add air and fuel to increase the output of the engine. When all throttle plates open and close simultaneously, the carburetor is called a single-stage carburetor. Some carburetors have two stages, called primary and secondary stages. In the primary stage, one or two throttle plates operate normally as in a single stage carburetor. The secondary stage throttle plates, however only open after the primary throttle plates have opened a certain amount. Thus, the primary stage controls off-idle and low cruising speeds, and the secondary stage opens when high cruising speeds or loads require additional air and fuel. The secondary throttle plates can be opened mechanically or by a vacuum source. Mechanically actuated secondary throttle plates are opened by a tab on the primary throttle linkage. After the throttle primary plates open a set amount, the tab engages the secondary throttle linkage, forcing the plates open. Vacuum-actuated secondaries have a spring-loaded diaphragm. Vacuum is supplied to the diaphragm from ports in the primary and secondary throttle bores. When the vacuum in the primary bore reaches a specific level, the vacuum supplied to the diaphragm overcomes the spring and opens the secondary throttle plates. The vacuum created in the secondary throttle bore increases the vacuum signal to the diaphragm, opening the secondary throttle valves still farther. nozzle I only. When the throttle valve is opened further, more suction moves cap E still further subjecting nozzle 2 also to engine suction along with nozzle 1. Now both nozzles I and 2 will deliver the fuel. But nozzle 2 is so adjusted that it gives lesser amount of fuel than nozzle l, thus providing compensation. Similarly, nozzles 3 and 4 which deliver still lesser amount of fuel are put into operation by further throttle opening.
(e) Suction controlled devices. These devices are operated by means of engine suction, which is applied to a sliding piston. The suction effect is increased as the engine speed is increased, which controls the movement of the piston which further controls the venturi area. Sometimes the engine suction is also used to actuate a needle which decreases the effective nozzle area as the speed is increased, thus providing compensation. This method is used in the S.U. Carburettor, which has been discussed in detail in Art 23 .
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