Carburettor System (Automobile)

9.13.

Carburettor System

In order to mix fuel and regulate speed, the carburettor has a series of fixed and variable passages, jets, ports, and pumps, which make up the fuel metering systems or circuits. There are six basic systems common to all carburettors :
(i) Float system
(ii) Idle and low speed system
(Hi) High speed, or main metering system
(iv) Power system
(v) Acceleration pump system
(vi) Choke system
9.13.1.

The Float System

Gasoline from the fuel tank is delivered by the fuel pump to the carburettor fuel bowl (main well), where it is stored. The gasoline must be kept in the fuel bowl at a precise, nearly constant level. This level is critical, because it sets the fuel level in all the passages and circuits within the carburettor. High fuel level produces a rich fuel mixture causing high fuel consumption and high emission level. Low fuel level produces a lean mixture, which leads to engine surge and misfiring. Because of these problems, fuel level is one of the most critical adjustments needed on a carburettor.
The main fuel discharge nozzle for the high-speed system is directly connected to the bottom of the fuel bowl. The fuel level in the bowl and the nozzle is the same. The float assembly (Fig. 9.42) has a light weight hollow brass or a foam plastic pontoon with a hinge and tang. As the fuel level in the bowl rises, the pontoon floats higher. It pivots on the hinge to move the tang against the needle valve. The needle valve is pushed against the seat by the float assembly tang to stop the incoming fuel into the bowl when the float reaches the set fuel level. The float lowers as the fuel level drops due to use, allowing the needle valve to leave the seat to refill the bowl with the fuel supplied by the fuel pump. During operation while fulfilling many operating
conditions, fuel flow into and out of the fuel bowl is almost equal. The needle valve stays in a partially open position to maintain the required flow rate. Fuel level is controlled and is maintained nearly constant by the float and the inlet needle valve. An air space is provided above the fuel in the bowl. The pressure in the bowl is atmospheric due to the vent to the carburettor air horn. The atmospheric pressure of fuel in the bowl provides the pressure differential needed for precise fuel metering into the venturi vacuum area of the carburettor barrel.
Float and needle valve design.
Fig. 9.46. Float and needle valve design.
Float and needle valve design and location in the fuel bowl vary with different carburettor designs (Fig. 9.46). Small springs are attached to some floats to keep them from bobbing up and down when the car travels over rough roads. Many fuel bowls have baffles to keep the fuel from sloshing on rough roads and sharp turns. The needles and seats in most carburettors are made of brass, and the needles often have plas­tic tips that conform to any rough spots on the seat and still provide a good seal when the valve is closed.
When the engine is shut-off, engine heat evaporates the fuel in the bowl. The amount of evaporation from a large bowl system can easily overload the canister used in emission control. Therefore, the modern carburettors incorporate a somewhat small float bowl of moulded plastic. Other install an insulator between the carburettor and intake manifold to reduce heat.
9.13.2.


The Idle and Low Speed System

This system completely controls supply of petrol during idle and light load speeds up to 32 km/h. At low speeds, a very small amount of air flows through the venturi causing slight venturi effect and consequently the throttle is nearly closed. This is not sufficient to produce fuel flow in the main metering jet system. Therefore carburettors are equipped with an idle system, illustrated in Fig. 9.47, which picks up fuel from the main well and carries it through restrictions to an elevation above the fuel level where air enters the fuel system through idle air bleeds, producing a mixture of fuel and air. This mixture follows another passage to an opening just below the throttle plate where the mixture flows through a manually adjustable idle port and discharges into the air stream. The idle mixture, which provides idle smoothness, is controlled by turning a manually adjustable needle screw, called the idle mixture adjuster screw.
One adjustment screw generally is used for each primary barrel. The screw tips stick out into the idle system passages and are turned inward (clock wise) to create a lean mixture, or outward (counter clockwise) for a rich mixture. Some carburettor mixture screws have plastic limiter caps (Fig. 9.48). These caps restrict the amount of adjustment to prevent excessively rich idle mixtures. Idle speed is the result of the amount of air going through the carburettor, which is controlled by throttle position. The throttle position is set by an idle air adjustment screw (Fig. 9.49).
Additional small openings called transfer ports (Fig. 9.47) are located just above the closed throttle plate in the carburettor barrel. At idle, the transfer ports suck air from the barrel, which
is at atmospheric pressure, into the fuel flow in the idle system. When engine is under slight acceleration, the engine needs more fuel than the idle port alone can provide and hence the transfer port comes into operation as the low speed system (Fig. 9.50). As the throat opens, the transfer port is exposed to the intake vacuum and flow reverses in the transfer port. Extra fuel flows out of the transfer port to meet the engine’s need during the switch over from idle to low speed operation. Fuel continues to flow from the idle port, but at a reduced rate. This permits an almost constant air-fuel mixture during this transition period.
Typical idle circuit.
Fig. 9.47. Typical idle circuit.
Idle limiter caps.
Fig. 9.48. Idle limiter caps.
The most common problem in the idle system is plugging of idle restrictions and air bleeds, needing through cleaning. This is noticed when a change in the mixture screw adjustment has no effect on engine idle.
Idle air adjusting screw.
Fig. 9.49. Idle air adjusting screw.
 Low-Speed operation.
Fig. 9.50. Low-Speed operation.

9.13.3.

The Main Metering or High Speed System

As the vehicle speed reaches more than 32 km/h the throttle is opened, wide enough, to provide sufficient air flow to create pressure slightly less than atmospheric at the tip of main discharge nozzle. At the same time, the partial vacuum area of the intake manifold moves up in the carburettor barrel. The air flow and pressure change strengthen the venturi effect, causing gasoline to flow from the main discharge nozzle (Fig. 9.51). With further increase in speed the main metering system continues to cut in till it takes over the entire load while the idle system cuts out. The main metering system ensures supply of sufficient petrol for the operation of engine above idle running to a maximum speed when the throttle is almost completely open.
 High Speed or main metering system.
Fig. 9.51. High Speed or main metering system.
 Multiple venturi system.
Fig. 9.52. Multiple venturi system.
For better mixing of the fuel and air, most carburettors have multiple, or boost, venturies placed one inside another (Fig. 9.52). The main discharge nozzle is located in the smallest venturi to increase the partial vacuum effect on the nozzle. Fuel flows from the bowl, through the main jet and main passage, and into the discharged nozzle. A high-speed air-bleed (Fig. 9.52) mixes air into the fuel before it is discharged from the nozzle.The primary or upper venturi produces vacuum, which causes the main discharge nozzle to spray fuel. The secondary venturi creates an air stream, which holds the fuel away from the barrel walls where it has a chance to slow down and condense. This results in air turbulence, which causes better mixing and finer atomisation of the fuel.
9.13.4.

The Power System

The high-speed system delivers the leanest air-fuel mixture to all the carburettor systems. When engine load increases during high-speed operation, this mixture is too lean to deliver the necessary power required by the engine. The extra fuel needed is provided instead by another system called the power system, or power valve. It supplements main metering fuel delivery. The power system or valve can be operated by vacuum or mechanical linkage. The exact type of power valve differs according to carburettor design, but all provide a richer air-fuel mixture.
One type of power valve (Fig. 9.53) is located in the bottom of the fuel bowl with an opening to the main discharge tube. A spring holds the small poppet valve closed, while a vacuum piston holds a plunger above the valve. Since manifold vacuum decreases as the engine load increases, a large spring moves the plunger downward. This opens the valve and allows more fuel to the main discharge nozzle..
Another type of vacuum-operated power valve uses a diaphragm (Fig. 9.54). Manifold vacuum operates the diaphragm which holds the valve closed. As vacuum decreases under an increased load, a spring opens the valve, which sends more fuel through the power system to main discharge nozzle.
Metering rods also can be used as a power system (Fig. 9.55), which is controlled by vacuum pistons and springs, or by mechanical linkage connected to the throttle. The ends of the rods are tapered or stepped to increase the extra fuel flow gradually and are installed in the main jet opening. The rods restrict the area of the main jet and reduce the amount of fuel that flows
through them during light load operation of the main metering system. Extra fuel for full throttle power is provided by moving the rods out of the jets to increase the flow through the jets.

Power system operated by vacuum controlled piston
Fig. 9.53. Power system operated by vacuum controlled piston
Power system operated by vacuum controlled diaphragm.
Fig. 9.54. Power system operated by vacuum controlled diaphragm.
Vacuum controlled metering rods, also called step-up rods, are held in the jets by manifold vacuum applied to pistons attached to the rods. When vacuum drops under heavy load, springs, working against the pistons, move the rods out of the jets. Mechanically operated metering rods are controlled directly by mechanical linkage connected to the throttle linkage.
9.13.5.

Accelerator Pump System

The system provides additional fuel for some engine operating conditions. If the throttle is opened suddenly from a closed position, or nearly closed position, air flow increases more rapidly than fuel flow from the main discharge nozzle. This dumping of air into the intake manifold reduces manifold vacuum suddenly and causes a lean fuel mixture. This excessively lean mixture results in a stumble, some times called a flat spot. For enough richness of the mixture, extra fuel is provided by the ac­celerator pump.
The accelerator pump (Fig. 9.56) is a plunger or diaphragm in a separate chamber in the carbutettor body. It is operated by a linkage connected to the carburettor throttle linkage (Fig. 9.57). When the throttle closes; the pump
 Metering rods based power system operated by mechanical or vacuum linkage.
Fig. 9.55. Metering rods based power system operated by mechanical or vacuum linkage.
draws fuel into the chamber through an inlet check valve, shown in Fig. 9.58A, and an outlet check valve closes so that air is not drawn through the pump nozzle. The pump moves down, or inward, when the throttle is opened quickly, to deliver fuel to the nozzle in the barrel (Fig. 9.58B) through outlet check valve. During delivery of fuel inlet check valve closes. The pump outlet check valve may be a steel ball or plunger and the inlet check valve is a steel ball, rubber diaphragm, or part of the pump plunger.
Typical plunger-type accelerator pump.
Fig. 9.56. Typical plunger-type accelerator pump.
Accelerator pump linkage.
Fig. 9.57. Accelerator pump linkage.
Most pump plunger or diaphragms are operated by a duration spring. The throttle linkage holds the pump in the returned posi­tion. When the throttle opens, the linkage releases the pump, and the spring moves the plunger for a steady and uniform fuel delivery. The accelerator pump operates during the first half of the throttle travel from the closed to the wide-open position.
During high-speed operation, the vacuum at the pump nozzle in the carburettor barrel may be strong enough to unseat the outlet check and siphon fuel from the pump. This is called pump pullover or siphoning. In most carburettors, air bleeds are placed in the pump discharge passages to prevent the siphoning. In some carburettors, an extra weight is added to the outlet check to resist the siphoning. The pump plungers in some carburettors have anti-siphon check valves.
Acceleration system problems cause en­gine stumble or hesitation which is due to damaged synthetic rubber piston or
Accelerator pump operation. A. Pump intake stroke B. Pump discharge stroke
Fig. 9.58. Accelerator pump operation. A. Pump intake stroke B. Pump discharge stroke
diaphragm requiring replacement. Sometimes dirt gets on the check valve seat or plungs the discharge nozzle requiring cleaning or replacement.
9.13.6.

The Choke or Starting System

During cold start, only the light, volatile part of the fuel vaporises at low temperature. Cold manifold walls cause gasoline to condense from the air-fuel mixture, and less vaporised fuel reaches the combustion chambers. A choke system is used during cold start to supply a large amount of fuel to the carburettor barrel. The choke plate (valve) is located in the air horn above the main discharge nozzle and venturi as shown in Fig. 9.59. The choke plate can be tilted at various angles to restrict air flow. Cranking the engine with the choke plate in closed position creates a partial vacuum throughout the carburettor barrel below the plate. This airflow reduction and partial vacuum area work together to allow more fuel to be drawn into the mixture.
Choke system.
Fig. 9.59. Choke system.
Automatic choke system. A. Integral choke. B. Remote choke.
Fig. 9.60. Automatic choke system. A. Integral choke. B. Remote choke.
The choke plate can be operated manually by a cable running to the driver’s compartment or automatically by a thermostatic spring. The choke plate shaft is connected to spring by linkage. The bimetal thermostatic spring is normally located in one of the two places. In one type, it is placed in a round housing on the carburettor air horn (Fig. 9.60A). This is called an integral, or piston type choke. On the other type, it is located off the carburettor in a well on the intake manifold (Fig. 9.60B). This is called a remote, a well type, or a vacuum-brake choke.
Regardless of type and location, the thermostatic spring closes the choke when the engine is cold. When the cold engine is cranked, the choke is completely closed. As soon as the engine starts, the choke is opened slightly for sufficient air flow. The manifold vacuum pulls the diaphragm or piston, which opens the choke slightly. As the engine warms, the choke thermostatic spring gradually relaxes its tension, allowing vacuum to slowly open the choke as well as slowly release the fast idle cam. When the engine is warm, the choke is fully released. The choke shaft is offset to give another opening force. If the throttle is suddenly opened on a cold engine, the offset choke plate tip opens allowing more air to enter the carburettor. The thermostatic spring for remote choke is located either at the intake manifold exhaust crossover or on the exhaust manifold where it quickly senses heat. In case of integral choke, heat is transferred from a manifold stove through an insulated tube to heat the thermostatic spring.
A sticky choke plate shaft, a stuck vacuum piston, bent linkages, improper adjustment, and plugged or a burned choke heat tube usually cause problems in the choke system requiring replacement of damaged parts, cleaning of shaft and bushings, and correct adjustments.

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