CARBURETTOR-3

BASIC CARBURETOR DESIGN

A carburetor is a metering device that is used to mix air and fuel in the proper ratios to accommodate the engine's needs during a variety of operating conditions. The mixing of air and fuel takes place when the fuel is released in fine particles into the incoming air. When two different pressures are exposed to each other, the higher of the two will move quickly toward the lower pressure in an attempt to balance the pressure. The more pressure differential there is, the faster the air will travel. In a carburetor, as airflow speed increases, more fuel is mixed with the air. The carburetor has two basic functions: to regulate engine speed by controlling the amount of fuel and air the engine receives, and to deliver the correct ratio of air and fuel to the engine's cylinders. Not only is a pressure differential formed by the pistons during their intake stroke, but one is also formed by the carburetor itself. In the throat or bore of a carburetor is a venturi. This restriction causes an increase in air velocity and lowers the pressure of the air passing through it. In many carburetors, a smaller boost venturi was positioned inside the main venturi to provide increased airflow speed and vacuum. A carburetor controls engine speed and air/fuel ratios by controlling air velocity. Actually the carburetor's throttle plate assembly controls the velocity of the air. A throttle plate is a circular disc that is placed directly in the flow of air and fuel, below the venturi. It is connected to the driver’s throttle pedal so that it opens to a vertical position as the pedal is depressed. When the throttle plate is all the way open, there is very little restriction of air. This is a maximum speed condition. As the driver's foot is removed, a spring closes the throttle plate. This restricts the amount of air going into the engine. This is a low speed condition. The throttle plate is nearly closed at idle speed, preventing much of the airflow into the cylinders. As a result of this reduced airflow, there is not much pressure in the combustion chambers during the compression and power strokes, and this causes the engine to run at idle speed. When the throttle is opened wider the airflow into the cylinders increases, creating more pressure on the pistons during the compression and power strokes and causing the engine to run faster. If air passes rapidly through a tube or cylinder that has a small hole in it, air will be drawn through the hole to combine with the moving air. Again, this is the result of a higher pressure moving toward a lower pressure. If a column of air is moving quickly, there must be a large pressure differential. Therefore, outside air will enter the small hole and join with the moving air in an attempt to balance the pressures. If a hose were connected to the small hole and to a container filled with a liquid, the liquid would be drawn out of the container and into the moving air if the liquid in the container was exposed to atmospheric pressure. The greater the air velocity, the more fuel will be drawn into the moving column of air. This is another principle that must be understood before studying how a carburetor works. As the throttle plate is opened, engine speed increases. This allows more air to be drawn into the carburetor. As a result, venturi vacuum increases because the greater the velocity of air passing through the venturi, the greater the vacuum. Venturi vacuum is used to draw in the correct amount of fuel through a discharge tube. As the air flows through the venturi, vacuum draws the fuel from the carburetor bowl into the stream of air going into the engine. More fuel is drawn in as venturi vacuum increases, and less as the vacuum decreases.
Venturi vacuum is created by the airflow through the venturi. This vacuum increases in relation to engine speed. Remember that an increase in vacuum is a decrease in pressure. Manifold vacuum is created by the downward piston movement on the intake strokes that attempt to pull air past the restriction of the throttle. Manifold vacuum is highest with the throttle closed at idle and on deceleration. When the throttle is opened, manifold vacuum decreases. This vacuum is lowest at wide-open throttle.

SIMPLE CARBURETTOR

To understand the principle on which a carburettor works, consider a simple carburettor. The main parts are a float chamber, fuel jet, venturi, nozzle and a throttle valve. The float in the float chamber is made of deep drawn brass sheet and is kept hollow for lightness. Such floats have a tendency to leak along the joint seams. Due to this reason floats are now made of nylon plastic or expanded synthetic rubber. The needle valve attached to the float lever serves to close or open the fuel inlet to the float chamber depending upon the requirements. The needle valve consists of a cylindrical stem with a conical tip made of steel or else a solid steel stem with a rubber seat tip. Alternatively, there may be a three - piece valve with a rubber seat tip and a spring - loaded ball in the body of the stem. The later two types maintain the liquid - tight seal during vibrations, which is not possible with ordinary single piece needle valve. When the fuel level falls below a definite predetermined value, the float also falls along with fuel level, thus opening the passage for the fuel supply. The fuel starts flowing in and the float rises gradually till the fuel level reaches the desired value. At this time, the float needle closes the fuel inlet passage. Thus a constant head of fuel is maintained in the float chamber. This constant level of fuel is slightly below the nozzle outlet, so that the fuel may not drop all the time from the nozzle, even when the engine is not working. This provision also prevents the fuel from spilling out when the car is tilted on account of hilly or highly cambered road. In practice fuel level in the float chamber is maintained about 5 mm below the nozzle outlet. A small vent in the float chamber keeps the pressure inside atmospheric. In modern practice it is preferred to vent the float chamber to the air intake of the carburettor. Such an arrangement prevents dust particles from mixing with the petrol through the vent as there is always an air cleaner on the intake side of the carburettor which filters the incoming air. Moreover, in case of blocking of air cleaner, the pressure at the air horn will not remain atmospheric. In case the float chamber is vented to atmosphere, this may cause change in the net pressure causing fuel flow from the nozzle and hence faulty working of the carburettor. The fuel supply from the float chamber at any time is metered by means of the fuel jet from where the fuel flows to the venturi through the discharge nozzle. The venturi is simply a restriction in the air passage. It is so called after the Italian engineer G.B. Venturi famous for his studies on the fluid flow. Here the passage area is minimum. Thus due to less area, the air velocity increases and because of this increase in velocity, decrease in pressure is caused at the nozzle which is located in venturi itself. Due to this depression being applied at the nozzle, the fuel comes out and is vaporized by the coming air stream. The mixture then goes through the inlet manifold, to the engine cylinders. The amount of fuel delivered depends upon the jet size, float level and venturi vacuum. The purpose of, a throttle valve is to control the quantity of air fuel mixture. It is attached to the accelerator pedal by means of suitable linkage so that when the pedal is depressed, the valve opens out. The throttle valve used is of two types, the butterfly valve and the cylindrical valve. The butterfly valve is more commonly used because of its simplicity; it is merely a disc hinged at the centre. The cylindrical type, however, is advantageous in that it does not obstruct the passage when fully open. Butterfly type, on the other hand, does obstruct some passage even when it is fully open. When the throttle valve is completely closed, no engine suction is applied to the nozzle the whole of the suction in that case, is acting below the throttle valve, in the region of the inlet manifold. A small throttle opening would still have a large depression on the engine side of the throttle permitting only a small amount of depression on the inlet side of the throttle valve. When the throttle opening is increased, however, the depression on the engine side of the throttle is reduced while in the venturi region it is increased. Thus the depression at the venturi and hence the amount of fuel flow is Controlled by the throttle opening which itself depends upon the position of the accelerator pedal.