Cutting on Carb(s)uretor
It is the device responsible to prepare the homogeneous air-fuel mixture to the engine under all the working conditions. With this, the carburetor also keeps a small amount of fuel as a reserve. To understand how the carburetor performs all these functions let’s look at a simple carburetor.
- Float chamber – The float in float chamber is made of deep drawn brass sheet and is kept hollow for lightweight but, such floats have a tendency to leak along the joint seams. Therefore the floats are now made up of nylon plastic or expanded synthetic rubber.
- Needle valve – The needle valve consists of a cylindrical stem with a conical strip made of steel or a solid stem with rubber seat tip. It is attached to the float lever and serves the function of opening and closing the fuel inlet to the float chamber depending upon the requirement. Alternatively, there may be a three-piece valve with a rubber seat tip and a spring-loaded ball in the body of the stem. They maintain a liquid-tight seal during vibrations, which is not possible with ordinary 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 fuel supply. The fuel starts flowing in and the float rises gradually to the fuel level reaches the desired value after which the needle closes the inlet passage. This helps to maintain a constant head of fuel in the float chamber. This level is below the nozzle outlet, so that the fuel may not drop all the time from the nozzle, even when the engine is not working. It also helps to prevent the spilling when the car is tilted on a hilly or highly cambered road.
- Vent – A small vent located in float chamber to keep the pressure inside atmospheric. In modern systems, it is preferred to vent the float chamber to the air intake of the carburetor as it prevents the dust particles from mixing with petrol as the air intake has an air cleaner. When the air cleaner has blocked the pressure in the air horn is no more atmospheric and if the vent is made to the atmosphere this creates a pressure difference and hence faulty working of the carburetor.
- Fuel Jet – The fuel supply from the chamber at any time is measured from where the fuel flows into the venture through the discharge nozzle.
- Venturi – It is the restriction in the air passage, it causes a decrease in the area which in turn causes the increase in velocity using Bernoulli’s theorem. Due to this depression being applied to the nozzle, the fuel comes out and is vaporized by the coming air stream. The amount of fuel delivered depends on the jet size, float level, and venturi vacuum.
- Throttle Valve –It controls the quality of air-fuel mixture. It is attached to the accelerator pedal. It is of two types-
- Butterfly Valve –It is just a disc hinged at the center, Due to its simplicity it is more widely used.
- Cylindrical valve – It doesn’t obstruct the path of the flux.
When the throttle valve is completely closed no engine suction is applied to the nozzle, the whole 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 engine side of the throttle valve. When the throttle opening is increased, the depression on the engine side of the throttle is reduced while in the venturi is increased.
The depression at the venturi and amount of the fuel flow is controlled by the throttle opening which depends on the position of an accelerator pedal.
REASONS FOR DEFECTS IN SIMPLE CARBURETOR
- Carburetors are designed to work at high speeds, it will not work at low speeds as the suction created at the venturi will not be sufficient to draw fuel from the nozzle at part throttle. Similarly, if the carburetor is designed to work at low speeds it will deliver rich mixture at high speeds.
- The coefficient of discharge for air and fuel varies in different amount with the depression, For air, it becomes constant at a certain value of depression, but for fuel, it increases gradually. So, carburetors are set at a particular speed above which it gives rich mixture and below which it gives a lean mix.
- Starting Difficulty – At the time of start engine requires a rich mixture, but the mixture provided by the carburetor is very lean.
- Ticklers – These are the devices used to cause flooding of carburetor at the start. By depressing the tickler, the float is depressed, thereby providing more fuel. They were used in old motorbikes.
- Choke – It is a simple butterfly Valve fitted at top of the air horn. It can be manual or automatic. For starting, choke is closed so that very small amount of air gets past it and the throttle valve is open due to which whole suction is applied to the nozzle, which delivers sufficient fuel to provide a mixture rich in quality, though small in quantity. But the choke must be closed immediately after the engine starts, otherwise, the engine will be flooded by fuel. To do so
- Eccentric mounting of choke – Due to the unequal pressure on the sides of the spindle apply a turning effect on them and open the choke.
- Strangler Valve – As soon as the engine starts the air pressure forces the strangler against the spring and air enters the carburetor avoiding the mix from being over rich.
- Adjusting Screw – A long tapered needle is used to control the jet area feeding the fuel to the engine.
- Automatic choke – The choke is mounted eccentrically. A bimetal thermostatic coil is located on the exhaust manifold to quickly sense the heat. A vacuum diaphragm unit in which diaphragm is subjected to inlet manifold vacuum is provided. When the engine is cold, the thermostatic spring holds the choke closed. In this position, the fast-idle cam maintains a small opening when the engine is cold. After the engine is started the vacuum diaphragm pulls the choke valve open. This tendency is helped by the eccentricity of choke valve. As the engine warms up, the thermostatic spring gradually winds up and allows the choke valve to open.
- Idling Difficulty –During idling the engine runs at low rpm and the suction is insufficient to draw the fuel from the nozzle. Therefore a separate supply circuit of fuel is provided. It is done by providing an idle jet and an air bleed hole, so the air and fuel combine to form an emulsion, which is then sent to the engine. The amount of air and fuel is controlled by the idle management screw. When the engine is accelerated the supply shifts from the idle circuit to the nozzle by the transfer ports.
- Operation at Different Speeds –A simple carburetor is set to work at a specific speed above which it gives rich mixture and below which it gives a lean mixture. The process of adjusting the mixture strengths at all speeds throughout is called compensation. It is achieved by-
- Extra air Valve – This is a spring loaded valve arranged to open by the engine suction and the stiffness of the spring.
- Compensating jet –It includes an additional fuel supply nozzle. This nozzle is attached to a well which is vented to the atmosphere so that when the engine is not running the fuel level in the compensation jet is same as the float chamber and hence no fuel is allowed to flow through the compensating jet. The main nozzle is connected directly to float chamber and will supply the richer mix at high speeds. But the case is different for the compensating jet. When the engine starts the throttle valve is open a bit, small engine suction is applied, which draws the fuel from the well through the delivery nozzle, until well is empty. At this point the flow through the main nozzle is maximum. Further opening of throttle doesn’t affect the fuel flow, however, the air flow increases. This results in a uniform mixture of nearly constant strength at different operating speeds.
- Air bleed compensation – Consider A)- fuel level in the nozzle under no load. B)- A jet tube with air bleed holes around its periphery. C)- Holes communicating the air bleed holes into the atmosphere when the nozzle is partly empty. As the throttle opens gradually, due to depression at the venturi, the fuel in the tube and the nozzle around it is consumed, providing a gradually richer mixture. The level of fuel in the jet tube and nozzle drops, exposing the uppermost holes in the jet tube to direct contact with the atmosphere through holes C. This destroys the depression to some extent and fuel flow is thus decreased. At high speeds the fuel level in the jet tube and around drops further, so as to expose the next row of air bleed holes, increasing the amount of air bleed to compensate the increased fuel flow from the jet. This results in a stable supply. The size and location of the air bleed holes decide the degree of compensation.
- Multiple Jet Compensation – In this method, 3-5 Jets are provided in the carburetor. A single cap is used to open different jet passages. At low speeds, only one nozzle is open to engine suction. As the speed increases the different nozzles will be engaged. But for compensation, the consecutive nozzles will have less supply of fuel.
- Suction Controlled Devices – They are operated by means of engine suction, which is applied to a sliding piston. The suction effect increases with speed. It can be used to actuate the needle used to decrease effective nozzle area.
- Difficulty at high speeds –Weak air-fuel mixtures supplied by the single jet carburetors will not give enough power at high speeds. Therefore we use a metering rod with stepped diameter end in the main jet. At lower speeds the larger diameter part of the radius in the jet which gives less fuel flow. At higher speeds, the metering rod is pulled up to the small diameter part is in the jet.
- Acceleration Difficulty – At sudden acceleration, there is a tremendous flow of air instantly but the fuel supply lags, causing engine to stumble due to the weak mixture. To reduce the lag-
- Accelerator Pump –A separate pump is attached to provide with the fuel supply momentarily. The pump is linked with the accelerator pedal. As the pedal is pressed the outlet valve opens and fuel is forced out of the acceleration jet. When the pedal is released the pump sucks the fuel from float chamber for next discharge.
- Influence of Weather – The viscosity of fuel is affected by the temperatures. To control the strength of the mixtures special devices are used to vary either the fuel jet area or air intake.
- Influence of Altitude – The density of air is affected with altitude due to fall in atmospheric pressure. The method used to rectify this defect in some Carburetors is –
- Zenith Carburetors – Two pipes connect air horn to the top of the float chamber. A pipe 1 is fitted with a valve which is used to use the engine suction to decrease the pressure in the float chamber which results in decreased fuel supply and hence weaker mix at high altitudes.
- Icing Troubles –Atomization and evaporation of fuel in the carburetor causes cooling of surrounding areas, in cold areas, there is always a probability of ice formation and choking of the venturi tube. To prevent it –
- Exhaust heat – The parts of carburetor like throttle valve and idle ports are heated using the exhaust gasses.
- Water Passes –By providing the water passes through the carburetor body. The engine cooling system provides hot water for heating the carburetor thus avoiding the formation of ice.
MULTIPLE VENTURI SYSTEMS
The advantages of providing multiple venturi systems are-
- It increases the air velocity and the depression and jet orifice portion, without affecting the maximum airflow capacity of the carburetor at high speeds.
- The annular blanket of air between the outer venturis which keeps the wet fuel off the walls for a greater distance. Thus more fuel particles reach the hotspot areas of the inlet manifold.
But, the manufacturing cost is high.
They are used in the engines having large no of cylinders 6 or more. A dual carburetor contains two sets of venture and fuel jets. The float chamber and acceleration pump are common. Each venturi supplies fuel to half of the cylinders.
Advantages of the dual carburetor are-
- Uniform quality of fuel mixture.
- High volumetric efficiency.
- Better distribution of mixture.
- Compact Design.
But, we can’t use two individual carburetors as it would be difficult for us to synchronize the two individual carburetors.
TYPES OF CARBURETORS
- Up-Drought type
- Horizontal type
- Down Drought type
The down drought type is the most commonly used as-
- In this gravity assists the flow of the mixture.
- High Volumetric Efficiency of Engine.
- Carburetor position is rendered.
- Throttle Return Check – Closing of throttle suddenly on an engine running at very high speeds causes a very high intake manifold vacuum, which will draw exhaust into the engine intake during the valve overlap. This will dilute the intake charge to cause misfiring and even stall. To prevent this, a throttle return check is connected to the throttle linkage. The return check diaphragm slows down the closing movement of the throttle valve and thus gradually reduces the speed to idle.
- Anti Dieseling Solenoid – Modern emission controlled engines run hotter, it results in some hot spots in the combustion chamber. They ignite the charge even after the ignition is turned off, which results in engine continuing to run. This is known as Dieseling. A solenoid is de-energized on turning the ignition off, it allows the throttle valve to close completely. This keeps air from getting into the engine which results in a mixture which is too rich to after-fire or diesel.
- Automatic mixture control – A Plunger shaped valve operated by a solenoid and a spring and controls a separate jet in the float chamber. When the solenoid is turned on, the valve is lifted which increases the fuel supply through the jet. When the solenoid is turned off the spring pushes the valve down to decrease the fuel supply. Such carburetors are called Feedback –Controlled Carburetors.
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