Carburetor and Components of Carburetor | IC Engine | Thermal Engineering

In this article we will discuss about the carburetor and components of carburetor.

The carburetor is provided with additional devices to perform proper operation.

The devices used are: 1. Main Metering System 2. Idling System 3. Starting System Or Choke 4. Compensating Jet 5. Acceleration Pump 6. Economizer.

Type # 1. Main Metering System:

The main metering system consists of two sub-systems. The float chamber and main jet are two sub-systems. The float chamber maintains the constant reserve fuel in the float chamber. It consists of float in the small chamber. The pipe from the fuel tank is connected to the float chamber.

The inlet is closed and opened by a needle valve and the needle valve is operated by the float. It maintains a constant reserve of fuel in the float chamber. When the level in the float chamber falls down, the float goes down and opens the needle valve which allows the fuel to flow in the float chamber.

The fuel in the float chamber reaches to the given level the needle valve closes the inlet pipe and the flow of fuel is stopped. This helps in maintaining the constant reserve of fuel in the float chamber. Fig. 13-45 shows main metering system.

The float chamber at the bottom is connected to the main metering jet. The inlet manifold has venturi and at the throat of the venturi the main jet has been placed. The main jet supplies the fuel to the inlet manifold from the float chamber. When the air flows through the intake manifold and carburetor, venturi because it is sucked by the engine cylinder from the atmosphere.

The air flows through the venturi and at the throat of the venturi, the air velocity becomes higher. This creates a vacuum in the throat and the fuel is sucked from the float chamber through the main jet. The pressure difference across the float chamber and throat of venturi is responsible for flow of fuel through the main jet.

The fuel from the main jet flow in the atomised form and mixes with air flowing through the intake manifold. This is the way the mixture of air and fuel is prepared in the intake manifold in the carburetor and goes to the engine cylinder during the suction stroke.

The carburetor has a throttle valve which controls the quantity of air fuel mixture in the engine cylinder. If the throttle valve is in the straight position, then it is fully open and complete vacuum of the engine is open to atmosphere to such the air fuel mixture. The throttle valve can be opened partially which provides resistance to flow and reduces the quantity of air fuel mixture going to engine cylinder.

The air-fuel mixture is then aspirated into the engine. By rotating the throttle valve placed in the intake tube, the amount of air-fuel mixture entering the engine can be controlled. The .throttle valve can be operated either manually or by governor.

The carburetors used with variable speed engines such as automobile engines are also provided with a choke valve, starting-and running jets and compensating device.

Type # 2. Idling System:

When the petrol engine is started and is running at slow speed the suction is not sufficient to get more petrol from the jet at the venturi and so an idling jet is provided. It is connected as shown in the fig. 13-46 near the throttle valve and when the engine is running at slow speed the throttle valve is nearly closed and so there is sufficient pressure drop near the starting jet to suck the petrol via idling jet. When the engine speed increases as a result of the opening of the throttle valve the idling jet goes out of action.

The idling is the motion of the engine at no load condition; In the idling condition the engine requires rich air fuel mixture. The ratio of the mixture required during idling is 8:1. The air fuel ratio supplied by the main jet is of the order of 14:1. Therefore at the time of idling it is necessary to supply more quality of fuel.

The idling system is as shown in fig. 13-46. There is a idling jet and a passage which connects directly between the inlet pipe before the venturi to the portion ahead of the throttle valve. The idling passage is also connected to the float chamber and the fuel can directly flow from the float chamber to the idling system.

The part of air flows in the idling system and since the throttle valve is partially closed’ in the main system, large air flows in the idling system and such more quantity of fuel. The rich fuel air mixture goes in the engine cylinder from the idling system.

This results into very rich mixture from the carburetor during idling. When the throttle valve is largely open then less amount of air flows in the idling system. The supply air and fuel mixture is reduced from the idling system.

Type # 3. Starting System or Choke (Fig. 14-47):

The choke valve, is used to get a mixture rich in petrol for starting the engine from cold. For this purpose the choke valve is closed to allow only a limited supply of air.

Type # 4. Compensating Jet:

It has been found with the variable speed engine that the increase in speed makes the mixture richer and richer, when the carburetor has a single jet. To avoid this, different compensating devices are used. One of them is to use two jets as shown in the figure.

One is the main jet and the second one is the compensating jet. Both these jets supply fuel to the pipe leading to the venturi. In the compensating jet, supply fuel to the pipe leading to the venturi.

In the compensating jet, a contraction is provided which does not allow the increase in flow of petrol at the same rate as in the main jet and so correct mixture is supplied at all speeds. Fig. 13-48 shows the compensating jet.

Type # 5. Acceleration Pump:

The acceleration pump supplies the extra fuel when the throttle valve is opened suddenly and there is violent acceleration of the system. If the throttle valve is opened suddenly, then the air flows and accelerates at faster rate whereas the fuel droplets are heavier which cannot flow and accelerated like air particles.

This results into supply of lean mixture to engine cylinder. This can be compensated by use of the acceleration pump. The acceleration pump consists of the piston and cylinder connected to the float chamber. The piston is held in the cylinder against the spring force.

If the accelerator is pressed suddenly then the inertia force on the piston by the pedal is more which results into sudden compression of piston against the spring force. This results into supply of more quantity of fuel to intake manifold. Fig. 13-49 shows the acceleration pump.

Type # 6. Economiser:

The economiser is the device used for development of most maximum possible power from the engine cylinder. This can be done by sending large quantity of fuel or very rich mixture in the cylinder by use of the economizer. The economizer consists of a orifice controlled by a needle.

The economizer is engaged only when the engine has reached to 90% to 95% of its load. The needle of the economizer is pulled when the engine has reached to its full load condition allowing the economizer to send additional quantity of fuel and engine gets very rich mixture. Fig. 13-50 shows the economiser.