 Myself, Ganesh B. Aglavi working as an assistant professor in Department of Mechanical Engineering Valchinist of Technology, Sallapur. In this session, we will see theory and design of carburetor, learning outcome. At the end of the session, the students will be able to apply venture meter working principle to the carburetor and you will be able to draw the schematic diagram of simple carburetor. Now, definition of carburetor, so carburetor is the device in which the fluid and air is mixed and this mixture is connected to the engine. So, carburetor is used usually in the SI engine. The functions of the carburetors are to meter the fuel and to mix the air and fuel. So, both these functions are done by the carburetor. Now, this is the schematic diagram of the venture meter. Now, how the venture meter works? As you know, this is the convergent portion, divergent portion. These convergent, divergent portions are connected to each other and this is called venture meter. Now, what is happening when any fluid flows through the venture meter, which is the insulated one? Continuously, there is increase in the velocity and pressure decreases, though in the divergent portion, it increases by small amount, but that exit pressure is always less than inlet pressure. Now, the minimum cross sectional area is here. It is called the throat, now the throat is having the minimum pressure. So, what we will do now, we will try to connect this throat to one tank. Now, if we connect the manometer to this venture meter, you will find this pressure, downstream side pressure is less than upstream side pressure. So, if I connect now this throat to one tank to one tank, then that pressure will be acting on that tank. So, based on this venture meter, our carburetor works. So, before this, can you tell me the practical example of venture meter applications, I am asking? Yes, you are correct. In the steam jet refrigeration, it is used whenever you want to carry any material and want to maintain the low pressure, you could connect this venture meter to that application. Majorly, it is also used in the civil sector, ok. Now, so, usually what is done as this is the tank, just now I have spoken about this tank is communicating with the throat, if I puncture this time, then what is going to happen? The atmospheric pressure is going to act on this fluid. So, as this is the atmospheric pressure, as you know, this is the standard atmospheric pressure, which is equal to 1.01325 bar. So, this pressure is higher than the throat pressure, because of that, that this fluid will flow because of the vacuum generated at the throat on its own. And if we connect here the small convergent portion, there will be injection, injection of that fluid, ok. So, what I will do? I will pass the air through this venture meter and here instead of other, any other fluid like water except I will pour the petrol. Then this petrol is going to inject over here. So, there will be injection of fuel called petrol and this air is coming. So, this process is a throttling process. The spraying of the petrol at the throat is the throttling process because of that, the petrol droplets are exposed to lower pressure. So, vaporization of petrol is going to occur in the downstream side. This is called atomization process. Now, this working principle we are going to use in the simple carburetor. Now, this is the schematic diagram of the simple carburetor. So, what I did? The float chamber is taken. In the float chamber, I am going to maintain the petrol level constant. For that purpose, this float is used which is a hollow device which is sealed from all the sides. So, this float on the top side, there will be needle wall. That needle wall is connected to the fuel pipe which is connected to the fuel tank. So, depending upon the float level, the fuel is allowed. It may be stopped. It may be allowed depending upon the petrol level. If it increases, it will be stopped. If the float flows in the downward direction, more fuel is going to enter in the float chamber. Now, as this float chamber is communicating with the atmosphere, there is air vent. Now, this float chamber is connected to this venturi. This is the venturi. And at the throat, I am going to open the fuel jet. Now, the petrol level in the float chamber is at a lower height than this mouth of this jet one. Because here also, I am going to make it the convergent portion of the fuel jet. Due to the convergent portion, the velocity increases and pressure decreases. So, usually jet later is represented, is used for describing the height difference between this nozzle tip to the petrol level in the float chamber. That is jet. And it is always negative. Negative in the sense, if I take this as the reference line, then this will be in the negative side. This is the neutral condition and this is the positive. If you keep the petrol at par with the tip of the nozzle, then there are chances of flowing of the petrol as the vehicle is experiencing some inclinations. If the petrol level in the float chamber increases, there is overflow. So, to avoid this, the level is usually less than the tip of the nozzle. During the suction stroke, there is a suction of air. Air is going to flow through this venturi. And as air flows, the vacuum pressure is generated at the throat. Because of that, this petrol is sprayed, is injected in the upper direction and it will flow in the downward side because of the movement of the air in the downward portion. Because this is the downward type of the carburetor. There are two types are there, downward and up-draught. For up-draught, the air is going to flow in the upper direction, bottom to top. And in this schematic diagram, movement of air is in the downward direction. That is why it is called down-draught type of the carburetor. So just now we have seen this vacuum is responsible for injection of the petrol. Now that mixture is happening in the downstream side and this during suction stroke is supplied to the SI engine. Now in the simple carburetor, other devices like modifications are absent. In the sense, for low speed, the engine is getting the lean mixture. So it will give the trouble for the starting. And as the engine speed goes on increasing, it supplies more rich mixture, richer and richer. As we know, 9 to 19 is the recommended air-fuel ratio for any engine. If it decreases below 9, then it is too rich to burn, it is too rich to burn. So this is just introduction of the simple carburetor. We have seen the working principle of the venturi meter also. Now you are supposed to derive the relation for finding the mass of air, how it is calculated and also for mass of air. So if you know mass of air and mass of fuel, you will be able to calculate the air-fuel ratio. Once again, the another important point is that the nozzle tip here, it should be getting opened at the throat itself, otherwise vacuum may not be enough for suction of this petrol. So this is about the simple carburetor and this is the throttle valve. When you are throttling, means you are controlling this valve, opening and closing of the valve is happening. Normally, it is partially open. Partially open in the sense, you can consider 10 percent is the partially open side. So also now one activity is there, you just observe the type of carburetor used in the two-wheeler. It may be 100 cc vehicle two-wheeler, 150 cc or 200 cc. In BS4 norms and the carburetor was there, it was used in BS6 now instead of carburetor where shifted to the injector. So just observe the location, size and shape of the carburetor. So this activity you complete. In the assignment side, just draw the schematic diagram of the simple carburetor and do the survey of the carburetor type of a different manufacturer's two-wheeler. For further study, you could refer IC engine fundamentals by Heavill and by Echan Gupta.