 Well friends, I am Dr. Prakash Sarunke from Valchand Institute of Technology, Sulapur Mechanical Engineering Department. Today, let us discuss fuel system in SI engines third session, say from our IC engine series. At the end of this session, students will be able to describe working of simple carburetor and to explain the modifications required in order to satisfy the demands under different operating conditions of IC engine. Let us first try to understand the simple carburetor. A carburetor is a device basically that mixes air and fuel for internal combustion engine in the proper ratio required for the combustion process. To understand design of modern carburetor, it is necessary first to know the working of elementary or simple carburetor. An elementary carburetor is one which is designed to provide an air fuel mixture for cruising or normal range at a single constant speed. Figure one shows a simple carburetor. Let us see the figure. Now here in this figure you can see observe here. So this is the air fuel mixture. So that will be taken inside to the cylinder. And this is the throttle valve which controls this particular quantity. Here say this is the shape that is venturi shape. This is the main jet which is supplying the fuel. So this is the orifice and say this fuel is coming from this chamber. So this is known as float chamber. So here there is a float and attached to this float there is a needle valve. This is a needle valve. So this is the opening valve sheet and say this is connected to the fuel supply. Which is I mean coming from the tank. Okay this is the, I mean this is the choke and say this is the air flow, air flow. Now air flow from the atmosphere so that will come inside. Now let us try to understand the working of this particular carburetor. Now as we have seen it consists of a float chamber nozzle, nozzle with metering orifice, venturi and throttle valve. For constant fuel supply level inside the float chamber should be maintained at constant level. So how is that maintained constant? How that level is maintained constant? You have seen the figure? Can you tell? Just think. Now the float and needle valve mechanism so that is present. So that maintains a constant level of fuel in the float chamber. Let us see. Yes again this. So if you see this float which floats say in the cement fuel and a needle valve is attached. So as the level drops so this float and subsequently the needle valve so that will move downwards and it will allow more fuel to enter inside by opening more this particular port. And if the level increases then the float will move upwards and needle valve will close more area and lesser amount of fuel so that will be cement admitted inside. So thus depending upon level say it will control the quantity of fuel coming inside and accordingly a constant level nearly constant level so that will be maintained with this particular mechanism. Now here we have already seen the float chamber is vented to the atmosphere. If we see again so this is the vent say to the atmosphere into the atmosphere. Now as shown in figure 1 intake air is controlled by the throttle opening. We have seen the throttle valve. The shape of the intake passage is of venturi that we have seen. So velocity of incoming air during suction stroke increases while passing through the venturi and because of which pressure will decrease. So as per the Bourg-Knoll's theorem that we have seen studied already. Now due to low pressure in the venturi fuel from the float chamber which is subjected to the atmosphere pressure will be sucked inside through that discharge jet or metering jet or main jet with the different names. The pressure difference between venturi section and that in the float chamber so that is the normal atmospheric it is called as carburetor depression. It is around 4 to 5 centimeter of Hg below atmospheric Hg means I mean mercury. Now petrol engines are quantity governed. So throttle valve opening will control the quantity of air as well as fuel because that will I mean govern that carburetor depression. So a simple carburetor which is working like this it supports from certain drawbacks. Firstly it provides increasing reach mixture with the speed. This happens due to the fact that increasing speed say that will thereby decreasing density of air. So as the flow of air will be at speed so density will be lesser and say with throttle opening say this will cause higher carburetor depression and higher carburetor depression will suck more fuel inside the carburetor or that device so that will naturally increase the richness. So this is one drawback. Similarly it provides linear mixture at low speeds or part throttle opening part throttle opening. So during the I mean lower in I mean in the beginning say the fuel flow will be lesser because of the lesser amount of carburetor depression. In order to fulfill the demands of an engine for different working conditions say we have to modify the design of the simple carburetor say also we have to remove the drawbacks. So there are the additional systems are like this one is the main metering system then idling system then power enrichment system acceleration pump and choke and choke. So let us study these systems one by one. Firstly main metering system, main metering system is designed to supply a constant air fuel ratio over a wide range of speed and load and it is with a purpose to achieve economy. Now for achieving economy air fuel ratio of 15.6 to 16 so that is required to be maintained at full throttle it is maintained so that it will cover the major I mean increasing range. Now to correct the tendency of supplying richer mixture at high speed some compensating devices are incorporated. So the main types of I mean incorporating in this compensating devices are say like this. So first compensating jet then second immersion tube third back section control and fourth auxiliary valve or port. So these are the commonly used devices. Let us study one by one. So first compensating jet. In this mechanism an additional compensating jet is provided through a compensating well as shown in figure 2. Let us see the figure yes. So this we have seen this is the venturi. So here there are two jets one is the main jet and other is the compensating jet. This compensating jet is coming from the compensating well. Compensating well is attached to the main float chamber through a restricted orifice. Now the mechanism works like this that total both of these I mean jets will together fulfill the requirements of the I mean fuel. Now coming to say this working compensating well gets the fuel supply through a restricted orifice that we have seen. As the air flow increases with the speed the fuel level in the well that I mean compensating well so that will decrease. And since it is getting the supply through restricting so it immediately the fuel will not say flow inside. And because of which the level in the well decreases and that causes reduced reduction in fuel supply in this compensating jet. So thus sum of increasing fuel supply through main jet so that I mean happens because of the I mean increasing air flow that we have seen. And decreasing one through compensating jet so total will remain constant if we see the figure here. So this is the graph so this shows the this is the air speed and this will show us the say I mean fuel flow or air flow ratio. So in case of compensating jet sorry main jet so this is the main jet. So fuel flow will increase and air fuel ratio so that will become richer so air will be lesser and fuel will be more. While in case of compensating jet exactly reverse happens say that air will be increasing and fuel will be decreasing because of this compensating jet. So as a effect net flow of the I mean fuel so that will be maintained constant so air fuel ratio will be maintained nearly constant. Okay so that is the combined effect combined effect. So this is how this I mean compensating jet works so sum of increasing so that will maintain constant air fuel ratio. Now another device say is immersion or air bleeding device. In this method the main metering jet is submerged 25 mm below the petrol level at the bottom of well if we refer this figure 4 then we will understand say this working. So this is the I mean venturi where this main jet is there but the main jet is not at this level but it is say below 25 mm. So this is the main jet level. Now this is again say this jet so it is open to atmospheric pressure atmosphere and there are holes inside say this particular tubing tubing. Now what happens so this is the main jet level. Now in this method what happens so there are holes on the sides of well we have seen through which atmospheric air is drawn inside and the petrol is emulsified. Increasing the linnness of the mixture because air will be mixing additionally. Now initially the petrol levels both in the well and in the float chamber are equal as throttle opens the petrol in well is sucked inside and holes on the sides are progressively uncovered. Now this results in decreasing richness of the mixture due to addition of air which is rushing from this. So main jet then provides the normal flow normal flow so which will help to maintain the some in constant air flow constant air flow as the speed increases. So that way say these two devices we have studied today compensating jet and emulsion to bar air bleeding device. So it will I mean help us to maintain the constant air flow ratio throughout all the speeds all the speeds. The other systems will be discussed in subsequent session. Now references for this are taken all the figures as well as the matter. Say we have taken the reference from the book of IC engines by Mathur Sharma and Mathur and Sharma. I thank to them and so thank you.