 Hello students, I am Dr. Prakash Sarunke from Mechanical Engineering Department from Alchane Institute of Technology, S.O. Lappu. Today we are going to discuss about fuel systems in SI engines, so this will be the second part from the IC engine series. The outcome expected is students will be able to explain the requirements of air fuel mixture at different operating conditions of IC engine. What are the mixture requirements for the combustion? There are two essential requirements for the mixture, supplying device or carburetor. One is that it should meter the liquid fuel in proper quantity to produce the required air fuel ratio at different load and speed conditions. And second requirement is that it should be able to produce the homogeneous mixture of air and fuel which is possible by atomizing the fuel. So what are different conditions of engine operation? Let us try to understand. There are mainly two categories of engine running condition. One is steady state and another is transient for which the requirements of the mixture are different, I mean air formation. Steady state refers to continuous operation at a particular speed and power output with normal engine temperature. And the transient state refers to changing or dynamic conditions where either one or more parameters such as speed, load of the engine are changing. Now let us see more about steady state condition. Steady state occurs under three circumstances. One is idling and low loads. Second is normal power range and third is high or maximum power range. Number one letter study, idling and low loads that is 20 percent of the power. So engine is running with no load, it means idling. In both of these conditions lesser throttle opening will be there, throttle opening and it causes low suction pressure which results in backflow of exhaust gases and air leaks from other parts. So this leads to poor thermal efficiency and higher exhaust emissions. To overcome this say there is need of reach mixture say that is required say in this particular condition. So air fuel ratio should be around 12.5 is to 1, so that is the requirement. So if we see this table and graph we will find say there are different conditions. So this is idling and low load 0 to 20 of the I mean rated power and here the problem is dilution of mixture due to products of combustion. So the air fuel ratio should be richer 12.5 is to 1 and say this is I mean written here. So if we see graphically so this is the state. So where there is a requirement 0 to I mean 20 load is I mean smaller and say richer air fuel ratio so they are required. The second condition is normal power where we produce 20 percent to 75 percent of the rated load and naturally this is the major operation area the requirement should be of the economy. Economy should be the main consideration and for achieving economy air fuel ratio are required to be leaner say 16.5 is to 1 or 17 is to 1 practically up to 75 percent of the load beyond that so it is a range of maximum power. In maximum power say again say there is a richer fuel air I mean fuel air mixture should be there. So air fuel ratio should be more I mean higher so they are around 14 is to 1 like that. So here that for normal power it is also called as cruising range air fuel ratio requirement should be 17 is to 1. And maximum power range so it is in the range of 14 is 14 is to 1 rich mixture. Now what happens in case of multi-cylinder engines mixture from the carburetor is supplied through intake manifold and they are in less to each of the cylinder. So there are chances of variation in air fuel ratio throughout different cylinders or between the cylinders because outer cylinders will be getting richer mixtures why the liquid droplets which are carried because of their inertia they will not turn immediately in the earlier cylinders and they will be carried out say in subsequent cylinders. So outer cylinders they get relatively richer mixtures. Now transient mixture condition these are the state state conditions. The main transient conditions of operations are starting and warm up acceleration and deceleration. Requirements in transient state are different than those of steady state because the evaporation of fuel may not be complete. Petrol is a mixture of hydrocarbons and these are having different boiling temperatures. Because having high vapor pressure and low boiling point are called as light ends while those which are opposite to it means lower vapor pressure and higher boiling point. So they are called heavy ends. In different transient conditions let us see how this affects. The first is starting and warm up requirements while starting from cold speed and temperatures both are low especially in cold weather or early in the morning. Hence much of the heavy ends do not vaporize do not they remain in the liquid form. Therefore ratio of evaporated fuel to air will be too lean to ignite. Hence much richer mixtures air fuel ratio of 3 is to 1 or sometime 1.5 is to 1 are to be supplied so that enough light ends in the vaporized form are available for combustion. Too high volatility leads to formation of vapor bubble in the carburetor and disturbs the metering of the property. While too low volatility causes condensation of fuel or cylinder walls leading to carbon deposits and dilution of engine oil so that reduces the lubrication power. Then there is an acceleration requirement for accelerating that is to increase the engine speed throttle is open fully. As a result fuel evaporated in the intake manifold moves at much faster speed and fuel in the liquid form so that forms a film on the walls of the induction system so that is not carried out. What happens the liquid lags behind temporarily and cylinder receives a leaner mixture rather than rich mixture requirement. Therefore to supply additional fuel to make the mixture rich a separate arrangement is needed and is normally provided in the form of acceleration pump. Figure 2 shows the construction of simple or basic carburetor design which is normally useful to fulfill the requirements of normal power running or in steady state conditions. Let us see this but to figure figure of simple carburetor as you can see here so this is the air inlet air inlet this is the choke valve we shall study it later. And here so this is the say reduction in area venturi is formed and say this is the throttle valve throttle valve so with which we can control the quantity. So this is the fluid I mean fuel supply chamber here there is a float from here say there are tubes or pipelines small pipelines. So we shall read say here the fuel in the supply manifold. This is air vent this is trainer and this is the fuel incoming from the tank. Now as the air comes inside say fuel will be supplied from the tank and when the pressure is low so due to suction I mean vacuum created say the fuel will be sucked here say through this jet. So by opening the throttle we can control the speed as well as the quantity of air fuel mixture inside and say accordingly say we shall be getting the required. Now this is the preliminary design of the carburetor the detailed working and modifications required will be continued in our next session. So the discussion on that because it is not possible to cover in this particular session. Now for this particular session I have taken the references from book of IC engine by Mathur and Sharma similarly this is the web site from which say I have taken certain figures so I thank to them and thank to you also.