 Today we shall study fuel system in SI engines so this is going to be the first slide or first lecture in this IC engine series. Now there are learning outcome at the end of this session students will be able to describe the terms carburation and air fuel ratio and they will be able to explain effects of air fuel ratio on the performance of the engine. Engines are normally classified in two types IC engines SI system and CI SI means spark ignition. In spark ignited engines air fuel mixtures are formed with the help of the device which is known as carbulator. So carburation is the process of producing a mixture of air and fuel in the correct proportion for required for combustion in internal combustion engines and say this process is performed in a device known as carbulator. So carbulator is a device that mixes air and fuel for internal combustion engines in the proper rate or proper ratio which is required for the combustion. Now air fuel ratio what do you understand by term air fuel ratio can anybody yeah air fuel ratio as the name suggests is the ratio of mass of air to mass of fuel present in the combustion process. If the air provided it just enough to completely burn all of the fuel this ratio is known as stoichiometric air fuel ratio or chemically correct air fuel ratio. Ratios lower than stoichiometric means where air is less and fuel is more so they are considered as rich mixtures or rich air fuel ratios while ratios higher than stoichiometric so they are considered as lean. Now what are the requirements of this air fuel ratio for performance or better performance of the engine and for that let us try to understand what are these semirequirements. Firstly there is a limited range of air fuel ratios which can be ignited in a spark ignition engines so that below which and above which there may not be an ignition. So for petrol as a fuel these limits are 7 is to 1 on the rich side means 7 parts of air and 1 part of semi fuel and above 20 is to 1 on the lean side so where 20 is the proportion of the air while 14.7 is to 1 being the stoichiometric or chemically correct ratio. Figure 1 next figure so that we shall see later shows that the maximum power is obtained at 12.5 is to 1 air fuel ratio at full throttle. So this is the graph and this graph if you see that air fuel ratio so that is on the x axis on the x axis and here percent of the maximum say power or efficiency so that has been plotted. If we see the curve of maximum power we will find that say the maximum power say goes on increasing as the mixture becomes slightly leaner from very rich I mean limit and at this particular ratio that is around 12.5 if you see the figure 12.5 so we obtain the maximum power later on as the mixture becomes leaner say you find that the power goes on dropping while in case of economy if you consider economy economy will be lesser because fuel consumption will be more say in this rich air fuel ratio so as the ratio becomes more and more leaner here the proportion of air goes on increasing so that efficiency will go on increasing so it will reach to its maximum peak say at somewhere 17 is to 1 and then it will be further drop say because the mixture becomes too lean while the stoichiometric correct ratio is somewhere here so that is around 14.7. So if we consider the maximum efficiency occurs for air fuel ratio at about 17 is to 1 so the same is reflected again in this sphere figure so this is the power and instead of efficiency now this graph is of the fuel consumption so fuel consumption say for richer mixture is high and it will go on dropping and this is stoichiometric ratio and this is the lowest so where say we obtain the maximum efficiency and again it goes on increasing now air fuel ratio power and fuel consumption we shall discuss more I mean details about this you will find that air fuel ratio for maximum power and maximum economy vary with load figure 3 will show the effect of varying load on requirements of air fuel ratio for economy and power so we find that this is the figure so this will give us the relative load at any given speed so this is the maximum load and this is the zero load while here air fuel ratio so which is say shown in this limits so this is the reach limit mixture and this is the lean limit mixture now these goes on varying with the load load if you consider that max power curve you will find that say for richer mixtures this is I mean around 11 or 11 point something so that will give us the maximum power at zero load zero means idling say no load and say this requirement goes on becoming increasing means more leaner mixture is required to produce maximum power say somewhere around 12 is to 1 say that is the requirement while for economy the condition again is similar say that mixture should go on becoming leaner as say we want to achieve economy with the load say here we require each mixture and then say it should go on becoming leaner or weak and say it will stabilize somewhere here say from here to here said to achieve the maximum economy even the load goes on increasing say the ratio will be semi stabilized now the same is reflected here say we find that at lower loads richer mixtures are required while as load increases leaner mixtures can achieve both maximum power as well as economy however the ratios for maximum economy are higher than that for maximum power now again if we highlight that fuel consumption and air fuel ratio further figure show figure 4 shows the change in properties of air fuel mixture by conducting a test with air fuel ratio as variable and keeping speed throttle opening and ignition timing constant so these are the variables so that are kept constant say if we see this figure sorry this figure we find that here say this is the mean effective indicated mean effective pressure in bar and this is the fuel consumption say in kg per kwh and requirements of air fuel ratio say they go on changing so this is the weakest mixture or leaner and say you will find that as we want more and more pressure or more and more power say you find say that mixture should go on becoming still seven weeks so this is the appropriate appropriate and fuel consumption will be list fuel consumption will be list here again as we want that more mean effective pressure then you will find that say the requirements goes on increasing that towards richer mixture richer mixture so here though we get the maximum efficiency the power consumption power produced is less this is the point C where we get the maximum power as well as maximum economy say which are 4 percent away from their maximum values and D is the say point where we achieve the maximum power again it goes on decreasing so this is the condition now if we see that further reducing the ratio to 15 based compromise between economy and power is achieved as both of them are away by 4 percent from their maximum value this is point C this is point C now effect of varying air fuel ratio so further reduction in air fuel ratio in the range of 12 is to 1 to 13 is to 1 give the point of maximum power D but specific fuel consumption is 25 to 30 percent more than maximum air fuel ratio affects the emission pattern also as seen in figure 5 so if you see figure 5 you will find that so this is giving us that air fuel ratio and this will give us the same in proportion of the different components in the exhaust and we find so this is the curve for hydrocarbon this is the curve for CO and this is for NOx nitrogen oxide and we find that as air fuel ratio goes on increasing both this hydrocarbon and CO say go that their proportion goes on decreasing but for NOx it is different say it increases with linear mixture achieves at the top and again goes on falling goes on falling so this is because the inner mixture produces higher temperature so that condition is favorable for formation of the NOx so if we see that at lower air fuel ratio that is for each mixture percentage of toxic components like Hc and CO is higher and it reduces as the ratio increases however proportion of NOx increases with air fuel ratio peak at the ratio of 17 is to 1 and then further falls so this is because of high temperature created due to lean mixture thus we have studied say effect of air fuel ratio say on the performance of the engine say today we shall stop say references for these figures and these were taken from the book of Icinges Mathur Sharma so