 In this lecture, we will be talking about ice engine operations with specific reference to the aircraft engines. Ice engines as you know are used in all kinds of vehicles and many other operations, but when it comes to their application in aircraft engines as basic power plant and power supplying unit, there are certain things that need to be done to satisfy the needs of aircraft engine and for the needs of flying the aircraft. Now, as you know an aircraft typically flies or cruises at a high altitude quite often 5 kilometers or even higher than that and as a result of which at that altitude the air density is very low and hence the air which is the working medium of this kind of engines is very thin. So, the air supply into the engine is actually much lower in terms of mass flow and hence we need to do something about the fact that the air supply in mass flow is rather low. Now, as we have seen in the earlier lectures when the air mass flow supply is less or charge as we call it is less, the power production also goes down. So, if the power production goes down in aircraft engine, if it falls short of the power, the propeller which produces thrust would not be able to produce the thrust that is required for the aircraft to fly and in such a case aircraft would not be able to fly at all. In fact, the problem starts much earlier when the aircraft starts climbing from takeoff from let us say sea level altitude to a higher altitude the as the air becomes thinner and thinner the power supply goes down because of the fall in the mass flow. Now, this is something which is typical of aircraft engine this is not a problem with any other land based application and hence the aircraft engineers had to do something to ensure that this problem does not create a problem where the aircraft is flying. So, aircraft engine ice engines used in aircraft do have slightly different configuration than the ice engines used in many or various land based operations. Let us take a look at some of these things that are typically done in aircraft engines. We will also look at how the aircraft engine or ice engine is operated under part load conditions. You do not use an ice engine all the time at its full throttle or full load conditions quite often you do not need so much of power. So, you can operate it under off of design or what is also known as part load condition. Under these conditions how do they operate and how do they produce power and what are their characteristics. These are also few things that we will be looking at today along with as I said certain augmentation that is required for making this kind of ice engine useful as aircraft power plant. Let us take a look at some of these issues today in today's lecture. Now one of the things that we have seen is the density of the fresh charge which is going into the cylinder is an important issue that determines the amount of mass flow that is going in and at the end of it the mass flow actually determines the amount of power that is being produced by the engine. The other important parameter that affects the volumetric efficiency which we have defined earlier is the pressure and temperature of the outgoing burned cast. And then of course there are engineering issues which are the design of the intake and the exhaust valves or the manifolds and the timing of the opening and closing of the intake and exhaust valves. Now these are the issues which have been engineered into the engine. So these are engineering issues and they need to be done properly to ensure that you have a good volumetric efficiency which means it should be as close to 100 percent as possible. So that you are getting the best out of the engine shape and size that you have. So volumetric efficiency essentially tells you how good you are making use of the engine given its shape and size already available with you. So these are the things that the designers in terms of engineering the product has to be you know very careful about before the engine is made operational to achieve high efficiency during its actual operations. Now let us take a look at some of the things that we need to bother about. For example when the full throttle operation is going on as you can see in this diagram the air is now being compressed along this. And as we have seen earlier quite often the process of ignition or burning of the fuel can be initiated may be initiated a little earlier. And if it is initiated much earlier this is how the graph would go the operation of combustion would be shown in the P V diagram which means it has been started quite a lot earlier before the T D C and hence the graph would go in a curved manner like that instead of straight line as we normally see in a ideal cycle. On the other hand the exhaust for example if it opens a little earlier the exhaust may have opened a little earlier somewhere over here and as a result of which you could have this exhaust operation initiated earlier and this would take a curved path and hence the exhaust would be occurring at a higher pressure at a substantially higher pressure than the intake operation. This as we know results in a certain amount of area that shows up over here and that area would actually be lost as far as the power production is concerned. So the opening and closing of the throttles is an important issue. Now under the part load condition or part throttle condition similar things would be happening the work done over here in part throttle is actually much less now than compared to the full throttle operation and in this situation if the opening or closing of the intake or the exhaust occur much before the actual time of their opening or closing then a good amount of work would actually be lost in the process of the differential between the exhaust operation and the intake operation. So this loop that you see over here is something which is not available for as a power output of the engine. This is the area that goes into the operation of exhaust and intake and as a result that would not be available in terms of BHP. This is one of the issues that comes up when the operation of the exhaust the ignition and the intake do not quite happen exactly ideally as we have seen in the ideal diagram. This is this actually shows the loop which I was just talking about in this as you can see here that in the full throttle condition if it happens nearly ideally you have a very small loop over here and as a result of which you can say that the power lost in the process of intake exhaust is rather small. On the other hand what can happen is under part throttle condition this loop could become very big. Now as it is as we have seen in the earlier diagram the part throttle condition the actual operational power available is actually on the lower side and then if the power lost in the process of intake exhaust is of much higher order this could happen due to the fact that the opening of the intake valve could be earlier or closing of the exhaust valve could be earlier. Now some of these issues then affect the loop or the area of the loop that is shown over here and if the area of the loop is higher that means more and more energy is being wasted by the fact that the operation of the intake and exhaust valves are not happening properly. So some of those issues need to be taken care of by the engineering aspects of the engine design and hence it shows up in the thermodynamic diagrams and as a result of which the engine power suffers. This is one of the issues that again we were just talking about that the delayed ignition what happens when you have a delayed ignition we just saw what happens when you have early ignition supposing you know this is what we would call a normal ignition a little early so that even when the compression process is being completed the ignition is initiated and as we have seen this goes along this path. Now supposing the ignition is delayed as a result of delayed ignition the ignition process is much later and then the path that it takes it does not take along this it quickly goes into this path and as a result of which this huge area that you see here shaded area is the loss of work with relation to the indicator diagram or the ideal process that one would have expected. So this loss of work is now due to the fact that the ignition is delayed and not initiated at an appropriate time. So there is an exact time at which the ignition needs to be initiated and if it is delayed it is going to incur a lot of loss of work from the engine. So that is another issue that the ignition needs to be timed exactly otherwise large loss of work by the power stroke is most likely to happen. Let us look at some of the issues again due to the late opening or poorly designed exhaust valve. This happens when for example this is your normal exhaust and now suppose your late exhaust valve opening shows that it is occurring around this as a result of which this is the now the exhaust path. Now this exhaust path is as you can see well above the exhaust path over here and this is situation that means this much of extra work needs to be done during the exhaust operation and this extra work would now not be available as a output of the engine. This is this could be due to either late opening or poorly designed exhaust valve. There is a one small issue here that due to the late exhaust the path that it takes actually creates a slight bit of gain in work which is shown here and the rest of the work is actually due to the loss of work. So there is a very small gain in work which is shown here, but the loss of work is of much higher order and as a result of which there is a large amount of loss of piston work and a very small amount of increase of intake work and a very small amount of gain in exhaust work. So the total loss is of a much higher order and as a result of which total output of the engine could go down substantially. This is due to the fact that you have a late opening or poorly designed exhaust valve operating within the engine. This may happen due to various reasons and some of those reasons need to be looked into either by the designer or by the operator during its functioning. The other issue is the poor intake design. We need to look at the fact that the intake operates ideally at a constant pressure and if the intake operates at a pressure much lower than what it is scheduled to, we get a inlet intake loop that is now much bigger than what it should have been ideally and this intake loop now takes away work from the main engine in the process of pumping as it is shown here and this intake pumping work now, extra intake pumping work now is not available as an engine output. So the poor intake design often also leads to or poor operation of the intake valve often leads to a loss of work which would not be available to the engine BHP or engine output. So this is another issue which needs to be looked into by the engineers who design the engine. So the important performance parameters that we need to look into are the heat release per unit mass of air. This is the quality of the fuel that you are putting into the engine. So the choice of the fuel needs to be taken care of by the heat release capacity of this fuel and the other is the quantity of charge which is a mixture of air and fuel mixed in the carburetor per stroke of the engine. Now heat release per unit mass of air it is decided by the chemical composition and the working fuel air ratio. Chemical composition is something which one needs to analyze before one makes a choice of the fuel and we have talked about the kind of fuel or aviation fuel that is normally used these days high octane fuel and that was chosen because of their chemical properties. The other is the working fuel air ratio and as we have seen the working fuel air ratio actually can change from one operating point to another. This is something which the engine operator has some control over and the normal or the ideal fuel air ratio is what we call normally the stoichiometric ratio, but quite often the operation happens at a ratio which is slightly different from the ideal stoichiometric ratio. It could be slightly higher or it could be slightly lower and as long as it is within a certain zone of operation a safe zone of operation of fuel air ratio and as long as it operates in that ratio the engine and the ignition process would continue to happen in a normal fashion. Now this is with reference to the heat release and all this will decide the heat release rate per unit mass of air as it is inducted into the cylinder. The other thing that will decide the amount of power that is coming out is the quantity of charge. The thermodynamics always gives us values in terms of power produced per unit mass flow. However, the total power produced is always dependent on the mass flow that is going inside the cylinder or inside the engine as a whole and hence the quantity of charge that is going in decides the amount of power that we would actually get. Now this is one of the issues that we would need to look into with specific reference to aircraft engines because in aircraft engines the quantity of charge that is going in goes down with the increase of altitude. As you go to higher and higher altitude you may be using the same fuel you would be using the same fuel. So, it is heat release rate probably would remain of the same order, but the mass of air is going down and the quantity of charge is going down. So, we need to look into this issue with specific reference to aircraft engines. In aircraft engine this issue of charge or mass of charge is taken care of or compensated for with the change of altitude by an additional unit called supercharger. This is basically a booster which is used before the charge or the air fuel mixture enters the cylinder and this cylinder is filled above the ambient pressure with the help of the supercharger and hence the density of the air is now higher than the ambient density and hence the weight or mass of air that is introduced inside the cylinder per cycle is greater than in the unsupercharged case. So, if you do not have supercharging this mass of air that is going in would continuously go down to when you reach higher altitude and it would continue to go down when you increase the altitude. So, you need a supercharger to compensate for the decrease of ambient pressure and ambient density and this is where the supercharger for an aircraft engine comes into the picture. Now, as you know the volume of the operation would remain same for the same engine. So, when you apply the volumetric efficiency the net work done would be higher than that of a naturally aspirated engine which means unsupercharged engine. So, you need a supercharger to get more and more work done for the same engine as it goes to higher and higher altitude. Let us see how that actually happens. If you have a supercharged engine typically you would be using some kind of a centrifugal blower to boost the density or the pressure that is going inside the cylinder and this boosting is done after the air and charge has been mixed and before it is of course, it enters the main cylinder before it is supplied to the main cylinder. Now, this is a separate unit and supercharger is separate unit from the basic IC engine and as I said it is used specifically for aircraft engine. Now, let us look at this diagram and we will see that when you have unsupercharged or what is often called naturally aspirated engine your basic indicator diagram would actually go along the dotted line, but as soon as you apply supercharging your basic intake now has gone to a higher pressure and then your compression would take it to a much higher pressure and hence your power stroke would be actually occurring at a higher pressure. So, the mean effective pressure which we have talked about would now be occurring at a much higher pressure the value of MEP would be in much higher for this cycle compared to this dotted cycle which is a unsupercharged engine and as a result of which you can well imagine now that the power produced by the supercharge engine would be of a much higher order and this is exactly shown in this thermodynamic PV diagram that supercharger actually boosts the performance to a much higher level how high it can be depends on the amount of supercharging that is available with the engine. In all this time the fuel used is same and is introduced or injected into the cylinder in the same manner. So, there is no change in the way the fuel is used or the kind of fuel is used it is only that the air charge that is being used as a working medium is supercharged or boosted in its pressure and density to a higher value and that allows us to produce more power that may be available to the aircraft power plant. Let us take a look at some of the issues that are related to the effect of supercharging at sea level and at altitude. Now, as I have mentioned you can use an engine both at full throttle or at part throttle depending on the amount of power you actually need to make use of or you need to fly the aircraft in case of an aircraft engine. Now, what happens is when you are at sea level if you have unsupercharged full throttle your characteristic would move along this line and then it will kind of plateau off and when you go to high altitude the unsupercharged full throttle characteristic would continue to go down along this line with the increase of altitude and as a result of which your brake horsepower would be continuously coming down. So, when you are at sea level at full throttle unsupercharged as your manifold absolute pressure goes up your power production goes up and then as soon as you move to high altitude and the aircraft starts climbing and altitude is gained the power production continuously starts going down and at let us say a cruise your power production is somewhere over here and one has to very quickly figure out whether this power production is sufficient for flying the aircraft. In most cases this power production would probably found insufficient for flying of an aircraft and that is where the supercharger now comes in. If you have a supercharger the supercharged power production would go along this line it would go to higher values and then as you move to the altitude you could have high supercharging or low supercharging and you could it could move along this line along which you have constant manifold absolute pressure MAP and then this is you are still working at part throttle and then you can open up the throttle and go to full throttle because you need to conserve power you need to get more power to make the aircraft fly and with change of altitude your air is thinning down. So, even with supercharging air will continue to thin down and as a result their mass flow going into the engine will continue to go down but it will go down now at a much higher value than what we had seen in the unsupercharged case and as a result of which this full throttle now allows you to produce more power far more power than you can get for unsupercharged engine and if you are able to create a high supercharging you can boost the power somewhere over here to a even higher value at full throttle. So, you can apply higher supercharging this was with low supercharging and you can go to even that means a second booster for example, you can have two supercharges and then you can go to even higher power production and take the power production somewhere over here at a very high altitude if the need arises. So, the combination of supercharging and use of a low supercharging or a high supercharging which means you can have two supercharges allows you to create more power during the aircraft operation at high altitudes. In all this what we have considered that the engine is operating at constant rpm and they are operating at constant failure ratio those also can be varied, but we have not considered those variations in this particular diagram and as a result of which as you can see now the supercharger gives a immense amount of power boosting to the aircraft engine starting at sea level going to higher altitudes and you can produce more power either through single supercharger or through a double supercharging operation to get a lot of power during its high altitude operation. So, the supercharging produces additional work that may be extracted from the exhaust gases by expanding them to the atmosphere through a turbine. So, the question now is how do you run the supercharger which produces this boosting of intake density and pressure it is done by running it with the help of a turbine and the question is how do you run the turbine. The turbine is run by the exhaust gas that is coming out of the engines and that is fed into the turbine remember the exhaust gas that is coming out of the engine is still at a reasonably high temperature and at reasonably high pressure. So, if you allow it to run through a turbine it will produce certain amount of power and this power then can be used to run the supercharger or the booster which as I mentioned earlier is basically a centrifugal compressor and hence the power supply to the supercharger can be done with the help of the turbine which is run with the help of the exhaust gas that is coming out. The aircraft then when it goes to high altitude increases the pressure and this allows the manifold pressure to be either held to a design value close to the design value for which the engine was designed for or something very nearby that value and as a result of which you continue to get a good amount of power that the engine is originally designed for. So, many of the superchargers are often called turbo superchargers and this supercharger plus turbine configuration the rpm of this can be varied by adjusting the turbine discharge nozzle to produce pressure ratio of the turbine. So, that the turbine power production can be varied to run the supercharger. So, the whole supercharger turbine combination can also be controlled separately by controlling the turbine discharge nozzle and so that the pressure ratio produced by the supercharger can also be varied depending on the need of the engine at that particular altitude. So, the turbo supercharger has a control system of its own independent of the engine control and as a result of which the amount of supercharging that you can do to boost the engine can also be varied depending on the need of the aircraft when it is flying. The supercharger delivery pressure is given by supercharger pressure ratio and this is the pressure at the turbo supercharger exit divided by the ramp pressure outside of the air scoop. This arrangement can be can maintain a constant engine BHP or almost constant engine BHP from sea level to a very high altitude. So, that the engine continues to give its nearly its design power produce a power production and this operating altitude is determined by the maximum allowable rpm of the supercharger. So, which means that the aircraft engine can now operate at a higher and higher altitude because of the supercharging available and the final altitude at which it can operate an aircraft can fly is now determined by the supercharging capacity and the maximum allowable rpm of the supercharger turbine combined. So, not only the power is boosted now you can fly the aircraft at a even higher altitude which has the advantage as we all know that higher the altitude at which you fly the aircraft lesser is the drag experience by the aircraft and hence lesser is the power actually required to fly the aircraft and if you require less power to fly the aircraft your continuous fuel consumption is also going to be low. So, all thus all those gains can be actually factored into the aircraft mission or aircraft flight schedule if you have a supercharger. So, the supercharged aircraft engine can actually fly at a higher altitude to effect the need gains that come from flying at a high altitude and this is something which is typical of aircraft engine normally not necessary in most of the land based operation. So, aircraft engines typically have a supercharger which is designed to operate at high altitude it is normally not necessary when it is you know cruising at low altitude or when it is taking off it is necessary when it is flying at high altitude and as I mentioned to what altitude it can aircraft can fly would actually be determined then by the availability of the supercharger and the capacity of the supercharger. So, supercharger is designed separately to fit into an aircraft engine and fit into the need of the aircraft and take the aircraft to high altitudes. Let us look at what happens if you have a supercharger and what happens to the p v diagram of the cycle. You have a supercharger which again is some kind of a aerothermodynamic unit. Hence, it needs to have certain amount of thermodynamic basis of its own for it to be included in the overall engine configuration. So, if we look at this we will find that the basic engine which operates let us say at a b which is the intake to the compression system and the compression line is b c as we have seen normally in most of the cycles that we have looked at and the work required to do this compression process is then given by a b c d and that is the area that is the compression work that needs to be done in the compression process of the engine. Now, the source of this work is the turbine. Now, this turbine is supplying power to this compression process that is being supercharged. Now, the turbine work is it extracts energy from the outgoing mass of burnt gas that has been exhausted from the main engine and the turbine work can also be represented in the same p v diagram and let us say that the turbine work is now represented by z y x w or this particular area and as we can see now that this work done by the turbine then needs to be equal to this work that is needed for the intake. So, as long as this work done by the turbine as represented in the p v diagram here is equal to this work shown by a b c d. We have a combination of turbine and compressor that can do the supercharging job and aid the engine in operating at high altitude. So, this is the supercharging p v diagram separate from the engine p v diagram that we have seen before and this needs to have a comply to the laws of thermodynamics that you have done in detail earlier. So, supercharging operation by itself has to conform to the basic laws of thermodynamics that you have done before in the earlier lectures. The exhaust gas which enters the turbine along the line let us say w x then expands along the x y line this is how the turbine operates. So, the intake starts from a b and then goes up to c and d the exhaust starts from w and then it enters through w x it expands along the line x y and does the work that is the working of the turbine and the working of the compression is from b to c working of the turbine in this p v diagram is from x to y and then pushes out the work of the gas along the y z line. If the turbine is used only to drive the compressor the areas as I mentioned the areas a b c d and the areas w x y z must be equal. If the turbine work falls short then the turbo compressor combination would slow down if the turbine produces more work then the turbo compressor combination will speed up to higher speeds whether that is required or whether that is warranted or allowed is to be decided by the engine operator. So, if the turbine work is excessive the turbine discharge nozzle may be throttle raising the line y z until the area y x w x y z is again equal to that is one way of reducing the turbine work. If it comes out to be doing more work as I mentioned earlier you have the discharge nozzle to control the amount of work that is being done by the turbine which runs the compressor which is the supercharger. So, that the turbine and the supercharger work are equal to each other no more and no less. This is how the supercharger actually functions and that is what is shown here thermodynamically in this p v diagram. If we look at the way it is shown you have a supercharger over here before the flow actually gets into the cylinder. As I mentioned it is normally done after the carburetor where the air and fuel are mixed in the correct proportion or the required or wanted proportion for that particular operation and then the fuel air mixture which we call charge is supercharged one of the possible or more used supercharging is of the order of 6 to 1 ratio at high altitude that is the kind of supercharging you probably would need and then boost the density before it is fed into the cylinder. Now, what happens is the power output of the brake horsepower as we call it can be now shown with the altitude it is expected that it will all the time go down, but the unsupercharged power goes down along this line and the supercharged power goes along the upper line and in between there is a line which is actually used for the purpose of supercharging itself and as a result of which one can say that the net output is given along this line. So, supercharger nearly restores the power output of the engine and typically a double supercharging or what is often known as high supercharging is used essentially only for the climb operation of the aircraft. It may not be necessary actually during the cruise operation of the aircraft it is necessary for the climb operation where you have to take up take the aircraft from low altitude to high altitude along with its passenger or cargo and you need excess power over and above the drag of the aircraft. The differential between the thrust produced by the engine or power plant and the drag experience by the aircraft takes the aircraft to high altitude through the climb operation. So, during the climb the thrust produced by the engine or the power plant has to be more than the drag experience by the aircraft and this differential produces the climb operation. So, during the climb you need that excess power to produce excess thrust and hence during the climb operation one may use double supercharger or high supercharging. Once you reach the cruise operation you do not need the high operation high supercharging anymore and you need only as much power as the aircraft is experiencing as thrust because during cruise as you know the thrust is equal to the drag for straight and level flight. So, during the cruise operation you need to produce only as much thrust as is experienced by the aircraft as drag and extra thrust is not required anymore and hence during that operation the second supercharger can be switched off and aircraft can operate only with a single supercharger or what we can call may be a low supercharger. So, that is how the supercharger is used for various aircraft operations during climb and then later on during the cruise flight of the aircraft. So, let us sum up and say that you can have a single stage supercharger which is good enough for the aircraft to fly at high altitude. You may have two stage supercharger which allows additional boosting during the climb operation or you can have variable speed supercharger where the supercharger speed can be varied to high or medium and as we were discussing just now that you may need to do at high speed supercharging to get high supercharging during climb and then settle down to medium supercharging for medium to low supercharging during its cruise operation of the aircraft. Most of the superchargers are centrifugal flow machines or centrifugal blowers or compressors and often compression ratio as we mentioned of the order of 3, 4, 5 or 6 and this kind of machine is being used for superchargers right from the beginning. However, it is possible that if you need low supercharging you can probably use axial flow compressor which in two or three stages can produce a sufficient pressure rise inside the supercharging facility to boost the performance of the aircraft. So, you can have a number of choices in the in your choice of supercharger depending on the size of the engine, depending on the kind of aircraft power scheduling that you need to do and of course, depending on the altitude at which finally, you expect the engine to be used for aircraft thrust making device. So, these are the various choices that engine designer has in choice of his supercharger and as you can see he has a number of choices for making a choice for supercharging of the aircraft engine. One of the choices that typically an engine designer would have is a turbo supercharger and one of the possibilities is that he can use a turbo supercharger with what is also known as intercooler. Now, in this diagram this arrangement is shown you have the engine we are showing let us say only you know two cylinders you have the basic engine which of course, runs the propeller which as we know produces the thrust and then this is your carburetor which of course, you know combines the air and the fuel and produces the charge that goes inside the cylinders and then you have normally a gear box which runs the supercharger which is of course, as we know as a normally a centrifugal compressor. Now, what can be done is as the supercharger is operating and it is the supercharger as we have seen is run with the help of a turbine this turbine is run with the help of the gas that is coming out from the exhaust pipe. So, instead of the gas going straight away out to the atmosphere the exhaust pipe takes it to the turbine and runs the turbine and then from the turbine it goes out through a small nozzle which as we mentioned can have a variable geometry capacity or variable throttle capacity to control the operation of the turbine and through the operation of the turbine control you can have control the over the supercharger operation. So, this is how the turbine supercharger combination is brought to a certain amount of control through the outlet control of the turbine. Now what can be done is you see from the supercharger the air is coming into the supercharger let us say from the atmosphere and then it is being fed into this pipe through which it goes to the carburetor and then through the carburetor it is fed into the cylinder. Now on the path of it is going into the carburetor the air can be further cooled with the help of ambient atmospheric air which is at high altitude quite cool. So, this cool air can now be used to cool the supercharged air because in the process of supercharging as you know if you have a compression the air actually gets heated up. So, along with the rise in pressure the thermodynamic tells us very clearly that through the supercharging process the pressure has gone up the temperature also has gone up. Now from the thermodynamics we know that if the temperature has gone up the performance of the engine would actually suffer a little the higher the temperature of the intake air lower would be the performance as a result of which in fact the lower would be the density of the air coming in. So, one of the ways of further boosting the engine performance is by cooling and this is often called an intercooler that it is a cooler which is intermediate between the supercharger and the carburetor and this intermediate cooling is simply called intercooler and this intercooler cools the air the air you remember is still compressed it is compressed to a high pressure compensating for the loss of altitude. And now it is cooled and it is passed through this intercooler it is just passed through it and it just gets cooled and this now compressed but cooled air is now fed into the carburetor and then the fuel air mixture is created which then goes inside the cylinder. And as a result of which we have a kind of double boosting of the engine performance first it is boosted by the supercharger and then it is boosted by cooling the air and all these concepts as you know comes from the basic understanding of the thermodynamics of the engine. So, you need to always refer back to the basic thermodynamics to understand why these things are done and of course, finally they have to be engineered into the engine configuration so that they operate properly during the actual functioning. So, intercooler is a possibility that has been used sometimes to boost the engine performance a little more if the engine is operating at very high altitudes. So, intercooler is an additional method by which turbo supercharger can be further compensated for its heating that occurs through the turbo supercharger unit. So, these are some of the combinations through which the aircraft engine often gets its operation boosted or augmented in its flight during climb during high altitude flight and some of these as I mentioned are typical of aircraft engine they do not normally happen or necessary during the land based operations and so you probably would not see them in most of the land based engines or land based vehicles sometimes you may have heard of them being used in the racing engines where of course, for racing purposes these kind of boosters are used to augment the power for the racing engines. In the next class we will look at all the things that we have done with reference to the various aircraft engines and try to solve a few problems using the basic thermodynamics and basic engineering parameters that we have defined and see whether those things can be put together in solving of realistic problems and I will also bring in a few problems for you to solve by yourselves and this is what we will do in the next class. So, it will be some kind of a tutorial where we will indulge in a little bit of problem solving using the basic thermodynamic cycles and the aircraft engine parameters that we have discussed in the earlier classes.