 In contrast, in the case of a control volume analysis, defining a control volume is relatively straightforward. Basically, whatever device we are looking at, we take the device, the boundaries of the device to be the control surface which is the boundary of the control volume and the device itself to be the control volume as shown in this case. So, here the first example as we can see looks at filling of a tank from a line. So, basically we take the tank to be our control volume and the control surface is shown in a dashed line here, very straightforward, very easily defined, mass, energy and heat, I am sorry, mass, energy, heat and work may cross the boundary of a control volume and the boundary of the control volume is usually referred to as the control surface. Now, in this case, we are looking at filling an evacuated or partially evacuated vessel from the atmosphere. Once again, we simply define the vessel to be the control volume. Here we are looking at emptying a vessel into the atmosphere and again, we define the vessel to be our control volume. So, in the case of a control volume, the definition of the control volume, in the case of a control volume analysis, the definition of the control volume is relatively straightforward. The device itself is the control volume. However, the complexity of the analysis is now pushed into the equations, the governing equations and further simplifications from there. In the case of the system, we do all the thinking upfront and then define the system. In the case of a control volume, the definition of the control volume is very straightforward, but the difficulty is now pushed to the analysis side. That is the difference between these two. An important point is that it is usually advantageous to have the control surface to be rigid and not deformed during the process. Although technically, there is nothing that prevents you from defining a control volume with a deformable control surface, it is usually not advantageous to do so, rather than doing that, you might as well adopt a system approach. So, for instance, if you recall in this particular example, which was the intake stroke of reciprocating air compressor, we may define a control volume as shown here. It is perfectly valid, mass enters through the side and this part of the control surface deforms as the piston moves. That is perfectly alright. But if you look at the system that we used for analyzing this problem, you will notice that this actually is not that much different from the system that we used. So, we gain no advantage by defining a control volume like this because we are now pushing the complexity into the analysis, whereas by using the system itself, which is not that much different from this, we can actually, so the system is not that much different from this. So, by using that, we actually simplify the analysis. So, it is not generally advantageous to have control surfaces which are deformable. So, the advantage of using a control volume is that it is easy to define. So, we take the device and define it to be the control volume, that is the general idea. Now, when do we use control volume, when do we use system? That question, the answer to that question depends very much on the problem that we are looking at or the device that we are looking at, but some distinctions may be drawn. For example, here we have two devices. This is a four stroke internal combustion engine which is driving an automobile. So, you can see the gearbox, this is connected to the drive shaft. So, this drives an automobile. Here we see a gas turbine engine which actually is driving a propeller. So, this is like the engine that you see in a turboprop aircraft. Now, let us say that we want to do a thermodynamic analysis of this and this, what would we choose? We would be choose a system approach or a control volume approach in this case. Now, let us isolate one cylinder from these four cylinders. Let us say we isolate this cylinder and if you look at the processors that the air undergoes. In both these examples, you will notice that the processes, thermodynamic processes are the same. So, air is taken into the engine here and here, air is compressed next and then fuel is injected and combusted and then the air undergoes expansion and produces work or power. So, the air undergoes the same process in both these engines but the manner or sequence in which the processes are accomplished is different between the two. So, here a fixed mass of air is taken into this cylinder. The valves are then closed and then the air is compressed. The piston moves up. The air is compressed and then fuel is injected and burned and the air then expands downwards pushing the piston down producing power and then the piston starts moving back up. The exhaust valve is opened and the exhaust gases are pushed out into the atmosphere. So, the fixed amount of air that is taken into the cylinder executes the four processes in sequence. And each one of this cylinder, the air in each one of the cylinder executes the same process in the sequence. We add more cylinders when we need more power and a smoother flow of power. The power produced here is not smooth because the power is produced only one out of every four strokes. So, it is intermittent. So, to smooth out this intermittency, we add more cylinders which gives us more power and also smoother flow of power from the engine. If you see the air is taken in and the air continuously flows through the compressor which is spinning like this which compresses the air and then it enters the combustor where the fuel is injected continuously and burned so the air gets heated up, then it flows through the turbine where it expands and generates power continuously it then leaves through the exhaust into the ambient. So, the big difference between this and this is in the manner in which we execute the process. Here we take a fixed amount of air in each cylinder and execute the process in sequence. So, it is a cyclic process or we execute these four processes one after the other in this case with a fixed amount of air. Here the air flows continuously and each one of this section in the engine executes one process and one process only. Here all the four processes are executed in each cylinder. But here the compressor does only compression. The combustor does only combustion and heat addition. The turbine does only expansion and extraction of energy and then the exhaust is used for say I am pushing the air out. So, the flow here is continuous, the power generation is continuous. Here the power generation is intermittent because we take a fixed quantity work with it to you know work with it to execute the four processes. So, if I want to analyze this device or the or a single cylinder in this device because I am taking in a fixed quantity of air a system approach would be best for this. Whereas here a thermodynamic analysis can be carried out very efficiently by using a control volume approach. I will simply draw a control volume around this entire device and then carry out my analysis. How much heat am I adding and how much power am I getting out of the device or if I want to do an analysis of each one of these components I can simply draw a control volume around them and then carry out the analysis. So, control volume is better for this because of the manner in which the operations are sequenced and a system approach is better for this again because of the manner in which the operations are sequenced in this device. So, one can use a system or a control volume approach for any problem most of the times but which one we choose is dependent upon which one is easier for analysis. The system requires as I said in a much more upfront thinking before it can be defined control volume is easier to define but the complexity now is in the equations and the analysis and the simplification of the equations. So, it is problem dependent but these are some general guidelines which we may use. In general if there is flow of matter into and out of the system then control volume approach is probably is definitely preferable and in other cases system approach is probably acceptable.