 The last gas power cycle that we're going to take a look at in this lecture is that of the Stirling engine and the Stirling engine is a little bit unique from the other two that we've looked at the auto and the diesel in that the combustion or the heat addition takes place outside of the engine itself and so that's why sometimes this is called an external fuel engine. The Stirling engine itself has been around for quite a few years and theoretically it can have an efficiency to be the same as that of the Carnot cycle given that it operates in an externally reversible manner. So the unique aspect of the Stirling is the fact that very high thermal efficiencies can be reached theoretically provided that we can implement some sort of mechanical device to be able to implement the cycle as designed. So what we'll begin with just like the other cycles that we looked at what we'll do is we'll start by looking at a PV diagram then I'll draw out a bit of a schematic in terms of how the Stirling engine may be constructed. So let's start with the PV diagram. So this is the PV diagram for the Stirling cycle and what is happening is we are going through a process that is isothermal and from one to two is where we are adding the heat. So we have isothermal heat addition at the hotter temperature the higher temperature within the cycle and the way that that is achieved is we are adding heat while expanding at the same time. Normally when you expand a gas it gets cool but we're adding heat at the same time which then maintains the temperature at a constant temperature. At the same time when we look at process three to four that is a constant temperature process whereby the pressure is going up and that is a process whereby we are rejecting heat and it is a compression process and we know when we normally compress a gas it will get hot but we're rejecting heat at the same time enabling us to stay at an isothermal or in an isothermal condition. Now another aspect of the Stirling that we have not yet seen in any of the cycles that we've been analyzing is there is what we call a regenerator and what the regenerator does is it takes thermal energy from one part of the cycle and transfers it to another part and so that is shown in the middle and what is happening is heat through the process two to three is being transferred to process four to one and I'll show you in a second how that can be implemented it's using a device that we call the regenerator. So let's take a look at a schematic of what the Stirling may look like and then hopefully that will start to bring some clarity in terms of what this regenerator actually is. So what we have are two piston cylinder devices and they're connected by a common regenerator piping system which goes into the regenerator and so there we have our two pistons or two cylinders I should say. I'll draw in the pistons now. So this here will label as being our hot cylinder and this is our cold cylinder and they are connected to one another through the centerpiece which is our regenerator. Now typically regenerators have the characteristic of high thermal mass they're able to store thermal energy it could be constructed as something as simple as steel wool or a porous metal matrix that does not have high pressure drop when the gas goes through it but it can still maintain thermal energy. So the characteristic that we would look for in the regenerator is high mass times the specific heat capacity and we refer to MC sub P as being thermal mass. So the reason why we want high thermal mass is because when the gas goes through the regenerator we want it to exchange or deposit I guess you could say transfer its thermal energy to the regenerator and then when the gas comes back the next time we want the thermal energy to be transferred back to the gas. Now the working fluid that you'll be dealing with you can have many different working fluids but common ones that are used for stirrings are helium and hydrogen. You can also use air although helium and hydrogen will yield the highest thermal efficiencies or practical operation. Now what is happening here is the gas is going from the hot cylinder to the cold and back and forth so we have gas here and we have gas here some of the processes that is two through three and one through four those are constant volume processes and when those are taking place the piston and the cylinder are moving in a manner the two pistons are moving in a manner that constant volume is preserved within our system and so the gas is essentially going from one to the other. It's being transported back and forth and when it goes through the regenerator it's either depositing or leaving thermal energy or it's coming back the other way it will be picking up thermal energy. So that is the example or the PV diagram as well as the configuration of the sterling.