 In Chapter 9, we'll be talking about gas power cycles. Note that gas in that name is referring to the fact that the working fluid remains in the gas phase the entire time. This is in contrast to the vapor power cycle, where the working fluid is vaporized, it goes from liquid to vapor over the course of the cycle. Within our analysis of gas power cycles, what we are doing is establishing models of types of heat engines and analyzing those models. And there's a couple of ground rules that we need to establish in our conversation about these models. First of all, we are looking at idealized versions of the real thing. We're going to be simplifying them a lot for the purposes of our analysis, and we are also going to be making some pretty broad sweeping assumptions, like for example the fact that there are no internal irreversibilities. Remember that in this context, irreversibility is referring to losses. We are treating the inside of the model as perfect, the inside of our processes as being as perfect as they can be. This is to try to get a good idea of the differences between the cycles and how changes to each of the cycles affects the result, whatever it is that we're calculating, be it power or thermal efficiency. We aren't necessarily trying to determine the actual parameter for the actual engine. That's not what we're doing right now. For example, if we were to try to analyze my engine in my 2013 Honda Accord, we wouldn't necessarily calculate a power output that was reflective of its actual power output. We might be 15 or 20% off, and we could correct that by trying to look for where our assumptions are simplifying reality, especially in situations like frictional losses, but it allows us to do analyses like, what happens if I were to change the compression ratio of my engine? If I were to swap out the connecting rods to the piston, make them a little bit longer so that I had a smaller big and small volume, which means that I would have a higher compression ratio. Or what would happen if I were to increase the diameter of the pistons and grind down the cylinders, make them a slightly wider diameter, a greater bore? How would that affect the power output of my engine? And the changes in the model reflected by those changes in the input conditions are going to be reflective of the actual thing. That's why we are setting up these analyses, so that we can look at broad generalizations between the models as well as changes within the model affecting the results. While we're here, let me remind you that we can use PV and TS diagrams to keep track of work and heat transfer visually. That means that we can look at a PV diagram and deduce what's happening in a cycle or vice versa. We can analyze a cycle and then create a PV and TS diagram from them to be kind of a visual representation of the cycle as a whole. For pretty much all of our cycles, we're going to be determining a thermal efficiency. And again, while we're here, let me remind you that for our analyses, we are assuming a lot of ideal operations. And in our determination of the most ideal consideration possible, we are comparing against a Carnot heat engine. Because a Carnot heat engine will have the highest thermal efficiency for a given set of thermal reservoirs. So in our establishment of the ideal model for a given power cycle, we started the Carnot cycle and then we adapted for the situation in front of us.