 A reciprocating engine refers to the fact that our piston is going up and down. It is going back and forth, and within this analysis we need to be able to describe volume as well as mass in relation to the geometry of the engine. So we'll establish a couple of relevant parameters to that reciprocating engine. First of all, the diameter of the cylinder in this reciprocating engine is referred to as the bore. The stroke is the region swept by the piston, the distance between the piston at bottom dead center and top dead center. Also note we use bottom dead center and top dead center as indications of position as well as volume. For example, the volume at bottom dead center is this entire volume above the piston, and the volume at top dead center is this volume when the piston is in its highest position. And by the way, bottom dead center and top dead center are referring to the fact that our piston is connected to a connecting rod on the crankshaft. So if I just draw it like this for a moment, I know, I know it's not super accurate. As the crankshaft goes around, the piston goes up and down. So when the crankshaft is in a position where the connecting rod attaches to it at bottom dead center, that is where the piston will be in its lowest position. When it's attached at top dead center on the crankshaft, the piston is in its highest position. We are analyzing the piston going back and forth between top dead center and bottom dead center, and sometimes we'll refer to those as TDC and BDC. Also note the region of volume that is swept by the piston, that is pi over four times the bore of the cylinder multiplied by the stroke. That volume here is called the displaced volume, abbreviated VD. And then the volume that is left over when the piston is at top dead center is called the clearance volume. So these volumes are all related to one another. We could say VBDC is equal to VD plus VC, and VTDC is equal to just VC. I know all those letters sound the same. A parameter that we often keep track of in reciprocating engines is the proportion of volume at bottom dead center to volume at top dead center. This gets a special name. It is called the compression ratio, and it is abbreviated with the letter R. The compression ratio represents the proportion of volume at its largest to its smallest. So if the total volume at bottom dead center, VBDC, is eight times larger than the total volume when the piston is at top dead center, that would be a compression ratio of eight, also sometimes described as eight to one. And we can also rearrange this and write it in terms of other things, like for example, we can write this as VD plus VC divided by VC if we wanted to. This is useful because the displaced volume is the one that is typically talked about. For example, if you went out and bought a car that had a two liter engine, that two liter volume is the displaced volume. And if it's a four cylinder engine, that would mean that each cylinder actually has a displaced volume of half a liter. If you knew that the displaced volume was two liters and you knew that the compression ratio of the engine was nine, and you wanted to figure out the volume at bottom dead center, you would have to use this relation to calculate VC, and then you would have to use VC and VD to calculate the volume at bottom dead center. Another parameter that gets tossed around when we talk about reciprocating engines is something called the mean effective pressure. And the mean effective pressure is the average pressure that would yield the same network out if it was pushing the piston from top dead center to bottom dead center. So if you had an air supply supplying air pressure at a constant pressure and pushing the piston down, what air pressure would yield the same network? You could think of this as, if you were plotting this on a PV diagram, what pressure would yield the same area under the curve at horizontal line that was the same area as the region enclosed by that power cycle itself? Mean effective pressure is not particularly useful anymore. I mean, it's very niche, but it's a useful parameter here because it gives us another thing to calculate. It also is another way of indicating how changes in the input conditions can affect outputs of the model. The engine with the higher mean effective pressure is generally the better performing engine. When we talk about reciprocating engines, we are going to be splitting them into two different models. Those models are distinguished from one another based on their ignition type. The idealized spark ignition internal combustion reciprocating engine is modeled with the auto cycle. The idealized compression ignition internal combustion reciprocating engine is modeled as the diesel cycle.