# Reciprocating Engines Most small aircraft use reciprocating engines, which operate on the basic principle of converting chemical energy (fuel) into mechanical energy. This conversion occurs within the cylinders of the engine through combustion.  The graphic above may look complicated, but we can break it down to understand the prupose. The goal of this engine, and most aircraft engines, is to turn the fuel that we have into rotational energy that can turn the propeller. Let's take a look at the individual components. # ![[Engine Parts]] # ![[Four Stroke Engines]] # Video Explanation Watch the beginning of this video explaining the four strokes of an engine. This explanation is focused on a car engine, but the concept is the same. > [Video: How a Car Engine Works](https://youtu.be/ZQvfHyfgBtA?si=cZoCGTmXNxOOOW0f) # Horizontally opposed engines The most common type of engine found in small aircraft is the horizontally opposed engine. These engines always have an even number of cylinders which face in opposite directions. They are generally air cooled. Using this design allows for a higher power to weight ratio as the crankshaft can be smaller. Below is a depiction of a horizontally opposed engine:  # The 172S Engine While there can be a variety of different engines installed in the 172, all of ours are fitted with the Lycoming IO-360-L2A. It's important to note that the name of the engine can be thought of as a code to make it easier to remember the different components. - I: fuel injected - O: horizontally opposed - 360: 360 cubic inches of displacement - Displacement here can be understood as the total amount of volume that all of the cylinders combined make up.