The engine that moves it is one of the most fascinating and talked about of all the complex machines we use today. In this chapter we will explain briefly some of the operational principles and basic mechanisms of this machine. As you study its operation and construction, notice that it consists of many of the devices and basic mechanisms covered earlier in this book. We define an engine simply as a machine that converts heat energy to mechanical energy. The engine does this through either internal or external combustion.
Combustion is the act of burning.
Internal means inside or enclosed. In external combustion engines, such as steam engines, the burning of fuel takes place outside the engine. Figure 12-1 shows, in the simplified form, an external and an internal combustion engine. The external combustion engine contains a boiler that holds water. Heat applied to the boiler causes the water to boil, which, in turn, produces steam.