Internal Combustion Engine
Combustion is the burning of fuel. When fuel is burned it gives off energy, in the form of heat, which creates the expansion of gas. This expansion can be rapid and powerful. The force and movement of the expansion of gas can be used to push an object. Shaking a can of soda is a way to see what happens when gas expands. The shaking motion causes a reaction of carbon dioxide—the soda's fizz—which, when the can is opened, pushes the soda's liquid from the can.
Simply burning fuel, however, is not very useful for creating motion. Lighting a match, for example, burns the oxygen in the air around it, but the heat raised is lost in all directions, and therefore gives a very weak push. In order for the expansion of gas caused by combustion to be made useful, it must occur in a confined space. This space can channel, or direct the movement of the expansion; it can also increase its force.
A cylinder is a useful space for channeling the force of combustion. The round inside of the cylinder allows gases to flow easily, and also acts to increase the strength of the movement of the gases. The circular movement of the gases can also assist in pulling air and vapors into the cylinder, or force them out again. A rocket is a simple example of the use of internal combustion within a cylinder. In a rocket, the bottom end of the cylinder is open. When the fuel inside the cylinder explodes, gases expand rapidly toward the opening, giving the push needed to force the rocket from the ground.
This force can be even more useful. It can be made to push against an object inside the cylinder, causing it to move through the cylinder. A bullet in a pistol is an example of such an object. When the fuel, in this case gunpowder, is exploded, the resulting force propels the bullet through the cylinder, or barrel, of the pistol. This movement is useful for certain things; however, it can be made still more useful. By closing the ends of the cylinder, it is possible to control the movement of the object, making it move up and down inside the cylinder. This movement, called reciprocating motion, can then be made to perform other tasks.