combustion

[[image:http://www.grc.nasa.gov/WWW/K-12/airplane/Images/combst1.gif width="709" height="531" align="bottom" caption="A graphic showing the process of combustion; fuel plus oxygen plus a source of heat combine to produce exhaust plus more heat."]] During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called**exhaust.** Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in [|air,] which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The [|temperature] of the exhaust is high because of the [|heat] that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. **Soot**, for example, is a form of solid exhaust that occurs in some combustion processes. During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some **source of heat** is also necessary to start combustion. Gasoline and air are both present in your automobile fuel tank; but combustion does not occur because there is no source of heat. Since heat is both required to start combustion and is itself a product of combustion, we can see why combustion takes place very rapidly. Also, once combustion gets started, we don't have to provide the heat source because the heat of combustion will keep things going. We don't have to keep lighting a campfire, it just keep burning. To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat.