Combustion Introduction
Combustion is an exothermic reaction that involves a kind of fuel and oxidants, and in the process, producing heat and light. A complete combustion process involves complete oxidation of compounds in the mixture; i.e. fuel and air.
A complete combustion will yield products like carbon dioxide and water. If sulphur is present, sulphur dioxide may be formed too. Nitrogen, gas that makes up the most of air's composition, will also be present. Therefore the products of complete combustion, are, by principal, oxygen, water (g), nitrogen and carbon dioxide. Theoretically, the reactions of oxygen and nitrogen, forming noxious oxides, are excluded in approximate calculations, due to the minute concentration levels in the reaction. Practically, an engine runs at a compression ratio low enough to reduce noxious oxide formation. Carbon monoxide may be formed too, as this is resulted from a lack of oxygen for complete combustion.
Combustion only applies to fuels in the gas phase, not in the liquid phase. The only event that the liquid starts to burn is at the flash point. The flash point is the lowest temperature possible for combustion to occur in fuels.
The temperature of combustion may be obtained by equating the difference of heat of combustion and the heat loss by conduction and radiation to the internal energy change in the mixture at constant volume; or the enthalpy change in the mixture at constant pressure. Reactions are thermodynamically favourable for high temperatures.
The amount of air needed to fulfil complete combustion is determined by dividing the ratio of the relative molecular mass of oxygen to the relative molecular mass of the furl by the percentage coefficient of oxygen from air. Typical composition is approximately 0.232 of oxygen and 0.768 of nitrogen by mass; or 0.21 of oxygen and 0.71 of nitrogen by volume.
Combustion can get unstable as components are prone to volatile pressure oscillations in combustion chambers. These oscillations are detrimental to the serviceable period of engine components, as the components are imposed with cyclic pressure and thermal loads. Droplet size and distribution of the fuel in engines play an important part of reducing combustion instabilities. Many ground-based engines are prone to emitting noxious oxides and this problem can be circumvented by lowering the combustion temperature, however, the combustion lean can make engines more susceptible to combustion instabilities. The Rayleigh index is a measure of thermoacoustic combustion instabilities over one cycle of instability, with the Rayleigh criterion indicating the severity of the instability.
Some of the common fuels are methane, ethane, propane, acetylene, ethylene, benzene, toluene, methyl alcohol and ethyl alcohol.
