 Welcome back Mechanical Ear. Did you know that the theoretical and actual combustion process make up majority of the new miracles in combustion? This makes us wonder, what is theoretical and actual combustion? Before we jump in check out the previous part of this series to learn about what combustion is. Now, theoretical or stoichiometric combustion is the ideal combustion process where fuel is burnt completely. A complete combustion is a process burning all the carbon to carbon dioxide, all the hydrogen to water, and all the sulphur to sulphur dioxide, with unburnt components in the exhaust gases such as carbon, hydrogen and carbon monoxide. The combustion process is incomplete and not stoichiometric. To determine the excess air or excess fuel for a combustion system, we start with the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct. When burned, all fuel and air is consumed without any excess leftover. In actual combustion process, however, the theoretical quantity of air is not enough. So, it is common practice to use more air than the stoichiometric amount to increase the chances of complete combustion or to control the temperature of the combustion chamber. The amount of air in excess of the stoichiometric amount is called excess air, with actual combustion process it is impossible to predict the combustion of the products on the basis of mass balance alone. Hence, we first saw where the theoretical combustion is and then went on to see what actual combustion is.