The combustion process typically involves the burning of a [[Hydrocarbon|hydrocarbon]] along with on [[Oxidizer|oxidizer]]. Most commonly the oxidizer used is atmospheric [[Air|air]], which consists of 21% oxygen and 79% nitrogen on a [[Mole|molar]] basis. As such, for every mole of oxygen in air there are a matching 3.76 moles of nitrogen. The presence of nitrogen in air can become a problem at high [[Temperature|temperatures]] where it can form harmful [[Substance|substances]] with other reacting chemicals. The general form for a combustion reaction with air is as follows: $C_xH_y+a(O_2+3.76N_2)\to bCO_2+cH_2O+3.76aN_2$ The moles of air required per mole of fuel is therefore: $n_{air}=4.76(x+0.25y)$ The air-fuel ratio can be defined in terms of moles or [[Mass|mass]]: $AF_{mass}=\dfrac{m_{air}}{m_{fuel}}$ $AF_{mol}=\dfrac{n_{air}}{n_{fuel}}$ $AF_{mass}=AF_{mol}\dfrac{M_{air}}{M_{fuel}}$ **Theoretical Air:** The amount of air required to theoretically complete the combustion process. **Excess Air:** The amount of extra air required to actually complete the combustion process. Can also be used to control the reaction temperature.