Editing Combustion (section)

===Trace combustion products===

Various other substances begin to appear in significant amounts in combustion products when the [[Adiabatic flame temperature|flame temperature]] is above about {{val|1600|ul=K}}. When excess air is used, nitrogen may oxidize to {{chem|link=nitric oxide|NO}} and, to a much lesser extent, to {{chem|link=nitrogen dioxide|NO|2}}. {{chem|link=carbon monoxide|CO}} forms by [[disproportionation]] of {{CO2}}, and {{chem|link=hydrogen|H|2}} and {{chem|link=hydroxyl radical|OH}} form by disproportionation of {{H2O}}.

For example, when {{val|1|ul=mol}} of [[propane]] is burned with {{val|28.6|ul=mol}} of air (120% of the stoichiometric amount), the combustion products contain 3.3% {{chem|O|2}}. At {{val|1400|ul=K}}, the [[Chemical equilibrium|equilibrium]] combustion products contain 0.03% {{chem|NO}} and 0.002% {{chem|OH}}. At {{val|1800|ul=K}}, the combustion products contain 0.17% {{chem|NO}}, 0.05% {{chem|OH}}, 0.01% {{chem|CO}}, and 0.004% {{chem|H|2}}.<ref name="EquiWeb">{{cite web |url=http://www.crct.polymtl.ca/equiweb.php |title=Equilib-Web |date=8 March 2022 |first1=Christopher W. |last1=Bale |first2=Eve |last2=Bélisle |access-date=15 May 2023 |publisher=Centre for Research in Computational Thermochemistry, Polytechnique Montreal}}</ref>

[[Diesel engines]] are run with an excess of oxygen to combust small [[Particle|particles]] that tend to form with only a stoichiometric amount of oxygen, necessarily producing [[NOx|nitrogen oxide]] emissions. Both the United States and European Union [[Emission standard|enforce limits]] to vehicle nitrogen oxide emissions, which necessitate the use of special [[catalytic converter]]s or treatment of the exhaust with [[urea]] (see [[Diesel exhaust fluid]]).