gas Stoichiometric air-to-fuel ratios of common fuels In the combustion reaction, oxygen reacts with the fuel, and the point where exactly all oxygen is consumed and all fuel burned is defined as the stoichiometric point. With more oxygen (overstoichiometric combustion), some of it stays unreacted. Likewise, if the combustion is incomplete due to lack of sufficient oxygen, fuel remains unreacted. (Unreacted fuel may also remain because of slow combustion or insufficient mixing of fuel and oxygen – this is not due to stoichiometry). Different hydrocarbon fuels have different contents of carbon, hydrogen and other elements, thus their stoichiometry varies. Note that oxygen makes up only 20.95% of the volume of air, and only 23.20% of its mass. The air-fuel ratios listed below are much higher than the equivalent oxygen-fuel ratios, due to the high proportion of inert gasses in the air. Gasoline engines can run at stoichiometric air-to-fuel ratio, because gasoline is quite volatile and is mixed (sprayed or carburetted) with the air prior to ignition. Diesel engines, in contrast, run lean, with more air available than simple stoichiometry would require. Diesel fuel is less volatile and is effectively burned as it is injected. See also Non-stoichiometric compound Biochemical systems equation References Zumdahl, Steven S. Chemical Principles. Houghton Mifflin, New York, 2005, pp 148–150. Internal Combustion Engine Fundamentals, John B. Heywood External links Engine Combustion primer from the University of Plymouth Free Stoichiometry Tutorials from Carnegie Mellon's ChemCollective Stoichiometry Add-In for Microsoft Excel for calculation of molecular weights, reaction coëfficients and stoichiometry. Reaction Stoichiometry Calculator a comprehensive free online reaction stoichiometry calculator. Stoichiometry Plus a stoichiometry calculator and more for Android. Chemical reaction engineering