The Stoichiometric Fuel/Air Mixture

An "ideal" fuel/air mixture in which both the fuel and the oxygen in the air are completely consumed is called the "stoichiometric" mixture.  The stoichiometric mixture is usually specified as the ratio of the mass of air to the mass of a particular fuel.  For example, if a fuel required 10 kg of air to precisely burn 1 kg of the fuel (that is, all the fuel burned and there was no oxygen left in the air afterwards) then the stoichiometric ratio for that fuel would be 10 : 1.

Chevron give the stoichiometric ratio of air mass to fuel mass for gasoline as 14.7 : 1, but don't specify the composition of this hypothetical gasoline.

From http://www.faqs.org/faqs/autos/gasoline-faq/part2/ comes the following table of typical stoichiometric air-fuel ratios:
 

Fuel

Stoichiometric air-fuel ratio

methanol

6.4

ethanol

9.0

MTBE

11.7

ETBE, TAME

12.1

gasoline (without oxygenates)

14.6

An oxygenated gasoline with 89% gasoline and 11% MTBE would then have a stoichiometric ratio of 14.281.


Last edited June 24, 2002.  All material Copyright © 2001, 2002 Graham Davies.  No liability accepted.