# Stoichiometric fuel/air ratio

#### mohhasco

##### New Member
what is the meaning of "stoichiometric fuel/air ratio"? is it a desired ration or something measured in certain unit or what?

• #### Kelly B

##### Senior Member
Hello and welcome,
Did you find it in a sentence? Could we please see the sentence, if you did?

#### mohhasco

##### New Member
Kelly B said:
Hello and welcome,
Did you find it in a sentence? Could we please see the sentence, if you did?

In fact, there is an equation that has this term in it and they define it as stoichiometric fuel/air ratio. the equation is modeling the NOx emission in an engine.

#### mohhasco

##### New Member
In fact I am concerned about the meaning of "stoichiometric" that fits this contest

#### panjandrum

##### Lapsed Moderator
I think, but you would need to confirm this from a textbook, that a stoichiometric ratio for any pair of substances is the ratio with which they mix in an idealised reaction.

So a stoichiometric air/fuel ratio is such that all the fuel burns and all the oxygen in the air is used.

Could be completely wrong.

#### panjandrum

##### Lapsed Moderator
Confirmed HERE.

It's important in the context of emission control because too little air results in fuel in the exhaust; too much air results in a different set of combustion compounds - and reduced efficiency.

#### panjandrum

##### Lapsed Moderator
Air consists, mostly, of oxygen and nitrogen.
Fuel is a hydrocarbon.

In complete combustion of the appropriate quantities of fuel and air (in the stoichiometric ratio), all the carbon and hydrogen from the fuel combine with all the oxygen in the air to form CO2 and H2O. The nitrogen in the air goes in one end and out the other.

Unfortunately there are lots of reasons why this cannot be achieved in practice, with the result that the products of combustion include the above plus unburnt fuel, CO, and various oxides of nitrogen - NOx .

#### DaleC

##### Senior Member
Yes, it's the optimum ratio. The key concept is definite proportions. Chemical reactants combine with each other in definite proportions. The composition ratio of a mixture of reactants is stoichiometric (for a given reaction) when the relative proportions of each match the coefficients in the reaction equation.

Stoichiometry is a term in beginning chemistry. In addition to all the other links provided so far, you can check a chemistry textbook or search the Web for <chemistry "stoichiometric ratio">.

When the mixture's composition ratio is not stoichiometric, there is a relative deficiency (excess) of some reactants. Therefore, a given volume of mixture won't react (be consumed) totally, there you won't get the maximum possible reaction outcome (whether that be heat or a product chemical), which is an inefficient state of affairs.