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Understanding AFR: Stoichiometric AFR

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Stoichiometric AFR


00:00 Every fuel has an ideal air fuel ratio where enough fuel molecules are provided to properly mix with all of the available oxygen molecules.
00:09 This provides theoretically ideal combustion where all of the air and fuel is consumed during the combustion process.
00:18 This ratio is known as the stoichiometric air fuel ratio and it is dependent on the chemical composition of the specific fuel.
00:27 For normal pump fuel, this ratio is 14.7:1 which means that for 14.7 kilograms of air, we need to supply 1 kilogram of fuel.
00:38 A table is included as a file below which gives you the stoichiometric ratio for a range of the more common fuels you are likely to see.
00:47 When we discuss air fuel ratio, we often use the terms rich and lean to describe how the engine is running.
00:54 A rich mixture is defined as one which has an excess of fuel, while a lean mixture is defined as one that has an excess of oxygen.
01:04 Rich and lean are both referenced to the stoichiometric AFR of the particular fuel.
01:10 For example if we are tuning on pump fuel, numbers higher than 14.7:1 are considered lean and numbers lower than 14.7:1 are considered rich.
01:22 Another term you may hear in tuning circles to describe a rich mixture is ‘fat’.

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