Combustion Stoichiometry

Burning one gallon of gasoline (about 2.7 kg) produces roughly 8.9 kg of CO2. The exhaust weighs more than three times the fuel. That is not a measurement error. It is stoichiometry: the mass balance of chemical reactions.

Every carbon atom in a fuel produces exactly one molecule of CO2 when it combusts completely. Carbon has an atomic mass of 12; each CO2 molecule has a mass of 44 (12 + 16 + 16), because two oxygen atoms from the atmosphere attach to the carbon. The 3.67x mass multiplier (44/12) is fixed by atomic weights. No engine redesign or combustion improvement can change it. This ratio is why fuels with different hydrogen-to-carbon ratios produce different amounts of CO2 per unit of energy.

If every carbon atom carries the same 3.67x mass tax, why do fuels differ in CO2 per unit of energy?

Find the hydrogen bonus. Hydrogen-rich fuels release part of their energy by forming water, which carries no carbon penalty. Methane gets about a quarter of its heat from hydrogen; coal, nearly none. So per unit of energy delivered, gas emits far less. The EIA's standard emission factors put natural gas at about 50 kg CO2 per GJ and bituminous coal at about 88. No engine redesign moves either number; the floor is set by the molecule.

This stoichiometric floor is why efficiency improvements alone cannot eliminate combustion emissions. The only exits are fuel switching, or capturing CO2 after it forms.

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