CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O
Complete combustion of a hydrocarbon CₓHᵧ in excess oxygen produces carbon dioxide and water: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O. This is an exothermic reaction and the basis of energy production from fossil fuels. For example, propane (C₃H₈) combustion: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O releases −2220 kJ/mol. Incomplete combustion (limited oxygen) produces carbon monoxide (CO) or soot (C) instead of CO₂, which is both wasteful and toxic. The stoichiometric air-fuel ratio ensures complete combustion — for gasoline (≈C₈H₁₈) this is about 14.7:1 by mass. Combustion calculations are essential in automotive engineering, power plant design, environmental science, and any process involving fuel burning.