Which condition leads to incomplete combustion?

• A. Limited supply of oxygen
• B. Excess oxygen
• C. High fuel quality
• D. Low temperature

Answer: (A)

Incomplete combustion happens when there isn’t enough oxygen to fully oxidize the fuel. With limited oxygen, carbon in the fuel can only partially oxidize, forming carbon monoxide or carbon (soot) instead of carbon dioxide, and the flame often looks smoky. Because the oxidizer is scarce, you don’t get the full combustion reaction that produces CO2 and H2O, so less energy is released overall. If oxygen were plentiful, combustion would proceed to completion, producing CO2 and H2O. The other factors listed don’t determine completeness in this way: excess oxygen promotes complete combustion, while high fuel quality or low temperature don’t by themselves cause incomplete combustion—the key is the amount of oxygen available.