What is a common indicator of incomplete combustion?

Incomplete combustion occurs when there is not enough oxygen available to combust the fuel fully, leading to the production of byproducts such as carbon monoxide. One of the most visible signs of incomplete combustion is the presence of yellow or orange flames rather than the desired blue flames.

Blue flames indicate efficient combustion, where the fuel is burning completely and producing minimal byproducts. In contrast, yellow or orange flames signify inefficiency and the production of soot or other materials as part of the combustion process. This change in flame color can be attributed to the presence of unburned carbon particles and the generation of carbon monoxide, which is a harmful gas.

The other options represent different characteristics that are not direct indicators of incomplete combustion. For instance, a consistent burning smell might indicate normal operation or other issues but does not directly reflect incomplete combustion. High notes of gas odor could imply a gas leak, while blue flames are a strong indicator of complete combustion, which means the fuel is burning efficiently. All these aspects highlight why yellow or orange flames serve as the key signal of incomplete combustion.