uncombusted

Uncombusted is an adjective describing matter that has not undergone combustion, meaning it has not been burned or oxidized by exposure to an oxidizer such as oxygen. In chemical and engineering usage, it typically refers to fuels, vapors, or other hydrocarbons that remain in their original chemical state rather than converting to combustion products such as carbon dioxide and water.

In combustion systems, uncombusted material can result from incomplete combustion. This occurs when there is insufficient oxygen, too low flame temperature, inadequate mixing of fuel and air, or insufficient residence time for the reaction to proceed. The unburned portion may appear as droplets, vapors, or low-temperature gases and is often accompanied by partially oxidized species and sometimes soot formation.

In vehicle and industrial emissions, uncombusted hydrocarbons (UHCs) are a common concern because they contribute to smog and can be hazardous in high concentrations. They are typically studied in the context of tailpipe emissions, gas burners, and aircraft engines, where optimizing air–fuel ratio and residence time reduces losses.

Mitigation strategies include improving combustion efficiency through precise control of air flow and fuel delivery, maintaining flame stability, and using aftertreatment technologies such as catalytic converters or oxidizers to convert remaining hydrocarbons to harmless products.