What Is Combustion?

Combustion is a rapid, exothermic chemical reaction between a fuel and an oxidizer that produces heat, light, and a range of product species including carbon dioxide, water vapor, and, depending on the reaction conditions, pollutants such as nitrogen oxides and unburned hydrocarbons. As a field of study, combustion spans thermodynamics, chemical kinetics, fluid mechanics, and heat transfer, and it underpins the design of most energy conversion systems in use today, from internal combustion engines and gas turbines to industrial furnaces and rocket propulsion. The discipline is organized around understanding the rates at which fuel molecules oxidize, the conditions that sustain or extinguish a flame, and the composition of the exhaust gases that result.

Combustion research addresses both fundamental science and engineering application. On the fundamental side, detailed chemical kinetic mechanisms describe the hundreds of elementary reactions that convert, for example, a methane molecule into CO2 and H2O through a chain of intermediate radicals. On the engineering side, those mechanisms inform computational fluid dynamics models used to design burners that maximize energy output while minimizing pollutant formation.

Flame Structure and Chemistry

Flames are broadly classified as premixed or diffusion, depending on whether the fuel and oxidizer are mixed before ignition or mix at the reaction zone. Premixed flames, such as those in a household gas burner or a spark-ignition engine cylinder, burn at a well-defined propagation speed called the laminar burning velocity and are susceptible to flashback if the flow velocity is too low. Diffusion flames, common in diesel engines and candle flames, are limited in their reaction rate by the rate at which fuel and oxidizer diffuse toward each other. NIST research on chemical kinetics and fire provides detailed kinetic models for both flame types, covering the elementary reaction steps that determine burning rate and flame temperature. The temperature at which a flame stabilizes depends on the equivalence ratio, the ratio of the actual fuel-to-oxidizer proportion to the stoichiometric proportion required for complete combustion.

Combustion Diagnostics and Sensing

Measuring the internal state of a flame requires non-intrusive optical and electrical diagnostics that do not disturb the reaction zone. Laser-induced fluorescence (LIF) and Raman spectroscopy provide species concentration maps inside laboratory flames, while ion current sensing measures the ionization level in the combustion zone to infer flame state in practical engines and industrial burners. The Combustion Institute coordinates international research in these diagnostic areas, and real-time flame monitoring using ion current sensors has demonstrated applicability to power plant boilers, vehicle engines, and aviation gas turbines, providing feedback for combustion control loops. Exhaust gas sensors that measure oxygen, carbon monoxide, and NOx concentrations provide post-combustion data that closed-loop engine management systems use to optimize ignition timing and fuel injection.

Exhaust Emissions and Environmental Impact

The exhaust gases from combustion carry the direct environmental and health consequences of energy conversion. Nitrogen oxides, formed when high temperatures cause atmospheric nitrogen to react with oxygen, contribute to urban smog and acid deposition. Particulate matter, produced in diffusion flames where fuel-rich zones promote soot formation, is linked to respiratory disease. Carbon dioxide from hydrocarbon combustion is the primary anthropogenic greenhouse gas. The Combustion Research Facility at Sandia National Laboratories conducts foundational research on soot formation pathways, NOx reduction strategies, and the combustion of alternative fuels intended to reduce net carbon emissions.

Applications

Combustion has applications in a range of fields, including:

• Power generation in thermal power plants using natural gas, coal, or oil
• Aircraft and rocket propulsion through gas turbines and liquid-fueled engines
• Automotive internal combustion engines and diesel systems
• Industrial process heating in metallurgy, glass, and cement manufacturing
• Fire safety research and suppression system design