A combustion reaction is an exothermic chemical reaction between a fuel and oxidant that produces heat and conversion of chemical species, which can result in the production of light in the form of glowing or flame. The fire triangle is a model used to show the three elements necessary for most fires: oxygen, heat, and fuel. A combustion engine uses the combustion of fuel and oxidizer in its combustion chamber to produce expanding gases that directly apply force to engine components to generate mechanical energy.