concept

Flammability limits

The range of concentrations over which a fuel-and-air mixture will still carry a flame. Too lean and there is not enough fuel to sustain the reaction; too rich and there is not enough oxidiser. The important part is that these limits are not constants of the fuel but depend on what else is in the air: adding an inert diluent — especially a triatomic molecule like carbon dioxide, which stores heat better than nitrogen does — drains the heat a flame front needs to propagate into the next layer of gas, narrowing the window from the rich end until it closes altogether. Inert-gas fire suppression works on exactly this principle.