When using a foam extinguisher on a Type B fire, what is its primary function?

The primary function of a foam extinguisher on a Type B fire is to smother the fire. Foam creates a barrier between the fuel and the oxygen in the air, effectively suffocating the flames. This separation crucially interrupts the combustion process, which requires three elements: heat, fuel, and oxygen. By cutting off the oxygen supply, the foam prevents the fire from continuing to burn.

While a foam extinguisher may also have secondary effects such as cooling (reducing heat) or helping to absorb some heat, its main purpose in the context of Type B fires—typically involving flammable liquids like gasoline or oil—is to smother and extinguish the flames.