An onboard inert gas generation system includes an evaporator comprising a vessel that receives a hydrocarbon fuel from a fuel tank, separates vapor fuel components from liquid fuel components, establishes a nearly constant fuel vapor composition, and outputs the fuel vapor to be mixed with air prior to combusting the fuel vapor and air mixture in a catalytic reactor. Water is separated from the inert gas produced and the inert gas is introduced into the ullage space of the fuel tank to prevent or reduce possible hazardous conditions in the fuel tank.